UNIV

C. LIBRAf

00126 2515

'^■••A'

OM POSITION

^W«m!^&

SIOKAGE ITiiAJ
PROC£SSI]^G-CN£

l.pl~F19G

U.B.C. LIBRARY

Acctffii^tun i!Cff. ^

iU

il\xli Kxt. bF Z^h ^G

MILK

ITS XATUEE AXD COMPOSITION

Cow's Udder (after Fiirstenberg). — The left side of the udder of a Dutch cow. The skin has been
removed to show the course of the superficial arteries and veins, as also the IjTnphatic vessels
and nerves, a, External pudic artery ; h, external pudic vein ; c, branch of artery supplying
the posterior part of railk-gland, the lymphatic gland, etc. ; r7, posterior mammary artery,
posterior artery of the milk gland ; c, continuation of external pudic artery, ending in the
skin and subcutaneous connective tissue near the breast bone ; g, large vein from posterior
part of mammary gland, from the lymphatic gland, the pudenda, etc. ; h, posterior mammary
vein ; I, lymphatic gland situated on the posterior outer surface of the mammary gland ; m,
peritoneal or milk vein (vena subcutanea abdominis) ; o, anterior mammary vein ; r, a lym-
phatic vessel recei\ing part of the superficial mammary lymphatics ; t, mammary nerve
formed by the union of the posterior branch of the iliac nerve (nervus ilio-hypogastricus) and
a branch of the external spermatic nerve ; v., posterior dinsion of this nerve ; x, continuation
of (, ending in the skin at the navel.

MILK

ITS NATUEE AND COMPOSITION

A HAXD-BOOK OX THE

CHEMISTRY AND BACTEEIOLOGY OF MILK.
BUTTEE, AXD CHEESE

BY
C. M. AIRMAN, M.A., D.Sc.

LONDON

ADAM AXD CHAELES BLACK

1895

PEEFACE

Much interest has been evoked during the last few
years in dairy education. This has manifested itself
in the institution of a number of dairy schools in
various parts of the country. Unfortunately, the
literature available to English readers on the science
of dairying is still of a very limited and imperfect
nature. Most of it deals rather with the art or
practice of dairying than with the scientific side of
the question.

The aim of the present volume is to give a short
popular statement of the more important facts of the
Chemistry and Bacteriology of Milk.

Xo attempt is made to deal with the practice of
butter and cheese-making ; but it is hoped that the
scientific principles (so far as they are known) under-
lying these processes are stated in such a manner
that they may be of assistance in explaining the
operations of the dairy.

viii MILK

It is hoped that the work may possess interest for
the general reader as well as for the farmer and
student of agriculture, as milk is such a widely used
article of diet, and exercises such an important
influence on public health as a propagator of disease.

The science of dairpng is largely dependent on
the science of bacteriology, and this latter science is
as yet in its infancy. As, however, the strides at
present being made in the study of bacteriology are
enormous, — as witness the fact that between the
years 1890 and 1892 some 1,013 papers on the
subject were published, — we may hope that the
mystery still surrounding many problems in dairying
will soon be dispelled. Bacteriology has already
done much for the butter-maker in furnishing
him with " pure cultures " of bacteria, and enabling
liim to secure a more uniform product than was
hitherto possible. These pure cultures are not
as yet used, so far as the Author is aware, in
this country, but their use is now general in such
countries as Denmark, Germany, Sweden, and
Holland. It is much to be desired that our farmers
should not be behind the foreigner in this matter.

Readers interested in the practice of dairying are
referred to the Em^lish edition of Professor Fleisch-

PREFACE IX

mann's elaborate treatise on Tlie Science and Practice
of Dairying (Blackie), translated and edited by
Professor Wright and the Author.

The sources from which the Author has drawn are
too numerous to admit of detailed acknowledgment.
At the end of the book a short list of some of the
books and treatises on the subject are given. He
would, however, take this opportunity of specially
acknowledging his indebtedness to the works of
rieischmann, Kirchner, Ftirstenberg, Duclaux,
Freudenreich, and Grotenfelt, and the numerous
valuable Bulletins issued from time to time by the
United States Government. Too great praise cannot
be paid to the last-named Government for the en-
lightened policy which they pursue with regard to
the circulation of valuable Bulletins, dealing with
various aspects of agricultural science.

For the illustrations the Author is indebted to the
writings of Dr. Fiirstenberg, Professor Elrchner, Pro-
fessor P. F. Frankland, Dr. James Bell, F.E.S., and
Dr. Freudenreich.

Analytical Laboratory,
128 Wellington Street, Glasgow,
September 1895.

CONTEXTS

CHAPTEE I

The Structure of the Cow's Udder, and the Secretion

of Milk

Definition of Milk — Structure of the Udder — The Formation of
Milk Page 1

CHAPTER II
The Percentage Composition of Cows' Milk

Constituents of Milk — Percentage Composition of Milk — Variation
in Composition of Milk ..... Page 9

CHAPTER III
The Constituents of Milk

Milk Fat — The Condition of the Fat Globules in Milk — Chemical
Composition of Milk-Fat — The Albuminoids of Milk — Casein —
Albumin — Lactoglobulin — Lactoprotein — Milk-Sugar — Ratio
of the different Constituents of Milk — Mineral Constituents —
Gases in Milk — Reaction of Milk — Sheep's, Goats', and Mares'
Milk — Specific Gravity of Milk — Colostrum or First Milkings

Pase 18

xii MILK

CHAPTER IV

Causes and Conditions Influencing the Quality and .
Quantity of Milk

Causes influencing Quantity of Milk Secreted — Difierence in
Morning and Evening Milk — Conditions influencing the Quality
of Milk — Period of Lactation — Food — Influence of Excitement
— Illness Page 42

CHAPTER V
The Changes which Milk Undergoes

Creaming of Milk — Souring — Amphoteric Reaction — Coagulation
of Milk — Effect of Heat — MUk Faults — Means of preventing
Changes in Milk — Preserved Milk . Page 53.

CHAPTER YI
The Bacteria of Milk

Occurrence of Bacteria — Contamination of Milk by Micro-organisms
— Importance of a Kno^vledge of Bacteriology to the Farmer —
Description of the Different Micro - organisms — Method of
Development of Bacteria — Size of Bacteria — Conditions in-
fluencing the Development of Bacteria — Number of Bacteria
in ^lilk — Classification of Bacteria infesting Milk — Bacteria of
a Useful Nature — Bacteria of an Indirectly Injurious Nature
— Bacteria of a Directly Injurious Nature — Blue Milk — Red
Milk— Yellow MQk— Slimy or Ropy Milk— Bitter Milk-
Lactic Fermentation of Milk — Butyric Fermentation — Casein
Ferments — Alcoholic Fermentation — Kephir — Nature of
Kephir Fermentarion — Koumiss — Pathogenic Germs — Bacillus
of Tuberculosis — Cholera Bacillus — Typhus Bacillus — Methods
of Destroying or Regulating Bacterial Life in Milk — Sterilisa-
tion of Milk — Pasteurisation of Milk — Importance of Cleanli-
ness in Handlinor Milk — Chemical Agents . . Pacje 63

COXTENTS xiii

CHAPTER VII
Butter — Importance of Bacteria for Butter-Making

Objects of Churning — Conditions inflnencing the S-;r aratior: of Fat
— Centrifngal Separators — Preliminary Souring of Milk or
Cream before Churning — Conditions influencing Churning —
The Bacteria of Butter — Aroma and Flavour of Butter —
Xumber of Bacteria in Butter — Influence of Lactation, Breed,
Age, and Food on the Quality of Butter — Chemical Composi-
tion of Butter — ]Margarine .... Page 114

CHAPTER VIII
Rennet and its Action

Occurrence of Eennet Ferment — Actlre Principle of Bennet — Coaga-
lating Power of Eennet — Diflierence between Hennet-Cnrd
and Acid-Curd — Action of Rennet — Forms in which Bennet
is Used Page 137

CHAPTEP IX
Cheese

Important R61e of Bacteria in Cheese-Making — Conditions deter-
mining Quality of Cheese — Bacteria in Cheese — Chemical
Changes Cheese undergoes during the Ripening Process —
Cheese-Faults Page 148

CHAPTER X
Testing of Milk

Testing Fat by the Amount of Cream — Optical Methods of Deter-
mination of Fat — Determination of the Fat by Churning —

XIV MILK

Determination of the Fat by the Addition of Reagents —
Determination of the Specific Gravity — Formulae for Calculat-
ing the Composition of ^lilk from certain Data — Chemical
Analysis of Milk Page 157

CHAPTER XI
Milk as a Food

Classification of Food Nutrients — Functions of Food — Digestibility
of Food Nutrients — The Food Value of Milk — Digestibility of
Milk— Effect of Boiling on Milk— Suitability of Milk as a
Food — Value of Butter and Cheese as Food — Milk as a Food
for Invalids Page 164

Appendix Page 175

Index Page 177

CHAPTEE I

Ttte Struchtre of the Coics Udder, and the Secretion

of Milk

Definition of Milk. — Milk may be described as the
secretion of the mammary glands of the female
mammal. It is a fluid which is secreted for a longer
or shorter period after giving birth.

Before dealing with the native, composition, and
properties of milk, it will be well to say a word or
two on the mode in which milk is secreted, and on
the structure of the mammary glands. As we are
here concerned almost entirely with cows' milk, it
will be sufficient for our purpose to deal with the
structure of the cow's udder.

Structure of the Udder. — The cow's udder, of
which a diagram is given as the frontispiece of this
book, consists of two milk-glands, — a right and a left,
— which, in the adult cow, vary from about 9 to 12
inches in length, 6 to 12 inches in breadth, and 4 to

-^ I

*

f

MILK

[I

8 inches in depth (Fleischmann). These milk-glands
are provided with outlet tubes, which are commonly
known as the tmts. As a rule, each milk -gland is
provided with two teats, but sometimes with three ;
the third, when present, however, being in what is
scientifically termed a "rudimentary form." The
two glands are separated from one another by a

Fig. 1. — Gland Lobules. Magnified 60 times, e, Outlet tube.
(Ftirstenberg.)

fibrous partition, running longitudinally, one of the
chief functions of which is to support the udder.
Each gland may be described as made up of a group
of glandular structures, of different sizes, which are
known as lobes, lohules, and alveoli, and each of
which is smaller than the preceding one. The size
of the alveoli varies from -0047 to '0078 of an inch
in length, and from '0035 to "0043 of an inch in
breadth (Fleischmann).

I]

STRUCTURE OF THE COIF'S UDDER

The lobes, lobules (see Figs. 1 and 3), and alveoli
(see Figs. 2 and 4) are provided with excretory
ducts of corresponding size, which flow into the

Fig. 2. — Gland Alveoli. Maguiiied 200 times. (Fitrsteuberg.)

so-called milk -cisterns or milk - reservoirs, y^hioh. qxq
situated immediately above the teats. When a duct,
therefore, is traced into the gland, it will be found to
become sub-divided into smaller ducts, and these, in
their turn, into still smaller ones ; while round the

MILK

[r

smallest of the ductlets are clustered several alveoli.
Of these milk-cisterus there are four, one above each
teat. They thus act, as their name indicates, as
cisterns for supplying the teats with milk.

The alveoli or pouches above mentioned are sur-
rounded by a structureless membrane {tunica -propria),
the internal surface of which is thickly covered with

Fig. 3. —Half Diagrammatic View of a Sectiox through a Lobule of
THE Mammary Gland (after Klein, Atlas of Histology, Plate 40, Fig. 1).
Magnified 45 diameters, a, A duct dividing into two branches ; h, b, h,
connective tissue surrounding and going between the ultimate pouches of
the gland ; c, c, c, the pouches or alveoli of the gland, the dots representing
the cells lining them.

epithelial cells (see Figs. 5 and 6). The mem-
brane is further covered with a system of capillary
blood-vessels (see Fig. 7), which brings the blood
near the cells, and thus nourishes them. Each
alveolus possesses a minute cavity in its centre (see
Fig. 4), in which the cells or their products can
accumulate. When milk is not being formed

I]

STRUCTURE OF THE COW'S UDDER

the cells possess a granular appearance, and the
central cavity above referred to is small ; but during
the process of secretion the cavity becomes dilated.

The teats consist of two parts : the upper part,
which is knqwn as the hasis, and the lower, which is
known as the nrp-ple. The nipple of the teat con-

FiG. 4. — A portion of the mammary gland, magnified about 400 diameters,
showing one complete and two incomplete alveoli, o, a, a, Short, columnar
epithelial cells lining the alveolus, each having an oval or rounded nucleus ;
t, &, 6, epithelial cells containing next the interior of the alveolus a milk
globule ; c, c, c, milk globules which have been set free from epithelial cells.

tains numerous blood-vessels, and has at the base
muscular fibres (see Frontispiece).

It has been calculated that the two milk-glands
and the four milk-cisterns in the udder of an average
milk cow, when the udder is not in any way dis-
tended,— that is, as it would be after the cow has been
milked, — have a cubic capacity of from lOi to 11 J
pints.

MILK

[I

The Formation of Milk. — The exact method in
which milk is formed in the udder is, as yet, far from
having been clearly demonstrated. Two important
theories have been advanced. According to the
older one, which held sway in scientific circles up to
the year 1840, milk is formed directly from the
blood — it is, in fact, a sort of filtered blood. Accord -

Fig. 5. — Uxco>rsECTED Epithelial Cells.
Magnified 600 times, b, Protuberance rest"
ing on basement membrane ; /, nucleus.
(Fiirstenberg.)

Fig. C— Epithelial Cells.
Magnified 600 times, a,
Attached together ; b,
free cell. (Fiirstenberg.)

ing to this theor}', the milk-glands merely act as a
filter. The other theory regards milk as the product
of the decomposition of the epithelial cells. There
can be no doubt which of these two theories com-
mands most support from the data available. In the
first place, if milk be merely filtered blood, its
quantity and quality will be directly influenced by
the food of the cow. That this is not so, practical
men have long known. Xo doubt food has a certain

I]

SECRETION OF MILK

Fig. 7.— Capillary Blood-ve.ssels. Magnified
ISO times. (Fiir.stenberg.)

indirect influence on the quality of milk, as we shall
point out farther on ; so far, however, as we at
present understand the subject, the direct influence
of food, as has been again and again proved, — as, for
example, by changing the diet, — is insignificant in
extent. That milk is not filtered blood is further
supported by the
fact that none of
the organic con-
stituents of milk
are present in the
blood. Again, we
find the propor-
tion of the differ-
ent ash constituents in blood and in milk to be
different. In the former it is found that sodium salts
predominate ; while in the latter potassium salts are
more abundant. On the other hand, considerable
support to the correctness of the second theory seems
to be afforded by the fact that in colostrum, the name
applied to the milk yielded immediately after giving
birth, are to be found certain minute corpuscles, the
so-called colostrum corpuscles (corps granuleujc), which
exhibit traces of cell structure.

How far, however, even this second theory satis-
factorily accounts for the formation of milk is
doubtful. It may be, indeed, that the various con-
stituents of the blood and the lymph bodies, as well

MILK

as the substance of tlie gland cells, are all utilised in
the formation of the constituents of milk. With regard
to the fat of milk, at .any rate, it seems that Ft is
partly formed from the fat of the blood and partly
from the cell substance.

CHAPTEE II

The Percentage Corn/position of Milk

The composition of the milk of different animals is
practically the same, although a considerable varia-
tion occurs in the proportion in which its different
constituents are present.

Constituents of Milk. — The chief constituents
are icatei\fat, ijrotein, or albuminoid matter (of which
casein is by far the most abundant), milk-sugar, and
mineral matter. The variation in the percentage of
these constituents may, under exceptional cuxum-
stances, be considerable.

The following table, after Konig, gives the average
composition of the milk of different animals : —

Water. , Casein. Albumin. , Ash.

Milk-
sugar.

Fat.

Human

Cow

Ewe

Goat

Mare

Ass

Sow

Bitch

Buffalo

Camel

Cat

Mule

Lama

Elephant

Porpoise

87-41
87-17
80-82
85-71
90-78
89-64
84-04
75-44
81-41
86-57
81-63
91-50
86-55
67-85
41-11

1 -03
302
4-97
3-20
1-24
0-67

6-10
5-85

3-12

-23

4-00

1-64

3-00

1-26
0-53
1-55
1-09
0-07
1-55

5-05
0-25
)
5-96

0-90

3-09
11-19

0-
0-
0-
0-
0-
0-
1'
0-
0-
0-
0'

0-

0-
0-
0-

I-31

6^21

3-

•71

4^88

3^

-89

4-91

6^

-76

4^46

4-

•35

5^67

!•

•51

5-99

!•

-05

3-13

4^

•73

3-08

9^

-87

4-15

7"

-77

5-59

3-

•58

4^91

3-

-38

4-80

!•

•80

5-60

3-

•65

8-84

19^.

•57

1-33

45^^

•78
•69
•86
78
•21
•64
•55
•57
•47
-07
-33
•59
■15
•57
•80

K

10 MILK [ii

From the above it will be seen that a considerable
variation in the composition of the milk of different
animals exists, the milk of the porpoise being the
richest in solids, while that of the elephant comes
next ; whereas that of the mule contains the largest
percentage of water. The most wide variation, it
wdll be seen, is in the fat, the other constituents
being present in comparatively uniform amounts.

As w^e have said, considerable variation in the
composition of the milk of the same species of
animal may exist, and this is the case with regard to
cows' milk, with the composition of which we are
most familiar.

At first sight, therefore, it would seem rather
difficult to say exactly what amount of these different
constituents constitutes normal cows' milk.

The following tables, drawn up by diiBferent
authorities, illustrate this variation, and give, at the
same time, what is considered by their respective
authors to be the composition of average milk : —

M

Cm

O

o

V2

o

Pi

B

O

O

o

o

O

0

35

00^

^3,-=

"so

iJE

X
X

p

_-} 0 •>•

X

I— 1

^

33 -^ ^-
X »^ ;5

;= 0

Solids not Fat.

•—

^ —

—

II

-

^^

c 5

— .

0 L^t Ci

X

05 .—

^ X

"^ lt;

1—

-1-3 +3

^B

^

*S.2

~ — ~

^

^ ;-i

■^— '

s

0

s^

i^X ~ '^

w

X

X

3

C

<J

0 0

C u- 0

0

0

p p

re c; r'

0

0

S-i

0

r- rj<

CC '^

6

CO

f>;

X

0

I-H

<1

TZ

CC

0
a X

L^

ii

02 •■— 1

0 0

^ c 0

■~ 5^

-;-> -^

--i -^ -J

^^

!

' ^

•^ i

CC X

zc ^ -^

■^

s

^;>

X •

CM

(>i

^

dq

r^

»

_^

S

1— 1

tf.

L- —

^

I~~ Tt

c: ut r-

0

^

z,

X

CC -T

6

<N

^

0

^M

<

0

^ C

X ^f*

0

^ »h ?i

!-H

-"^ '^

0 0

000

^

"Z' .'J^

-iJ -^

-U -iJ -u

0

^

~ i-

■+J

—

•— zt

jp 1>

CO 0 0

^

— >

i?* G<1

CO

Ci

^

X

Ci

^^

y.

Cj

p:-

i

»c 0

000

0

0

Jt- rt

0 :p t^

0

(N

s-

0

J?' eo

ft -r-

0

oq

>

X

l-H

<i

^

• • ^

^

— i.

i.

^

^

i ^

■i^

OD

0

0

-+j

02

03

. . :=!

r^ .—

^ ^ •

r-r-l

0

r^

of

X.

5 a

'o

3
d ^ ^

u

^■6

4.3

^^Z

M

^■""v. -^

^iiJ.

0
H

o

8
^

i

00

l-H

o

s

Pi

12 MILK [li

With reference to the above figures, it may be
added that, as far as one ingredient is concerned,
viz. the fat, limits even wider than those above
cited have been published. Thus a sample of milk
has been found to contain only '25 per cent ^ of fat ;
while another sample has been found to contain as
much as 11-06 per cent.^ These are respectively the
lowest and highest percentages of fat found in
samples of genuine milk, so far as the author is
aware, that have yet been published.

The results above quoted may be summarised as
follows — the average composition being obtained by
averaging the results : —

Composition of Milk.

Average. Limits.

Water . . . 87-34 81 to 91

Total Solids . . 12-66 7-2 to 18-9

100-00

Fat . . . . 3-72 -25 to 11-0

" Solids not Fat ''■ ^ . 8-93 5-6 to 12-6

* Consisting of —

Casein and- Albumin . . . .3-594

Milk-sugar 4-614

Ash -730

Variation in Composition of Milk. — With

respect to the variation in the amounts of the
different constituents of milk, it is not to be inferred

1 See Milch-Zeitunrj 22 (1893), p. 804.
' See The Analyst (1893), pp. 1-12.

ii] PERCENTAGE COMPOSITION OF MILK 13

that such wide limits, as have been just indicated,
actually occur under normal circumstances. Indeed,
it need scarcely be pointed out that, if this were the
case, detection of the adulteration of milk would be
rendered a well-nigh impossible task. Where any
wide variation from what is above stated as the
average composition of milk is found, the conditions
under which such milk is obtained may be safely
assumed to be abnormal; for, under normal con-
ditions, the composition of milk is comparatively
uniform, with the exception, perhaps, of the amount
of fat. Furthermore, it must be borne in mind that
this uniformity of composition is rendered more prob-
able by the fact that milk sold for consumption is
generally " dairy " milk — that is, the mixed milk pro-
duct of several cows. Wide variation in the com-
position of such milk is not likely ever to occur. In
such a case, even supposing the milk of any single
cow should exhibit an abnormal composition, the
fact that it is mixed v;ith the milk of a number of
other cows, several of which may yield milk richer
in quality than average milk, serves to a large extent
to counteract the influence of the abnormal milk in
lowering the quality.

In forming an opinion as to the genuineness of a
sample of milk, the more important data to be taken
into account are the "total solids," the fat, and the
" solids not fat " ; and the Society of Public Analysts

14 MILK [II

has drawn up, as a result of a thorough consideration
of the whole subject, the following standards, which
express the minimum amount in Avhich these in-
gredients should be present in a genuine sample of
milk : —

Total Solids . . .11-5 per cent
Fat 3-0 „

Solids not Fat . . .8-5 „

In support of the reasonableness of the above
standards, it may be mentioned that Sir Charles
Cameron, M.D., the Public Analyst for Dublin, has
never found the total solids in mixed milk to fall
below 12 per cent. The practical importance of the
point is very great, since on it depends the efficient
working of that portion of the Sale of Foods and
Drugs Act which refers to the adulteration of milk :
a form of adulteration wdiich, we may add, is most
commonly practised. For example, if a sample of
milk should be found to contain only 2 per cent of
fat, on what grounds, it may be asked, would a Public
Analyst be justified in regarding it as adulterated, since
the analyses of milk known to be genuine have shown,
as we have above pointed out, an amount of fat less
than this ? A superficial consideration of the subject
seems to point to the conclusion that the condemning
of such a sample would be unjustifiable. Now, as
the law at present stands, there is no doubt that
there is something; to be said for such a contention.

ii] PERCENTAGE COMPOSITION OF MILK 15

Genuine milk containing such a low percentage of
fat is no doubt of very rare occurrence, yet it may
occur ; and not to take this into consideration may
seem to savour of injustice. But the question
arises : Ought it to make much difference, where the
quality of the milk is so poor, whether the sample
has been adulterated in the technical sense — that is,
been submitted to a watering process subsequent
to its being obtained from the cow's udder — or not ?
for it is obviously unfair that a person who expects
to obtain ordinary milk should be provided with
abnormal milk. A man sellino; such abnormal milk
should be treated very much in the same way as a
man selling adulterated milk. ]\Iilk, therefore,
should be defined under the Sale of Foods and
Drugs Act as the normal secretion of the mammary
glands of the cow. How far this is the interpretation
of the existing law on the subject is a question open
to opinion. Certainly, if this were more widely
recognised, many of the vexatious disputes which
the sanitary authorities too often experience in their
attempts to obtain convictions in the case of un-
doubtedly adulterated samples of milk would be
done away with. A satisfactory solution of the
difficulty would be effected by selling milk on the
basis of its analysis ; and this, there can be little
doubt, while accompanied by certain difficulties, will
be the method sooner or later adopted.

1 6 MILK [ii

The variation in the composition of the milk of
different cows is due to a number of causes : among
them may be mentioned the individuality and hrccd
of the cow, the iKviod of lactation, and the fcediivj.
Before, however, considering the conditions which
influence the quantity as well as the quality of milk,
a short description of its different constituents may
be criven.

Before concluding the Chapter, a list of some of
the standards adopted in some of the States of
America may be given. ]^Iilk showing a poorer
composition than the standard is regarded as
adulterated.

II]

PERCENTAGE COMPOSITION OF MILK

17

State, City, etc.

Percentage by Wei|

gbt of SoUds.

Solids not
Fat.

Fat.

Total SoUds.

iSTew York. Law 1893

9-00

! 3-00

12-00

Maine. Law 1893

9-00

3-00

12-00

Michigan. Law 1889

9-50

3-00

12-50

Iowa. Law 1892

■ 3-00

. . .

New Hampsliire. Law 1883

13-00

Ohio. Law 1889

9-38

312

12-50

Oregon. Law 1893 .

. . .

3-00

Vermont. Law 1888 .

9-25

3-25

12-50

Wisconsin. Law 1889

3-00

Pennsylvania. Law 1885 .

9-50

3-00

12-50

New York. Law 1884

9-00

3-00

12-00

New Jersey. Law 1882

9-00

3-00

12-00

Massaclinsetts. Law 1886 .

9-30

3-70

13-00

May and June

• • •

12-00

Minnesota. Law 1889

9-50

3-50

13-00

Columbus, Ohio .

9-37

3-12

12-50

Baltimore, Md. .

12-00

Denver, Col.

• • .

12-00

Lansing, Mich.

3-00

12-50

Madison, Wis.

3-00

Des Moines, Iowa

3-50

1313

Portland, Oregon

. . .

12-00

Omaha, Nebraska

3-00

12-00

U.S. Treasury Department .

9-50

3-50

13-00

Philadelphia, 1890 Ordinance

8-50

3-50

12-00

CHAPTEE III

The Constituents of Milh

The constituents of milk are fat, casein, albumin,
ladoglohidin, lacto])rotein, milk-siLgar, mineral matter,
certain gases in solution, as 'svell as minute traces of
otlier bodies.

Milk -Fat. — The fat may be considered as the
most important constituent of milk, not only
because its commercial value, as well as the com-
mercial value of one of its great products, — cheese, —
depend on it, but because it is the chief constituent of
the other and still more important milk product, butter.

The colour and opacity of milk are largely due
to the presence of its fat. It exists in the milk in
the form of minute globules, — known as the fatty or
milh globules, — so minute as to be quite invisible to
the naked eye. They are apparent, however, if we
submit a drop of milk to microscopic examination.
It will be seen from the accompanying diagram (see
Fig. 8) that they differ very considerably in their

Ill]

THE CONSTITUENTS OF MILK

19

size, tlie largest being about 6^ times the size of
the smallest. Careful investigations carried out by
Fleischmann have shown that the diameter of the
former measures "00039 inch, and fliat of the latter
'0000624 inch. Between these two sizes we have

.0"o,

fc^'

'o-t^c^Q

^^^o
%?^'^

fO a, "Oft ft

Fig. S. — Microscopic Appearance of Milk, containing 3"o per cent
of fat. (Kirchner.)

a large variety of globules. It would seem as if
some definite ratio existed between their size and
their number, the smaller ones being more abundant
than the larger ones. Indeed it seems probable that
the weight of all the globules of one size is equal to
the weight of all the globules of another size ; or, to

20 MILK [ill

put it in another way, the number of globules of
different sizes is in inverse ratio to their size. It is
also to be noted that the size of the fatty globules in
the milk of the same cow is not at all times, and
under all conditions, the same ; and probably even a
greater difference exists in those of the milk of cows
of different breeds.

Some conception of how numerous these minute
fat globules are in milk may be inferred from the
statement that it has been calculated that their
number in a kilogram of milk (about 2 J lbs. or IJ
pints), containing 4 per cent of fat, amounts to from
eighty thousand millions to twenty billions, according
as we take the globule to be of the largest or of the
smallest size ; or, to put it in another way, their
surface area would amount to from 25 square metres
(about 30 square yards) to 157 square metres (about
188 square yards).

While the opacity of milk is chiefly due to the
fatty globules, it is also due to the fact that a portion
of the mineral and nitrogenous matter is present in
a state of suspension. Milk in thin layers, however,
is not absolutely opaque, and it has been proposed
to test the percentage of its fat by taking advantage
of this fact and estimating its degree of opacity. A
thin layer of milk is examined, and the amount of
light it retards is noted. But this so-called " optical
method" of estimating fat is very unreliable, since

Ill] THE CONSTITUENTS OF MILK 21

it has been found that the same weioht of fat retards
more li^ht when it is in the form of small Q-lobules
than when it is in the form of larger globules ; so
that two samples of milk, containing equal amounts
of fat, but the fat in the one in the form of larger
globules than in the other, might give different
results when thus tested.

The Condition of the Fat Globules in Milk. — A
point of the very highest interest, and one which
has engaged considerable attention, is the condition
in which these fat globules are present in milk. It
was till comparatively recently believed that they
were surrounded by a solid albuminous membrane
which became ruptured when milk was churned.
This theory seemed to derive considerable support
from the behaviour of the fat under different cir-
cumstances. For one thing, it was found almost im-
possible to obtain fat from milk absolutely devoid of
nitrogenous matter. Another reason for believing in
the existence of a membrane was the isolation which
the fatty globules maintained in the milk. If there
is no albuminous envelope, why do the globules not
coalesce ? it may be asked. A further striking anomaly
seemed also to point to the existence of a membrane,
viz. that the fat is in a liquid condition, although
its temperature may be far below its melting-point.-^

^ A proof of the liquid condition of the fat in the globules at
even fairly low temperatures, viz. those approaching the freezing-

22 MILK [in

But all these seemingly plausible arguments in
favour of the existence of a membrane can be easily
explained on grounds which do not require the
assumption of a membrane. In the first place, the
difficulty of obtaining milk-fat free from albuminous
matter is due simply to the fact that minute
quantities of these bodies cling to the fatty globules,
and, although not forming any solid membrane, may
yet form a thin liquid envelope. Again, the isolation
of the fatty globules, although not so easily explained,
does not need the supposition of a membrane. It
may be explained by the semi-solid condition of the
casein and of a part of the mineral matter. According
to Babcock, milk, after being kept for a short time,
has formed in it a body resembling blood-fibrin.
This body has the effect of rendering the milk less
limpid, and at the same time helping to prevent
coalescence of the globules. A further argument in
support of the membrane theory is based on the
behaviour of milk towards ether, which, it is well
known, is an excellent fat solvent. If milk be
shaken up with ether, it is found that the ether
does not dissolve out the fat globules, as is proved
by the fact that there is no diminution in the opacity

point of water, is afforded by submitting them to microscopic
examination, when it will be seen that they maintain their globular
form. On solidifying them at a considerably lower temperature,
however, they will be seen to lose their globular form and assume
different shapes, due to solidification.

Ill] THE CONSTITUENTS OF MILK 23

of the milk. If, however, in addition to ether, a
potash or soda solution be added, the ether speedily
dissolves the fat, and the milk becomes much clarified.
According to the supporters of the membrane theory,
the alkaline solution dissolves the membrane and
permits the ether to act upon the fat. That this,
however, is not the true explanation has been
strikingly proved by Soxhlet, who found that milk
to which potash or soda solution had been added is
not clarified by the addition of benzene or chloro-
form— equally powerful fat solvents. It seems clear
that the peculiar action of ether is due to another
cause, which need not here be discussed. Lastly,
the liquid state of the fat in the globules may be
explained as due simply to their isolation, and
furnishes an example of what is chemically known
as superfusion. It is a well-known fact that any
liquid, if it is in a sufficiently fine state of division,
may be cooled down below its freezing-point without
becoming frozen. A sudden impact, however, will
convert it into the frozen state.

If, then, it may be said that the fatty globules have
an envelope, it can amount to nothing more than a
mere liquid envelope, such as the film of a soap-
bubble.

Chemical Com2Josition of Milk- Fat. — Milk -fat
differs from all other known kinds of fat in its
complex and highly characteristic chemical com-

24

MILK [iiL

position, and may be said to be characterised among
fats by its fine flavour. There can be no doubt, also,
that the fine state of its division in milk or cream
makes it, when taken in either form, more easily
digested than other fats. On the other hand, it is
more susceptible to decomposition than they are.

Fats are bodies made up of gli/cerides— organic
bodies, which present in their composition a certain
resemblance to the salts of inorganic chemistry. These
glycerides are composed of glycerin and a fatty acid}
Milk-fat, like most other fats, is chiefly made up of
three of these glycerides, viz. stearin, 'palmitin, and
olein, which together constitute about 91 per cent of
its composition. But what is especially characteristic
of milk-fat is the presence of some seven other fats,
viz. hutyrin, capronin, capnjlin, caprin, laurin,
myristin, and hutin. Of these, butyrin and capronin
predominate in amount, the^other five being present
in very small quantities. The melting-point of these
different glycerides varies ; that of stearin being 55'^ C.
(123' Fahr.), palmitin 62-8' C. (144-6" Fahr.), and
myristin 31= C. (88' Fahr.). All the other fats,
with the exception of butin and caprin, which
are present in very minute quantities, and which,
therefore, have little effect in influencing the

^ In butter-fat glycerin may be said to be present to the amount
of 4 "5 per cent, while the fatty acids are present to the extent of
9-4 '5 per cent.

Ill] THE CONSTITUENTS OF MILK 25

melting-point of butter, are liquid at ordinary
temperatures. Olein, indeed, only becomes solid at
the freezing-point of water. Since the proportion in
which the nine fats are present in milk-fat is a vari-
able one, its melting-point is also variable, and may
be stated at between 29' and 41' C. (84' to 106=
Fahr.) The nature of the fat is largely determined,
however, by the^ relative percentage of stearin,
palmitin, and olein.

The fatty acids which go to form the above fats
may be divided into . two classes, viz. those which
are insoluble and non- volatile — and this class is by
far the largest in amount, and includes palmitic,
stearic, oleic, butic, and myristic acids — and those
which are soluble and volatile. The respective
amounts of these two classes of fatty acids in milk-
fat, as we have just indicated, varies at different
times. Amoncr the conditions which influence their
amount is the stage of lactation, the age, breed, and
feeding of the cow. The percentage of volatile fatty
acids was found by Duclaux in nineteen samples of
butter-fat to vary between o"77 and 7'95 per cent,
-the average being 7*3. They consisted almost
entirely of butyric (4-58 per cent) and capronic
(2*7 per cent) acids. The presence of these volatile
acids is of importance, as it affects the quality of the
milk-fat and influences its flavour. It may be said
that the higher the percentage of fatty acids in the

26 MILK [hi

milk-fat, the better the quality of the butter. It has
been thought that the composition of the fatty-
globules of different sizes in milk varies, the fat in
the larger globules being of finer flavour than the fat
in the smaller globules (see Butter, Chapter YIL,
p. 114). In experiments carried out at Gottingen
by Goetz, the influence of breed on the size of the
fat globules has been investigated. Thus it was
found that in the milk of a Jersey cow their size
varied between -009 and -0042 millimetres ; in that of
an East Friesian cow (German breed) from '0063 to
•0021 millimetres ; and in that of a Simmenthal
cow (German breed) from -004 to '0027 millimetres.
With respect to the influence of food in causing a
variation in the percentage of the fatty acids, a
number of experiments have been carried out,^ which
go to show that this is considerable; while with
regard to the influence of lactation, it has been found
that the maximum percentage offatty acids in the milk-
fat occurs in from five to seven days after calving.

It may be added, in conclusion, that the colour of
the milk-fat varies from white to yellow. If pure
milk-fat be kept from the access of air and light
for some time it becomes rancid. This is caused
by the decomposition of the bodies out of which it is
formed, with the result that small quantities of fatty
acids, particularly butyric acid, are set free. When

^ By Mayer, Kirchner, and Ladd.

Ill] THE CONSTITUENTS OF MILK 27

freely exposed to the air, and in the presence of
sunlight, this decomposition goes on more quickly.
It may be here pointed out that not merely are
certain fatty acids set free, but actually new acids,
such as formic acid, are formed by the absorption of
oxygen. In this process of decomposition the fat
assumes a white colour. The conditions which
influence the percentage of fat in milk will be
discussed in the next Chapter, under the wider
question of the conditions which influence the com-
position of milk.

The Albuminoids of MUk. — The albuminoids of
milk, from a chemical point of view, are the most
interestincr of the milk constituents. ^lore discussion
has taken place with regard to their nature than with
regard to the nature of all the other constituents of
milk. Some, like the eminent French investigator
Duclaux, have held that there is only one albuminoid
substance in milk. This theory, however, seems to
be at variance with the beha\dour of milk when
treated with certain reagents ; and there can be little
doubt that there are several albuminoids in milk.
Of these, casein, or what has been called caseous
matter, or caseinogen, to distinguish it from pure
casein, is the most abundant, and forms about 80 per
cent of the total nitrogenous matter. The other
albuminoids are albumin, which is next abund-
ant in amount to casein ; lactoprotein, which has

28

MILK

[in

also been called albiiminose or galactine ; and lado-
f/ldbulin.

Casein. — According to Kirchner, casein {i.e. casein
proper) varies from 2 to 4-5 per cent, and is, on an
average, present in milk to the extent of 3*2 per cent.
Its percentage composition is as follows : —

Carbon

53-00

Hydrogen .

7-12

Nitrogen .

15-65

Oxygen

22-60

Sulphur

•78

Phosphorus

•85

lOO^OO

From the above it will be seen that milk casein
contains both sulphur and phosphorus. Casein is
characterised by the presence in it of a substance
called nuclein, a body not found in albumin. What
is usually known as the casein in milk is not, as we
already mentioned, pure casein, which is a body
insoluble in water. There seems to be no doubt that
the casein, in milk, is combined in some way with
lime. This compound, which, as we have said, is
sometimes called caseous matter or caseinogen, differs
from pure casein by being more soluble. It is not,
however, strictly speaking, soluble in water, but
forms with it a bulky, colloidal (or glue-like) mass.
This is proved by the fact that if milk be filtered
through porous clay the caseous matter does not pass

Ill] THE CONSTITUENTS OF MILK 29

through, like the other albuminoids, which are in
true solution. The presence of this glue-like caseous
matter in milk exerts an important influence on the
state of the fatty globules, as we shall see later on,
by preventing the more minute ones from separating
out from the main body of the milk by rising to its
surface. The caseous matter of milk may be regarded,
then, as being of the nature of a salt, in which the
casein proper takes the part of the acid, and the
lime that of the base. The proportion in which
the casein and lime are respectively present are 100
parts of the former to 1'55 of the latter. As we
shall see farther on, the action of rennet or dilute
acids is to decompose this caseous matter, and to
precipitate it from its colloidal form into an insoluble
one.

The caseous matter in its semi-dissolved condition
possesses a certain amount of opacity. The opacity
of milk is, therefore, due to its caseous matter as well
as to its fatty globules. This is seen from the fact
that creamed milk, which contains as little as *! per
cent of fat, presents considerable opacity. It is this
body which forms the chief constituent of the curd
or coac^ulum of milk, the nature of which, accordino^
to the conditions under which it is produced, will be
discussed when dealing; with cheese.

It has been noticed that, when milk is allowed to
stand for some time, an alteration of the casein into

30

MILK

[III

peptones may take place. Thus, a German investi-
gator has found that, in milk standing eight hours,
a loss of caseous matter amounting to '25 per cent
(that is, 10 per cent of the original quantity) took
place, owing to such conversion. A consideration of
this fact points to the importance of coagulating milk
destined for the manufacture of cheese as speedily as
possible.

Albumin. — The albumin of milk does not seem
to be identical with blood or serum albumin, and
hence it has been called lactalhumin. Its composition
(Sebelien) is as follows : —

Carbon

52-19

Hydrogen .

7-18

Nitrogen .

15-77

Oxygen

23-13

Sulphur

1-73

100-00

Albumin differs from casein in not containing
phosphorus, and from the other albuminoids of milk
in containing sulphur. Its average percentage in
milk, according to Kirchner, is '6 ; its amount
varying from '2 to '8 per cent. It is, as has
already been mentioned, in a state of solution in
milk. It is soluble in dilute acids and rennet, and
is not coagulated along with the casein in cheese-
making. It thus chiefly passes into the whey under
such circumstances. By heating the milk to from

Ill] THE CONSTITUENTS OF MILK 31

70" to 75' C. (158' to 167"' Fahr.), however, it is
coagulated. While present in normal milk in but
small quantities, its amount in colostrum is very
much greater.

Lactoglobulin. — This body is present in very
minute traces in ordinary milk (according to Sebelien
only a few parts per million) ; but in colostrum it may
amount to as much as 8 per cent. Like albumin, it
is in a state of solution, and can be precipitated from
milk after the caseous matter has been removed by
treating it with magnesium sulphate. When the
milk is heated to a temperature of from 70° to 76° C.
(158° to 169° Fahr.) it is coagulated,

Lactoprotein. — If milk be treated with acetic
acid to precipitate the casein, then boiled to remove
the albumin and lactoglobulin, it will be found that
there is still another albuminoid substance left.
This body, which is present to the extent of from
•08 to '19 per cent — on an average "IS per cent — in
fresh milk, belongs to the peptone group of substances.
It has been variously named lactoprotein, alhuminose,
and galadine. According to Babcock, there is yet
another albuminoid present in milk, viz. fibrin,
which amounts to "1 per cent.

Milk-Sugar. — ]\Iilk-sugar is characteristic of milk,
since it does not occur elsewhere. It is very easily de-
composed when in a state of solution, — as it is in milk,
— the result being its conversion into lactic acid. This

32 MILK [hi

change is effected by bacteria. The class of bacteria
effecting it are of common occurrence, and have been
found in great numbers on the cow's udder, in the
byre, in milk- vessels, etc. They thus obtain easy
access to the milk, and the souring of milk through
their action follows sooner or later. The following
is the composition of milk-sugar : —

Carbon 40-00

Hydrogen .... 6'10
Oxygen . . . .48-90
Water 5-00

100-00

Its chemical formula is C^^H^^Oj^, H^O. The con-
version of milk-sugar into lactic acid may be ex-
plained by the following equation, in which one
molecule of milk-sugar is converted into four mole-
cules of lactic acid : —

Ci,H,,OiiH20 = 4(C3H,03).

With regard to the bacteria and the conditions
under which they develop, further information will
be given in dealing with the bacteria of milk. Milk-
sugar was discovered in milk as early as the end of
the seventeenth century. It is a crystalline body
(four-sided prisms) of a w^hite transparent colour,
and contains, as has been indicated by the formula,
one part of water of crystallisation. It is difficultly

Ill] THE CONSTITUENTS OF MILK H

soluble in water or alcohol, and lieuce possesses only
a slightly sweet taste. The brown coloration which
milk is seen to assume, when heated to a compara-
tively high temperature, is due to the decomposition
of this body. When heated between 100^ and 131'
C. (212' Fahr. and 268= Fahr.) it shows signs of
decomposition and becomes brown. At 131" C. (268'
Fahr.) it loses its water of crystallisation and under-
goes further decomposition, which gives rise to the
formation of galactine and perhaps also grape-sugar.
With an increase of temperature the brown coloration
deepens, and at 175° C. (3-17' Fahr.) the formation of
a dark brown substance called lactocaramel takes
place. In a watery solution of sugar, such as in
milk, this, decomposition takes place when the
temperature rises above 70° C. (158' Fahr.). .

The percentage of milk-sugar in milk may be said
to vary between 3 and 6 per cent, amounting, on an
average, to 4*68 per cent. Another carbohydrate is
supposed also to be present in milk, but regarding it
we know very little.

Ratio of the different Constituents of the MilJc. —
With regard to the variation in the different con-
stituents, it may be added that the relation of the
casein to the albumin is more or less variable ; the
fat is rarely less than the casein and albumin. The
average proportion is 1*2 lb. of fat to 1 lb. of casein
and albumin, or 1*5 lb. of fat to 1 lb. of casein. A

3

34

MILK

[ill

milk containing less than 1"3 lb. of fat for each 1 lb.
of casein has in all probability been skimmed. Milk
^vith 3 per cent of fat will usually contain less than
12 per cent of total solids.

Mineral Constituents. — The mineral constituents
of milk, despite their comf)aratively small proportion,
have an important influence on its nature and pro-
perties. Their amount may be said to vary less than
that of any of the other ingredients, and may be
stated, on an average, at '75 per cent, the limits of
variation being from '5 to "9 per cent. They consist
oi jpotashy soda, lime, magnesia, and iron, in combina-
tion with lohosiolioric, sulphuric, liyclrocliloric, and car-
Ionic acids. The three largest ingredients are potash,
lime, and phosphoric acid, which are each of them
present to the extent of from 20 to 26 per cent of
the total ash. The following analysis (Schrodt) repre-
sents the average composition of the ash of milk : —

Potash .

25-42

Soda .

.

10-94

Lime

21-45

Magnesia

.

2-54

Ferric oxide .

-11

Sulphuric

anhydride

4-11

Phosplioric

anhydride

24-11

Chlorine

. •

14-60

Deduct oxygen replaced by chlorine

103-28
3-28

100-00

Ill] THE CONSTITUENTS OF MILK 35

AVith regard to the condition in which the mineral
matter is present in milk, it may be of interest to
point out that the sulphuric anhydride present in
the ash is derived from the sulphur, which consti-
tutes a part of the caseotis matter and albumin,
while a .portion of the phosphoric acid is formed by
the oxidation of the phosphorus present in the casein
to the extent of "8 per cent. Again, a portion of the
lime is, as we have already pointed out, in a state of
combination with the casein ; while lastly, a portion
of the potash and magnesia is in combination with
citric a.cicl or other orcranic acids. AYith regard to
the presence of this last-named acid in milk, it may
be mentioned that Henkel has found it to amount
to 1 per cent, while according to the same authority
the entire Cjuantity of organic acids calculated as
citric acid amounts to '24 per cent. A portion of
the magnesia, lime, and phosphoric acid are in a
state of suspension in the milk.

The influence of the mineral matter in causing
the so - called amphoteric reaxtion of milk will be
referred to later on.

In addition to the above-mentioned substances,
minute quantities of the following bodies have
been found in milk, viz. direct, lecithin, hypoxanthin ,
kreatin, chlorestin, leucin, and tyrosin ; no doubt
formed by a partial decomposition of the albumin
in the process of digestion in the animal body. The

0

6 M/LA^ [ill

presence of these substances, however, is of little
importance.

Gases in Milk. — Lastly, we have oxygen, nitrogen,
and carbonic anhydride in varying amounts. Of
dissolved nitrogen the amount may be stated at '75
per cent ; dissolved oxygen "1 per cent ; free carbonic
anhydride 7"5 per cent, and in a combined form from
'01 to '2 per cent (Kirchner).

The constituents of milk are thus partly in a state
of emidsion (the fat), partly in a state of suspension
(caseous matter, lime, magnesia, phosphoric acid), and
partly in a state of solutio7i (albumin, milk-sugar,
and most of the mineral matter).

The Reaction of Milk. — The reaction exhibited
towards different vegetable colouring substances by
milk varies. Human milk is normally alkaline in
its reaction, while that of carnivorous animals is
generally acid. With regard to cows' milk, how-
ever, the very curious fact has been observed that it
exhibits the so-called amphoteric reaction ; that is,
both an acid and alkaline reaction. This is due to
the presence in it of acid and neutral phosphates and
carbonates of the alkalies.

Sheep's, Goats', and Mares' Milk. — With regard
to sheep's, goats', and mares' milk, it may be men-
tioned that the first named possesses a yellowish-
white colour and is characterised by a high per-
centage of total solids, which consist chiefly of fat

Ill]

THE CONSTITUEiVTS OF MILK

37

and caseous matter ; that of the second is almost
pure white, and possesses a curious smell and flavour,
being also slightly richer in fat and albumin
than cows' milk; while that of the last named is
bluish in colour, and possesses an aromatic sweetish
taste.

The following table gives the average composition
of the milk of the above-mentioned animals : —

Sheep.

Goat.

Mare.

Water

Fat .

Casein

Albumin .

Milk-sugar

Mineral Matter .

Total Sohds

83-0
5-3
4-6
1-7
4-6
•8

85-5
4-8
3-8
1-2
4-0
•7

90-7
1-2

5 7
•4

100-00
17-0°/

• o

100-00
14-57

/ o

100-00
9-3-

o

Spe^iific Gravity of Milk. — The specific gravity of
cows' milk may be said to vary from 1'028 to 1*035
at 15^ C. (water being taken as 1). This is the
specific gravity of mixed milk. Occasionally, in the
case of the milk of single cows, it may sink as low as
1-0263 or rise as high as 1"0380. As a rule, however,
most milk will have a specific gravity varying
between 1-030 and 1-033.

38

MILK

[III

Colostrum or First Milkings. — As a general rule,
the milk which the cow gives immediately after
calving differs very materially in its nature from
ordinary milk. So widely different is it in composi-
tion that it is known by a different name, viz.
colostrum. This colostrum, or " beastings " as it is also
called, is secreted, not merely after calving, but also

Fig. 9.— Colostrum Bodies (x 300). a', Cells with nucleus; o, cells under-
going fatty degeneration; b, cells containing large drops of fat; c, cells
with a partially destroyed cell membrane ; d, e, and /, cells which have
entirely lost the cell membrane ; rj, cell masses from the milk canals.

for a short time before it ; and not only does it differ
from normal milk by the very much larger per-
centage of total solids which it contains — more than
twice as much — but by the presence in it of bodies
not found in normal milk. What is especially
characteristic of colostrum is the presence of certain
grape -shaped bodies, the so-called colostrum cor-
imscles (corijs yranulevx) (see Fig. 9). These corpuscles,

Ill] THE CONSTITUENTS OF MILK 39

wliich are four or five times tlie size of the ordinary
milk globules, resemble iu a^Dpearance white blood
corpuscles, from which they have been considered by
some to be derived. They disappear in milk, as a
rule, in from three to fourteen days after calving, but
sometimes not till the lapse of a much longer period.
The most striking difference, between the composition
of colostrum and normal milk, is to be found in the
very much larger percentage of nitrogenous matter
the former contains. Indeed, the richer quality of
colostrum is due almost entirely to this fact, since
the other constituents are not in excess of what they
are in normal milk. The followincr ig the averao;e
composition of the colostrum of 22 cows (Eugling) : —

"Water .

.

71-69

Total. Sohds .

,

28-31

Fat

3-37

Casein .

4-83

Albumin

15-85

Sugar

2-48

Ash .

1-78

100-00

From the above analysis, it will be seen that the
increase in nitrogenous matter is due to the albumin,
the casein being pretty much the same as in normal
milk. Another point in which colostrum differs
from normal milk, is the fact that it does not contain
milk-sugar, or, if it does, only in mere traces. The
milk-sugar is replaced by other kinds of sugar, viz.

40 MILK [hi

grape-sugar or lactose. Colostrum further differs from
normal milk in containing more ash, and in the com-
position of this ash. Nearly one-half of it (41 '43
per cent) consists of phosphoric acid, as against
about 27 per cent in the ash of normal milk.
Globulin has been found in colostrum to the extent
of 8*3 per cent (Emmerling), and such bodies as
lecithin, cholesterin, urea, and nuclein have also been
found in traces. Its composition, however, is a very
variable one. It has been found immediately after
birth to contain from 24 to 32 per cent of total solids.
In three or four days, however, it becomes very
similar to ordinary milk. Occasionally, even after
the lapse of a fortnight, its peculiar properties do not
entirely vanish. In appearance it is a turbid liquid,
of a viscous or slimy nature, possessing a yellowish
colour, a strong and peculiar smell, somewhat saltish
in flavour, and, as a rule, a somewhat weak acid re-
action. AYlien boiled it coagulates, on account of the
high percentage of albumin it contains. Rennet
does not coagulate it, or, if it does, only very imper-
fectly. The fat of colostrum has a higher melting-
point than the fat of normal milk, and is also dis-
tinguished from it by its peculiar smell and flavour.
The large percentage of albumin it contains makes it
a very nutritive and easily digested food for the
young calf. Indeed, it seems to exert a special
action on the alimentary canal. There are no grounds

Ill] THE CONSTITUENTS OF MILK 41

whatever for the belief, which has soraetimes been
popular, that colostrum is not suitable as a food for
the young calf. The fact that, when allowed to stand,
a cream layer separates out, which amounts to about
50 per cent of the whole, has given rise also to an
erroneous but popular idea that it is very rich in fat.
This has led to its use for churning. It need scarcely
be pointed out that such a use of colostrum is to be
condemned. As we have already pointed out, it is
not so rich in fat as normal milk. It is, furthermore,
highly unsuited for either butter or cheese making.
For these reasons, the milk from cows which have
recently calved should not be used, for from four to
seven days, for churning, or for cheese -making, for
double that period. Indeed, it is safer to keep the
milk apart for even a longer time.

CHAPTER IV

Causes and Conditmis Influencing the Quality and
Quantity of Milk

The causes which influence the quality and
quantity of milk have been much studied, and
although many points still remain obscure, a good
deal of information has been collected on the
subject. In the first place we shall discuss the
causes which influence the quantity of milk secreted.
Causes Influencing the Quantity of Milk
Secreted. — The amount of milk yielded by a cow
depends on the activity of the milk-glands in the
udder ; and this, in its turn, is influenced by a
number of conditions. For one thing, it seems to
be dependent on the amount of milk the udder
contains. Thus, so long as the udder is compara-
tively empty, and other circumstances are favourable,
the secretion of milk by the milk-glands seems to
take place unhindered. But whenever the udder
becomes full of milk, the rate at which the secretion

iv] CONDITIONS INFLUENCING MILK 43

goes on is diminished. It still goes on, however, as
is evidenced by the distension of the udder which
takes place under such circumstances ; but it
is no longer at the normal rate. The pressure to
which the udder is subjected seems to affect the
nature of the secretion, the quality of the milk
secreted being different.

Now, what has been above stated seems to throw
much light on the interesting and well-known fact
that, if a cow^ which has been in the habit of being
milked tw^ice a day is milked oftener, the result is
not only that more milk, but milk of richer quality,
is obtained than was formerly the case. But when
the secretion of milk is forced to go on under
pressure, not merely is the quantity of the milk
lessened, but, owing to this pressure, more frictional
resistance is offered to the passage of the fat globules,
and the rest of the solids of the milk, through the
secretins^ vessels and ducts in the udder. It is for
this reason that milk secreted under pressure is
poorer in quality. The above facts also explain
another interesting phenomenon familiar to all
interested in dairying, viz. that the milk wdiich is
first drawn when the cow is beino- milked is in-
variably poorer in quality than that last drawn — the
so-called strippings, which, it is well known, are
always very rich in quality. Indeed, the following
figures, which contain the results of an experiment

44 MILK [iv

carried out by Boiissingault, will illustrate that there
is a steady increase in the total solids during the
whole period of milking. In this experiment the
milk drawn from the cow's udder was divided into
six consecutive portions and analysed. Tlie follow-
ing were the results found : —

(1)

(2)

(3)

(4)

(5)

(6)

Solids . 10-47

10-75

10-85

11-23

11-63

12-67

Fat . 1-70

1-76

2-10

2-54

3-14

4-08

Solids not Fat 8-77

8-99

8-75

8-69

8-49

8-59

From the above figures, it will be seen that the
increase is limited to the fat, the solids not fat
remaining practically uniform. A view which was
formerly held to explain this was that the milk
underwent a sort of creaming process in the cow's
udder ; the last drawn being the surface layer of the
milk, and therefore richer in fat than that first drawn.
In view, however, of our present knowledge of the
structure of the udder, it is not likely that any such
creaming process could take place. The explanation
first given commends itself as a far more probable one.
Difference in Morning and Evening Milk. —
If, in the case of a cow milked twice a day, the
intervening period between each milking is the same,
and if other conditions are similar, it may be said
that there will be approximately no difference be-
tween the morning and evening milk, both as regards
quantity and quality. If, on the other hand, the

iv] CONDITIOXS IXFLUENCING MILK 45

periods intervening between the times of milking be
unequal, it will be found that the milk obtained
after the longer interval is greater in quantity, but
poorer in quality, than that obtained after the shorter
interval. We may add that experiments have shown
that when a cow is milked three or four times a day
an increase in its amount to the extent of 20 per cent,
and an increase in its fat to the extent of 25 j)er
cent, may be obtained over that gained when it is
only twice milked.

Another condition which influences the activity
of the milk-glands in the udder is the age of the cow.
The maximum development in respect of milk pro-
duction in a cow is generally to be found between the
fourth and fifth calf it gives birth to. The influence
of food is also important, but we shall treat it under
the conditions which influence the quality of milk.

Conditions Influencing the Quality of Milk.^
From what we have pointed out it will be seen that
the condition which influences not merely the quan-
tity, but also the quality, of milk to the greatest
extent, is the nature of the milk-glands — that is to
say, the individuality of the cow. Xext to the
individuality of the cow may be said to be the hreed.

Breed. — In choosing milking cows it is important
to choose good milking breeds. Although it has
undoubtedly been found that those breeds which
give a large amount of milk do not, as a general rule,

46 MILK [iv

give as rich a milk as those giving less, it does not
necessarily follow, as is too commonly believed, that
cows yielding large quantities of milk yield poor
milk ; for it is quite possible to have both a large
yield of milk, and one of good quality.

All farmers are aware that, so far as the quality of
milk is concerned, no cows yield so rich a milk as
the Jersey and Guernsey cows. Analyses of the milk
of cows belonging to these breeds are on record which
show an extraordinarily large percentage of total
solids. As an example of such milk, reference may be
made to a sample analysed by the author, in which the
total solids amounted to no less than 18"39 per cent.
The milking records made in connection with the Lon-
don Dairy Show (1880-89) also illustrate this. Thus :—

Total Solids.

Fat.

Shorthorns .

. 12-87

3-73

Jerseys

. 14-65

5-02

Guernseys

. 14-23

4-90

Ayrshires

. 13-43

4-15

But a more convincing evidence of this difference
is afforded by comparing the average of the results of
a large number of analyses of the milk of different
breeds ; and interesting statistics, recently published
in America, may be cited in this connection.'^ The
results have been drawn from hundreds of analyses
of samples of milk obtained from six different breeds.
They are as follows : —
* American Experiment Station Record, vol. v. Xo. 10, p. 945.

Daily
Milk
Yield.

x

r- C O O O O

c 9 o -^ 'sf o

'T i: ■>! X M C?-l

-— r- — — ,— 1 CM

-^
^

s

O Ci :r: ut i-- 3>1

C3 CO q^i c: rc o
-^ o CO o r^ r^

X X X X X X

A "= X c ir: M o c
."Tip r c^ooootc
•^ 5. COOOOO

'S

<:

"H rt oc c X X o

3 — O CC Ci CT. CC

" r- t^ t^ CO o r-
t^

» :^ :^ 3 3 ^ ^

5 &

3

7^ T" 9 H C X

<^ O ct iC 1^ t}<

'3

Oi 9 r^ ^ ro CO

CO CO CO CO CO CO

1l

-4-5

s

o

■— 'M u-; t^ O i2

^ r— 1 r— ip O -}<

i^ o ^^ ^*^ '■*^ ^^

O -tJ

-(J

3
o

ft

o r^ C i-': X r^
X y cp CO o o
c: c; c: c: c: Ci

31

-g O o r- :r rt -
g y :f t- o ir r:i
j; L-: -f ^: ft 4i ^T

<

X ^1 -^1 -M ^ oq
CO 1— r^ ir: 'M CO

71 — ■?! : i

O

O o

• . .^s

r> CJ 3 ^

>^ « ^ :^ 1 1

i o ^ £ 2 ^

^ S a >> ;= O

48 MILK [iv

From the above table it will be seen that the total
solids ranged from 15 '40 per cent in the case of the
milk from the Jersey, to 12 '3 9 per cent in the case
of the Holstein — a difference of 3 per cent. As
regards the fat, it will be seen that there is a difference
of 2 '15 per cent, the difference in total solids only
amounting to 3 per cent, which leaves less than 1
per cent for the variation in all the other constituents ;
or, to put it in another way, the percentage of fat in
the total solids ranged from 28 to 36'4 per cent,
while the percentage of " solids not fat " ranged from
83*2 per cent to 63"6 per cent. The variation in the
solids not fat only amounted to "71 per cent, viz.
from 9"80 to 9'07 ; and this variation was equal in
amount in the case of the casein and milk-sugar,
which varied between 3*39 per cent and 3*91 per
cent, and 4*84 to 5*33 per cent respectively. The
constituent which was most constant was the ash,
which only varied between '76 and '69 per cent.

Period of Lactation. — Another influence affecting
the quality of milk is the period of lactation — that
is, the length of time after calving. And here it
may be pointed out that the duration of the lactation
period varies very considerably in the case of different
cows. In very rare cases, the secretion of milk may
actually continue without intermission from one
calving period to another ; while, on the other hand,
it may occasionally cease a comparatively short time

iv] CONDITIOXS IXFLUENCING MILK 49

after caMng. On an average it may be said to last
some three hundred days. An American expert who
has studied the question at considerable length comes
to the conclusion that ^ " on an average cows give the
thinnest milk just after calving ; it becomes slightly
richer durinfy the next two weeks, and then it holds
almost uniform in quality for four or five months,
after which it gradually increases in richness as the
cow comes near to calvinc^ a^ain, and bv the ninth
month from last calving is only about one-seventh
richer than it was during the earlier months." The
difference in the quality of the milk due to tliis cause
is manifested almost entirely in the fat — " the solids
not fat " remaining practically the same. E. H. Dean,
another American expert, has found that, dividing
the lactation period into three parts of ninety-one
days each, there was an increase in fat of only 17
per cent in the second period, and 46 per cent in
the third period, over that of the first period.

Food. — The nature of the food may be mentioned
as a condition influencing the quality of milk. This
influence has been in the past both over and under-
rated. The old belief was that food influenced, in a
very direct manner, the composition of milk. The
modern tendency, on the other hand, is to rather
underestimate the effect that feeding has on milk

^ W. "W. Cook in Agricultural Science, vol. 7 (1893), pp. 253,
265.

so . MILK [iv

secretion. While it is true that experiments have
shown that change of diet seems to have little
immediate effect on the milk, it is a mistake to
imagine that milk is independent of feeding. There
is no doubt that such conditions as individuality and
breed, under ordinary conditions of feeding, count for
more than food ; still the importance of having a
properly adjusted diet for milk cows is very great,
and unless cows are properly fed they cannot be
expected to yield their maximum quantity of milk,
since under an ill-assorted diet the milk - yielding
capacity is bound to suffer just as much as any
other function of the body. Unfortunately, the ex-
periments carried out on the influence of food have
been comparatively few in number, and the results
obtained very contradictory. There are, however,
certain conclusions which we are warranted in mak-
ing on the influence of food.

With regard to the three food nutrients, — the
albuminoids, fat, and non-nitrogenous bodies, — the
first named have by far the most important in-
fluence on milk. Whatever theory may be adopted
as to the formation of milk in the cow's udder,
there can be no doubt that the nitrogenous
bodies in the milk are derived from the nitro-
genous bodies of the cell substance. The food
nutrient from which this cell substance is built
up is bound, therefore, to have a very important

iv] COXDITIOXS INFLUENCING MILK 51

influence on the composition of milk. The more
abundantly the cell .substance is built up, the
more abundant will the secretion of milk be. An
insufiO-cient proportion of albuminoids, therefore, in
the food will have the effect of diminishing the yield
of milk. On the other hand, neither the fat nor the
non-nitrogenous bodies in food are of such import-
ance in influencing the composition of milk. So far
as we can see, the fat has no effect on the production
of fat in the milk. It may, however, — no doubt does,
— exercise an important function in protecting the
albuminoids from oxidation, and thus leaving them
free to carry out what is their proper work, viz. to
build up the cell substance. The influence which
insufficient feeding is bound to exercise on milk
formation, is towards impoverishing the milk yield
in quality as well as in quantity.

Influence of Excitement. — Other causes influence
the composition of milk, such as the condition of the
cow when milked. At a meeting of the Society of
Public Analysts in London, two years ago, a number
of analyses of abnormal milk samples were read and
discussed. AmonfT the cases cited was one of a cow
which had been milked at a fair when in an excited
condition. The milk, on analysis, was found to con-
tain only 10-85 per cent of total solids, and only
1-85 per cent of fat. That this was due to the
excited condition of the cow was shown by the fact

52 MILK [iv

that, when milked next day, when quiet and in
normal condition, the milk showed 12-75 per cent of
total solids and 3-64 per cent of fat. That the
tendency of excitement may not always be in the
direction of giving milk of an abnormally poor
quality is illustrated by two other samples of milk
obtained from cows at a fair, which showed, on
analysis, respectively 19 '5 per cent of total solids and
11-06 per cent of fat, and 16 per cent of total solids
and 7'37 per cent of fat.

Illness. — Another cause which affects the quality
of milk is illness. To what extent milk may be
affected by this cause is illustrated by a case quoted
by a German expert in 1893. Three samples of
milk, obtained from a cow suffering from illness,
were found to have as poor a composition as the
following : — The solids ranged from 8-31 per cent to
916 per cent, the fat from '25 per cent to 1'55 per
cent, and the milk-sugar from 3'82 to 4-10 per cent.

Among other causes which influence the general
quality of the milk may be mentioned exertion of a
severe kind and the time of day. The season of the
year has also an important influence, milk being
generally poorer in spring than in autumn.

CHAPTEE V

The Changes ichich Milk undergoes

The number of changes which are constantly going
on in milk are borne witness to by the rapidity with
which that valuable food sours, coac^ulates, chans^es
its colour, etc., when kept for any time. These
changes are, many of them, of a very complicated
nature, and as yet little understood. Although not
always apparent, they start from the moment the
milk leaves the udder. Most of them are connected
with the action of the ubiquitous micro-organic life,
which is being shown more and more every year to
be of the highest importance in many operations.
In milk, of all fluids, they find their most congenial
home and an abundance of food, and they are not
long in establishing themselves in such congenial
surroundings. Some limit their attention to one
milk constituent, and some to another, so that in a
very short time a slow transformation of these
different constituents is begun, which eventually

54 MILK [v

leads to the changes in the properties of milk so
familiar to all. In this Chapter we shall only casu-
ally refer to the micro-organisms which eft'ect these
changes, and shall leave a more full treatment of
this important subject to a following Chapter.

Creaming of Milk. — Milk, as it comes from the
udder, may be described as of practically uniform
composition. If, however, we let it stand at rest for
some time, we find that the uniformity of its com-
position is disturbed by an accumulation, which
takes place more or less quickly on its surface, of
its minute fatty globules. Concurrently with this
separation of the fat, a change in the colour of the
main body of the milk will be observed. By the
removal of the fat the opacity of the milk is
diminished, and it is rendered more transparent.
This has the effect of imparting the bluish tinge,
so characteristic of skim milk. The surface layer
of milk, which is thus enriched in fat, is known as
cream. ^ Now there is a popular belief to the effect
that the richness of milk in fat is indicated by the
depth of this cream layer ; and while, no doubt, this
is true, within certain limits, it is, as we shall im-
mediately proceed to show, not necessarily so. The
tendency which the milk globules possess to rise to
the top exists to such an extent that if a quantity of
milk six inches deep be allowed to stand undis-
turbed, at a temperature of 15° C. (60° Fahr.), for

v] CHANGES WHICH MILK UNDERGOES 55

twenty-four hours, from 80 to 85 per cent of the fat
will be found in the surface cream layer. The rate at
which the fat globules find their way to the surface
is dependent on their size. The larger globules rise
first. The very small globules never rise at all, as
the amount of nitrogenous matter which adheres to
them is too Q-reat for them to assert their lighter
specific gravity. This accounts for the fact that all
the fat is not found in the cream layer, even after
milk has stood for a long time. Xo doubt, with the
help of centrifugal force, more complete separation
may be effected ; but even under such circumstances
complete separation is not obtained, although as
much as from 90 to 96 per cent of the fat may be
separated in this manner.

Souring. — After a time milk spontaneously co-
agulates and develops a sour taste. Before this
takes place, however, a careful inspection of the milk
would show that it had undergone very considerable
changes. From the moment of its lea^dng the udder
it is taken possession of by a class of bacteria which
are known as lactic bacteria, and of which no less
than one hundred different kinds have been identified
already. These bacteria produce lactic acid in milk.
This they do by decomposing the milk - sugar, which
is present to the extent of about 4| per cent, and
which is a very easily decomposable substance. Now
these bacteria, like all other bacteria, are very much

56 MILK [v

influenced in the rate of their development by tem-
perature. This, indeed, is the reason wliy the
influence of heat is so enormous in regulating the
changes which take place in milk. Kapid develop-
ment of these classes of bacteria may be said only to
take place at temperatures above 15^ C. (60^ Fahr.).
It is for this reason that milk ought to be cooled do^vn,
immediately after milking, below this temperature,
if it is desired to keep it for any time. Similarly,
the temperature ought not subsequently to be allowed
to rise above this point. But, just as a low tempera-
ture is unfavourable for the development of bacteria,
so also is a high temperature. Thus, heating the
milk to 50° C. (122" Fahr.) checks this kind of fer-
mentation. The effect of the formation of this lactic
acid, when it reaches certain proportions in the milk,
is to cause the coagulation of another important con-
stituent of milk, viz. the caseous matter.

And here reference may be made to the widely
spread belief that electricity in the air has the effect
of souring milk. There can be no doubt that in
*' thundery " weather milk or cream sours more
quickly than under ordinary conditions ; but this
cannot be traced to the effect of electricity. In fact,
experiments have shown that electricity, instead of
accelerating the souring of milk, actually retards it.
The true explanation is probably to be found in the
fact that the temperature of the air is generally

v] CHAXGES WHICH JJILK UNDERGOES 57

increased before thunder, and that this promotes a
more active development of bacteria in the milk.
That it is not due to electricity is proved by the fact
that sterilised milk will not sour, no matter how
violent the thunderstorm is.

Amphoteric Reaction. — And here it may be well
to say a word or two on rather a curious property of
milk. This is the amphoteric reaction it possesses —
a property which we have already referred to. By
warmins the milk, it is found that the alkaline reac-
tion becomes more pronounced. Warming, however,
has no influence on the acid reaction. By the
gradual formation of free lactic acid in the process
of fermentation, milk loses its amphoteric reaction,
the alkaline reaction disappears, and the acid re-
action alone remains and gradually increases in
strength. This takes place, after a time, to such an
extent that, although the milk remains liquid at
ordinary temperatures, a slight increase of tem-
perature, or the addition of carbonic acid, causes
immediate coagulation. Finally, the casein, even at
the ordinary temperature, becomes coagulated.

Coagulation of Milk. — The coagulation of the
casein is thus due to the development of acidity in
the milk. The higher the milk is heated — up to a
certain point — the more quickly will coagulation
take place. The coagulation of the casein may be
effected by the addition of various precipitating

58 MILK [v

reagents, such as dilute acids, salts, alcohol, etc., and
by rennet. It is worthy of note that the coagulation,
which is formed by spontaneous souring of milk,
and that formed by different coagulating reagents,
differ in their nature — a point which will be discussed
in a subsequent Chapter.

Effect of Heat. — The extent to which coagula-
tion of the casein in milk takes place and the
relative effect of different coagulating reagents is
influenced by temperature and certain other con-
ditions. Thus the higher the temperature (up to
boiling-point), the less the quantity of acids or other
precipitating reagents required to effect coagulation.
Again, coagulation seems to be greater the sooner
the milk is treated after milking. The amount
of precipitant to be used, therefore, will largely
depend on the temperature. The action of heat
on the properties of milk is altogether of a most
interesting nature. When milk is heated to boiling
or to higher temperatures, it develops a peculiar
flavour — a flavour especially unpleasant to many
people. Indeed, heating milk to even a considerably
lower temperature than boiling temperature imparts
this *' cooked " flavour to milk. Thus if milk be
heated to 158" Fahr. for fifteen or twenty minutes it
acquu^es it. If, however, the milk be cooled again
it loses it. It is possible that both the smell and
flavour of milk which has been strongly heated is

v] CHAXGES WHICH MILK UNDERGOES 59

due to the presence of small quantities of sul-
phuretted hydrogen formed by the decomposition of
such albuminoids as contain sulphur. The colour
of milk is also slightly changed under the action of
heat ; and this is due to the decomposition of the
milk-sugar and the formation of small ciuantities of
yellow and brown substances of the nature of lado-
caramel. The result of heating milk is to change
the fine state of division of the fatty globules, many
of which run together and form larger globules.
Again, on heating milk, even to the comparatively
low temperature of oO~ C. (122° Fahr.), a skin is
formed on the surface, caused by the coagulation of
the albumin.

Milk Faults. — Milk has been found in the past
to be liable to strange diseases, or, as they are
technically known, '-faults." Thus, for example,
it has been found to develop strange coloured
patches — blue, yellow, or green — or to have its
whole colour changed. These milk faults, as a
rule, only become apparent in the milk some time
after it has been drawn from the cow's udder. The
practical importance of the subject consists in the
fact that a serious disturbance in the quality of the
milk products is the result. These changes, as we
shall see in the next Chapter, are generally caused
by bacteria. Milk, again, is sometimes obtained in
which the cream rises very slowly. The milk of

6o MILK [v

COWS which have been long milked often shows this
property. It arises from tlie fact that the original
condition of the nitrogenous matter in the milk be-
comes changed in an extraordinary manner, and a
large proportion of the fatty globules become free.
It has also been noticed that such milk contains less
calcium phosphate than normal milk.

Means of preventing Changes in Milk.~It may
be asked, How may these changes be prevented or
retarded ? Now, in doing so, the great agent is heat.
Cleanliness is not a less valuable instrument : clean-
liness in every way — on the hands of the milker, on
the teats of the cow, in the milk-pails and other
receptacles used for holding the milk, in the bpe,
etc. Immediately after milking, the milk should be
cooled down : the lower the temperature the better.
On the other hand, it may be sterilised by heating ;
and, in order to avoid imparting the disagreeable
boiled flavour to the milk, this may be effected by
heating it to 70^ to 80° C. (158^^ to 177^' Fahr.). The
addition of chemicals, so-called "preservatives,"
cannot be too strongly condemned. Even such
comparatively harmless preservatives as bicarbonate
of soda, boracic acid, salicylic acid, and peroxide of
hydrogen, ought not to be used.

The methods of sterilising milk and the import-
ance of cleanliness will be further referred to at the
conclusion of next Chapter on the bacteria of milk,

v]" CHANGES WHICH MILK UNDERGOES 6i

SO that all further consideration of the subject may
be postponed till then.

Preserved Milk. — The sterilisation of milk by
submitting it to a high temperature, while not always
necessarily effective, is accompanied by the objection
that it imparts to the milk a flavour peculiarly dis-
agreeable to many persons. On the other hand,
sterilisation by " intermittent sterilisation '' is an
extremely inconvenient method, and is utterly un-
suited for practical work. Many attempts have
been consequently made in the past to convert milk
into some form in which it will keep without under-
going decomposition. These attempts have all taken
the form of preparing milk in a condensed form. At
first preserved milk was sought to be prepared by
evaporating milk to dryness. The solids thus left
behind, after being mixed with a small quantity of
bicarbonate of soda, were then pressed iuto the form
of cakes. This method, however, did not meet with
success. In the first place, the cakes were found
not to keep well, since the fat soon acquired a rancid
flavour. It was also found that such cakes did not
dissolve properly in water, the drying process having
destroyed the colloidal condition of the caseous
matter. It was subsequently discovered that, by
simply condensing the milk to about one-half or one-
third of its volume, a condensed milk which kept
fairly well could be prepared, and that the addition

62

MILK

[v

of cane - sugar still further increased its keeping
properties.

As at present manufactured, condensed milk is
prepared both in a sweetened and unsweetened con-
dition, the sweetened variety having about 12 per
cent of cane-sugar added to it. In preparing the
unsweetened variety the milk is generally first
sterilised by heating and then condensed in vacuum
to a half or a third of its bulk. The following
analyses of both varieties will illustrate their com-
position (Hehner) : —

Sweetened variety—
Nonvegian
Nestle's Swiss Milk

Water.

Fat.

Album- Milk-
inoids. Sugar.

Cane-
Sugar.

Ash.

28-85
15-30

9-21

8-85

8-98
9-98

14-14
13-62

36-74
50-08

2-08
2-17

Unsweetened variety —

American (mean of )
ten analyses) \

45-59

15-67

17-81

15-40

2-53

CHAPTEE \1

The Bacteria of Milk

Although the functions performed by micro-
organisms in the dairy have only comparatively
recently come to be recognised, it has long been
realised, by the help of experience, that a very
necessary condition of successful dairying is cleanliness.
By working in a cleanly fashion in the dairy, con-
tamination with bacteria is avoided to a larsje extent ;
for the only dangerous constituents of dirt are these
minute living organisms — dead dust is a harmless
enough article.

The changes which micro-organisms may effect in
milk have been again and again exemplified by the
mysterious development of strange milk '•faults,"
already referred to in the preceding Chapter. Among
these may be mentioned such as manifest themselves
by the milk becoming ropy, stringy, or viscous ; by
its assuming a blue, red, yellow, etc., colour, — or, at
least, the development of coloured patches in it, — a

64 MILK [vi

bitter taste, and by its becoming liable to fermentative
curdling, etc. It was, till lately, customary to regard
such faults in milk as due to the nature and condition
of the food, the nature of the soil, and, above all, to
illness or disease in the cow — to everything, in short,
except the true cause, viz. dirt. "VVe now know,
however, that such faults in milk are due to dirty
and careless handling, or, more exactly stated, to
micro-orfranisms which <]fain access to the milk when
it is dirtily treated. Indeed, the best proof of this is
to be found in the fact that improved dairy practice
has rendered such faults of extremely rare occurrence.
In large and modern dairies they are practically
unknown, and it is only in small dairies, where
unfortunately extremely primitive methods too often
obtain, that they still occur.

Occurrence of Bacteria. — Before more particu-
larly dealing with these micro-organisms, let us see
where they are to be found. To this a simple answer
may be given. Modern investigations have shown
that their presence is well-nigh universal. In the air,
in the soil, and in water they are widely distributed.

In the air they are to be counted by thousands in Ci

every cubic yard, while in like quantities of w^ater
and the soil millions of them exist. It is true
that, under certain circumstances, air may be, if not
absolutely, yet comparatively, free of them. For
example, it would seem highly probable that in air

vi] THE BACTERIA OF MILK 65

above the sea, far from land, they do not occur at all ;
while air on the top of high mountains is correspond-
ingly pure. In uninhabited spaces their number
is enormously less than in inhabited spaces. In the
air above the streets of Paris it has been calculated
that they are present to the extent of about 4000 per
cubic yard.

Their tendency is to obey the law of gravity, and
to subside from the air. This, however, can only
completely take place when the air is kept absolutely
undisturbed — a condition which, we need scarcely say,
hardly ever obtains. But, numerous as they are in
outside air, their number in that of enclosed spaces,
such as the dairy, and more especially the byre, is
even greater. Thus, one investigator has found 120
bacteria and moulds in a quart of the air of a byre.
In the litter, in the manure, and in the dirt on the
floors of byres they are very abundant, and whenever
this is stirred or disturbed in any way large numbers
of them are sent into the air. It is for this reason
that the hands of milkers, the teats of cows, and the
milk vessels are liable to become so rapidly con-
taminated with bacterial life.

Contamination of Milk by Micro-Organisms. —
The thousand-and-one ways in which milk may thus
become contaminated makes it a very difficult task to
obtain it from the cow absolutely sterile, — that is,
devoid of bacterial life, — or even to believe that milk

5

66 MILK [vi

when it comes from the udder is devoid of it. lu
many cases, even where the most elaborate precautions
have been taken by experimenters, tlie attempt to
draw sterile milk from the cow's udder has failed.
And yet the doctrine, that milk as originally formed
in the cow's udder is sterile, is a true one ; that is,
except when the udder is in a diseased state. The
failures above referred to are not to be wondered at
when we remember the difficulties to be contended
with. In the first place, we have the difficulty of
keeping the hands absolutely free from bacterial
contamination. Then the comparatively wide surface
of the milk brought into contact with the germ-laden
air in the process of milking renders the task still
more difficult. And lastly, assuming that such
difficulties are successfully overcome, we have the
difficulty of keeping the teats absolutely free from
micro -organic life. It has been found, for example,
that bacteria present in a drop of milk at the opening
of the teat may even work their way into the milk
cistern, and, thanks to the high temperature which
there prevails, may very rapidly develop. Their
complete removal from the udder may, consequently,
not be effected till milking has progressed for some
time.

As illustrating this point, Lehmann has found that
the first 10 oz. of milk drawn contained from 50,000
to 100,000 per cubic centimetre (broadly speaking,

vi] THE BACTERIA OF MILK 67

the -^th of an ounce), the main portion of the milk
about 5000, and that it was only the last 10 oz. drawn
that were practically free from bacteria.

In view of what has been now stated, we may
afQrm that milk, as usually obtained, contains micro-
organisms, and that these organisms will develop and
increase in milk, unless it is specially submitted to
the action of heat, or is treated with antiseptics.

Importance of a Knowledge of Bacteriology to
the Farmer. — Xow, while bacteria may be regarded,
as far as milk itself is concerned, as undesirable, yet
we must remember that the manufacture of dairy
products, such as butter and cheese, are dependent on
their action ; so that a consideration of the subject is
doubly interesting to all engaged in dairying, not
merely on account of the harm they do in promoting
undesirable changes in milk, but also for the indis-
pensable role they perform in the manufacture of
butter and cheese. We should try to know, therefore,
as much about the nature of these minute org^anisms,
and about the conditions which influence their de-
velopment, as possible, so as to enable us to take
precautions either iu checking or killing them, or in
regulating and fostering their development when
required. The success of the dairy industry depends
on properly controlling certain fermentative processes,
such as that potent in cream- souring for butter
manufacture, and that active in starting and develop-

68 MILK [vi

ing certain ripening processes in cheese manufacture.
We want, therefore, as much information as to how
we can best foster these kinds of fermentation as can
possibly be obtained. If the information we already
possess regarding these minute workers in the dairy
is meagre, we must recollect that it is always being
added to, and that its value is great.

Milk as a Propagator of Disease. — There
is another aspect of milk, considered from a bac-
teriological point of view, which merits attention,
viz. the risk it runs of acting as the propagator of
various kinds of disease. The peculiarly rich pro-
perties of milk as a food renders it admirably adapted
for forming a nutritive medium for the development
of various disease (the so-called pathogenic) germs,
which are constantly to be found, along with other
micro-organisms, in the air and elsewhere. There can
be no doubt whatever that such diseases as typhus,
diphtheria, cholera, and, above all, consumption, which
are caused by germ life, are too often disseminated
in this way. Indeed, it would seem highly prob-
able that a very large quantity of milk used is infected
with the tubercular bacillus (see note ^\ p. 113), and
that in this way the seeds of that most insidious of
all diseases, consumption, is implanted in the system,
more especially in the case of children. According
to certain authorities, about 5 per cent of the samples
of town milk contain tubercular bacilli. As to the

vi]" THE BACTERIA OF MILK 69

method in which the tubercular bacillus obtains
access to the milk there can be no doubt. It does
not come into the milk, as a rule, subsequent to
milking, but is already in the milk '^hen it leaves
the udder. The enormous prevalence of tubercular
disease among cows — it has been calculated that 2\
per cent of all cows butchered are infected with
tuberculosis — renders the danger of milk actings as a
propagator of this disease very great. It must not be
inferred, of course, that all milk containing tubercular
bacilli is equally capable of producing the disease.
The probabilities are that where it is present
in a sample of milk, this sample, before it is
consumed, becomes mixed with a large quantity of
milk free from bacilli of this type ; but the resulting
dilution, as has been very properly pointed out by
Freudenreich, while it diminishes the risk of infection,
does not do away with it entirely. Typhus, again, has
often been spread by milk. In fifty typhus epidemics
which Hart investigated in England, no less than
twenty-eight were found to be due to infected milk.
Here, of course, the infection would be imparted to
the milk after milking, in any of the many numerous
ways in which milk becomes contaminated. '^\\h
regard to cholera, it has also been proved again and
again that milk has been the means of propagating
this terrible disease. While lastly, with regard to
scarlet fever and diphtheria, Hart has traced no less

70 MILK [vi

than fourteen epidemics of the former, and seven of
the latter, to this source in England ; and while we
have not the same evidence with regard to other
infectious diseases, there can be little doubt that
many of them in the past have been spread in this
manner. There would seem to be a certain class of
bacteria that are quite harmless when taken into the
system, under ordinary conditions ; if, however, the
vitality of the body is in an impaired state they
become a source of disease. Such bacteria are prob-
ably common in milk, and are doubtless the cause
of gastric and intestinal disturbances so common in
young infants during the summer months (summer
diarrhoea and cholera infantum). They are able to
produce certain by-products which have a poisonous
effect when taken into the susceptible digestive tract
of the infant.-^ The importance, therefore, of a know-
ledge of the bacteriology of milk, in the interests of
public health, will thus be seen to be enormous.^

Although the existence of the world of micro-
organisms was drawn attention to as far back as
1675 by Leeuwenhoek, who discovered them first in
the saliva of the mouth, still it is only within the
last thirty or forty years that any strides in the
science of bacteriology have been effected. Still
more recent is our knowledge of their enormous
distribution in air and water, and our conception

^ See Bulletin 44, Ag. Exper. Stat., University of "Wisconsin.

vi]' THE BACTERIA OF MILK ji

of the important role tliey perform with regard to
human life. In view of this, the progress already
made is most encouraging, and is a happy augury
for the future.

A word or two of a more or less general nature may
be in the first place said with regard to bacteria.

Description of the Different Micro- Or ganisras. —
These tiniest members of organic life, which repre-
sent its lowest form, and of which about a thousand
different species have been already studied, were
formerly regarded as belonging to the animal king-
dom, and were called by Ehrenberg in 1828 Infusoria
or infusion animals. The tendency of the most
modern research, however, is to classify them rather
along with the plant world. They consist of a single
cell, and grow like the cells of plants, which in many
other respects they resemble. Indeed they are related
by many links to the Algae. They may be divided
into different classes. This classification is generally
based upon their shape. Under the microscope
they are seen to be joined to one another in chains,
bundles, or heaps, and occasionally as firm glutinous
masses. Those possessing a round globular form are
known as cocci, micrococci, mcicrococci (when they occur
singly), cliplococci (when they occur in pairs), and
streptococci (when they are arranged in chains), and
lastly staphylococci (when they appear in grape-like
bunches). Sometimes they are united together by a

72 MILK [vi

jelly-like material ; and in such species as thrive in
moist or fluid surroundings, the outer layer of the
cell wall is of a jelly-like nature, and is provided in
some cases with minute whip- like filaments, the
cilia, by means of which they are enabled to swim
about, and in this respect resemble animal life. The
way in which such bacteria move differs considerably,
some having a circular spinning motion, while others
have a spiral motion, and others, again, glide along.
When the organism is exposed and dried, this outer
jelly-like layer becomes a hard crust. Those of a
rod -like shape are known as hacilli or bacteria, the
former being longer than the latter. Those possess-
ing a spiral or corkscrew shape are known as spirilla,
or when in a shorter form as comma hacilli. The
term bacteria, however, is generally used in a wider
sense to denote the micro-organisms above enumerated.
In addition to these there are two other classes of
micro-organisms : viz. yeasts, which are comparatively
large oval bodies ; and the moulds so familiar to all
as giving rise to hairy patches on various kinds of
food, especially jam (see Fig. 10, p. 73).

Method of Development of Bacteria. — A point of
much interest, and one also of great importance, is
the method in which bacteria develop. This is done
in a variety of ways. Some grow in length, some
increase by splitting up into two parts, which is known
as fission (Fig. 11, p. 74), and some by throwing off

VI]

THE BACTERIA OF MILK

11

spores (rig. 12, p. 74) — small round or egg -shape
bodies which are formed inside the bacterium, which

CO

%AW»il|||»

fr

if

dO

M

^ w/

Fig. 10. — DIFFERE^•T forms of Micro-Organisms. A, Micrococci (Frankland) ;
B, diplococci (Freudenreich) ; C, streptococci (Freudenreich) ; D, staphy-
lococci (Freudenreich) ; E, F, G, bacilli (Frankland) ; H, spirilla (Frank-
land) ; K, mould (penicilUum glaucum, Freudenreicli) ; L, yeast cells
(Frankland) ; M, comma bacilli (Frankland).

in turn, under suitable conditions of heat and mois-
ture, develop into full-grown bacteria. Xow in this
process of reproduction or development one or two

74 MILK [vi

points are worthy of note. For one thing, it may

take place at an enormous rate. A bacterium may

develop in twenty minutes' time the power of repro-

^ o ducing itself, so that under favourable

• «* s circumstances, in a few hours' time,

'' • * o

• it may give rise to a progeny number-

Fio. 11. -Micrococci, ^j^„ millions. Indeed, accordiuf^ to

SHOWING MODE OF *^ JO

MULTIPLICATION- BY Conu, tlic microbcs descended from

FISSION. (Frank-
land.) a single individual, if permitted to

develop under the most favourable circumstances,
would, in less than five days, occupy a space equal to
that of the entire ocean. It is needless to say that
such favourable circumstances never do exist.

Again, with regard to spores, it has been found
that they possess very much greater powers of re-
sistance than the developed
bacterium. The greater vital-
ity possessed by the spores,
in comparison with the adult
cells, is due to the harder
nature of the covering mem-

1 rm 1 j_i. • 1 Fig. 12. — Multiplication by

brane. The latter is, as a rule, spores of bacilli. (Frank-
comparatively easily killed by ^^°^-^
unfavourable circumstances, but the former in some
cases possess extraordinary powers of resistance.

^ize of Bacteria. — The size of bacteria is so minute
that they require a high power of the microscope to
distinguish them. They are only visible to the

vi] THE BACTERIA OF MILK 75

naked eye when they occur in colonies in some fixed
medium. Many of them are less than ^^ ^ ^ ^th
of an inch in length, and few of them greater than
-g-J-Q-th of an inch. Hundreds of millions of these
organisms could be spread over a square inch in
a single layer. A bacterium which causes lactic
fermentation is only about o 5 q q oth of an inch
broad and g Q^Qpth of an inch long. No less, there-
fore, than twenty -five thousand of these bacteria
could be placed side by side without occupying more
than an inch, and nine hundred billions of them
would only weigh ^^th of an ounce.

Conditions influencing the Developmeiit of Micro-
organisms.— The conditions of life of micro-organisms
is a point which it is very important we should con-
sider, as our knowledge of this subject furnishes us
with the means of regulating their development.

Bacteria may be divided into different classes,
according to their products. Thus a large class give
rise to pigments, another set up fermentation, and a
third class give rise to putrefactive decomposition.

We can, however, divide them into two great
classes, according as they live on dead matter or
on living organisms. To the latter class belong the
pathogenic — the disease-producing — germs. Among
diseases that have been proved to be due to bacterial
life may be mentioned tuberculosis (consumption),
diphtheria, erysipelas, lockjaw, pneumonia, typhus,

76 MILK [vi

cholera, glanders, malaria, and leprosy. The accom-
panying illustration shows the form of some of these
pathogenic germs (see Fig. 13, p. 77). Their pernicious nfl '^.
action is due to poisons, the so-called ptomaines,
which are produced within the body. The functions
performed by the other class of micro-organisms
are highly useful, and it is with them we are
mostly concerned when dealing with the bacteria
of milk. Among the most important conditions in
regulating their development is temperature. For
most the range of temperature is generally between
15'^ to 40'^ C. (59'- to 104' Fahr.), about 32' C. (90*^
Fahr.) being the most favourable temperature.
Comparatively low temperatures (50" to 60' C. ; 122^
to 140' Fahr.) are sufficient to kill most bacteria.
Similarly, low temperatures exercise a fatal effect.
On the other hand, the spores of many bacteria have
been found to possess great powers of resistance.
Some of them are able to resist the lowest possible
obtainable temperatures. Thus Pictet found that
the spores of certain bacteria were able to survive in
frozen oxygen at a temperature of — 213° C. ( — 353^
Fahr.). Some can survive boiling temperature ; while
a few have been discovered that are able to resist
even considerably higher temperatures, viz. dry heat
of 150' C. (302' Fahr.). The important bearing
which these facts have on the practice of dairying
is obvious. They explain why heating milk to be-

Vl] THE BACTERIA OF MILK -jj

tween 60'^ to 65' C. (140° to 150'' Fahr.) for some time

A

' /

E

/&'' f

.,,•"*. -••

.#•

o-^-

>~-^

X

\,

Fig. 13.— Pathogenic Germs. A, Bacillus tuberculosis (Koch); B, Spirilla
of Asiatic cholera (Koch); C, Bacillus of typhoid fever (Migula); D,
Streptococcus of erj-sipelas (Frankland) ; E, Bacilli of tetanus or lockjaw
— «, individual bacilli ; h, sjjores (Kitasato).

will do much to destroy bacterial life. And here it
may be pointed out that there is a considerable differ-

78 MILK [vi

ence between damp and dry heat, the former being
very much more deadly than the latter in its effect. In
order to make sure of effecting the complete destruc-
tion of the bacteria in any object, it is necessary to
submit that object to a very high temperature for
some time — a temperature of from, say, 160^ to 163^
C. (320° to 326° Fahr.). The fact that the spores
possess greater powers of resistance than adult germs
explains the efficiency of the method largely used
for reducing bacterial life in such fluids as milk.
This consists of heating the fluid from time to time
at a comparatively low temperature, but sufficiently
high to destroy adult germ life. AVhen such a fluid
is heated for the first time, many spores present in it
escape destruction. As, however, the fluid is kept
at a temperature favourable for germ development,
the spores in due time develop into adult organisms,
and are destroyed in the course of heating. Such a
method of treatment, which is known as intermittent
sterilisation, while it may be said to ensure complete i
destruction of micro - organic life, is yet too trouble-
some a method to have alw^ays recourse to. It is a
fortunate circumstance that most pathogenic germs
are comparatively easily killed. Generally speaking,
it may be said that bacteria do not develop below
4° C. (39° Fahr.), and much above 50° C. (122° Fahr.).
By cooling a liquid containing bacteria, therefore, to
4° C. (39° Fahr.), bacterial development is completely

vi]" THE BACTERIA OF MILK 79

stopped; but, as a rule, by cooling to V C. (45'' Fahr.)
the same end ^Yill be effected.

Amono' other conditions which regulate the de-
velopment of micro- organic life are the reaction and
nature of the liquid or other medium in which they
exist, the absence or presence of the oxygen of the
air, and the absence or presence of liglit. With
regard to the reaction of the liquid, it is a noteworthy
fact that some bacteria only develop in the presence
of a neutral or alkaline reaction, while others can
develop in an acid reaction. Most bacteria require
the former condition. An example of the latter kind
are those producing lactic acid — the bacteria which
induce the souring of milk. Acid reaction, we may
add, is especially favourable for the development of
moulds.

But one of the most important factors in regulat-
ing bacterial development is the nature of the
nutritive medium in which they find themselves.
Like higher organisms, they have their preference in
the matter of food. They require oxygen, carbon,
water, and mineral salts. Most of them also, how-
ever, require nitrogenous diet. It is hardly necessary
to observe that, of all media, milk is the best, and
hence it is that it becomes the happy hunting-ground
of so many of them.

Their behaviour in the presence of oxygen serves
to divide bacteria into two great classes. Some —

8o MILK [vi

and to them the term aerohies has been applied —
require air, and perish if they do not have access to
it. Others, on the contrary, can develop witliout
air. Eecent researches of a most interesting kind
have shown the important influence which sunlight
exerts on bacteria. It has been found that, for most
kinds of bacteria, sunlight is highly inimical. Indeed,
putrefactive liquids may be actually rendered sterile
by simply submitting them to the action of sunlight.
A free supply of air at the same time helps sunlight
to exert its full sterilising effect. Lastly, it may be
added that moisture, as a rule, is a necessary condition,
although its absence does not in all cases prove fatal.
With regard to the development of yeasts and moulds,
it may be explained that the former multiply by
" budding " or " sprouting," while the latter develop
by the formation of long threads (Jiyphae).

Having thus described the general nature and
conditions of life of micro-organisms, we may next
proceed to deal with those met with in milk. And
before dealing with the different species, it will be
well to say a word or two regarding their number.

Number of Bacteria in Milk. — We have already
pointed out that in milk, even drawn under the most
careful conditions, hundreds of thousands exist in
every ounce. From 60 to 100,000 have been found
in 1 cubic centimetre (-2^^^ ^^ ^^ ounce) of milk a
few minutes after milking. Milk investigated by

vi]

THE BACTERIA OF MILK

Uhl in different cities slio\Yed from 3,338,775 to
55,365,800 per litre (1| pints). Their number
rapidly increases on keeping, and it has been
estimated that in milk which has stood six hours
there may be from two to six millions, on an
average, in one cubic centimetre. The tempera-
ture, of course, will affect their number. Conn
has found, for example, that in a sample of milk
kept for four days in a cold cupboard there were
only 10,000 bacteria per cubic centimetre ; while the
same milk, taken from the cupboard and left in a
warm room for six hours, contained 1,000,000. The
following table shows the influence of temperature
on their rate of increase : —

IXCREASE IX XuilBER OF BaCTERIA (CnOPF).

At 34

C.

At 12-5' C.

(93' Fahr.)

(oi'o" Fahr.)

74

fold .

. none

23

3?

4 fold

64

;?

6 „

215

j»

8 „

1830

?>

. 26 „

3800

1;

. 435 „

In 1 hour

„ 2 hours

3) 3 „

4

33 ^ 3)

33 O 33
„ 6

As further illustrating the influence of tempera-
ture, it may be mentioned that a difierence of lO'' C.
(18" Fahr., viz. from 59^ to 77" Fahr.) made a differ-
ence in milk kept for 15 hours of 71,900,000 (viz.
from 100,000 to 72,000,000) bacteria per cubic centi-
metre (Miquel). In milk kept for 21 hours at 25' C.

6

82 MILK [vi

(77" Fahr.) uo less than the gigantic number of
677,500,000 bacteria per cubic centimetre have been
found (Freudeureich).*^

Classification of Bacteria infesting Milk. —
The bacteria infesting milk may be classified in
various ways. The classification adopted by
Grotenfelt ^ is the following : —

I. Bacteria luliose Action is Indifferent. — In the
first place, we have a large number of bacteria whose
action is quite indifferent; that is to say, which
exercise, so far as we know, neither a harmful nor a
beneficial action.

II. Bacteria vjhose Action is of a Useful Nature. —
Secondly, we have bacteria whose action in milk is
of an indifferent nature, but which are active in
milk-products — cheese, etc.

III. Bacteria vjJiose Action is of an Indirectly
Injurious Nature. — Thirdly, w^e have bacteria which
exert an indirectly injurious action on milk. Such
bacteria produce conditions which are favourable for
the development of bacteria whose action is directly
injurious. This class includes bacteria which cause
an alkaline reaction in milk — a reaction favourable
to a large class, indeed the majority, of ferments.
Under this class also come certain bacteria which
exert a directly injurious action. For instance, the

^ See Principles of Modern Dairying, by Grotenfelt, translated
by F. W. WoU (Jobn Wiley and Sons), p. 94.

vi] THE BACTERIA OF MILK 83

bacteria causing acidity in milk may be included
under this class, since they are the necessary fore-
runners of other bacteria which only thrive in
strongly acid media, and which are therefore most
frequently found in sour milk. As an example of
this class may be mentioned the butyric bacilli,
which impart to milk a strongly bitter flavour, but
which are most familiar through their action in
rancid butter.

IV. Bacteria ivhose Action is of a Directly Injurious
Nature. — Fourthly, there are bacteria which exert a
directly injurious action on milk, and which are
therefore of greatest interest in this connection.
This last class of bacteria may be divided into
different sub- classes. Thus we have one kind which
produces acidity, and of which the lactic bacteria
and certain colour-producing bacteria, such as the
bacillus prodigiosus, may be cited as examples. To
this sub-class also belong bacteria producing butyric
fermentation, which develop in the absence of air,
and which produce a slight coagulation of the
caseous matter of the milk, which becomes subse-
quently dissolved ; as also bacteria producing volatile
acids, under the action of which milk generally
assumes a grayish colour and becomes rather viscous,
while a lively generation of gas often takes place,
consisting of carbonic acid, sulphuretted hydrogen,
and, it may be, small quantities of alcohol and acetic

84 MILK [vi

acid. Another sub-class are those which produce no
acidity in milk, but which effect the coagulation of
the caseous matter, or which effect the fermentation
of the caseous matter without coagulation. This class
includes such bacteria as the 'potato hacilli, a group
of bacilli to which Duclaux has given the name
tyrothrix, and those bacteria which were formerly
known as "putrefactive" bacteria, since they
decompose albumin and produce an unpleasant
smell.

Lastly, we have micro-organisms present in milk
other than bacteria, viz. yeasts and moulds.

Before treating of the action of certain classes of
bacteria which effect the common kinds of fermenta-
tion going on in milk, such as its souring, it will be
convenient to refer to certain milk " faults " which
have been shown to be generally caused by the action
of bacterial life. Of these, the following are the most
important : —

Blue Milk. — This fault consists in the develop-
ment in the milk, after a lapse of from twenty-four
to seventy-two hours, according to the temperature
of the milk, of patches on its surface of a blue
colour. It was the first kind of fermentation of
milk to be studied, having been investigated by
Foulkes in 1841. The formation of these blue patches
only takes place — to any extent, at any rate — after
the milk has assumed a slight, though distinct acid

vi] " THE BACTERIA OF MILK 85

reaction. It completely ceases as soon as the milk
has become coagulated, or, to speak more correctly,
as soon as the caseous matter has become coagulated.
It was on this account formerly thought that the
cause of blueness in milk was connected with the
casein, and that it was produced by some chemical
ferment which acted upon the casein. We now
know, however, that blueness in milk is due to the
action of micro - organisms. The history of this
discovery is an interesting one, and much research
has been devoted to its elucidation. The organism
chiefly implicated in the action has been called the
hacilhts cyanogenusf' It cannot produce a blue colour
apart from milk, and seems to act on the casein and
to have no effect on the milk-sugar. Indeed, it
would seem that the explanation of blue milk is a
double one, and that it is the joint work of the
above-mentioned bacillus in conjunction with the
lactic organism, since it has been found that if the
hacillus cyaiiogemos be inoculated into sterilised
milk, it does not produce a blue colour, this being
only produced by the addition of a little acid,^ or by
inoculating the milk with lactic organisms. It thus
seems to be an example of what is known as
symbiosis, that is, the association of several species
of micro-organisms, working together to produce
some special effect. We have further examples of
such " feeding " associations, in other departments of

86 MILK [vi

agriculture, such as in the case of the organisms
which enable leguminous plants to utilise the free
nitrogen of the air. Further evidence in support of
this explanation is to be found in the method in
which the blue patch first appears.

Where this organism comes from is not known,
but there can be little doubt that its source is
filth. Blue milk possesses an acid smell. It may
be churned, but does not yield good butter ;
although it is a mistake to suppose, as was formerly
done, that it possesses any poisonous properties.
It may be added that it is killed by exposure to a
temperature of 80° C. (176° Fahr.).

Red Milk. — This fault may be caused by red
colouring matter in the food, or by the red colouring
matter of the blood, which passes into the milk, or
it may be caused by the action of micro-organisms.-^
Where the food causes redness in milk it is generally
due to the presence of madder in it. When the
colour is due to blood, this may enter the milk from
some small rupture in the udder. Lastly, where
redness is due to the action of micro-organic life, it
is generally caused by the development of a blood-
red micro-organism, the micrococcus j)rodigiosns^
which develops especially abundantly on potatoes.
Among other micro - organisms which have been
identified as causing it are the hacillus Icictis
erythrogenes and the sarcina rosea. The occurrence

VI]

THE BACTERIA OF MILK

87

of such micro-organisms in milk is a very rare one.
The bad influence they exert on the milk consists in
the fact that they cause it to rapidly coagulate.
In other cases they lead to the formation of
trimetliylamin, and produce a herring-like smell and
flavour.

Yellow Milk. — This may be produced by the
hcccilbts si/nxanthus —
a micro-ors^anism be-
lonsfinc^ to the casein
group of ferments, to
be subsequently con-
sidered — or other
micro-orcranism. The
organisms producing
it do not act alike.
Some produce a bril-
liant yellow colour,
while others resemble
rennet in their action,

5,^-

-'.tL -£'i

and first curdle the ^^^' i^- — colonies of bacillus violaceus.

(After Grace C. Frankland.)

milk, the curd being

then dissolved into a yellow or orange or amber-
coloured liquid.

There are also certain kinds of organisms which
turn the milk gj^en, and some which turn it violet
As one of the causes of this last fault, the hcccillus
violaceus has been identified (Fig. 14).

88 MILK [vi

Slimy or Ropy Milk. — Milk occasionally, in-
stead of exhibiting its ordinary limpid nature, be-
comes thick and slimy. Such milk either does not
cream at all, or else very imperfectly. It cannot be
churned, and is not suited for drinking purposes.
In some countries, however, — as, for example, in
Norway, where it is really liked as an article of
diet (Tattemyelk) — it is produced artificially by im-
mersing the stem of the butter -wort {Pinguicula
vulgaris) in the milk, or by feeding this plant to the
cows. Slimy milk is used in the manufacture of one
kind of cheese, viz. Edam. On the whole, however,
it is regarded as an unmixed evil, and has caused
great trouble in dairies. Various theories have been
put forward with regard to the cause of slimy milk ;
but, like most other faults which we have mentioned,
it has been finally traced to the action of micro-
organisms. Pasteur was the first to discover that
there was a special yeast which had the power of
giving rise to a slimy fermentation of milk-sugar,
and thus gave a proper direction to the investigation
of the subject. Lister subsequently studied the
question of slimy milk, and attributed it to the
growth of bacteria. More modern researches on the
subject seem to show that slimy or ropy milk is
caused by a large variety of micro-organisms, among
which may be mentioned the bacillus mesentericus,
leuconostoc mesenteroides, actindbacter du lait visquetix,

vi] " THE BACTERIA OF MILK 89

actindbacter polymorphus, hacillus viscosus^ bacillus
lactis pihdtosi, potato hacilli, streptococcus Holland-
icus, micrococcus Freudenreichii, and hacterii'.m Hessii,-
as well as others.

It must not be imagined, however, that the
nature of the fermentation is the same in all
these cases. They produce a widely different
effect on milk. Some induce only a slight amount
of sliminess, while others have a very much more
marked effect. The length of time they take to
effect this slimy condition differs widely, and the
products are also of a varied character. In some
cases the substance produced seems to resemble
cellulose in its nature, in others it seems to be of the
albuminoid order. Mannit, carbonic acid, lactic acid,
butyric acid, peptone, etc., are among the products.
Altogether, no less than eighteen distinct organisms
have been identified as associated with tliis slimy
fermentation.

Bitter Milk. — A very common fault in milk is
what is known as bitter milk. This may be, Like
many other faults, due to a number of different causes,
but it is probably most commonly due to micro-organic
life. In the first place, it may be caused by the
nature of the food, which may contain some bitter

^ In length from the .^ ;> ^ ^, „ tli to Tiiroth of an inch, and in
breadth from the ^ .> > 0 0 th to ^tstiFu th of an inch.

- From the g-rtrcrth to the 317517 th of an inch long, and -sTj-^iTTr th
of an inch broad.

90 MILK [vi

principle. This is probably the case only when the
bitter taste is present in the milk immediately after
milking; when it is developed only after standing
for some time, it is due to some other cause. In
such a case the cure is obviously to change the
feeding. It may be yielded by cows which have
been a long time in milk, and in such a case it is
due to imperfect secretion of the milk in the milk-
glands. Lastly, it may be due to inflammation of
the udder.

The older theory that bitter milk is always
accompanied by the production of butyric acid has
not been shown to be absolutely correct ; for, while
it is at times associated with the production of
butyric acid, it is also found to be produced by
organisms which do not produce butyric acid. There
are quite a number of species of bacteria which
effect this result, and it would seem that in all cases
the bitter flavour of the milk is independent of
butyric acid. Some of these organisms resemble in
their appearance the j)toUus vulgaris. To what,
precisely, the bitter taste is due is not as yet known.
One theory is (Hueppe) that it is connected with
the decomposition of the albuminoid matter of
milk, and with the production of peptones ; and it
is to these peptones and similar products that the
bitter taste may be ascribed.^ The fact that these
bacteria chiefly belong to the casein group of ferments,

vi] THE BACTERIA OF MILK 91

which are characterised by their great powers of
resistance, explains why it is that the bitter taste is
most frequently found in boiled milk which has been
allowed to stand for some time. In such cases, by
boilincf the milk, the bacteria effectincr lactic ferment-
ation are killed, and the spores of the bitter-milk
bacteria only are allowed to remain. Such treat-
ment is favourable to their development, since the
presence of lactic acid in unboiled milk is inimical
to their growth. Among the micro-organisms which
have been identified as causing this fault may be
mentioned "Weigmann's Mtter - wAlk hacillus, which
possesses a length of f^-yoo^^ ^^ 1 s s 0 0^^ ^^ ^^ moh.
long, and from 21 \q q^^ ^*^ ^ .^ I q ^th of an inch
broad ; and Conn's hitter-milk micrococcus.

There are a number of other faults to which milk
is liable, and which are due to micro-organic life,
such as premciticre curdling, and the development of a
salt taste, regarding which not much is as yet known.

Having discussed the different " faults " to which
milk is liable, we may pass on to describe the cause
of the more common changes which milk undergoes
in keeping. The bacteria causing these changes may
be most conveniently classified according to their
products.'^ "We have two principal groups, known
as Lactic ferments and Casein ferments.

Lactic Fermentation of Milk. — The most com-
mon fermentative change in milk is its souring.

92 MILK [vi

which takes place on keeping milk for any length
of time, and which results sooner or later in
its so - called spontaneous coagulation. This sour-
ing is due to the formation of lactic acid, derived
from the decomposition of the milk-sugar in the
manner explained in a former Chapter. The pro-
duction of lactic acid from milk-sugar may be
effected by quite a large class of bacteria ; and it has
been found that while one ferment may be the
common exciting cause in one district, another
ferment may be the exciting cause in another
district. In the case of some of these bacteria the
lactic acid is merely incidentally formed along with
other products. Such bacteria are those producing
different colours in the milk, which we have already
referred to. Another interestincr fact with re^jard to
this class of fermentation is that it is highly doubtful
whether any two of the organisms effecting it act on
the milk in exactly the same manner. An interesting
observation made by Grotenfelt has shown that the
lactic bacilli lose their power of producing lactic
acid if they be cultivated for a time in a medium
free from sugar. Again, the amount of lactic acid
produced depends on the original condition of the
milk, as well as the temperature. One of these
ferments splits up the molecule of milk-sugar com-
paratively easily into four molecules of lactic acid,
producing at the same time an extremely slight

vi] THE BACTERIA OF MILK 93

evolution of carbonic acid. Other lactic ferments
produce small quantities of secondary by-products,
especially alcohol. The most important practical
application of lactic fermentation is in the souring
of cream in the manufacture of butter ; and as it is
known that the various different lactic bacilli are not
all equally adapted to act as ferments in effecting this
change, attempts have been made to isolate and
cultivate, in pure cultures, such as will produce the
best butter with the finest aroma. A point of much
interest in lactic fermentation is that it only continues
for a certain time, this being the case with many
other kinds of fermentation. When it has proceeded
to a certain extent, the acid generated becomes fatal
to the bacterial life causing it, and it ceases ; con-
sequently when much over one per cent of lactic
acid is generated fermentation ceases. Curdling, it
may be added, only takes place after the lactic acid
has reached a certain amount.

A number of the bacteria causing lactic ferment-
ation have been isolated and studied by different
observers ; among others by Hueppe, Grotenfelt,
Marpmann, Krueger, and Weigmann. Thus Hueppe
has discovered a bacillus about ^ , ^ ^ ^th of an inch
long and gT^^th of an inch broad. It develops at
temperatures between 10" to 45° C. (50° Fahr. and
113' Fahr.), and most rapidly at 35° C. (95° Fahr.).
Other bacilli, micrococci, sphaerococci, streptococci,

94 MILK [vi

and bacteria have been studied by Hueppe and the
above-mentioned investigators, which cause lactic
fermentation. Lactic bacteria do not, as a rule,
develop spores, and consequently have not very
great resistant powers ; a temperature of 70° C.
(158° Fahr.) usually killing them. The chief diffi-
culty of keeping milk is due to the presence of this
class of bacteria ; and hence it is that heating milk
even to a comparatively low temperature has such
an effect in preserving it.

Lactic ferments play a very important role in the
dairy in the case of making butter from slightly
soured cream. This is generally done by adding
small quantities of sour milk to the cream, and then
permitting it to stand for some time before churning.
But here there is a danger, since it often occurs
that the milk thus added to the cream contains
bacteria which give rise to disagreeable odours and
a bad taste. From a bacteriological point of view,
therefore, it may be said that butter should only be
made from sweet cream. Weigmann has attempted
to prepare pure culture of cream -ripening ferment.
In this attempt he has been successful, and he has
succeeded in isolating a micrococcus which sets up
normal lactic fermentation.

Certain of the lactic bacteria exert, as we have
just indicated, a harmful action. This is seen in the
manufacture of cheese. They split up the milk-

vi] ' THE BACTERIA OF MILK 95

sugar with such energy that gaseous products are
evolved, and the cheese becomes perforated with holes.
To one such class of bacteria Freudenreich has
applied the name Schaffer bacillus. It seems to be
closely related to a bacterium common in the intestine
— 'bacterium coli commune. It is hence of the greatest
importance that milk should not become contamin-
ated, in the process of milking, with cow-dung.

Butyric Fermentation. — Occasionally it happens
that milk or cream coagulates without any previous
lactic fermentation. This is seen in the coagulation
of milk which has been boiled, and the reaction of
which is neutral. In this type of fermentation,
which is accompanied by the development of an
alkaline reaction, and not an acid reaction, the
changes are similar to those effected by rennet. It
is characterised by the formation of a bitter taste,
and by the solution of the coagulum into a somewhat
clear liquid, and the formation of butyric acid. It is
caused chiefly by the butyric bacillus, but here again
we seem to have a number of different bacteria
working. Indeed it would seem that there are quite
a number of bacteria producing butyric acid.^ It
has been suggested, therefore, to constitute a group
of butyric ferments. If, however, as Freudenreich
truly points out,^ butyric acid is rather to be regarded
as a residue resulting from the breaking down of
^ See Dairy Bacteriology, p. 88.

96 MILK [vi

casein and milk-sugar in various ways, and is con-
sequently not to be regarded as a uniform process, it
is best not to adopt this method of classification.

Casein Ferments. — Just as we have a class of
ferments which act upon the milk-sugar, so we have
a class that decompose the casein. To the latter class
the name casein ferments has been applied. This order
of bacteria have also the power of coagulating the
milk, not, like the lactic ferments, by the production
of lactic acid, but by the production of a rennet-like
substance. As belonging to them may be men-
tioned the Tyrothrix tenuis, which has been studied
by Duclaux ; and a ferment which Conn has suc-
ceeded in preparing in the form of a powder, and which
acts like rennet ferment. Some of these ferments have
the power of redissolving the curdled milk by means
of second ferments. They play an important part in
the ripening of cheese, and the decomposition of
the casein which they initiate is of a most com-
plicated character, and is accompanied by the
production of such substances as peptone, leucin,
tyrosin, and butyric acid. Among them are the
hay and potato bacilli, and a number of so-called
butyric ferments. Those best known are those which
have been studied by Duclaux, to which he has given
the name Tyrothria, and of which eight or nine differ-
ent kinds have been identified. The casein ferments,
unlike the lactic ferments, are for the most part spore-

VI] ■ THE BACTERIA OF MILK 97

forming, and are consequently extremely ditticult to
destroy, since many of their spores can resist a tem-
perature of about 115 C. (240' Fahr.). It is owing
to their presence in it that milk is so difficult to
completely sterilise.

There are one or two other forms of fermentation
w^hich do not come under any of the classes above
mentioned. Among them is alcoholic fermentation.

Alcoholic Fermentation. — This kind of fermenta-
tion does not often occur in milk ; but although it is
true that milk does not readily undergo alcoholic fer-
mentation, yet it can be induced'^; and in certain
parts of the world two beverages made from milk
which has undergone this kind of fermentation
have been long in use. These beverages are Iccpliir
and koumiss.

Kephir. — Kephir has long been used in the
Caucasus, and is made from cow's milk as a rule.
Investigations were first made into its nature as early
as the close of last century, and since then it has
been made the subject of further research. It is
made by the help of a special ferment known as
"kephir grains." This ferment is in the form of
hard, yellow, granular lumps about the size of a pea.
By soaking these kephir grains in w^ater and then
adding them to milk, alcoholic fermentation is
speedily induced, and the beverage is ready for use
in the course of two or three days. The object of

7

98

MILK

[VI

soaking the grains, which has the effect of making them
swell, is for the purpose of increasing their activity.
After the beverage is prepared, the grains are taken
out of it, dried, and kept for future use. When dried
they may retain their fermenting power for years.

A certain amount of mystery surrounds these
kephir grains. Their origin is unknown, since they
have been used from time immemorial. Their com-
position, although investigated at considerable length,
is also somewhat of a mystery. They seem to con-
tain a number of different kinds of bacteria and
moulds, among which yeast fungi predominate. The
other micro-organisms which have been found,
according to different investigators, in kephir grains
are the oiclium lactis, leptotlirix dispora Caucasia (?),
saccharomyces cerivisiae, hacilhts acidi lacti (lactic
bacilli), hacilhts hutyriciis. The following chemical
analysis of kephir will show its composition
(Hammarsten) : —

Water .

88

915

Fat .

3

088

Casein

2

904

Lactalbumin

186

Peptone Bodies

067

Sugar .

2

685

Mineral Salts

708

Alcohol

720

Lactic Acid .

•727

100-000

vi] ■ THE BACTERIA OF MILK 99

Struve found the kepliir grains contained 51 '69
per cent of albuminoids and 3"99 per cent of
fat.

Nature of Keinliir Fermentation. — Although the
exact nature of the kephir fermentation is not
known, there can be little doubt that it is a very
complicated one. It is certainly not, as was at first
believed, a simple case of the alcoholic fermentation
of milk-sugar by means of yeast fungi ; although it
has been found that certain kinds of yeast, present in
kephir grains, are able to produce alcoholic ferment-
ation. The number of fermentative products present
in kephir shows that a number of micro-organisms
are active in the process, some of them giving rise
to alcohol, others to peptone and a variety of
acids and other bodies, and others coagulating the
casein.

Koumiss. — Although not exactly identical with
kephir, koumiss is of a similar nature. In the steppes
of Eussia, where it has been long used, it is generally
prepared, not from cow's milk, but from mare's milk.
It is, like kephir, a foaming liquid, resembling
butter-milk or sour whey. It differs from kephir
slightly in composition, and from the fact that, while
kephir may be made direct from cow's milk, koumiss
can only be made after the addition of a little cane-
sugar. The following is an analysis of koumiss
(Fleischmann) : —

100

MILK

[VI

Mare's Milk

Water 91-53

Milk-Sugar .

1-25

Lactic Acid .

1-02

Casein

1-91

Fat .

1-27

Alcohol

1-85

Carbonic Acid

•88

Mineral Matter

•29

100-00

Koumiss may be made by adding a little cane-
sugar and yeast to skim milk.

Both these beverages have a high dietetic value,
and koumiss has of recent years been used by
medical men as a tonic. The dietetic value does
not seem to be due to the alcohol they contain, but
chiefly to the peptonised condition of the casein,
which is thus more easily digested.

Pathogenic Germs. — The bacteria which we have
just been discussing may be regarded as, for the most
part at any rate, normal inhabitants of milk. There
are, however, a class of microbes which may be
resrarded as abnormal. These are the so-called
pathogenic or disease-producing bacteria, which we
have already mentioned, but not yet discussed. It
may be well to discuss very briefly this question.

The method in which these pathogenic germs act
differs in the case of difi"erent germs. Some directly
absorb their nutriment from the surrounding sub-

VI] THE BACTEPJA OF MILK loi

stance by decomposing it ; while some produce
enzymes, which in their turn cause a chemical de-
composition, the products of which are utilised by
bacteria as food materials. We have such bodies as
ptomaines and toxin thus produced. Old cheese is
on this account rather a dangerous article of food.

Bacillus of Tuherculosis. — The pathogenic microbe
which undoubtedly is most commonly present in milk
is the bacillus of tuberculosis (see Fig. 13, p. 77). As
we have already pointed out, the presence of this
bacillus is probably nearly always due to the presence
of tuberculosis in the cow. According to Hirschberger,
10 per cent of the cows living in the neighbourhood
of towns, where they are not treated properly, suffer
from tuberculosis ; and 60 per cent of these yield
milk containing tubercle bacilli. The virulent char-
acter of the bacilli in such milk has been proved by
experiment. In Copenhagen four out of twenty-eight
samples of mixed milk proved virulent when injected
under the skin. Freudenreich ^ quotes a case, cited
by Brouardel, where five out of fourteen young girls
living together in a boarding-house became consump-
tive, subsequent to the daily use in the establishment
of milk from a tuberculous cow.

It is truly an unfortunate fact that tubercle
bacilli seem to be able to live in dairy products for a
very long time. In butter they have been found to

^ Dairy Bckteriology, \\ 43,

102 MILK [vi

be alive after the lapse of 120 days ; and in cheese
after the lapse of 35 days. After such an interval of
time as a month, they probably become so attenuated
as no longer to be very dangerous. It is noteworthy
that the tubercle bacillus is not killed at such a low
temperature as the majority of bacteria, which, as
we have already noted, are killed at a temperature
of under 60'' C. (140° Fahr.). Experiments have
proved that it is necessary to heat milk for thirty
minutes at a temperature of 65° C. (149° Fahr.),
fifteen minutes at 68° C. (155° Fahr.), and ten
minutes at 75° C. (167° Fahr.) ; but it w^ould be
better to boil the milk in cases where the presence
of tubercle bacilli is suspected.

Cholera Bacillus. — That milk may be the means of
spreading cholera has been proved beyond doubt in
several cases. The cholera bacillus (see Fig. 13, p. 77)
is not, however, capable of living so long in milk as that
of tuberculosis. In ordinary milk, indeed, it seems
to be killed in the course of twenty-four hours ; in
boiled milk, however, it is capable of multiplying
abundantly. The reason of this is to be found in
the fact that in unboiled milk the lactic acid formed
by the numerous lactic bacteria present exercises a
speedily fatal effect on the cholera bacilli ; in boiled
milk, on the other hand, the conditions are more
favourable, since the lactic ferments have been killed
by the process of boiling. In butter, it would seem

VI] THE BACTERIA OF MILK 103

that the cholera bacilli may live for four or five days.
In cheese, experiments show that they do not seem to
be a source of danger. Here again the effect of boiling
the milk would be to destroy the cholera germs.

Typhus Bacillus. — Another pathogenic germ which
has been found in milk is that giving rise to typhus
(see Fig. 13, p. 77). The number of typhus epi-
demics traced to this source has already been referred
to. The bacillus of typhus has been found to be
able to survive in butter for a period of from five to
seven days.

With regard to the behaviour of other pathogenic
germs in milk we know very little. That such
diseases as scarlet fever and diphtheria have been
propagated by milk we have abundant proof; but as
the microbes causing these diseases have not as yet
been isolated, we know little about the conditions
under which they develop.

As already pointed out, we have certain moulds
and yeasts which frequent milk or milk products.
Some of these moulds — as, for example, the widely
distributed green mould ijenicillmm glaucuni (see
Fig. 10, p. 73) — are familiar in cheese ; indeed,
the last-named mould plays an important part
in the ripening of Eoquefort cheese. It is also
familiar in Gruyere, Gorgonzola, and Brie cheeses.
Another kind of mould common in milk is the
oidium lactis, abundant in sour milk. It presents a

I04 MILK [vi

fine white velvety appearance. Among the yeasts
may be mentioned the saccharomyces lactis, the sac-
cliaromyccs acidi Icidici, and the saccharomyces richer.

Methods of Destroying or Regulating Bac-
terial Life in Milk. — Before concluding this Chapter,
it may be well to say a word or two on the methods
we have at our disposal for regulating or for entirely
checking the development of bacteria in milk.

At first sight the task of excluding bacteria from
milk, or even regulating their development, seems to
be a hopeless one. But this is not so. Perfect
sterilisation is not, under present practical methods,
possible, nor, indeed, is it of such great importance,
although in many cases highly desirable. In the first
place, we cannot expect to entirely prevent the
entrance of bacterial life to milk in the process of
milking. 'We may, however, by the exercise of
scrupulous care and the observance of cleanliness,
minimise contamination. If milk is dirtily and care-
lessly handled, bacteria, with spores possessing ex-
tremely resistant properties, are apt to take possession
of it, such as those of the butyric, the hay, and potato
bacilli, which render subsequent sterilisation by heat
an extremely difficult task.

Sterilisation of Milk. — A distinction should be
made between two terms which are often used
synonymously, viz. sterilisation and Pasteurisation.
Perfect sterilisation of milk can only be effected by

vi] THE BACTERIA OF MILK 105

submitting the milk to the action of continuous -
steaming for two hours at a temperature of 120' C.
(248' Fahr.), or for thirty minutes at a temperature
of 130' C. (266' Fahr.) (Fleischmann). Sterilisation
is the term generally applied to the employment of
temperatures as high as, or higher than, the boiling
point of "svater. Submitting milk to a high tempera-
ture is objectionable, however, for several reasons.
When so treated, the composition of the milk under-
goes a certain amount of change, which not merely
affects its biological condition, but also its physical
condition. The soluble lime salts which it contains
are converted into an insoluble condition. This
change prevents the milk from forming with rennet
a cohesive coagulation, the coacjulation under such
conditions being flocculent. Furthermore, the original
fine state of di^ision of the fatty globules is partially
destroyed, and a large number of them come to
the top. The milk also assumes a dirty brown or
yellowish colour, and a strong taste of boiled milk.

Pasteurisation of Milk. — The word Pasteurisa-
tion is applied to the use of heat at comparatively
low temperatures, those generally ranging from 60°
to 64° C. (140' to 147"' Fahr.). Heating milk even at
comparatively low temperatures, such as in the case
of intermittent pasteurisation, effects certain changes
in milk, but nothing like to the extent that is done
by heating to a temperature of 120' C. (248'' Fahr.).

io6 MILK [vi

Pasteurisation imparts to milk a slight taint of
the cooked flavour, but this flavour is removed on
the milk being cooled down. The maximum tem-
perature, therefore, to which milk may be submitted
in pasteurising should be below that which imparts
to the milk a permanent cooked flavour. Thus it
has been found that while heating the milk to a
temperature of about 65° C. (149° Fahr.) for twenty
minutes induces the flavour of cooked milk, this
flavour is not permanent, and quickly disappears on
cooling. According to Duclaux, milk suffers per-
manent change in taste when heated to 70' C. (158°
Fahr.). This temperature, then, would seem to be
the limit to which milk should be heated in pasteur-
ising. Pasteurisation, however, merely temporarily
checks fermentation, since it does not kill all the
bacterial spores in the milk. The curdling of milk
which has been pasteurised is rather different in its
character from ordinary sour milk curdling, and is
brought about by bacteria, which are able to excrete
rennet. Lactic acid bacteria, as a rule, do not form
spores.

For the above reasons intermittent sterilisation,
on theoretical grounds, is to be preferred to all
other methods. Unfortunately, however, it is such
an inconvenient method, and requires so much time,
and is so little suited for extended application, that
it cannot be carried out on a wholesale scale. At

vi] THE BACTERIA OF MILK 107

present, therefore, it is impossible to effect the perfect
sterilisation of milk. We must be content, accord-
ingly, with partial sterilisation, such as is effected by
pasteurisation. Intermittent sterilisation, we may
mention, may be carried out by heating the milk for
two hours at a temperature of from 70' to 75' C.
(158' to 167^ Fahr.), then keeping it at a temperature
suitable for germ development, viz. about 40"' C.
(104'' Tahr.), in order to permit of the spores which
are left behind to develop into adult bacteria. The
milk is again submitted for two hours to a similar
temperature, and then again allowed to stand for
several days at the same favourable temperature,
40^ C. (104"^ Fahr.). These consecutive changes of
temperature are repeated four or five times, and at
last the milk is brought to 100'' C. (212' Fahr.). But,
while complete sterilisation of milk may be described
as hardly within the range of practical dairying, partial
sterilisation is easily effected. It is a well-ascertained
fact that bacteria commonly occurring in milk, and
which excite its common fermentation, as well as the
most dangerous pathogenic germs, can be easily and
surely destroyed by heating for an hour at a tem-
perature of from 68' to 75' C. (154' to 167'' Fahr.),
or by heating for three-quarters of an hour at 100' C.
(212' Fahr.) with steam. It is not difficult, therefore,
to perfectly sterilise milk which only contains the
common bacteria, and at the same time not to affect

io8 MILK [vi

to any extent its colour, its chemical composition,
and the state of division of its fatty globules. Un-
fortunately, milk of this description — that is, free
from spores of a very resistant nature — is of rare
occurrence in ordinary practice. As a rule, milk will
be found to contain a number of resistant spores,
which enter it owing to careless handling.

Unfortunately the hacierium svMilis (hay bacillus),
a bacteriuQi possessing most resistant spores, is one of
the most common in the dust of the byre. Another
source whence resistant spores are apt to enter milk
is the proximity of fermenting foods kept in the
neighbourhood of the byre : such a food, for example,
as silage. It may be added that the pasteurisation of
milk is calculated to do much to reduce the spread
of disease among infants, whose sole food is milk.
As exemplifying this, it has been claimed that the
introduction of pasteurised milk among the poor
people of Xew York has been instrumental in greatly
reducing infant mortality during the hot summer
months. It is important also to point out that
pasteurisation has no injurious effect on milk in-
tended for butter-making purposes. Xo difficulty is
experienced in obtaining the required texture and
grain of the butter. The relative digestibility of
sterilised as well as pasteurised milk, as compared
with ordinary milk, is a question which has been
much debated. It seems to be highly probable.

VI ]" THE BACTFRIA OF MILK 109

however, that neither pasteurisation nor sterilisation'
of milk impair — to any extent at any rate — its
digestibility.

Importance of Cleanliness in Handling Milk. —
From what has been above said, it will be seen that
the sterilisation of milk is sometimes easily effected,
and sometimes with extreme difficulty.

As illustrating;- the effect which even heatinsf at a
comparatively low temperature has on the keeping
qualities of milk, it may be mentioned that Fleisch-
mann found by numerous experiments that milk
pasteurised at 70' to 75" C. (158' to 167' Fahr.), and
then maintained at a temperature of 12' to 14^ C,
(54^ to 57' Fahr.), kept at least thirty hours longer
than ordinary milk. If sterilisation is to be effectual,
it cannot be too strongly impressed upon the mind
of the dairyer that the greatest care should be taken
in handling the milk in as cleanly a manner as
possible. It is from the impurities which are apt
to enter into milk through dirty handling, such as
particles of manure, of skin, of food, hair, woollen
threads, cobwebs, etc., that the resistant spores
common to milk are derived.

As exemplifying the influence of cleanliness, an
experiment carried out by the eminent German ex-
perimenter Soxhlet may be cited. A cow which
had been kept in a badly ventilated byre was milked
without having its udder previously cleansed. It

no MILK [vi

was found that under such circumstances the milk,
kept at a temperature of 60^ Fahr., coagulated in
fifty hours. The milk drawn from the same cow, but
under different conditions, viz. in an orchard, i.e.
in the open air, and after the udder and hands of
the milker had been cleansed, only coagulated, al-
though kept at the same temperature, after the lapse
of eighty- eight hours.

Too great stress cannot be put on the importance
of cleanliness, since milk is such an admirable
medium for supporting bacterial life. It readily
absorbs bad gases, therefore precautions should be
taken that the situation of the dairy removes it from
any risk of contamination from such sources. The
danger of infection of milk from dust may be
strikingly illustrated by the statement that 1 gramme
(-2Vth of an ounce) of dust from the road at Turin
was found by an Italian investigator, Maggiora, to
contain no less than seventy -eight millions of
bacteria. The air of rooms in which milk is kept
should be cool. If the air is warmer than the milk,
the tendency of the milk to absorb impurities there-
from is very much increased. Air in milk-rooms
should also be kept dry. Of no less importance is
cleanliness in the milk vessels.

It has been pointed out by Hueppe, an eminent
bacteriologist who has investigated the subject, that
in effecting the sterilisation of milk destined for the

vi] ■ THE BACTERIA OF MILK in

use of cliildren — that is, where perfect sterilisation
is desirable — the milk should be previously submitted
to the action of the centrifugal separator. He asserts
that in the slimy residue which is separated from
the milk in this way most of the micro-organisms
present in the milk, and especially those possessing
great resistant properties, are to be found, and may
thus be removed from the milk and render its
subsequent sterilisation much easier. On the other
hand, it has been asserted that in separation by
centrifugal force, the cream receives the greatest
number, and that the milk becomes much more
largely charged with bacterial life, from the au\.
which is sucked in in such large quantities into the
machine when in operation. According to 0. Lugaer,^
one litre of milk containing; 2,050 millions of germs,
on being passed through a centrifugal separator, had
these germs distributed in the various products, in
the following proportions, viz. 1,700 millions in the
cream, 560 millions in the butter -milk, and 18
millions in the dirt. It seems to be true, however,
that one disease -producing germ generally separates
out in the dirt, viz. the tubercle bacillus. It is not
necessary to enter into a discussion of the various
methods which have been suggested by Soxhlet,
Hueppe, and others, for effecting sterilisation of milk
for household purposes, etc.

1. See Minnesota Experiment Station Becord, 1S93, p. 2S5.

112 MILK [vi

While the application of heat is the most favour-
able method of effecting sterilisation, the preservation
of milk may also be effected by the application of
cold. Cold, however, while it may retard the de-
velopment of bacteria, does not, unfortunately, entirely
destroy them. Certain pathogenic germs have been
found capable of resisting the lowest attainable
temperatures. If kept in refrigerators, milk remains
sweet for a long time. Unfortunately, a serious
objection to such treatment of milk is the separation
of the cream from the skim milk. A change in the
physical condition of the milk seems to be effected,
with the result that milk which has been frozen is
very different from ordinary milk. It is for this
reason that the attempts which have been made, on
a large scale, more especially in Paris, have resulted
in failure. We may add that cooling milk to a
temperature of 10° C. (50' Fahr.), while it does not
affect the physical condition of the milk, exercises an
important preservative influence.

Chemical Agents. — Various chemical agents
have again and again been used as preservatives.
Obviously the most effective disinfectants, such as
corrosive sublimate, carbolic acid, etc., cannot be used
on account of their poisonous nature ; whereas such
substances as salicylic and boracic acids, carbonate
of soda, quicklime, and hydrogen peroxide have to
be used in such quantities, if they are to be at all

vi] " THE BACTERIA OF MILK 113

effective, as to render them also unsuited.^" All
authorities, however, unite in condemning the use of
preservatives for milk.

(f) The diseases whicli may be directly transmitted from tlie cow
to man are — in addition to tuberculosis — scarlet fever, diphtheria,
and foot and mouth disease. (^) A few dairy bacteria are occasionally
known to have pathological characters. Four of these are Bacillus
coli coramunis, Sta2)hylococcus alhus, Staphylocociius 'pyogenes aureus,
and Streptococcus pyogenes aureus. {'^) It should be pointed out that
the actual number of bacteria in milk is of little significance. The
most wide variations have been found in the same milk under
different conditions. {'^) Gessard has quite recently obtained several
distinct varieties of this organism by cultivating it under different
conditions. These varieties differed widely in the type of pigment
they produced. (*) Gessard's work, above referred to, seems to in-
dicate that, in order to produce the typical blue colour, the acid
must be lactic acid. (/) It is somewhat misleading to speak of red
milk and violet milk in the same category as blue milk. Blue
milk is a dairy infection which attacks the milk rapidly, and pro-
duces in a short time a very prominent pigment. In the case of
the other types mentioned the pigment is slow in appearing, some-
times a few days, and sometimes several weeks elapsing before it
makes its appearance to any appreciable extent. (^) Freudenreich
has recently disproved this theory of Hueppe's. He has succeeded
in separating the bitter-tasting material in a partially free condi-
tion, and finds it is not a peptone. {^) Up to the present time con-
siderably over 200 dairy bacteria have been described. (*) Up to
the present time about a dozen organisms are known to produce this
kind of fermentation. There are many other kinds of fermentation
in which butyric acid is produced as a by-product. (•') Alcoholic
fermentation of milk is so well recognised now that it has been
suggested to use whey for the formation of alcohol, by inoculating
it with the proper species of bacteria. {^) Recently it has been dis-
covered that potassium bichromate exerts a very important anti-
septic influence on milk, from '3 to "4 of a gramme per 500 cc. of
milk, it is stated, being sufficient. Quite recently also, formalin,
viz. a 40 per cent solution of formaldehyde, has been used with
great success as a preservative.

8

CHAPTER YII

Butter — Im'portance of Bacteria for Butter-Making

Butter may be described as the most important of
milk products ; and although the conditions and
nature of the changes going on in the process of the
conversion of milk into butter have been made the
subject of considerable investigation, there can be no
doubt that we are still merely on the threshold of
our knowledge of this highly important operation.
The nature of the bacteria which effect the many
subtle changes going on in butter-making, and the
precise nature of the changes they bring about in the
butter-fat and other constituents of butter, are ques-
tions regarding which we are still very ignorant.

As we have already remarked, in milk, so far as
it is used by itself and not as the raw material for
the manufacture of butter or cheese, the presence of
micro-organic life must be regarded as undesirable.
We consequently direct our attention towards de-
stroying, or, as far as possible, limiting, the action

VI i] B UTTER MAKING 1 1 5

of such micro -organic life. In the case of butter and
cheese, however, micro-organic life, far from being
inimical, is absolutely necessary — indeed, on it de-
pends the whole success of butter and cheese making ;
and the chief object aimed at by the butter or cheese
maker — however unconsciously this object may be
pursued — is to produce conditions favourable for the
development of the proper kind of micro- organic
life.

Object of Churning.— The chief constituent of
butter is, as all are aware, butter-fat. But butter-fat
is not, as it is too common to suppose, the only con-
stituent. AVe have in butter, in addition to the butter-
fat, all the other constituents of milk — in small
amount, it is true, but exercising a not unimportant
function in furnishing nutrition for the development
of the bacterial life so necessary for the production
of good butter. The relative proportions in which
these constituents are present are similar to those in
which they are present in milk. But, while these
minor constituents of milk must not be regarded as
of no importance, yet the chief and most important
constituent of butter is the butter-fat, and the first
great object of the butter-maker is to obtain from his
milk the largest possible amount of the butter-fat it
contains.

Fat in milk, as we have seen, is in the form of
globules. In butter-making these globules are made

ii6

MILK

[VII

to coalesce by churning the milk, the fat globules
being converted from a liquid condition to a solid
one by the violent impact which they thus experience
(Figs. 15, 16, 17, and 18, pp. 116-119). Butter can
be obtained by directly churning the milk ; but it

Fig. 15. — Microscopical Appearance of a Drop of Milk, containing
3-6 per cent of fat, magnified 670 times. (Kirclmer.)

has always been regarded as more convenient to first
cream the milk, and thus effect a concentration of the
globules before churning. In the first place, there-
fore, it may be well to consider what the conditions
are which regulate the separation of the fat.

Conditions influencing Separation of Fat. — A

VIl]

BUTTER MAKING

117

number of influences regulate the rate at which the
fatty globules separate themselves from the rest of
the milk by rising to the surface. What impedes the
separation is the fact that we have not in milk an
example of " pure emulsion/' i.e. a mixture of a fat

Fig. 16.— Drop of CREAii (diluted with water) five minutes after
churning has begun. (Kirchner.)

with a liquid. The caseous matter in the milk is in
a semi-solid or colloidal form, as was pointed out in
a preceding Chapter, and this retards the separation
of the fat globules. The temperature of the milk is
another important factor. AMien warm milk is set
to cool, currents are set up, and it is difficult to

ii8

MILK

[VII

avoid them. The colder portion of the milk, being
of greater specific gravity, sinks to the foot ; and the
warmer portion, being lighter, rises to the top. In
this way the collection of fatty globules on the
surface is disturbed and impeded. The descending

Fig. 17. — Drop of Cream (undilute^l) fifteen minutes after churning
has begun. (Kirchner.)

currents carry away more fat with them from the
cream layer than the ascending currents bring back
to the surface. It is only after the entire mass of
the milk assumes the same temperature as the sur-
rounding air, and when currents due to temperature
are no longer induced, that the fatty globules can

VI i] BUTTER-MAKING 119

follow their tendency to collect on the surface without
disturbance. The higher the temperature of the milk
the less is the resistance offered to the separation of
the globules. It may be further pointed out that
lactic fermentation is unfavourable for separation.

Fig. 18. — Drop of Cream shortly before end of churning.
(Kirchner.)

The sooner, therefore, after milking, the milk is set,
the better, since the conditions for its separation are
more favourable during the first few hours than
subsequently.

The rate at which the globules separate is not
equal. During the first hour or two most of the

120 MILK [vii

fatty globules come to the surface, and separation
thereafter takes place very slowly. The percentage
of fat in the cream, i.e. the richness of the cream, will
be found to vary very considerably, according to the
different conditions under which it is formed. Thus,
for example, the lower the temperature at which
separation is effected the less will be the percentage
of fat in the cream. The shape of the vessel, again,
also affects the quality of the cream. Milk creamed
in narrow-necked vessels yields poorer cream than
milk creamed in wide-necked vessels. While, lastly,
the depth of the milk from which the cream is allowed
to separate influences its quality.

Centrifugal Separators. — Before the year 1877
the separation of the cream from the milk was en-
tirely effected by allowing the milk to stand at rest for
a lengthened period — varying, as a rule, from twelve
to forty -eight hours. But in the above-mentioned
year, a method for effecting this separation, by means
of centrifugal force, was introduced, and ever since
then the use of "separators" — as the apparatus in
which this method is applied are called — has steadily
increased. The special advantages of the separator
over the older methods are several. For one thing,
the saving of time they effect is great, seeing that
they effect a much more complete separation of the
fatty globules, in about a twentieth or so of the
time.

VIl]

B UTTER-MAKING

121

It is impossible to obtain all the fat bv creamiDg,
no matter how long the milk may be permitted to
stand. There are always a certain number of the
smaller fatty globules which never rise to the surface,
and which thus remain in the skim milk (Fig. 19).

Fig. 19. — Microscopical Appearance of Skim Milk. (Kircliner.)

Even by using centrifugal separators it seems im-
possible to separate all the fatty globules from the
milk. It has been calculated that the amount of fat
left in the skim milk by the old methods of separa-
tion may amount to, on an average, '8 per cent. By
the use of separators, on the other hand, not more
than, on an average, '2 per cent should be left in the

122 MILK [vii

skim milk. From calculations made, it has been
estimated that, working on a large scale, the separator
yields about 93 per cent of the fat. This is probably
about 20 per cent more than is obtained by the old
method. The saving effected by using the centrifugal
separator is thus seen to be considerable.

The efficiency of the centrifugal separator is de-
pendent on the rate at which it is worked, as well as
on the temperature, composition, and amount of milk
treated.

Milk transported to a distance, and consequently
submitted to a disturbance, or milk which has been
boiled, does not separate so well as fresh milk. It
has also been noticed that milk very rich in fat does
not separate so thoroughly as milk containing an
average amount. And here it may be well to point
out that skim milk produced by a separator forms
a better article of food — when used for the feeding of
pigs — than that obtained by the other methods ; for,
although it contains less fat, it is freer from lactic
acid, and forms a much more palatable food.

Preliminary Souring of Milk or Cream before
Churning. — It was early discovered that milk or
cream, in which the process of souring had gone on
to a certain extent, was more easily churned, and
yielded more satisfactory results, than when in the
fresh condition. It has hence been customary to
" ripen " cream before churning. Why this should

vii] BUTTER-MAKING 123

be so is not, as yet, clearly known. "We have
referred to Babcock's theory with regard to the
formation of a small amount of fibrin in milk. It is
possible that the effect of the ripening process is to
gradually dissolve the fibrin, and thus permit of
more easy coalescence on the part of the fatty
globules.

Conditions influencing Churning. — It has been
found that temperature is among the important
conditions which influence the ease with which
coalescence of the fatty globules takes place. It
would seem that the obstacles which retard their
union decrease with the increase of temperature.
"V\Tiere the process goes on too quickly, however, —
and this may be caused by churning at too high a
temperature, or by too rapid a motion of the churn, —
it has been found that the little lumps of butter do
not separate out readily, and are apt to include, in
addition to the solidified fat, fatty globules in the
liquid condition. The presence of liquid fat in the
raw material gives rise to a smeary condition in
the butter. It may be mentioned in passing, that
there is a slight rise of temperature in the churning
operation, amounting to a few degrees. According to
Professor Fleischmann, this should not exceed, in the
case of churning sour milk or sour cream, two to
five degrees Fahr. ; and, in the case of sweet cream,
six degrees Fahr. According to the same authority,

124 MILK [vii

the temperature at which churning should be begun
in sweet cream is 12*5° C. (54'5^ Fahr.), and for
sour cream 16° C. (60*8° Fahr.). Sweet cream
seems to require to be churned for a longer time
than sour cream, and the loss of fat is very much
greater in the former case than in the latter, —
indeed, the loss has been calculated by certain
American experts to amount to nearly 50 per cent
more. The length of time during w^hich churning
should last may be said to vary from thirty to forty-
five minutes. The effect of diluting cream with
water before churning is to increase the period of
time required. The loss of fat which, under such
circumstances, takes place is slightly greater, and the
resultincf butter is not so firm.

The Bacteria of Butter. — We now come to
consider a very important aspect of butter-making,
viz. the relation of bacterial life to butter. It is
beyond doubt that the flavour, aroma, and the many
qualities which good butter possesses, are due to the
action of bacteria. We have already pointed out
that, after separating the cream from the milk, it is
soured, or "ripened," for a number of hours before
being churned. This is effected by allowing it to
stand in a warm place for some time, and sometimes
by adding some sour cream. Xow one reason for
this is that the milk globules, under such circum-
stances, coalesce more easily than they do in sweet

VI i] BACTERIA IN BUTTER-MAKING 125

cream. But a more important reason is that such
treatment permits of the development of certain
kinds of bacteria which have a most important effect
on the subsequent aroma and flavour of the butter.
During this souring period, the initiatory steps of
various forms of fermentation are produced.

Aroma and Flavour of Butter. — Butter made
from ripened cream has always a superior flavour to
that possessed by butter made from nnripened
cream. Indeed, the obtainiug of the proper flavour
and aroma are among the chief objects aimed at by
the butter-maker in butter-making. How the aroma
in butter is exactly produced is not very clear. It
has been thought that it is due to some alcoholic-
like product formed during ripening, or, it may be,
to the presence of lactic acid itself ; but, so far as
this latter statement is concerned, lactic acid used
artificially for ripening cream has not been found
very successful. The flavour is probably due to the
presence of certain volatile acids in the butter which
are not present in fresh milk ; but, whatever the
cause, there can be little doubt that both aroma and
flavour are connected with decomposition products
formed by bacterial growth.

The enormous importance of having the proper
sort of bacteria developed will be at once seen from
what has been above stated. The experiments,
therefore, which have been carried out with a view

126 MILK [vii

to isolating the bacteria implicated are of the veiy
highest importance. The practical outcome of these
experiments has been the preparation of so - called
" pure cultures " for ripening cream. So far the use
of these pure cultures has been followed with the
very best results, and it has been found that the
quality of the butter improves as soon as these are
used. According to experts, the amount of the
improvement has been estimated at more than 20
per cent. At present it may be mentioned that
nearly every dairy in Denmark, Germany, Sweden,
and Holland, depends upon pure cultures of bacteria
to produce uniformly good qualities of butter, and it
is greatly to be desired that this practice should be
introduced into this country without loss of time.
Very favourable results have been found by using
such cultures for correctinsr certain defects in
butter, such as oiliness, fishy and hitter flavours,
and a tendency to become rancid. One bacillus has
been isolated which seems to produce the proper
aroma in butter, when used in cultures for ripening
cream, and this ferment has been used in some of
the creameries of Germany with excellent results.-^
It may be interesting to note that the proper aroma
in butter seems to be connected with the first pro-
ducts of decomposition, set up in the cream as the
result of bacterial growth; for the bacteria which

^ See Appendix to Chapter, p. 136,

vii] BACTERIA IN BUTTER-MAKING 127

have been found growing in ripening cream may
produce all sorts of disagreeable flavours. While
pleasant flavours seem to belong to the flrst products
of decomposition, those formed during the later
stages give rise to disagreeable ones.

Since, then, the great danger in ripening is to con-
tinue it too long, and thus induce a disagreeable flavour
in the butter, a point of first-rate practical importance
is to determine the proper length of time for ripening.
But this is just exactly what it is very difficult to
do. The butter-maker can have no certainty that
his cream is supphed with the proper species of
bacteria ; hence the importance of being able to
furnish to butter-makers the proper ferment, and
thus avoid all such risk. Investigation into this
subject is but in its initial stages, and only one
species of bacteria has as yet been discovered which
produces the proper results. It is along this line of
investigation that the most useful results from future
researches in the science of butter-making are to be
looked for. We are the more encouraged to expect
this on the analogy of the great benefits which have
accrued to the brewing industry from the application
of similar principles. The great uncertainty which
accompanied the manufacture of beer in former
times has been greatly reduced by the use of pure
yeast cultures. As illustrating the intimate connec-
tion between bacterial life and flavour in butter, it

128 MILK [vii

may be mentioned that a species of bacteria has been
found which produces an odour and taste similar to
that produced by the feeding of roots. Butter made
by the use of this kind of bacteria acquired this root
taste in about two weeks.

Number of Bacteria in Butter. — Interesting
investigations on the number of bacteria present in
butter have been recently carried out by Lafar on
different samples of Munich butter. In one gi'amme
(about the ^^th of an ounce), taken from the centre
of a pat of butter, nearly two and a half millions
were found. Equal amounts of butter, taken from
more exposed portions of samples, were found to
contain far larger numbers — in one case as many as
forty - seven and a quarter millions being found.
Taking the average of a number of investigations,
Lafar concluded that a gramme of butter contains
from ten to twenty million bacteria.

The few experiments which have been carried out
on the subject show that artificial butter contains
fewer bacteria, and those of a different nature, than
natural butter. The presence of pathogenic bacteria
in butter is dangerous, for the reason that butter
cannot be submitted to sterilisation, but has to be
eaten in the raw state. Lafar is of the opinion that
typhus and cholera bacteria would die in butter in
from five to seven days, and those causing tuber-
culosis lose their power of doing harm in twelve days.

viil BACTERIA IN BUTTER-MAKING 129

Influence of Lactation, Breed, Age, and Food

on Quality of Butter. — Among the other conditions
which influence the quality of butter may be
mentioned the lactation 'period, the 'breed, and the aye
of the cow. Butter made from the milk of cows
which have been milked for some time is generally
firmer than that made from the milk of newly milked
cows, and possesses a less fine flavour. With regard
to the influence of feeding on the condition of the
butter, it has been proved that both the colour, the
flavour, the smell, the keeping qualities, and, in a
very special degree, the solidity of the butter-fat, are
dependent on the nature of the food used. Especi-
ally intimate seems to be the relation between the
food and the fatty acids. The volatile fatty acids
are at their maximum with a new period of lactation
and at the beginning of the pasturage season, and
decrease with the advance of the season and period
of lactation. There seems to be no connection
between the fat in the food and the fat in the
butter. From extensive experiments carried out
on the effect of food on butter. Professor
Adolf Mayer has come to the conclusion that
rations rich in carbohydrates have a favourable
effect in increasing the volatile fatty acids. It
must be frankly admitted, however, that the evidence
on the effect of different foods on the composition of
butter is very conflicting. Some interesting experi-

9

130 MILK [vii

ments, having for their object the comparison of the
daily yield of butter obtained from 1 lb. of fat in
the milk of different breeds of cows, including
Jersey, Guernsey, Holstein (German), Ayrshire,
Devon, and Holderness (American), showed the
following results : —

lb. Butter.

lb. Butter.

(1) Guernsey

(2) Jersey .

(3) Holderness

. 1-07

. 1-04

•98

(4) Devon . -97

(5) Ayrshire . -93

(6) Holstein . -88

It may be added that the size of the globules
seems to affect the quality of the butter, since the
composition of large fatty globules seems to differ
from the composition of small ones (see Milk- Fat,
Chapter II., p. 9). Butter made from large globules
has a richer colour, a better taste and consistency, a
lower melting-point and point of crystallisation,
contains less insoluble fatty acids, and, finally,
possesses a lower specific gravity than butter made
from smaller globules.

Chemical Composition of Butter. — In conclu-
sion, a word may be said on the chemical composition
of butter. A point of considerable importance, and
one regarding which there has been much discussion
during recent years, is the amount of water which
butter should contain. On the Continent, the con-
census of opinion seems to be that the limit should
be fixed at 15 per cent, while in this country an(i

VI i] BACTERIA IN BUTTER-MAKING 131

in America a larger limit seems to be supported. A
large number of analyses of American butter showed
an average of about 19 per cent of water. The ques-
tion is of considerable importance, since the amount
of water undoubtedly affects the quality. The per-
centage of fat may be said to vary from 82 to 85 per
cent, the other constituents — the casein, albumin, and
ash — forming, on an average, not much more than
1 per cent. For preserving purposes salt is often
added to butter.

The following may be regarded as the average
composition of fresh and salt butter : —

Fresh Butter.

Salt Butter

Water

14-00

12-50

Fat .

83-50

84-50

Protein

•80

•50

Milk, Sugar, etc.

1-50

•60

Ash .

•20

•10

Salt .

1-80

100-00

100-00

The following diagram illustrates the appearance
of butter under the microscope (see Fig. 20, p. 132).

Margarine. — Before concluding this Chapter, it
may not be out of place to say a word or two on that
largely used butter substitute, oleomargarine, or, as
it is more commonly known, ma/rgarine. Having
regard to the present enormous dimensions of the
trade in margarine, it is striking to reflect that its

132

MILK

[VII

manufacture only dates from the year 1870. It was
in that year, a few months before the outbreak of
the great Franco-German war, that the Emperor
Napoleon TIL commissioned M. Mege-Mouries to
carry out experiments with a view to discover a

Fig. 20.— Bctter. Magnified 350 diameters. (Bell.)

good substitute for butter, for the use, in the first
instance, of the French marines and the poorer
inhabitants of Paris. The result was an article
which was first known as " Margarine-Mouries."
This substance was prepared in a simple manner
from the best animal tallow.

vii] BACTERIA IX BUTTER-MAKING 133

The preparation of tlie new butter substitute
rapidly extended and became established in America,
Piussia, Germany, Austria, and other countries. Up
to the end of the year 1880 nearly all the margarine
sold in Europe was prepared according to the French
process ; and as the new fat was found to be an
excellent cooking fat, more economical for cooking
purposes than butter itself, since it contained a
larger percentage of fat, and as it also kept better,
and was undoubtedly both in equality and flayour
superior to the inferior kinds of butter, its use and
popularity steadily increased. The large extension
in its manufacture, however, had the result that the
raw material, viz. fresh ox tallow, first exclusively
used in its preparation was soon no longer procur-
able in the necessary quantities. Other oils, chiefly
the cheaper plant oils, such as cotton-seed, earth-nut,
walnut, cocoa-nut, sesame, and the poorer sorts of
olive oil, in addition to ox tallow, bacon fat, goose
fat, fat from soap-boiling manufactories and from
slaughter-houses, etc., had to be used. Further, the
mode of preparation was considerably changed, and
the fat, in the process of extraction, was submitted
to a higher temperature and a gxeater pressure than
in the original process, with the result that an inferior
article in every way was prepared. Margarine,
therefore, from being an admirably cheap butter
substitute possessing many quahties to recommend

134 MILK [vii

it, soon became, under the altered conditions of
manufacture, an article to be viewed with suspicion.
This suspicion was further increased by the reflection
that the larger part of the raw material from which
it was manufactured in the margarine factories was
imported from America and Australia, viz. from
sources not under control. In this way, it is not
impossible that certain kinds of infectious diseases
may be propagated by its use ; and badly prepared
margarine may possibly contain the spores of animal
parasites originally present in the fat constituting
the raw material, and may thus be introduced into
the human system. Since, in the preparation of the
article, only a comparatively low temperature can
be used, these organisms may very easily escape
destruction. Lastly, among the least serious objec-
tions which can be urged against the use of margarine
made from plant fats is the fact that they are less
easily digested than animal fats.

There can be no doubt that during the early years
of the trade much of the marcrarine made was far
from a desirable article; and while the manufac-
turers of butter might naturally feel aggrieved at
having, as a rival of butter, an article which, in
many cases was of an inferior nature, yet, so long as
the article w^as sold in a fairly legitimate manner,
they had no recourse but to put up with it. The
case was altered, however, when this same article

Ml] BA CTERIA IN B UTTER-MAKING 1 3 5

was fraudulently sold in the place of butter, and
largely mixed "vritli pure butter. There can be no
doubt that the practice — at first, it is possible,
innocently followed — of producing in margarine an
article which resembled genuine butter in flavour and
external appearance, as well as in the form in which
it is packed and exposed for sale, led the way to
fraudulent practices. The very name, butterine,
under which it was at first permitted to be sold,
had a fraudulent ring about it.

The extent to which this fraudulent sale of
margarine was practised gave rise in the year 1885
to an attempt being made, in most countries where
the dairying industry was in an advanced state, to
deal with this practice. The result of the movement
was that laws were passed in most European countries
forbidding the sale of the article under the name of
butterine, and enforcing other restrictions. There
can be no doubt that this legislation has done much
to lessen the fraud ; yet it is equally certain that an
enormous amount of adulteration of butter with
margarine still goes on.

The chief difference, from a chemical point of
view, between margarine and butter is that the
former contains a larger percentage of fixed insoluble
fatty acids than the latter, which may be said never
to exceed 90 per cent of the total quantity of fat.
The specific gravity of the fat of margarine is also

136 MILK [vii

lower than that of genuine butter fat. The following

diagram illustrates the appearance of margarine
under the microscope (see Fig. 21).

Fig. 21. — Oleomargarine. Magnitied 350 diameters. (Bell.)

Appendix
Conn points out that an objection to the pure cultures at present
in use on the Continent is the fact that they consist of acid-pro-
ducing bacteria, and that when added to cream containing lactic
ferments they unduly hasten fermentation. The result is that
cream, before being treated with them, has to be pasteurised. This
is objectionable, since it imparts to the butter a "cooked" flavour.
He claims that by the use of an organism, which he calls Bacillus
No. 41, this is obviated. His bacteria, instead of hastening the
souring of the cream, actually retards it, so that it is not necessary
to pasteurise the cream before adding it.

CHAPTER VIII

Rennet and Its Action

Befoee passing on to consider cheese, it will be best
to discuss the nature and action of rennet.

The chief constituent of ordinary cheese is the
casein of milk, and the object of the cheese-manu-
facturer, therefore, is to separate this casein from the
milk. This is effected by coagulating it. From a
very remote period it has been customary to effect
the coagulation of milk in either of the two following
ways, viz. by allowing it to become spontaneously
sour, or by treating it with rennet. For long these
two processes w^ere regarded as essentially the same
in their nature, and although several observers
noticed that certain differences existed — in the re-
action produced, for example, by the respective kinds
of coagulation — little notice was paid to this fact.

Occurrence of Rennet Ferment. — Before dis-
cussing the action of rennet, it will be well to say a
few words on the nature of this substance. With re-

138 MILK [viii

gard to this, there is little precise information. It is,
as all are aware, a preparation usually made from the
stomach (the rennet stomach) of the calf; but it is
also found in the stomach of the sheep, and, although
in smaller quantity, in the stomachs of many other
animals, the deer and the chamois, as well as in
fishes and birds. "Whether it is always of identi-
cally the same composition when obtained from these
different sources or not, it is impossible to say. So
much, at any rate, may be said, that it is possible
to gain from the stomachs of the above-mentioned
animals, on treating them with salt or lactic acid, a
substance which exerts an action on milk similar to
that exerted by rennet from the calf's stomach.
Rennet has also been found in the human stomach.
It is a secretion of certain glands embedded in the
lining of the stomach, and is most abundant in the
very young animal, especially during the period of
suckling. As the young animal, however, gradually
ceases to depend for its food on milk, the production
of rennet decreases. It has also been found in a
number of plants. For example, the juices of the
fig tree, the artichoke, certain kinds of thistle, and
the melon tree, as well as other plants, have been
found to contain a substance, the action of which is
similar to that of rennet. Whether, however, this
substance is identical in all cases, and whether it is
the same as the rennet obtained from the calf's

viii] RENNET AND ITS ACTION 139

stomach, is also unknown. Pinally, it may be men-
tioned that rennet, or a rennet-actins^ substance, is
produced by numerous bacteria.

Active Principle of Rennet. — While it has been
found impossible hitherto to isolate rennet in an
absolutely pure condition, Hammarsten (to whose
researches is due most of our information on the
nature of rennet, as well as its action) has obtained,
in a comparatively pure state, what he regards as its
active principle. To this he has given the name /«&,
while the terms chymosin and pw:ine have also been
applied to it by other investigators. It belongs to
that class of ferments which are known as chemical
or unorganised ferments (also known as enzymes), and
which have only been distinguished from the organised
ferments for about thirty years. To the same class
belong dmstase, a substance which converts starch
into sugar, and pepsin, which dissolves the albu-
minoids. Both rennet and pepsin are found in the
gastric juices of the stomach, and both are to be re-
garded as digestive ferments of the highest import-
ance. These enzymes, or chemical ferments, are
produced by the growth of the organised ferments.
These two classes of ferments — the true or living
ferment, and chemical ferment or enzyme — are dis-
tinguished from one another by certain properties.
The former are insoluble in water, while the latter
are soluble. Again, while the chemical ferment can

140 MILK [viii

only effect a certain amount of fermentative work,
which can be definitely measured, there is no limit
to the amount of work the organised ferment may
effect. As already pointed out, a most important
relationship exists between these two ferments ; for
most true ferments give rise to a chemical ferment,
which is one of the products of their action. It is
on this account often very difficult to know whether
certain fermentative changes are caused by the true
or the chemical ferment.

Coagulating Power of Rennet. — As has been
pointed out, the amount of work which unorganised
ferments can effect is capable of being definitely meas-
ured. Now some conception of the coagulating power
of rennet is afforded by the following short description
of an experiment carried out by Soldner, a German
investigator. He prepared an extract of rennet by
treating the dry stomach of a calf with a 5 per cent
solution of salt. The rennet was then precipitated,
in the form of a grayish-brown powder, by adding
more salt. He found that one part of this powder in
the dry state was able to coagulate, in forty minutes,
at a temperature of 35'' C. (95° Fahr,), one million
parts of milk. On analysis, he found that this
brown powder only contained 36 per cent of organic
matter ; and as this 36 jjer cent was probably not
the pure rennet, the inference is that one part of
pure rennet can coagulate, at the above temperature

viii] RENNET AND ITS ACTION 141

and iu the above period of time, more than three
million parts of milk.

y Difference between Rennet -Curd and Acid-
Curd. — The nature of the action of rennet is a point
regarding which our knowledge is still very imperfect.
If the coagulation of curd produced respectively by the
formation of lactic acid or other acid (and this curd
may be distinguished by calling it " acid-curd,' V and
that produced by rennet ^ rennet-curd '3)are examined,
it will be found that these two curds differ in many
respects in their properties and composition. Eennet-
curd, for example, is an elastic substance scarcely
soluble in water, and not in the slightest extent
sticky or greasy. Acid-curd, on the other hand, is
not elastic, is less insoluble in water, and is sticky
and greasy. Again, acid-curd, will be found to
exhibit a strongly acid reaction; while, although
the reaction iu rennet-cuid is also acid, it is only
slightly so. It is for this reason that acid-ciu'd is
not so suited for the manufacture of such a wide
variety of cheese as rennet-curd, since its acid re-
action is not favourable to the development of a large
variety of micro- organic life. On the other hand, in
rennet-curd the conditions for bacterial development
are much more favourable. Hence in cheeses made
from acid-curd, with very few exceptions, the process
of ripening resembles in general the process of putre-
faction, inasmuch as it goes on from outside to inside.

142 MILK [viii

In cheese made from rennet-curd the ripening pro-
cess goes on throughout the whole mass. Further-
more in rennet- curd the calcium phosphates which
are in suspension in the milk are entirely enclosed
in it, whereas acid-curd, it will be found, only en-
closes a small quantity of these phosphates, and this
for the reason that most of them are dissolved by
the lactic acid. Lastly, acid-curd contains more fat
than rennet-curd.

Nature of Action of Rennet. — Xow, if we ex-
amine into the nature of the rennet coagulation we
shall find first of all, as Hammarsten found in his re-
searches, that it is entirely independent of the forma-
tion of an acid in the milk, and that the reaction of the
curd is not changed during the curdling. Although
the curd does, as a rule, exhibit an acid reaction, this
is due to the action of micro-organisms. Hammarsten,
secondly, proved that the action of rennet is entirely
independent of milk - sugar. This he proved by
curdling solutions of casein, which had been entirely
freed from sugar, with rennet. Now these two
points alone are sufficient to distinguish the acid
coagulation of milk from the rennet coagulation ; for
it is known that the acid coagulation of milk, as
effected in milk which is allowed to spontaneously
coagulate, is due to the action of true or living fer-
ments, to which the name lactic ferments has been
applied, which act upon the milk-sugar, decomposing

viii] RENNET AND ITS ACTION 143

it into lactic acid. This lactic acid coagulates the
milk when it reaches a certain amount. Acid coagu-
lation of this type, therefore, may be said to be
entirely dependent on the presence of milk-sugar.

That the curds respectively produced by rennet
and by acid are very different, Hammarsten further
proved by the following experiment. He took some
curd which had been produced by acid coagulation,
and, dissolving it in a little alkali and then neutral-
ising the solution, he obtained a solution of casein,
which he found he could not coagulate with rennet.
From this he concluded that there was something
present in the original milk which was a necessary
condition of the action of the rennet, but which was
not contained in the acid-curd. This something, he
argued, must be present in the whey which he had
separated from the acid-curd. To see if this was so,
he added to his casein solution some of this whey
and then tested it with rennet, when he found that
it was coagulable by rennet. Further investigation
demonstrated that this something, which was lacking
in the acid-curd, was lime salts. He thus discovered
the further interestinf]^ fact that rennet coacmlation
requires the presence of lime salts, and found that
other alkali salts can take the place of lime. In
fact, he came to the conclusion that the curdling by
rennet is similar to the clotting of blood.

It has also been found that milk possessing an

144 MILK [viii

alkaline reaction is not capable of being coagulated
by rennet. A very slight acid reaction, on the other
hand, assists its action, which is largely influenced
by temperature, the most favourable temperature
being 30" C. (86° Fahr.). A temperature of 60° C.
(140° Fahr.) destroys its action. Milk which has
been boiled generally loses its power of being precipi-
tated by rennet, but such milk regains its suscepti-
bility at once if calcium chloride or other soluble
lime salt be added, or if tlie lime salts precipitated
by boiling be again dissolved by the addition of a
dilute acid.

It has been sometimes noticed that even fresh
milk is not coagulated by rennet. Investigation
would probably show that such milk, owing to dis-
turbance in the milk-gland, exhibits a slightly alka-
line reaction, and does not contain soluble lime salts.

It has already been stated that casein in milk
exists in a semi-dissolved or colloidal form. This
semi-soluble condition of casein is due to the fact
that it is combined in some way with lime. Accord-
ing to this view, the casein present in milk may be
regarded as of the nature of a salt, in which the
casein proper takes the part of the acid, and the lime
that of the base. The coagulation which results on
the addition of an acid may thus be explained by
the decomposition of this body, the lime being
neutralised by the acid and the casein set free in the

viii] RENNET AND ITS ACTION 145

form of an insoluble clot. Be this as it may, the
casein in the milk, to which the term caseinogen has
been given, to distinguish it from the casein in the
curd, seems to be kept in its state of semi-solution by
the alkaline condition of the milk, seeing that the
addition of a small quantity of acid at once effects
precipitation.

The action of rennet, however, is much more com-
plicated, as it breaks up the casein into two albu-
minoid bodies, one of which is easily coagulated by
acids, and at a temperature of from 70° to 80° C.
(158° to 176° Fahr.) ; whereas the other is not pre-
cipitated at all by acids, and is only coagulated by a
temperature of from 95° to 100° C. (203° to 212°
Fahr.). ISTow it is only the first of these two bodies
that is recovered in the curd, since it forms along
with alkaline earths a solid body. The second is in
solution, and remains in the whey, and is thus lost.
According to this theory, then, rennet does not
directly possess the power of precipitating the casein
in the milk. All that it does is to decompose the
casein into two proteid bodies, one of which has the
power of becoming coagulated by the calcium salts,,
which are always present in the milk. This reaction
throws some light on the amount of time required
for the coagulation of milk by rennet. It may be
pointed out, that the amount of albuminoid matter,
lost in the coagulation of milk, is further increased

10

146 MILK [viii

by the fact, that certain bacteria have the power of
acting upon the curd and dissolving it ; hence it is
advisable to use rennet in such a manner that it will
produce its coagulation as quickly as possible.

Form in which Rennet is used. — Formerly a
solution of rennet was always used. This was of
home manufacture, and was made in small quantities
for immediate use. Now, however, rennet is largely
manufactured on the commercial scale, either in the
form of a solution or in a powder. These solutions
generally contain, in addition to the active principle
of the rennet, small quantities of pepsin, an unorgan-
ised ferment which produces lactic acid, and com-
paratively large quantities of slimy matter and other
organic matter, the composition of which is unknown.
They also contain salt or alcohol, and sometimes
other preservatives, such as boracic acid, salicylic
acid, benzoic acid, etc. All these substances increase
the keeping qualities of the rennet solution, but this
is effected at the expense of its strength, since they
render a portion of the rennet ferment inactive.

Extracts of rennet should be clear in appearance,
and should not possess a disagreeable smell. Their
coagulating properties should not lose, in the course
of a year, by more than 25 per cent. Eennet
powders are more concentrated than rennet solutions ;
they should be almost entirely white, and devoid of
smell, and should almost completely dissolve in

viii] RENNET AND ITS ACTION 147

water. Care should be taken, in keeping tliem, to
preserve them from dampness, and when used they
must be dissolved in water as thoroughly as possible.
Eennet solutions should be kept from the action of
licrht, as this tends to weaken their strength.

CHAPTEE IX

Cheese

Cheese-making may be described as a very ancient
art. Fresh cheeses, that is to say, non- ripened
cheeses, were probably made thousands of years ago ;
and although the art of cheese-making was doubtless
practised in a more or less crude form, much attention
seems to have been devoted from very early times to
the different methods of preparing cheese. Thus, for
example, in Aristotle's writings we find references to
the use of different kinds of rennet.

Important R61e of Bacteria in Cheese-
Making. — Cheese-making may be described as a
much more difficult art than butter -making, the
number of conditions which have to be reckoned
with in it being very much greater than in the case
of butter-making. The importance of bacteria in
the manufacture of cheese is, again, very much greater
than in the manufacture of butter. In the latter
case they are highly desirable allies, and, for a satis-

ix] CREESE .149

factory article, uo doubt necessary, since without
their help butter tastes flat. Yet butter made
without their aid is still capable of being used. On
the other hand, in cheese -making, bacteria are
absolutely indispensable, since there can be no doubt
whatever that what imparts to a cheese its char-
acteristic properties, and indeed renders it so
desirable an article of food, viz. its flavour, is due
to the action of the different kinds of bacteria or
classes of bacteria. The proper flavour of cheese is
the result of a ripening process in which the chief
agents are bacteria. This ripening process goes on
at different rates in different cheeses, and may last
for weeks or months. To the cheese-maker, there-
fore, the assistance which the bacteriologist is likely
to afford is of the very highest importance.

Conditions determining Quality of Cheese. —
As fat is the chief constituent of butter, so casein is
the chief constituent of cheese ; but while fat pre-
dominates in butter over all other ingredients, in
many cheeses the amount of fat present may be quite
equal to, and, indeed, largely exceed, the amount of
casein. Although this is the case, it is the changes
which go on in the ripening process in the casein that
are of chief importance. The means taken to produce
different kinds of cheeses are various. The nature of
the cheese will be determined by a number of con-
ditions, among which may be mentioned the method

ISO MILK [ix

in which the milk has been coagulated. This deter-
mines, to a certain extent, the composition of the
curd, its physical condition, such as its fineness.
Among other conditions are the degree of moisture
in the curd, the percentage of fat, the regulation of the
temperature during the numerous processes, and the
sourness of the curd. One point of enormous import-
ance in cheese-making is to obtain a similarity of
composition in the curd.

Bacteria in Cheese — That the ripening of cheese
is due to the action of micro-organisms has been
conclusively proved. Thus, for example, if means be
taken to prevent the growth of bacteria, by subjecting
the cheese, as has been done, to a stream of carbonic
acid gas, or if cheese b& made from sterilised cream,
it is found that no ripening takes place. Similar
results have been obtained by treating the cheese
with some disinfecting agent, which would prevent
bacterial growth in the cheese without affecting its
chemical condition. As to the nature of these
organisms, however, and as to the specific functions
they exercise, much doubt yet remains. Different
investigators have attempted to estimate their
number. Thus Adametz found, in a gramme of fresh
cheese, from 90,000 to 140,000, and Freudenreich as
many as 1,800,000. The former observer has found
850,000 in a gramme of Swiss cheese, and in the
same amount, taken from the outer layer, of soft

ix] CHEESE 151

house cheese, 5,600,000. Their number has been
found to increase slowly during the ripening process.
That these bacteria belong to various species seems
to be evident. It seems also pretty conclusively
proved that every kind of cheese has to be ripened
by a definite species ; but the difficulty is in identi-
fying the specific kinds of ferments connected with
the different kinds of cheeses, that perfect ripening
requires the united action of many different species
of bacteria, the action of which upon one another is
of a very complicated nature. Certain cheeses, again,
not only require various kinds of bacteria in their
preparation, but also certain moulds. Some of them
are aerobic, while others are anaerobic. How far the
characteristic properties of different kinds of cheese
are respectively due to the action of one specific class
of bacteria is not absolutely certain. There seems to
be strong evidence, however, that this is the case ; since,
as the ripening process goes on, one species generally
increases at the expense of others. As the bacteria
concerned in the ripening of cheese are always present
in milk, the art of cheese-making consists in producing
circumstances which w^ill favour the predominant de-
velopment, of that class of bacteria, specially impli-
cated in the ripening of the particular kind of cheese
to be made. A ferment, made by the bacteria which
ripen cheese, has been isolated and studied by
Duclaux : he calls it casease.

152 MILK [IX

Chemical Changes Cheese undergoes during
the Ripening Process. — But what, it may be asked,
are the chemical changes which cheese undergoes in
the ripening process ? Eipening, from a chemical point
of view, may be said to consist chiefly in the trans-
formation of the characteristic constituent of cheese,
\\z. the casein, from the insoluble condition in which
it is present in fresh cheese into certain soluble
albuminoid bodies. The unpalatable nature of fresh
cheese is due to the fact that the casein is in an
insoluble condition. By allowing the cheese to
ripen, therefore, a certain amount of decomposition
of the casein is effected, and the solubility of the
cheese in water is consequently largely increased.
It is, therefore, to the presence of these soluble
bodies that the flavour, quality, and condition of a
cheese are largely due. "What these decomposition
products exactly are is still doubtful. We know,
however, that the milk- sugar, which is present to a
small extent in fresh cheese, is no longer to be
found in ripened cheese. It has been decomposed
into such bodies as lactic acid and, it may be,
butyric acid. The fat in cheese seems, however, to
undergo very little decomposition in the ripening
process. Among other decomposition products, alco-
hol, oxalic acid, carbonate of ammonium, leucin, and
tyrosin have been identified. During ripening, too,
there is generally produced a certain amount of

ix] CHEESE 153

gases, consisting of carbonic acid, hydrogen, and
sulphuretted hydrogen. Indeed, the production of
gas often gives rise to a not uncommonly occurring
"fault" in cheeses, viz. an inflated or puffy condi-
tion. This is often caused by premature ripening of
the cheese ; but the presence of pores in certain
kinds of cheese, amounting sometimes to the size of
little cavities, is a desirable property. One investi-
gator, who has carried out researches on the swelling
of cheese, has come to the conclusion that the forma-
tion of holes or pores in hard cheese is due simply to
the action of a single bacillus, the hacillus diatrypeti-
cus casei. Indeed, the general process of cheese-
ripening seems to be similar in its nature to the
process of digestion by the various digestive fluids
of the stomach and alimentary canal. It may be
added that the percentage of water in the ripening
process becomes distinctly less. A fact of consider-
able interest, and one which has an important practi-
cal bearing, is the inimical influence which light
has on the development of the ripening bacteria.

Cheese-Faults. — AMiile the proper ripening of
cheese is thus undoubtedly due to the action of bac-
terial life, so also are most of the faults which
cheeses are liable to. Among these faults may be
mentioned fungoid growths on the surface of the
cheese, red, blue, or yellow in colour, sometimes
leading to the total discoloration of the cheese, as

154 MILK [ix

in the case of "black" cheese. Agaio, the bitter
flavour which characterises certain cheeses is -un-
doubtedly due to bacterial life. These faults have
been generally attributed in the past to the health
and condition of the cow, its food, the condition of
the byre, and other causes. These may, no doubt, be the
indirect cause, but the direct cause is bacterial life.

The difficulties with which the cheese -maker
has to contend, therefore, are great. He cannot
always be sure of a uniform product; for, in spite
of all precautions he may adopt, disastrous results
may be obtained. To bacterial life also is due the
poisonous properties which, unfortunately, cheeses
have been found not infrequently to develop. These
poisonous properties are due to the production of
j)tomaines, which are apt to occur in old cheese.

Imperfect and unsatisfactory as our knowledge at
present is of the relation of bacteriology both to
butter and cheese making, in this department of
research lies the hope of the future of the dairy
industry. We have pointed out how very uncertain
must necessarily be the results obtained in butter
and cheese making under present conditions. This
is due to the fact that we have to work so much in
the dark in these processes, and thereby incur great
risks. While bacteria are the useful servants of the
butter and cheese manufacturer, they can also prove
his most deadly foes. Unfortunately, it is not always

ix] CHEESE

o:>

in his power to decide which role they shall perform.
When, however, our knowledge of the bacteria impli-
cated in these processes has become sufficiently exact
to admit of our isolating them and preparing them
in pure cultures, we shall then be in a position both
to make butter and cheese from fresh milk which
has had little time to develop any undesirable kind
of fermentation, or from sterilised milk, by simply
adding the pure culture of the proper bacteria, and
thus secure uniformly good results.-'-

Composition of Cheese. — Cheese, as we have al-
ready said, varies very much in composition. Some-
times it contains but a small proportion of fat, while
in other cheeses the fat is the most abundant con-
stituent. There is nearly always present a small
Cjuantity of lactic acid. Good cheeses contain from
30 to 35 per cent of water : inferior kinds from 38
to 45 per cent. Eich cheeses contain from 25 to 35
per cent of fat, sometimes even more. The casein is
very variable in amount. In skim -milk cheese it
amounts to as much as 50 per cent ; while in some
of the soft cheeses it may be as low as 11 to 12 per
cent. The following table illustrates the composition
of the commoner kinds of cheese " : —

^ That there is great room for improvement in the mannfacture of
cheese maybe inferred from the statement that, according to Dr. Conn,
sometimes as high as 50 per cent of the cheeses made in a factory are
worthless, or comparatively worthless, owing to abnormal ripening.

^ Johnston's Elements of Agricultural Chemistry^ Seventeenth
Edition, by C. M. Aikman, p. 461.

156

MILK
Soft Cheeses.

[IX

Water.

Casein.

Fat.

Ash.

Neufchatel

37-87

17-43

41-30

3-40

Fro IP age de Brie

51-87

18-30

24-83

5-00

English Cream

2-30

50-68

Camembert

51-30

19-00

21-50

4-70

Roquefort

11-84

85-43

1-85

„ (two months old)

19-30

43-28

32-30

Hard Cheeses.

Water.

Casein.

Fat.

Ash.

American

22-59

37-20

35-41

4-80

J)

31-80

36-00

28-70

3-50

Cheddar

27-83

44-47

24-04

3-66

>j

28-34

47-03

21-01

3-62

Dunlop

38-46

25-87

31-86

3-81

Gloucester

21-41

49-12

25-38

4-09

)j

35-82

37-96

21-97

4-25

Parmesan

27-56

44-08

15-95

5-72

Stilton .

32-18

24-31

37-36

3-93

})

38-28

23-93

30-59

3-20

Gruyere

40-00

31-25

24-00

3-00

)j

34-68

31-41

28-93

3-85

Gorgonzola

43-56

24-17

27-95

4-32

Skim .

43-14

49-79

0-86

6-21

CHAPTEE X

Testing of Milk

A SHOET description of the methods in use for
testing milk may be given. To do so, various
methods may be employed. These may be divided
into two classes : (1) tests and (2) chemical analysis,
the former of which depend chiefly on the physical
properties of milk. Taking the former class of tests
first, these are as follow : —

1. Determination of the percentage of fat hy meas-
uring the thickness of the layer of cream thrown njp
on alloiuing it to stand for some time.

2. A determination of the fat hy opticcd tests.

3. Determination of the fat hy the amount ofhutter
the milk yields.

4. Determination of the fat hy the addition of
certain reagents, etc.

5. Testing the milk hy determiniiig its specific
gravity.

1. Testing Fat by Amount of Cream. — With

158 MILK [x

regard to tlie determination of the percentage of fat
in milk by the thickness of the cream layer, it must
at once be said that, although this method has long
been popular on account of its simplicity, it is a
most unreliable one. An apparatus which is largely
used for this purpose is the Chevalier CremonuUr,
which consists of a glass cylinder of about 20 centi-
metres deep and about 4 centimetres broad, possessing
a graduated scale which indicates the percentage of
fat corresponding to different layers of cream. The
milk is allowed to stand in this for twenty-four
hours, and the depth of the cream layer then read
off. The reason of the inaccuracy of such a method
of testing the fat in milk is due to the fact that, as
has been pointed out elsewhere, the same quantity
of fat in milk will, under different conditions, give
layers of cream of very varying depth. It may
be said that ordinary milk will give, on an average,
about 10 per cent of cream ; but so variable is this
that it may sometimes amount to from 30 to 40 per
cent, and at other times may be as low as from 4 to
5 per cent, and this despite the fact that the range
in the amount of fat present may be very trifling.
Experiments carried out by Kirchner exemplify
this. In four different samples of milk in which the
percentage of fat only varied to the extent of '03
per cent, a difference in the percentage of cream,
amounting to 4 per cent, was found, owing to the

X] TESTING OF MILK 159

difference in the conditions under "\;vhicli tlie tests
were made.

2. Optical Method of Determination of Fat. —
A variety of instruments have been devised for
applying this test. It is based on the fact that the
opacity of milk is dependent on the milk-fat globules.
As, however, this is only partly the case, and as the
same weicjht of fat retards more licrht when it is in
the form of small globules than when it is in the
form of larger globules, this method of testing is
very unreliable.

3. Determination of the Fat by Churning. —
Apparatus has also been devised for the determina-
tion of the fat in milk by the amount of butter it
yields on churning. This method, since it involves
the use of very expensive apparatus, is not suited for
general use.

4. Determination of the Fat by the Addition
of Reagents. — Of the various methods for the deter-
mination of the fat by the addition of reagents,
Soxhlet's is the best known. This method consists
in dissohong the fat from a measured quantity of milk,
to which some potash or soda has been previously
added^ by ether, and calculating its amount from the
quantity of fat dissolved in the layer of ether which
separates out, from the specific gravity of the latter.
The apparatus by which this test is carried out is
known as the ' areometer!

i6o MILK [x

Another admirable method for determining fat is
by the ladocrit, an apparatus devised by Dr. de
Laval. A measured quantity of milk is placed in a
glass tube, an equal amount of concentrated acetic
acid, containing 5 per cent of concentrated sulphuric
acid, is then added, and the mixture heated for a few
minutes at boiling temperature. The effect of the
acid is to dissolve the caseous matter, which retards
the movement of the fatty globules. The tube is
then placed in a specially constructed centrifugal
apparatus, and the mixture is separated by centrifugal
force, the depth of the layer forming a measure of its
amount.

Another method is that devised by Leffmann and
Beam. A quantity of milk is treated in a bottle with
a graduated neck with one-fifth of its volume of a
mixture of equal parts of amyl alcohol and strong
hydrochloric acid. Strong sulphuric acid is then
added to the mixture in nearly equal quantity, and
the whole is well mixed. The bottle is then placed
in a centrifugal separator, which is rotated for one or
two minutes. The fat layer, which collects in the
neck of the bottle, is then read off.

5. Determination of the Specific Gravity. — The
determination of the specific gravity of milk must be
reckoned as one of the most important of all the
tests to which it can be subjected. Alone, it is no
doubt apt to be misinterpreted. In milk we have

x] TESTING OF MILK i6i

not, as in a mixture of water and alcohol, merely two
factors determining specific gravity. In milk we
have vxtter, fat, and solids not fat. The tendency of
the fat, since it possesses a lower specific gravity
than water, viz. '93, is to counteract the tendency of
the " solids not fat," which possess a higher specific
gravity, viz. 1*6. Two interpretations, therefore, of
a low specific gravity in a sample of milk are possible,
viz. the addition of water, or excessive richness in
fat. It is for this reason that the determination of
the specific gravity alone must not be overrated in
value.

Formulae for Calculating Composition of Milk
from certain Data. — In conjunction, however, with
a knowledge of the total solids of milk, the specific
gravity furnishes a most useful indication of the
percentage of fat ; and similarly, if the fat be known,
it permits us to calculate the percentage of total
solids. This can be effected by using the following
formulae (Fleischmann), in which

T = percentage of total solids in milk.
F = percentage of fat in milk.
G = specific gravity at lb" C.

(«) For calculating the percentage of fat from the
specific gravity and total solids : —

lOOG-100
F = -SS.ST - 2-22

(jr
II

1 62 MILK [x

(J) For calculating the percentage of total solids
from the fat and specific gravity : —

(jr

The following is a formula which has been devised
by Hehner and Eichmond, and which has been largely
used in England : —

F=-859T- -21860

This formula suffices for normal milk, but for
skim milk it has been found necessary to modify it
as follows : —

The above correction has only to be applied when
the specific gravity (G) divided by total solids
exceeds 2'5.

H. Droop Eichmond has quite recently^ devised
a formula for calculation of total solids from the
specific gravity and fat. It is as follows : —

T = ? + ^F + -14
4 5

Chemical Analysis of Milk. — With regard to the
chemical analysis of milk, the subject is one which
does not admit of being discussed with any fulness
in a work like the present. It may suffice to mention

^ Analyst (March 1895), p. 57.

x] TESTING OF MILK 163

that, as ordinarily carried out, it includes a determina-
tion of the total solids, the fat, and the ash. The
\Yater and the solids not fat are calculated by
difference. The total solids are estimated by evapor-
ating at boiling temperature, on the water bath, a
weighed quantity of milk to dryness. The percentage
of ash is obtained by incinerating at a low red heat.
For the extraction of fat various methods are in use.
The most satisfactory one undoubtedly is that known
as Adam's coil process, which consists in absorbing
the milk by means of a coil of filter-paper, which is
then subsequently dried and transferred to a Soxhlet
extraction apparatus, where the fat is dissolved out
by ether and weighed.

CHAPTEE XI

Milk as a Food

The position which milk occupies as a food is unique.
That it acts in the capacity of a perfect or complete
food is well known to all, since it is the first and
often the only food of the young. It may not be,
therefore, without interest to say a word or two in
conclusion on its value in this connection, so far as
the subject has been investigated. In order to do so
it will be necessary to state shortly the principles
upon which the value of a food rests.

Classification of Food Nutrients. — All food
materials, whether animal or vegetable, generally
consist of two portions — the edible and the refuse.
The refuse portion consists of such parts as the bones
of meat and fish, the bran of wheat, and the skin of
potatoes. The edible portion, on the other hand,
may be divided up into a number of groups of
substances of different chemical composition, of
varying digestibility, and charged with different

xi] MILK AS A FOOD 165

functions. To the members of each of these different
groups the name nutrient is given. Of these
nutrients there are a large number; but they can
all be classified under a few groups, this classification
being based upon their chemical composition. Thus
we have the protem group, the characteristic feature
of which is the possession of nitrogen in its com-
position ; the carholiydrates, of which starch and
sugar are typical members ; and the fats. The first
group may be subdivided into several smaller groups,
such as albuminoids, gelatinoids, amides, and nitro-
genous extractive substances. Lastly, we have another
class of nutrients, which are also, in their way, of
importance, viz. mineral 7iutrients. Xow, while all
of these groups of different nutrients perform
important functions, they cannot be described as of
equal importance.

Functions of Food. — The functions of food are
various. Inasmuch as the body is constantly
experiencing change, is always undergoing, as we
say, a certain amount of "wear and tear," and is
also, at certain periods of life, viz. in youth, under-
going a process of growth, one obvious function of
food is to make good this wear and tear, and to build
up the tissue, bones, etc., out of which the body is
formed. Since animal life, in building up its tissue,
is only able to make use of foods more or less similar
in their nature to its tissue, it follows that the

i66 MILK [xi

muscles, tendons, and other portions of the body
which contain nitrogen in their composition can
only be built up by members of the protein group of
nutrients. But, as has been pointed out, the protein
group is a large one, and all its members do not
possess this tissue-forming power ; indeed, it is only
the albuminoids — so called from the typical member
of the group, albumin, or the white of an ^gg — which
possess it.

The albuminoids, then, are the most important
group of nutrients. From them also is the most of
the fat in the animal body formed; although both
the fats and the carbohydrates are also capable of
being used in this way. The proportion in which
the fat in the body is derived from the three sources
is not known, and, indeed, varies with circumstances,
more especially according to the proportion in which
the three different classes of nutrients are present in
the food. As the body, more especially in certain
of its parts, such as the bones, blood, teeth, hair, and
many of the fluids, contains mineral salts (phosphates,
sulphates, and the chlorides of potassium, sodium,
calcium, and magnesium), certain mineral substances
are absolutely necessary.

But another great function of food is to furnish
the body with the necessary animal heat. Although
the members of the protein group are capable of
acting in this way as heat-givers, and furnishing the

xi] MILK AS A FOOD 167

necessary fuel to keep the animal machine working,
this is more especially the function of the other two
groups of nutrients, \dz. the fat and the carbohydrates.
But since the carbohydrates are unable to discharge
any functions as flesh-formers, and can form fat
probably to only a very limited extent, their charac-
teristic function may be described as heat-givers.

And here it may be well to point out with regard
to the value of the three members of the three groups
of nutrients as heat-givers, that they are not all
equal in this respect, and that fats, when burned in
'C-JLv the body, yield 24 times as much heat as an equal
weight of a protein or carbohydrate nutrient. A
common method of comparing different foods is to
compare their heat-gi\^ng value. Another method
of comparing foods is by calculating what has
been named their albuminoid or nutritive ratio,
viz. the ratio which their flesh-forming nutrients
bear to their heat-gi\dng nutrients. The amount of
albuminoids or flesh-forming nutrients in almost all
foods is very much less than the amount of fat and
carbohydrates. It is customary, therefore, in cal-
culating the nutritive ratio, to take the amount of
albuminoids as 1. To obtain the other number of
the ratio, the amount of fat present in the food is
first multiplied by 2|-, and then added to the weight
of the carbohydrates. Thus, supposing that a food
contains per poimd, 2 ounces of albuminoids, 2

1 68 MILK [xi

ounces of fat, and 2 ounces of carbohydrates, the
nutritive ratio of such a food would be

2 : 2 X 2J + 2 = 7, or 1 to 3 J.
Digestibility of Food Nutrients.— But the value
of a food as a source of nourishment does not solelv
depend on its composition, viz. the percentage of
albuminoids and other nutrients it contains, but also
on the extent to which these nutrients are capable
of being digested. And here we are met with a
difficulty, since the digestibility of a food nutrient
cannot be absolutely stated, as much depends on the
digestion of the individual partaking of the food.
Indeed, no better example of this fact could be cited
than the case of milk, which is generally found to be
one of the most digestible of foods, yet which some
people find to be very indigestible. What the cause
of such an anomaly is, it is difficult as yet to say. It
may be, as has been suggested by recent researches
on this subject, that ferments in the digestive canal
of some people may cause particular compounds to
be changed into injurious, and even poisonous forms,
so that it sometimes may be literally true that " one
man's meat is another man's poison." But digestion
proper is a chemical process, and is capable of being
determined with fair accuracy by careful experiment,
and our knowledge on this subject has received many
valuable additions in the last few years. These have
shown that the digestibility of the different nutrients

xi] MILJ-C AS A FOOD 169

in different foods varies very considerably, and that
the feeding value of a food should never be solely
estimated by simply taking into account the per-
centages of the different nutrients it contains.

Value of Milk as a Food.— If we examine the
composition of milk, in the light of the above neces-
sarily brief statement of the nature and functions of
the different food nutrients, we shall find why it
should occupy such a unique position among foods.
In the first place, it contains no refuse portion ; and,
in the second place, it contains members of all the
different groups of nutrients. The albuminoids are
represented by casein and lactalhurain, which amount
to, on an average, 3 J per cent ; the fats are represented
by hutter-fat, which amounts to of per cent ; while
the carbohydrate group is represented by the railk-
sagar, averaging about 4| per cent ; and, lastly, we
have in the ash the necessary mineral ingredients.

Digestibility of Milk.— With regard to the digesti-
bility of these food nutrients in milk, broadly speaking,
it may be said that, so far as the small number of
experiments, carried out on this subject, go to show,
all the protein and all the carbohydrates are digested.
The same, however, is not the case with the fat,
which, it would seem, is only digested to the extent
of about 96 per cent. In cheese, again, the same
may be said, the fat being slightly less digestible,
viz. to the extent of 95 per cent. In butter, however,

I70 MILK [xi

it is probably the same as in milk. While lastly, iu
margarine, the fat is less easily digested, viz. only to
the extent of 95 per cent. Of course this refers to
the case of adults endowed with sound digestion. As
it has been already pointed out, milk in many cases
may prove far from an easily digested food. But while
a nutrient may be perfectly digested, the time required
for this operation differs in the case of different foods.
Thus, in certain experiments carried out to show the
relative digestibility of meat, mutton, milk, etc., in
the uncooked and cooked state, the following results
were obtained : —

Hours necessary

Eeef

for

Digestion,

Raw

2

Boiled, half done

,

21

Boiled, well done

, ,

3

Roasted, half done .

. ,

3

Roasted, well done .

,

4

Mutton —

Raw

. ,

2

Veal

Raw

,

2i

Pork-

Raw

,

3

Milk

Uncooked .

.

31

Boiled

,

4

Sour

.

3

Skimmed

.

31

Effect of Boiling on Milk. — AVe have already
pointed out the result the heating of milk has on its

xi] MIf.K AS A FOOD 171

nature. "We saw that among the changes which took
place was the coagulation of the albumin, which
forms a skin on the surface of the milk. Xow, while
such a change would not affect the ultimate digesti-
bility of the albumin, it would no doubt retard it. This
doubtless accounts for the fact that, as the above table
shows, milk in the uncooked state is more speedily
digested than in the boiled condition, as indeed is
seen to be the case with the other foods cited. The
difference, however, is slight. Again, what may
strike the reader as, at first sight, singular, is that
sour milk is more speedily digested than either fresh
or boiled milk. This is probably due to the fact that
the lactic acid generated in the souring process helps
the gastric juices in the stomach in their solvent
action on the different nutrients.

Suitability of Milk as a Food.— If it be asked
whether milk contains the different food nutrients in
the best possible proportions for sustaining animal
life, we may safely answer that it does, so far as
children are concerned. Certain foods we know are
better adapted for the digestive organs of children
than adults, and this is the case with milk. Again,
it has been found that the composition of the mineral
constituents of milk is very similar to the composition
of the mineral matter in the body of the sucking
animal, with slight exceptions. In one respect there
is a strange anomaly, and this is that the percentage

172 MILK [xi

of iron in the latter is about six times that in the
former. This has been explained on the ground
that the amount of iron in the body of the young
is relatively much gi-eater than in the body of the
adult ; that, in short, iron is stored up in the body of
the young for future use. Milk has also been found
to be richer in potash and poorer in soda than the body
of the sucking animal. Another point which renders
milk less suitable for adults than for young is its
extremely bulky nature, and the fact that it contains
an excess of fat. A greater proportion of water and
fat is required in the food of the young than in the
food of the adult. It has been calculated that, if one
were to live on a milk diet alone, eleven pints per
diem would be required to be consumed in order to
aftbrd proper nourishment.

Comparing it with such a food as lean beef, it
may be said that 1 lb. of beef contains about the
same quantity of actually nutritious materials as a
quart of milk. The food which comes nearest milk
in the amount of nutriment it contains is oysters,
which are practically of the same nutritive value as
milk. There can be no doubt, however, that while
milk is perhaps not suited to act as the sole food of
the adult, it is one of the best and cheapest articles
of diet we possess, and should be far more widely
used than at present is the case. It is one of the
most convenient, useful, and inexpensive sources of

xi] MILK AS A FOOD

16

albuminoids which we have. Indeed, skim milk
may be safely asserted to be the cheapest of all foods.

Value of Butter and Cheese as Food. — With
regard to the value of milk products as articles of diet,
as far as their heating power goes, butter and cheese
stand at the head of all foods. Indeed, butter has
twice as much heating power, weight for weight, as
any other food except pork and cream cheese ; while
skim-milk cheese, so far as flesh-forming constituents
are concerned, is the most concentrated of all the
common foods.

Milk as a Food for Invalids. — While the subject
hardly admits of discussion in this place, a single
word may be said in conclusion on milk as a food
for invalids. Milk cures are of very old date, and
have been practised for many years, more especially in
Switzerland, where they are carried on, on a large
scale, in establishments specially equipped for the
purpose. A large and wide experience has shown
the great value of milk as a diet for consumptive
patients, especially when living at mountain health-
resorts. Again, in such diseases as ulceration and
catarrh of the stomach it is found in many cases to
be the sole possible food. Special preparations of milk
have long been used. We have already referred to the
use of koumiss in this connection, a beverage made by
the alcoholic fermentation of milk. Butter-milk, again,
has been found to be of great use for diabetic patients.

APPENDIX

Works ox Dairying

The Science and Practice of Dairying^ by Professor Fleiscli-

mann. Translated and edited by Dr. C. M. Aikman

and Professor E. P. "Wright. (London and Glasgow :

Blackie, 1895.)
Principes de Laitiere, by E. Duclaux. (Armand, Colin, et

Cie., 1893.)
Dairy Industry and Dairy Farming in Denmark E. P. Ward.

(Crewe, 1893.)
British Dairying, by Professor J. P. Sheldon. (London :

Crosby, Lockwood, and Co., 1893.)
Dairy Farming, by Professor J. P. Sheldon. (London : Bell

and Son, 1893.)
Handbuch der Milchwirthschaft, by Dr. W. Kirchner. (Berlin :

Parey, 1891.)
Die Milchdriisen der Kuh, by Dr. M. X. F. Filrstenberg.

(Leipzig: Engelmenn, 1868.)
The Fermentation of Milk, by Dr. H. W. Conn. (United States

Department of Agriculture, Bulletin Xo. 9, 1892.)
A Review of Recent Work on Dairying, by E. W. Allen.

(United States Department of Agriculture, Experiment

Station Record, vol. 5, 1894, No. 10.)
The Souring of Milk. (L'nited States Department of Agri-
culture, Farmers' Bulletin No. 29, 1895.)
Bacteria in their Relation to the Dairy. [Annual Report

1893, Agricultural Experiment Station, University of

Minnesota.)

176 MILK

The Princijyles of Modem Dairy Practice, by Grotenfelt. Trans-
lated by F. W. Woll. (Xew York : John Wiley and
Sons, 1894.)

Bacteria in their Relation to the Dairy, by Freudenreich. Trans-
lated by Professor Davis. (London: Methuen, 1895.)

Dairy Bacteriology. (United States Department of Agricul-
ture, Bulletin No. 25, 1895.)

Die Neueren Massen-Fetthestimmungsverfahren fiir Milch, by
Dr. P. Vieth. (Bremen : M. Heinsius Nachfolger,
1895.)

Elements of Dairy Farming, by Professor J. Long. (London,
Glasgow, and Edinburgh : W. Collins, Sons, and Co.,
Limited, 1894.)

Theory, Practice, and Methods of Dairy Farming, by Professor
J. P. Sheldon. (London : Cassell and Co., Limited,
1891.)

INDEX

Acid-curd, 141

Aerobies, 80

Age of cows, influence of, ou

butter, 129 ; on milk, 45
Albumin, 27, 30
Albuminoids, 165, 166 ; of milk,

27-31
Albuminose, 31

Alcobolic fermentation of milk, 97
Algae, 71
Alveoli, 3, 1
American cbeese, composition of,

156
American Holdemess cows' milk,

47
Amides, 165
Ampboteric reaction of mUk, 36,

57
Aroma of butter, 125
Average composition of milk, 12
Ayrsbire cows' mUk, 46, 47

Bacilli, 72, 73

Bacillus, actinobacter du lait
visqueux, 88 ; actinobacter
poljTQorphus, 89 ; cyanogenus,
85 ; lactis pituitosi, 89 ; leu-
conostoc mesenteroides, 88 ;
mesentericus, 88 ; prodigiosus,
83; synsantbus, 87; Aiolaceus,
87 ; ■viscosus, 89

Bacteria of mUk, 63-113

Bacterium Hessii, 89

Beastings, 38-41

Bitter milk, 89

Blue milk, 84

Breeds of cows, milk from
different, 45

Budding of yeasts, 80

Butin, 24

Butter, 114 - 136 ; aroma and
flavour of, 125 ; bacteria of,
124 ; chemical composition of,
130 ; number of bacteria in,
128

Butyric acid, 26

Butyric bacilli, 83, 95

Butyric fermentation of mOk, 95

Butyrin, 24

Camembert cheese, composition

ol 156
Capillary blood-vessels, 7
Caprin, 24
Capronin, 24
Caprv'lin, 24
Carbohydrates, 165
Casease, 151
Casein, 27, 28
Casein ferments, 91, 96
Ca.seinogen, 27
Centrifugal separators, 120
Changes milk undergoes, 53-62
Cheddar cheese, composition of,

156
Cheese, 148 - 156 ; bacteria in,

150 ; composition of, 155 ;

conditions determining quality

of, 149 ; ripening of, 152
Cheese-faults, 153

12

178

MILK

Chemical analysis of milk, 1 62

Chlorestin, 35

Chlorine in milk, 34

Cholera propagated by milk, 68,

76
Churning, 115-124
Chymosin, 139
Cilia, 72

Citric acid in milk, 35
Cleanliness in relation to milk,

109
Coagulation of milk, 57, 137-147 >
Cocci, 71

Colostrum, 7, 38-41
Colostrum corijuscles, 7, 38
Comma bacilli, 72, 73
Condensed milk, 61
Conditions influencing composition

of milk, 42-52
Constituents of milk, 9, 18-41
Consumption propagated by milk,

68, 75
Corps granuleux, 38
Cream, microscopical appearance

of, 117-120
Creaming of milk, 54

Devon cows' milk, 47

Diastase, 139

Digestibility of foods, 168 ; of

milk, 169
Diphtheria propagated by milk,

68, 69, 75
Diplococci, 71, 73
Dunlop cheese, composition of,

156

English cream cheese, composi-
tion of, 156

Enzymes, 101, 139

Erysipelas propagated by milk, 75

Excitement, influence of, on milk,
51

Eat in milk, 18-27 ; condition
of, 21 ; conditions influencing
separation of, 116 ; determina-
tion of, 157-160

Fatty acids, 24

Fatty globules, 18 ; number of in
milk, 20

Faults of milk, 59

Ferric oxide in milk, 34

Fibrin, 31

Fission of bacteria, 72, 73

Food, influence of, on butter, 1 29 ;
on milk, 49 ; milk as a, 164-
173 ; nutrients, 164

Formation of milk, 6

Formic acid, 27

Formulae for calculating composi-
tion of milk, 161

Fromage de Brie cheese, composi-
tion of, 156

Galactine, 31, 33

Gases in milk, 36

Gelatinoids, 165

Glanders propagated by milk, 76

Gloucestershire cheese, composi-
tion of, 156

Glyceride.s, 24

Goats' milk, 37

Gorgonzola cheese, composition of,
156

Grape-sugar, 33, 40

Green milk, 87

Gruyere cheese, composition of, 156

Guernsey cows' milk, 46, 47

Heat, effect of, on milk, 58
Holstein Friesian cows' milk, 47
Hj-phae, 80
Hypoxanthin, 35

Illness, influence of, on milk, 52
Infusoria, 71
Insoluble fatty acids, 25
Intermittent sterilisation, 61, 106
Invalids, milk as a food for, 173

Jersey cows' milk, 46, 47

Kephir, 97-99
Koumiss, 99
Kreatiu, 35

INDEX

179

Lab, 139

Lactalbumin, 30

Lactation period, 48, 129

Lactic bacteria, 55, 91-95

Lactocaramel, 33, 59

Lactocrit, 160

Lactoglobulin, 31

Lactoprotein, 27, 31

Lactose, 40

Laurin, 24

Lecithin, 35

Leffmann-Beam milk-tester, 160

Leprosy propagated by milk, 76

Leucin, 35

Lime in ash of milk, 34

Lobes, 3

Lobnles, 3, 4

Lockjaw propagated by mUk, 75

Macrococci, 71

Magnesia in ash of milk, 34

Malaria propagated by milk, 76

Mammary gland, 3, 5

Mares' milk, 37

Margarine, 131-136

Micrococci, 71, 73, 74

Micrococcus prodigiosus, 86 ;
Freudenreichii, 89

Microscopical appearance of milk.
19, 116

Milk as a food, 164-173 ; bacteria
of, 63-113; causes and con-
ditions influencing quality and
quantity of, 42 -'52 ; changes
undergone by, 53-62 ; con-
stituents of, 18-41 ; percentage
composition of, 9-17 ; testing
of, 157-163

Milk-cisterns, 3

Milk-fat, 18-27

Milk-sugar, 31-33

Mineral constituents of milk, 34-
36

Moulds, 72, 73

MjTistin, 24

Neufchatel cheese, composition
of. 156

Nuclein, 28

Olein, 24

Optical method for determining
fat, 159

Paohtin, 24

Parmesan cheese, composition of,

156
Pasteurisation of milk, 105-109
Pathogenic germs, 68, 75-77,

100-104
Penicillium glaucum, 73
Pepsin, 139
Peptones, 30
Percentage, composition of milk,

9-17
Phosphoric acid in milk, 34
Pixine, 139

Pneumonia propagated by milk, 75
Potash in milk, 34
Potato bacilli, 84, 89
Preseryatives for milk, 60, 112
Preseryed milk, 61
Preyenting changes in milk, 60
Protein group, 165
Ptomaines, 76, 101
Pure cultures of bacteria, 126

Kaxch) butter, 126

Red mHk, 86

Rennet, 137-147 ; actiye prin-
ciple of, 139 ; coagulating
power of, 140 ; forms in which
used, 146 ; occurrence of, 137

Rennet-curd, 141

Ropy milk, 88

Roquefort cheese, composition of,
156

Sale of Foods and Drugs Act, 14
Sarcina rosea bacilhis, 86
Scarlet feyer propagated by milk,

69
Secretion of mUk, ^-'^. 42
Sheep's milk, 37
Shorthorn cows' milk, 46
Skim milk, 121

i8o

MILK

Skim - milk cheese, composition

of, 156
Slimy milk, 88
Society of Puljlic Analysts,

standard, 14
Soda in nulk, 34
Solnble fatty acids, 25
Souring of milk, 55, 122
Specific gravity of milk, 37, 160
Spirilla, 72, 73, 77
Spores, 73, 74
Standards for milk, 17
Staphylococci, 71, 73
Stearin, 24
Sterile milk, 65
Sterilisation of milk, 61, 104
Stilton cheese, composition of,

156
Streptococci, 71, 73 -
Strejjtococcus Hollandicus, 89
Structure of cow's udder, 1-8
Sulphuric acid in milk, 34
Superfusion, 23

Symbiosis, 85

Tattemyelk, 88

Teats, 2, 5

Testing of milk, 157-163

Toxin, 101

Trimethylamin, 87

Tuberculosis bacillus, 68, 77, 101

Tunica propria, 4

Typhus propagated by milk, 68,

69, 75, 103
Tyrosin, 35
Tyrothrix, 84

Udder, structure of, 1-8
Urea, 35

Variation in composition of

milk, 12
Violet milk, 87

Yeasts, 72, 73
Yellow milk, 87

THE END

Printed by R. & R. Clark:, Limited, Edinburgh.

WORKS Bit DR. C. M. AIKMAN

Published by Messrs. Blachxood d: Sons.

1. Manures and the Principles of Manuring. Crown

8vo. 6s. 6d.
'• The best scientific exposition of our knowledge of niannres and manuring
which has yet been written," — Bath and West of England Society's Jour^nal.

2. Farmyard Manure : Its Nature and Composition.

Crown Svo. Is. 6d.
"The volume is a model of its kind. It is comprehensive, yet concise. It
is clearly \mtten and admii-ably arranged." — Farming World.

3. Johnston's Elements of Agricultural Chemistry.

17th Edition. Revised and Enlarged. Crown Svo. 6s. 6d.
"As it stands now, this edition keeps the work up to its old reputation as,
of all existing text-books, perhaps the most generally satisfactory all round
that a student of its subject could use." — Scotsman.

4. Johnston's Catechism of Agricultural Chemistry.

92nd Thousand. Revised and Enlarged. Crown Svo. Is.
" Should be in the hands of all students of agriculture, and practical farmers
would derive great benefit from its perusal." — Schoolmaster.

5. Nitrogen : Its Sources and Uses in Agriculture.

With an Appendix containing Contributions bv Sir J. B.
Lawes, Bart., LL.D., F.R.S. ;' R. AVarington, F^.R.S. ; Pro-
fessor P. F. Frankland, F.R.S. ; J. Jamiesou, F.I.C. ; and Dr.
Stutzer, Bonn. Is. 6d.
" The work treats comprehensively the sources of nitrogen, its uses, and

relation in different fonus to plant life. . . . Every agriculturist should have

it." —Agricidtural Economist.

6. Sir John Bennet Lawes, Bart., LL.D., F.R.S., and

the Rothamsted Experiments. 6d.
" Au admirably compiled pamphlet." — Field.

By Messrs. A. d; G. Black

Milk : Its Nature and Composition. A Handbook
on the Chemistry and Bacteriology of Milk, Butter, and Cheese.

By Messrs. Vinton d- Co.

The Food of Crops and Ho-w to Apply. An Ele-
mentary Handbook on the Principles of Manuring. [SJiorfly.

By Messrs. Blaclie <h Sons.
(In conjunction with Professor R. P. Wright.)
The Science and Practice of Dairying. Translated
from the German of Professor Fleischmann. [Shortly.

By the Society for the Promotion of Christian Knoidedge.
Health Manual : Air, "Water, and Disinfectants. Is.

Ill Crown 8ro, CloUi. Illustrated with \^Z Figures. Price 3s. 6d.
AN INTRODUCTION TO

STRUCTURAL BOTANY

( FL 0 WERIXG PL A XTS )

By D. H. SCOTT, M.A., Ph.D., F.RS.

HONORARY KEEPER OF THE JODRELL LABORATORY, ROYAL GARDENS, KEW.

In Croicn 8vo, Cloth. Price 5s.
AX INTRODUCTION TO

CHEMICAL THEORY

By ALEXANDER SCOTT, M.A., D.Sc, F.E.S.E.

JACKSOXIAN DEMONSTRATOR IN THE UNIVERSITY OF CAMBRIDGE.

In Demy 8vo, Cloth. Illustrated ivith 12 Lithographic Plates.
Price 18s. net.

INVESTIGATIONS ON

MICROSCOPIC FOAMS

AND ON

PKOTOPLASM

By Prof. 0. BUTSCHLI

Translated from the German by E. A. Mesxhin, B.A. Oxon, Fellow of
Merton College, Oxford.

In Demy Sro, Cloth. Illustrated unth 26S Figures. Price 18s. net.

ZOOLOGY OF
THE INVERTEBRATA

A TEXT-BOOK FOR STUDENTS.
By AETHUR E. SHIPLEY, M.A.

FELLOW AND ASSISTANT TUTOR OF CHRIST'S COLLEGE, ANT) DEMONSTRATOR OF
C0MPARATI\T: anatomy in the university of CAMBRIDGE.

A. & C. BLACK, SOHO SQUARE, LONDON, W.

In Grown 8vo, Cloth. Price 2s. 6d.

PLEA FOR A SIMPLER LIFE

By GEORGE S, liEITH, M.D,, F.RC.P.E.

Super-Royal 8t"o, Cloth. With 185 Illustrations. Price 20s.

TEXT-BOOK OF

OPERATIVE SURGERY

By Dr. THEODOR KOCHER

PROFESSOR OF SURGERY AND DIRECTOR OF THE SURGICAL CLINIC
IN THE UNIVERSITY OF BERNE

[ auslatecl witli the si^ecial authority of the Author from the second
revised aiid enlarged German edition, by Harold J. Stiles, M.B.,
F.R.CS. Edin., Senior Demonstrator of Siu'gery, and formerly
Demonstrator of Anatomy in the University of Edinburgh ;
Assistant Surgeon, Royal Edinburgh Hospital for Sick Children.

ii Crown 8vo, Cloth, 800 2)ciges. Illustrated with Woodcuts. Price 5s.

THE SENILE HEART

ITS SYMPTOMS, SEQUELS, AXD TREATMENT
By GEORGE WILLIAM BALFOUR, M.D., LL.D.

In Two Volumes. Demy 8vo. With 426 Illustrations.

TEXT-BOOK OF

GENERAL PATHOLOGY

AND

PATHOLOGICAL ANATOMY

By Prof. RICHARD THOMA

Translated by Alexander Bruce, M.D., F.R.C.P.E., Lecturer on
Pathology, Surgeons' Hall, Edinburgh ; and Pathologist to the
Royal Hospital for Sick Children, Edinburgh.

[ Vol. I. in preparation.

A. & C. BLACK, SOHO SQUARE, LONDON, W.

THE

STANDARD EDITION

OF Tui:

WAVEELEY NOVELS

To be completed in T-wenty-five Volumes, Crown 8vo, bound in

Art Canvas, with gilt top, and published Monthly,

commencing 15th October 1895.

PRICE 9/R PER VOL.

2/6

T^HE Volumes will he illustrated \vitli frontispieces
reproduced from dra^\angs by the following artists
and printed on Japanese handmade veUum paper : —

Wm. Small.
Hugh Thomson.

Frank Dadd, R.I.
Gordon Broxsne.

C. M. Hardie, R.S.A.
Wm. Hole, R.S.A.

The Title-page and the Binding design have been
engraved from drawings specially prepared by A. A.

TURBAYNE.

A Glossary and Index will be appended to each
volume, and in addition the last volume will contain a
General Index to the whole series. This Edition will
be enriched with the Copyright Annotations of the late
David Laing, LL.D., who was an intimate friend of
Sir Walter Scott, and will also include all the Author's
Introductions and Notes.

Note. — The above may also be hcul bound in full limp leather^
gilt edges, inice 3s. 6c?. per volume.

en

03

^SS^.-

m