THE UNIVERSITY
OF ILLINOIS
LIBRARY
630.7
II6b
co
A6RI6ULIURAL
U18ABY
UNIVERSITY OF ILLINOIS '
Agricultural Experiment Station
BULLETIN NO. 237
EFFECT OF TEMPERATURE OF
PASTEURIZATION ON THE CREAMING
ABILITY OF MILK
BY H. A. HARDING
URBANA, ILLINOIS, DECEMBER, 1921
CONTENTS OF BULLETIN 237
PAGE
INTRODUCTION 395
ESTABLISHMENT OF PASTEURIZING TEMPERATURES 396
DISTINCTION BETWEEN CREAM LAYER AND CREAM LINE 397
THE PRACTICAL IMPORTANCE OP THE CREAM LAYER , 397
MEASUREMENTS OB' CREAMING ABILITY 398
CORRECTIVE EFFECT OF MOMENTARY HEATING ON CREAMING
ABILITY OF COLD MILK 399
THE DESTRUCTIVE EFFECT OF HIGHER AND MORE PROLONGED
HEATING 401
LIMITATIONS OF TEMPERATURE CONTROL 406
SUMMARY 407
RELATION OF THESE FINDINGS TO THE MILK PROBLEM 407
REFERENCES . . 408
EFFECT OF TEMPERATURE OF
PASTEURIZATION ON THE CREAMING
ABILITY OF MILK
BY H. A. HARDING, CHIEF IN DAIRY BACTERIOLOGY
INTRODUCTION
Until 1900 commercial pasteurization was a compromise between
the heating which the scientist considered necessary in order to free
milk from the danger of carrying disease germs, and the heating which
would leave the milk in a condition acceptable to the consumer.
Under these circumstances, milk was pasteurized in but limited quan-
tities by a few dealers, and the use of the pasteurized product was
practically restricted to infant feeding.
Since the recommendation by Russell and Hastings in 19001 that
pasteurization at 140° F. for thirty minutes be accepted as satisfactory
from the sanitary standpoint, commercial pasteurization has spread
over the country with ever increasing rapidity. In about 1913 pasteur-
ization began to be generally recognized as the most important safe-
guard of the healthfulness of the milk supplies of the country.
The principle of pasteurization has been considered settled for
more than two decades. However, during this period the practice of
pasteurization has undergone many changes. There has been little
question but that the bulk of pasteurized milk has been properly
treated in the various pasteurizing machines. On the other hand,
criticism regarding minor points has been rather constantly raised.
The three points in connection with which pasteurizing machines are
most open to criticism are: (1) the lack of exactness in heating
the first and last milk during the act of pasteurizing; (2) the occa-
sional escape of milk thru leaky valves; and (3) the results from
foam. This feeling of uncertainty regarding the practical details
of commercial pasteurization has finally led to a definite demand for
pasteurization at 145° F. for thirty minutes as a minimum tempera-
ture and time.2
This movement for a higher temperature in pasteurization has
arisen in a perfectly natural way as a method of increasing the margin
of safety surrounding a process which is extremely important from
the standpoint of public health. In view of the fact that the earlier
attempts to pasteurize milk at high temperatures produced a com-
mercially unacceptable product, and thereby sharply curtailed the
use of pasteurized milk, it is desirable that careful study be given
395
396 BULLETIN No. 237 [December,
the effect upon the milk of the suggested change in pasteurizing tem-
perature to a minimum of 145° F. The recent development of a
method by which the creaming ability of milk can be easily and
accurately measured opens the way for a study of this problem.3
Many of the typical pasteurizing machines have a capacity of 10,000
to 20,000 pounds of milk per hour. The high cost of such machines
and the necessity of a force of experienced attendants to operate and
care for the machinery, together with the need of a large supply of milk
in testing the machines under working conditions, made it practically
imperative that such a study be conducted in commercial milk plants.
Owing to the generous cooperation of the Gridley Dairy Company of
Milwaukee, the Sheffield Farms Company, Inc., of New York City, the
Detroit Creamery Company of Detroit, the Pevely Dairy Company
of St. Louis, and the Bowman Dairy Company of Chicago, it was
possible in this study to cover practically all the prominent types of
pasteurizing machinery. The author desires to express appreciation
of the generous assistance of these firms.
ESTABLISHMENT OF PASTEURIZING TEMPERATURES
In 1895 when Russell proposed pasteurization of milk for twenty
minutes at 155° F.,4 there was some uncertainty regarding the time
and temperature necessary to destroy the germs of tuberculosis. The
explanation for these variations in scientific results was found by
Theobald Smith in 1899.5
The problem of the destruction of disease germs in milk by pas-
teurization was again studied by Russell and Hastings in 1900,6 and
by Rosenau in 1908.7 In each of these three separate investigations
the results were practically identical.
The studies of Rosenau showed that the germ of tuberculosis is
much more resistant to heat than any of the other disease germs which
occasionally find their way into milk. In all the above mentioned
studies, the number of germs of tuberculosis added to the tested
samples of milk was vastly greater than would be present as the
result of natural infection. Even under these circumstances there
were times when living germs of tuberculosis could not be found after
the milk had been heated to 140° F. for one minute. Only in a few
cases were any of the germs alive at the end of ten minutes of heat-
ing, and in no case were any found after heating for fifteen minutes
at this temperature.
While the conclusions from these three investigations were iden-
tical, there are interesting differences in the suggestions of the investi-
gators regarding commercial pasteurization. Smith made no recom-
mendations on this subject. Russell and Hastings suggested a minimum
of 140° F. for twenty minutes. Rosenau, with identical scientific
results before him, recommended 145° F. for thirty minutes.
1921] TEMPERATURE OF PASTEURIZATION AND CREAMING ABILITY OF MILK 397
The feeling is common among those who have carefully considered
commercial pasteurization, that in view of the importance of the
pasteurizing process from the standpoint of public health and the
imperfections of the operation and supervision of pasteurizing
machinery, the margin of safety in pasteurizing time and temperature
should be as wide as the production of a satisfactory commercial
article will permit.
DISTINCTION BETWEEN CREAM LAYER AND CREAM LINE
At present the phrase "cream line" is used by milkmen in refer-
ring to two distinctly different things, one of which is properly
called the cream line, and the other is more accurately referred to as
the cream layer.
The fat globules are quite evenly distributed in freshly drawn
milk, but they tend to rise because of their specific gravity. On
standing there is soon formed an upper layer commonly called cream.
The relative volume of the cream and the fat-poor milk below
depends largely upon the fat content of the milk. However, there are
a number of factors which at times markedly change the normal rela-
tions. Important among these factors is the temperature at which
milk is pasteurized.
The facility with which milk develops this cream layer is referred
to as creaming ability. In bottled milk this creaming ability is com-
monly expressed in terms of the depth of the layer of cream in the
bottle or in the percentage of the volume of the bottle occupied by
the cream.
As the cream layer forms there is developed a more or less dis-
tinct line of demarkation between the layer of cream and the fat -poor
milk below. This line of demarkation is quite properly called the
cream line. Unless a cream line is present it is impossible to measure
the depth of the cream layer. Because the -cream line and the depth
of the cream layer are so closely associated, they are often confused.
It is important that they be considered as two clearly distinct phe-
nomena, because the influence of temperature upon the two is quite
different. The present study concerns itself with the effect of the
temperature of pasteurization upon the cream layer.
THE PRACTICAL IMPORTANCE OF THE CREAM LAYER
The cream layer is important for two reasons ; first, the housewife
uses the volume of cream in the bottle as an index of the richness of
the milk; and second, she has need of cream in her culinary opera-
tion and she is accustomed to obtain this cream from the top of the
bottle.
Until the studies of Theobald Smith in 1899 and of Russell and
Hastings in 1900 made it evident that milk would be safe when
398 BULLETIN No. 237 [December,
pasteurized at 140° F., commercial pasteurization made practically
no progress because the temperatures previously recommended were
such as to practically destroy the cream layer.
In New York City in 1914 the health authorities required pasteur-
ization at 145° F. for thirty minutes, but the difficulties encountered
by the milkmen because of the destructive action of this treatment
upon the cream layer were such that the requirement was soon modified
to that of 142° to 145° F. for thirty minutes. At present there are
only a few cities which require the pasteurization of their ordinary
milk supplies. In the remaining cities pasteurized milk is sold in
competition with raw milk. Because of the importance ascribed by
the consumer to the cream layer, the pasteurized milk which fails to
carry essentially as much cream as the raw milk does not meet with
popular favor.
It is agreed by practically all students of the question that proper
pasteurization is the most important step in the production of a safe
milk supply. An efficient pasteurization is exceedingly important, but
it is also important that the relation of pasteurization to the cream
layer be understood, so that in the zeal to provide as wide a margin
of safety as possible the increase in pasteurization temperature will
not be carried so far beyond the actual requirements of the case as
to reduce the use of safe, pasteurized milk.
MEASUREMENTS OF CREAMING ABILITY
The various methods of measuring the creaming ability of milk
are discussed at length in Circular 249 of this Station.
Three of these methods are adapted to the measurement of cream
which has risen in the milk bottle. The simplest of these is to stand
bottles beside each other and compare the depth of the cream layers.
Another method is to measure the distance from the top of the bottle
to the line dividing the cream from the milk. More accurate compari-
sons may be made by determining the volume occupied by the layer
of cream. Where the total volume of the bottle is known, the content
of cream may be expressed in percentage by volume. The first two
methods are quite inaccurate and the third is rather laborious.
In the present study the measurements of creaming ability were
made by filling round-bottomed test tubes, one-inch in diameter, to
a depth of 216 millimeters (81/2 inches), with the milk to be tested.
These tubes of milk were immediately cooled in ice water, and when
cool were held at 40° F. for approximately twenty-four hours. The
depth of the resulting cream layer was measured in millimeters, and
each millimeter of cream represented 0.47 percent by volume.
The volume of cream as determined in this way agrees closely with
the volume of cream developed in bottles under similar temperature
conditions.
TEMPERATURE OF PASTEURIZATION AND CREAMING ABILITY OF MILK
399
CORRECTIVE EFFECT OF MOMENTARY HEATING UPON
THE CREAMING ABILITY OF COLD MILK
The temperature of milk when drawn from the cow is about 100° F.
The best dairy practice aims to bring the milk promptly to a tem-
perature of 50° F. or below. During the colder months, the tempera-
ture of the milk often falls nearly or quite to the freezing point before
it reaches the milk plant.
TABLE 1. — EFFECT OF MOMENTARY HEATING UPON THE CREAMING ABILITY OF MILK
Date
Raw milk
Heated milk
Temperature
Cream
Temperature
heated
Cream
Change
op
%
o p
%
%
Apr. 12
39
11.32
85
13.21
+ 1.89
May 11
40
9.43
141
12.26
+ 2.83
Apr. 21
41
8.07
132
12.26
+ 4.19
Apr. 4
42
9.90
140
11.79
+ 1.89
June 16
45
11.32
142
13.21
+ 1.89
June 16
47
11.32
142
13.68
+ 2.36
June 1 7
48
11.87
142
13.21
+ 1.34
Oct. 25
48 t
11.79
143
13.21
+ 1.42
Feb. 19
49
7.55
90
10.85
+ 3.30
Oct. 26
49
13.68
143
14.62
+ .94
Feb. 4
50
11.79
96
14.15
+2.36
June 18
50
11.32
142
12.74
+ 1.42
Oct. 24
51
12.74
143
12.97
+ .23
Oct. 25
51
12.26
143
11.02
— 1.24
Oct. 26
51
14.38
143
14.15
— .23
Oct. 26
51
14.62
143
14.15
— .47
Oct. 26
51
13.68
143
12.97
— .71
Oct. 26
51.5
14.15
143
14.15
.00,
Oct. 25
52
12.97
143
13.21
+ .24
Oct. 26
52
12.97
143
13.91
+ .94
Oct. 26
53
13.91
143
13.21
— .70
Oct. 25
53.5
12.50
143
12.50
.00
Oct. 24
54
12.74
143
12.74
.00
Oct. 24
54
12.26
143
12.74
+ .48
Oct. 25
54
13.44
143
12.97
— .47
Oct. 26
54
12.74
143
13.91
+ 1.17
Oct. 26
54
13.91
143
13.91
.00
Oct. 26
54
14.62
143
14.15
— .47
Oct. 24
55
14.85
143
14.38
— .47
Oct. 25
55
12.50
143
11.02
— 1.48
Apr. 2
56
14.62
85
14.15
— .47
Oct. 24
56
13.91
143
13.68
— .23
Oct. 25
56
12.97
143
13.21
+ .24
Oct. 26
56
14.15
143
12.74
— 1.41
Apr. 22
57
14.15
129
14.15
.00
Oct. 24
57
15.09
143
14.85
— .24
Oct. 26
57
14.15
143
13.44
— .71
Apr. 23
58
11.79
131
11.79
.00
Oct. 24
58
14.62
143
13.68
— .94
Oct. 25
58
13.91
143
12.74
— 1.17
Oct. 25
58
13.68
143
12.26
— 1.42
Oct. 25
58.5
13.21
143
12.26
— .95
Apr. 21
59
12.26
133
12.26
.00
June 24
66
15.09
141
13.68
— 1.41
June 25
68
15.57
143
13.68
— 1.89
June 26
69
15.57
142
13.68
— 1.89
400 BULLETIN No. 237 [December,
Measurements of the creaming ability of cold milk as delivered
at milk plants, together with measurements of the creaming ability
of the same milk after momentary heating to various temperatures,
are given in Table 1.
These data were derived from observations at six widely separated
milk plants. While the fat content of the milk received at these plants
averaged between 3.4 and 3.6 percent, that of individual samples
varied more widely. The treatment of the milk previous to sampling
is not known in detail, but the conditions varied from that of milk
which was being delivered from nearby farms to that of milk which
had been delivered at country milk plants, cooled to about 40° F.,
and shipped by rail to the city plant. Accordingly, there was a wide
variation in the temperatures to which the milk had been cooled and in
the agitation to which it had been exposed while cold.
The results presented in Table 1 are arranged according to the
temperature of the milk at the receiving vat. There Is no evident
relation between the creaming power of the cold milk and the tem-
perature at which it was received, except that the three samples
received above 65° F. all show a high creaming power. These samples
represent milk coming directly from nearby farms. The four samples
of raw milk which developed less than 10 percent of cream by volume
came from four different plants. In each case they represent milk
which had either been held cold for considerable time at the plant,
with prolonged agitation, or had been subjected while cold to cor-
responding agitation during transportation.
After leaving the receiving vat the milk was heated, in some cases
by a forewarmer and in other cases by a pasteurizer. Samples were
collected and the creaming power determined after this momentary
heating.
In the case of the milk received at or below 50° F., a momentary
heating ranging from 85° to 142° F. resulted in a measurable increase
in the creaming power. On the other hand, when the temperature of
the milk on its receipt was between 50° and 60° F., a similar heating
usually produced little change ; and when the temperature on receipt
was above 65° F., such heating resulted in a distinct decrease in the
creaming ability.
In observing the effect of any given treatment upon the creaming
power, such treatment is frequently judged by the relative amounts
of cream obtained upon the raw and upon the treated milk. The
above data suggest that the amount of cream which will develop upon
raw milk depends quite largely upon the agitation to which it has
been subjected while cold.
The results given in Table 1 show that milk with a comparable fat
content, when moderately heated, is given a fairly comparable cream-
ing power regardless of the agitation to which it has been exposed
1921] TEMPERATURE OF PASTEURIZATION AND CREAMING ABILITY OF MILK 401
while cold. A more detailed study of this phase of the question is
necessary before it will be certain that the creaming power thus
attained is identical with the original creaming power of the fresh
milk. In fact, the results from the last three samples suggest that
the creaming power of fresh warm milk is slightly reduced by a
momentary heating to 141°-143° F.
This corrective effect of momentary heating upon the creaming
ability of milk is quite important in connection with the study of
the effect of the pasteurizing process. Thru this corrective action, the
effect of earlier agitation while the milk is cold is largely removed
and the milk is given a fairly uniform creaming power. Under such
circumstances the variations in the creaming power of pasteurized
milk having the same fat content are practically all due to the tem-
peratures used in pasteurization.
The measurements in this study were made in the plants earlier
mentioned, partly because they were representative plants and partly
because the observations could thus be made to include practically all
the systems of pasteurization now in commercial use. There was also
in mind the common belief that each different type of machine has its
own peculiar effect upon the volume of cream which will develop.
In this study samples were collected, at convenient intervals, all
the way from the receiving vat up to and including the bottles.
Samples were taken in many instances after the milk had been held
at the given temperature for periods ranging from five minutes to an
hour, but the study was centered upon the effect after a holding
period of thirty minutes.
The samples obtained in commercial plants were taken under
regular working conditions, except that the temperature of pasteuri-
zation was deliberately varied from the lowest temperature consistent
with safety and with the local ordinances, up to 145°-149° F. All
temperatures shown on the charts are those at the close of the thirty-
minute holding period. In some plants the lower limit was 140° F.,
while in others it was 142° F. In the plants of the Sheffield Farms
Company Inc. no samples were obtained after pasteurizing at a tem-
perature of less than 145° F.
Ideal conditions for testing the effect of the pasteurizing tempera-
ture upon the creaming power of milk would include a sufficient supply
of raw milk of uniform composition so that it could be run thru a
single pasteurizer with all conditions kept constant except the tem-
perature of pasteurization. In commercial plants the most common
difficulty is the variation in the fat content of the raw milk supply.
402
BULLETIN No. 237
[December,
o 13.68
s
> 12.74
>>
,0
S 11.79
P
5
-g 10.85
i
c
£ 9.90
. 1'
Fig. 1.—
t
utes at
now ever, very suus-
f actory conditions for
the study were found
in a plant in which
the milk was received
from two country sta-
tions, where it had
been cooled to about
40 F. before being
shipped to the city
plant. In the city
plant it was held cold
in large tanks until
needed. The results
1S of forty tests of the
result of pasteuriza-
tion for thirty min-
. to 146° F. are given in
L~^,
1^.
£ ..
N..
^1
Si i
\
V
X
N
10° 142° 144° 146° F
Forty Tests of the Effect of Pasteurizii
Temperatures on the Cream Layer
temperatures ranging from 140° F
Fig. 1.
In Fig. 1 each dot shows the result from a single test, and by its
position indicates the temperature of pasteurization and the percentage
of cream by volume which developed. For example, the dot at the
extreme left indicates that a sample of milk pasteurized for thirty
minutes at 140° F. developed 12.74 percent of cream by volume.
The line running across the chart connects the averages of the
results at each temperature and shows their relation. This line in
Fig. 1 indicates that there is a distinct difference between the effect
of pasteurization at 142° F. and at 144° F. upon the creaming ability
of the milk. As the temperature of pasteurization rises above 144° F.,
the volume of cream decreases rapidly.
A similar group of results was obtained in a milk plant in another
city, where the milk on receipt was held in large tanks before pas-
teurization. In this plant the observations extended over tempera-
tures ranging from 140° to 151° F. The results from forty-five such
measurements are shown in Fig. 2.
It will be seen from Fig 2 that between 142° and 144° F. there
is a distinct change in the direction of the line connecting the averages
at each temperature, and this sharp decrease in the volume of the
cream continues rather uniformly up to a pasteurization for thirty
minutes at 151° F. At this latter temperature, the resulting cream
amounts to less than 4 percent.
It should be noted in this connection that altho pasteurization at
151° F. had reduced the volume of cream more than 80 percent, a
distinct cream line was still formed. Such observations make it plain
that the temperature of pasteurization exerts a powerful influence
1981] TEMPERATURE OF PASTEURIZATION AND CREAMING ABILITY OF MILK 403
12.74
11.79
g 10.85
9.90
8.96
8.02
7.08
6.13
5.19
4.18
\
\
\
X
\
140C
142 c
144 c
146 c
148C
150° F.
upon the vol-
ume of cream
even before it
reaches the
point where
i t destroys
the cream
line entirely.
In the two
plants from
which results
have been
presented the
milk was of
r el a t ively
uniform fat
content b e-
fore pasteuri-
zation. In a
plant in a
third city the
conditions were equally satisfactory, except that owing to the nature
of the supply there was more variation in the fat content during the
day's run.
The results from
fifty-five tests of pas-
teurization at tem-
peratures ranging
from 140° to 146° F.,
in the third plant, are
shown in Fig. 3.
It will be seen from
Fig. 3 that the results
from this plant were
very similar to those
obtained from the
two plants already
mentioned. This fact
is particularly strik-
Fig. 2. — Forty-five Tests of the Effect of Pasteurizing Tem-
peratures on the Cream Layer
12.74
11.79
10.85
9.90
8.96
Fig. 3. — Fifty-five Tests of the Effect of Pasteurizing
Temperatures on the Cream Layer
140° 142° 144° 146° F.
ing because of the
fact that each plant
operated a distinctly
different kind of pasteurizer. It will be noted that there is again a
distinct decrease in the volume of the cream layer between pasteuriza-
tion at 142° F. and at 144° F., and this decrease continues as the
temperature of pasteurization is increased.
404
BULLETIN No. 237
[December,
11.79
9.90
140C
142
Fig. 4. — Ninety-eight Tests of the Effect of Pasteur-
izing Temperatures on the Cream Layer
All the foregoing
results were obtained
at city plants. At
country plants, close
to the point of pro-
duction, the pasteur-
izing machinery is
started as soon as a
small supply of milk
is at hand, and at
times the milk in the
receiving vat may be
that of only one or
two patrons. Under
such conditions there
is a maximum variation in the fat content and a corresponding varia-
tion in the results of the measurements.
The results of ninety-eight tests at such a country plant are shown
in Fig. 4.
Owing to a number of disturbing factors, the results shown in
Fig. 4 are somewhat less accordant than those previously shown. It
chanced that about one-third of the patrons of this milk plant used
milking ma-
chines, and
about one-half
of the tests were
made at the time
of the first hot
wave of the sea-
son, when the
milk was unduly
acid. The re-
maining obser-
v a t i o n s were
made about one
month later,
when conditions
were normal.
The results of
this second visit,
taken by them-
selves, would
show a normal
temperature re-
lation.
14.62
13.68
12.74
c, 11-79
8
.3 10.85
o
^ 9.90
fi
S 8.96
03
5 8.02
« 7.08
Ci
& 6.13
5.19
4.24
•
•
-• .
•i-j:
* If
-•: :
..
r* — ••
5 BJ
"^
*. -
.
• ::
C3
SL ..
~
• •
^
•
•
* •
N.
I S^
7
•'
\
'\
\
>
\
\
\
\
^
V
\
v^
t
\
\
\
140C
142C
144C
146 =
148°
150° F.
Fig. o. — Two Hundred Eight Tests of the Effect of Pasteur-
izing Temperatures on the Cream Layer
1921] TEMPERATURE OF PASTEURIZATION AND CREAMING ABILITY OF MILK 405
14.62
13.68
12.74
n7Q
— • i v
i
| 10.85
^ 9.90
S 8.96
02
V
O 8.02
41
I 7.08
6.13
5.19
4.24
A
N
x
\
\
140 c
142 c
144C
146 c
148 c
150° F.
Fig. 6. — Summary of 401 Tests of the Effect of Pasteurizing
Temperatures on the Cream Layer
These re-
sults empha-
size the fact
that while the
temperature
of pasteuriza-
tion may be,
and often is,
the largest
factor in de-
termining the
volume of
cream in the
milk bottle,
this cream
volume is also
influenced by
other factors
which are
still for the
most part lit-
tle under-
stood.
All the results thus far presented were obtained by the use of a
single pasteurizing machine in each plant. Many of the larger plants
are equipped with two or more machines. In Fig. 5 are given the
results from 208 measurements, which include samples from five dif-
ferent pasteurizers in two adjoining plants handling a common milk
supply. Three of these machines were of the same make, but these
three machines varied as widely in their effect upon the creaming
power of the milk handled in them as did the three machines of dif-
ferent makes. The differences in the effects from the five machines
were really very slight. The data from one of these machines have
already been presented in Fig. 2. The results shown in this chart
are more varied than those previously shown. This is due in part
to the larger number of observations, in part to the effect of the
different machines, and in part to the fact that these tests extended
over a period of about four months.
However, the general effect of pasteurization upon the cream is
practically identical with that found in the other plants already dis-
cussed. The volume of cream begins to decrease noticeably at tem-
peratures between 142° and 144° F., and this decrease grows rapidly
with the increase in the temperature of pasteurization.
The results presented in Figs. 1 to 5, showing the effect of the
temperature of pasteurization upon the volume of cream later de-
406 BULLETIN No. 237 [December,
veloped on the milk, were taken from five different plants. These
plants were selected from a larger number in which tests have been
made, because in each of these plants the tests were sufficient in num-
ber to provide a basis for a dependable estimate of the effect of the
pasteurizing temperatures used, and because in each of them the aver-
age fat content of the milk handled ranged between 3.4 and 3.6 per-
cent, altho the individual samples varied more widely. The data
from other plants, so far as they go, were entirely in harmony with
those here presented.
Since these data, while accumulated in widely scattered cities, are
really quite comparable, the results of the 401 tests are grouped
in Fig. 6.
While the results shown in Fig. 6 vary considerably, the fact stands
out clearly that the cream rising on the milk pasteurized at 142° F.
was distinctly more abundant than that rising on the milk pasteurized
at 144° F. As the temperature of pasteurization rises above 144° F.,
the decrease in cream becomes rapidly more pronounced. Taking the
volume of cream obtained at 142° F. as the basis of calculation, the
loss in volume as the pasteurizing temperature is increased to 145° F.
amounts to slightly more than 10 percent ; at 146° F., it has increased
to 16.6 percent ; and at 148° F., it has increased to approximately 40
percent by volume.
LIMITATIONS OF TEMPERATURE CONTROL
Milk pasteurization is frequently spoken of as tho it were con-
ducted at a fixed temperature. While this represents an ideal toward
which the industry is striving, it is one which has not been realized.
The details of temperature control vary with the different types
of pasteurizing machinery. The vat pasteurizers depend largely upon
direct control by the operator, while the continuous machines are
commonly provided with automatic devices. However, these automatic
regulators require continuous oversight and adjustment, and at best
the temperature fluctuations are merely kept within more narrow
limits.
In extreme cases the limits of temperature fluctuation may be as
wide as ten degrees; more commonly they do not amount to more
than five degrees, and in well regulated plants the variation during
the day's run may be held down to about three degrees. It is very
rare indeed that fluctuations in the temperature of pasteurization for
the entire day are consistently kept within any narrower limits.
The phrase "pasteurized at 142°-145° F., for thirty minutes" is
accurately descriptive of the work in many of the best milk plants.
In such plants the bulk of the milk is pasteurized at 144° F., with
occasional brief deviations in either direction. In the plants having
TEMPERATURE OF PASTEURIZATION AND CREAMING ABILITY OF MILK 407
the most perfect temperature control of any known to the author,
the temperature fluctuations amount to about two degrees. In addi-
tion to a constant and exacting personal supervision, this perfection
of temperature control has been preceded by an initial cash outlay
for plant and equipment of well above a half -million dollars.
Few milk companies have an amount of business and a financial
backing which will permit such heavy investment. Altho it is to be
hoped that the same accuracy of temperature control may be attained
thru less expensive installation, the fact seems to be that so far it
has not been thus attained.
SUMMARY
The layer of cream on the bottle of milk as delivered to the con-
sumer is important because the consumer uses it as an index of the
richness of the milk and as a source of cream.
Since 1900 a heating of the milk to 140°-145° F. for thirty minutes
has been generally recognized as proper pasteurization. Recently
there has been a growing demand that the temperature of pasteuriza-
tion be increased to 145° F. for thirty minutes as a minimum time
and temperature.
In the operation of practically all large commercial pasteurizers,
a variation of at least three degrees Fahrenheit is practically unavoid-
able. Many machines vary even more widely. Accordingly, the
requirement of 145° F. as the minimum temperature means pasteuriza-
tion at 145° to 148° F.
The data here presented show that the volume of creafti on milk
begins measurably to decrease when the temperature of pasteurization
rises from 142° F. to 144° F. As the temperature goes higher, the
decrease in the volume of cream becomes rapidly more pronounced;
at 145° F. it amounts to slightly more than 10 percent by volume;
at 146° F. it amounts to 16.6 percent ; and at 148° F. to approximately
40 percent.
RELATION OF THESE FINDINGS TO THE MILK PROBLEM
As has been pointed out, without the presence of a satisfactory
layer of cream in the top of the milk bottle it is difficult to induce
the public to purchase pasteurized milk.
The results here presented show that when the temperature of
pasteurization is raised to 144° F., the volume of cream begins
measurably to decrease. As the temperature of pasteurization is fur-
ther increased, the damage to the cream layer increases rapidly.
It has been further pointed out that there is an unavoidable fluc-
tuation in the temperature of pasteurization, and that this fluctuation
rarely amounts to less than three degrees and often ranges more
408 BULLETIN No. 237
widely. Accordingly, if pasteurization is defined as heating to 145° F.,
as a minimum, for thirty minutes, such pasteurization involves heating
the milk to 145°-148° F., and in many plants will involve even higher
temperatures.
Taking these two sets of facts together it is evident that the re-
quirement that milk be pasteurized at a minimum temperature of
145° F. for thirty minutes will result in a sharp reduction of the
cream layer. This conclusion is not only to be drawn from the fore-
going studies, but it is also in entire accord with practical experience.
The pasteurizing process is the most important protection which
the public has against the spread of disease thru milk. It is therefore
important that this process carry the broadest practicable margin of
safety. At the same time it is equally important that the margin of
safety should not be carried to such limits as to produce a milk
unacceptable to the public, and thereby reduce .the consumption of
safe, pasteurized milk.
Repeated, careful studies by different scientists have uniformly
shown that the most resistant disease germs which may enter milk
are destroyed by a fifteen -minute heating at 140° F. Under such
circumstances pasteurization at 142°-145° F. for thirty minutes pro-
vides a very considerable margin of safety, both in the matter of tem-
perature and of time of exposure.
The proposition to require pasteurization at a minimum of 145° F.
for thirty minutes involves so large a destruction of the cream layer,
which is highly esteemed by the public, that such requirement would
probably result in a decreasing consumption of pasteurized milk.
Accordingly, this increase in the margin of safety w7ould hardly seem
justified in the absence of any evidence that 142°-145° F. is insuffi-
cient, and in the absence of any attempt to provide increased safety
in other ways.
REFERENCES
1. RUSSELL, H. L., AND HASTINGS, E. p. Thermal death point of tubercle bacilli
under commercial conditions. Wis. Agr. Exp. Sta. Ann. Eept. 17 (1900),
147-170. 1900.
2. Report of Committee on Milk Supply, Sanitary Engineering Section, American
Public Health Association. August, 1920.
3. HARDING, H. A. The measurement of the volume of cream on milk. 111. Agr.
Exp. Sta. Circ. 249. 1921.
4. RUSSELL, H. L. Pasteurization of milk and cream for direct consumption.
Wis. Agr. Exp. Sta. Bui. 44. 1895.
5. SMITH, THEOBALD. The thermal death point of the tubercle bacillus in milk
and some other fluids. Jour, of Exp. Med. 4, 217-233. 1899.
6. See 1, above.
7. ROSENAU, M. J. Thermal death points of pathogenic microorganisms in milk.
Public Health and Marine Hospital Service of the U. S. Hvgicnic Laboratory
Bui. 42. 1908.
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