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Contribution from the Bureau of Plant Industry
WM. A. TAYLOR, Chief

Washington, D. C. PROFESSIONAL PAPER June 13, 1921

FREEZING INJURY TO POTATOES WHEN
UNDERCOOLED.'*

By R. C. WricHt, Physiologist, and GrorGE F. Taytor, Chemical Laboratorian,
Office of Horticultural and Pomological Investigations.

CONTENTS.

Page. Page.
Scope of the investigations_____-_-_ 1 | Inoculation of unde cooled potatoes_ 7
Physiology of the freezing process__ eT POPSET IG ge ee sia Bese oY a ea) 14
lantot the sworke:: bisa eee es 4

biterature> cited es) ees eee 15

SCOPE OF THE INVESTIGATIONS.

Each year the loss to growers, shippers, and carriers of potatoes
due to freezing reaches an enormous figure. This is particularly
true of the late or main crop produced in the Northern States. This
crop is usually in constant danger of exposure to freezing tempera-
ture from just before it is harvested until it reaches the consumer.
There are two general classes of frost-injured potatoes—those frozen
solid and subject to collapse immediately on thawing and those that
show evidence of injury only on being cut open a few hours after
being warmed. The first class of injured potatoes is easily identified
by the soft, wet condition that develops on thawing. This type of
injury is due to the potato being subjected to freezing at very low
temperatures or to prolonged exposure at higher freezing tempera-
tures. The second type of injury is not apparent on superficial ex-
amination. It is due to exposure to temperatures just below the
freezing point or to a very low temperature forashorttime. If after
the potato has been thawed a few hours it is cut open, evidences of
this type of freezing injury are apparent by the presence of vascular
discoloration of three types or a combination of any of the three.

1This bulletin gives the results of a portion of the work carried on under the Bpewect
“ Factors affecting the storage life of vegetables.”

11310°—21

2 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE.
PHYSIOLOGY OF THE FREEZING PROCESS.

The frost necrosis of potato tubers is described by Jones, Miller,
and Bailey (4)? as being characterized by three types, viz, ring, net,
and blotch. In the ring type only the vascular ring, an area near the
surface, is discolored. In the net type threadlike areas radiating
from the center are discolored. The third type is marked by irregular
blotched areas. Ordinarily no further change takes place after the
injured potato thaws, although in the most severe cases the tubers
soon begin to break down if kept warm.

Frost injury seems to be the result of actual ice formation within
the potato tissue. Abbe (1), in his investigations of the effect of freez-
ing upon plant tissue, considers that as the tissue cools water exudes
from the cells into the intercellular spaces. After sufficient under-
cooling this water freezes. The concentrated sap remaining in the
cells will not freeze until cooled below the freezing point of water.
On thawing, this intercellular water escapes by transpiration and the
plant wilts. G6dppert (3) and Sachs (6) both observed the presence
of ice within the cells and the intercellular spaces of plant tissue.
However, Sachs found crystals of ice usually present in the inter-
cellular spaces. Miiller-Thurgau (5) was one of the first to publish
(1880) upon frost injury of potatoes in his investigations upon the
freezing point of plant tissue. The phenomena of undercooling were
investigated by him to a considerable extent. He showed that plant
tissue required undercooling below the true freezing point before
actual freezing commences. He also showed that the expressed sap of
the potato tuber freezes at a higher temperature than the living tis-
sue. The ultimate freezing point of potatoes is placed at 30.21° F. by
Miiller-Thurgau and between 28° and 26° F. by Appleman (2).
Vaughan and Miller (7) state that “actual freezing does not begin
in potatoes until the temperature drops below 28° F.” Jones, Miller,
and Bailey (4) place the freezing point at 28.4° F.

_ The author of an unsigned article in the Potato Magazine (8) states
that exposure at 30° F. for 9 or 10 hours or at 16° F. for 1 hour
will develop signs of frost necrosis. In results published by Wright
and Harvey (9) the freezing point varied from 29.67° to 28.13° F.
according to variety and season. In this investigation the freezing
points of 18 different varieties were determined. The freezing
points varied in different varieties and according to family groups.
The early and medium-early potatoes froze at a higher point than
the purely late varieties. The subject of undercooling was discussed.
It was stated that potatoes, since their sap consists of salts, sugar, and
other soluble material, freeze at a lower temperature than pure water.

2 The serial numbers in parentheses refer to “ Literature cited’ at the end of this
bulletin.

FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 3

It was shown that potatoes could be undercooled several degrees
below the true freezing point without actual freezing taking place.
In addition, it was shown that when once cooled below the true freez-
ing point the process of undercooling can be terminated at any point
by inoculating the tuber. The term “inoculation” is used: in this
connection to indicate any disturbance which will start crystalliza-
tion or ice formation. Following inoculation the temperature of the
potato rises quickly to the true freezing point, accompanied by the
formation of ice within the tissue. Inoculation is caused by some me-
chanical disturbance, such as a jolt or jar sufficient to cause freezing
to commence. It is a well-known fact that water, for example, can
be undercooled below its freezing point. After a certain amount of
undercooling actual freezing will result from any slight disturbance
or by the addition of a crystal of ice.

In the potato, inoculation is readily accomplished by striking or
jarring the undercooled tuber. The degree of force necessary to
accomplish this is determined by the extent of undercooling. It
seems that a potato can be undercooled to such a point that the
slightest perceptible jar will instantly inoculate it. Water on the
surface or sap from a bruise will prevent a potato from reaching the
degree of undercooling it would attain if dry, since this free moisture
will freeze at a higher temperature than the living tissue and will in
this way inoculate the entire potato. Miiller-Thurgau (5) and Jones,
Miller, and Bailey (4) found that a freshly cut potato will freeze
more quickly; that is, it will not undercool as far as a dry potato.

Miiller-Thurgau was of the opinion that the undercooling point
varies with the air temperature to which potatoes are exposed. He
attempted to justify this view by data. In one instance freezing oc-
curred in two hours at an exposure of 23.9° F. and in another in five
hours at an exposure varying from 15.8° to 10.7° F. Jones and his
colleagues (4), commenting on these results, state that possibly the
earlier freezing at the higher undercooling temperature was due to a
more rapid fall in temperature in this case. Data submitted by them
and also in Department Bulletin 895, heretofore cited (9), indicate
that the degree of undercooling is dependent for one thing upon the
rate of fall of temperature. It is also entirely possible that a sudden
unnoticed jar might have caused earlier freezing where the under-
cooling temperature was higher. In Department Bulletin 895 (9,
p- 6) a diagram is presented in which results are illustrated when two
potatoes were exposed to different temperatures. One potato ex-

8It is considered that the term “inoculation” is rightly used here, as it is directly
comparable to the process in physical chemistry, known by the same term, in which
crystallization is started in a concentrated solution by adding a crystal of the solute or

when freezing is started by adding a crystal of ice to pure water that has been
undercooled.

4 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE.

posed to a temperature of 8.7° froze in 6 minutes when the actual
internal temperature had reached 23° F. The other potato was
exposed to 15.8° and froze in 12.5 minutes when the internal tem-
perature had reached 20.2° F. The actual freezing point of both
potatoes was 29.15° F. This substantiates the results obtained by
Jones, Miller, and Bailey (4). They cite an instance of a tuber freez-
ing in 80 minutes when undercooled to 24.44°, while another exposed
to a slowly diminishing temperature did not freeze until 13.1° F.
was reached. Another specimen froze in 40 minutes when under-
cooled to 26.4° while exposed to a rapidly diminishing tempera-
ture which had reached 12.2° F. They state that, in general, po-
tatoes do not freeze until exposed to 26.6° F. or lower. To judge
from the results submitted in this report it is possibly safe to state
that except in the case of accidental inoculation the degree of under-
cooling at which freezing occurs depends upon the variety, the
rapidity with which undercooling progresses, and the length of ex-
posure to a given degree of undercooling.

PLAN OF THE WORK.

In order to apply to the handling of potatoes the knowledge of the
process of undercooling and freezing already gained, a series of ex-
periments was conducted at the Arlington cold-storage plant of the
Bureau of Plant Industry. It has been noted by potato growers and
shippers that sometimes one lot of potatoes while being handled or
otherwise disturbed in transit will freeze when another left undis-
turbed at the same temperature remained uninjured. Some apple
handlers are very careful not to jar or jolt frosted apples, because
they say it will cause them to bruise and rot. It is a practice among
some onion growers to store their onions through very severe winter
weather in mows or buildings between layers of hay where they are
undisturbed during the winter. These persons will often guard
against loose doors or shutters which can be slammed by the wind,
as this is liable to cause the onions to freeze and subsequently to rot.
Numerous instances are reported where potatoes have been exposed
for hours or even days to temperatures below their freezing point
without injury. Miiller-Thurgau reported having stored potatoes
at 32° to 26.6° F. for two weeks without injury. Of course, internal
frost injury may have been present without its being detected unless
the potatoes were cut open and examined.

In the present work seven standard varieties of potatoes, all true
to name and grown under the same conditions by the Bureau of
Plant Industry at the Aroostook Farm of the Maine Agricultural
Experiment Station, were used. These varieties were the Triumph,
Irish Cobbler, Spaulding No. 4, American Giant, Rural New Yorker,

FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 5)

Russet Rural, and Green Mountain. They were all held at 40° F.
previous to experimental use.

Storage tests on the seven varieties held at temperatures below the
true freezing points were conducted in order to correlate, if possible,
the freezing points (Table I) of these varieties as determined by the
thermoelectric method described by Wright and Harvey (9) and the
actual freezing of the different varieties in storage. These tests were
conducted at 28° and 25° F. The higher temperature was chosen
because it represented about the minimum degree of undercooling to
which the potatoes could be subjected, since it was just below the
freezing points of most of the varieties used. The lower tempera-
ture was chosen because it was far enough below the freezing points
of all varieties to represent a definite degree of undercooling at which
freezing quickly follows a very light inoculation. When the potatoes
were selected for the freezing tests they were placed in small wooden
baskets holding from 8 to 10 specimens, in order that they might cool
down without much delay. At the conclusion of each test the potatoes
were removed, to be held at ordinary room temperature for about 24
hours before being cut longitudinally for examination. If cut imme-
diately after being removed from storage, no evidence of freezing
injury will be apparent unless they have been frozen solid. Unless
specially noted, all injury reported is of the vascular type, as de-
scribed by Jones, Miller, and Bailey.

TABLE I.—Freezing points of seven varieties of potatoes.

| ‘ | | :
Freezing Freezing

Variety. point. Variety. point.
°F. 9 ie
OIA Pe oc oo sas ee acincine nase oe 29704 | Rural New. Yorker. 02: 5.-. 022.5... n- 28.70
fish Cobbler. - Sick Bees ee 29°66)||SRuUsseb Rural ieee as ste ose nae eee 28. 32
SPAUILGIN CANO? Ae van seek ee ee ee a 29iaor eG ree Moulitalnees as see eee eee 28. 50

PMIMICTICAN GiaMbar eee eet Seneca uae 29.64

Tables II and III show the results obtained from these tests at 28°
and 25° KF. The data presented in Table II show that potatoes did
not freeze to any serious extent when exposed to 28° I. for many
hours. In experiment No. 1 two specimens of each variety were held
seven hours, one specimen of Rural New Yorker being injured. In
experiment No. 2 twenty specimens of each variety were held 24 hours.
Only two varieties showed injury. These included two specimens of
Trish Cobbler and one specimen of the American Giant. In the third
experiment two specimens of each variety were held 48 hours, one
Rural New Yorker being injured. In experiment No. 4 half-bushel
lots of Triumph, Spaulding No.4, American Giant,and Russet Rural
varieties were held in bags for 48 hours with no injury. In experi-
ment No. 5 eleven specimens of each variety were held for 70 hours.

6 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE.

Serious injury was apparent in this experiment. Injury occurred to
five specimens of Triumph, six of Irish Cobbler, five of Spaulding
No. 4, four of American Giant, one of Rural New Yorker, two of
Russet Rural, and one of Green Mountain. The most serious injury
occurred in the early-maturing varieties. In experiment No. 6 six
tubers of each variety were held 96 hours, with injury occurring to
two specimens of Triumph, two of Irish Cobbler, two of American
Giant, and one of Russet Rural. The Spaulding No. 4, Rural New
Yorker, and Green Mountain varieties suffered no injury. The aggre-
gate injury in this experiment was 17 per cent, while in experiment
No. 5 the injury totaled 31 per cent.

TasLe Il.—Freezing injury to seven varieties of potatoes held at 28° F. for
different lengths of time.

Number of experiment and duration of exposure.
|
No. 1, 7 No. 2, 24 No. 3, 48 No. 4, 48 No. 5, 7 No. 6, 96
hours. hours. hours. hours. hours. hours.
Variety.
Num- |} Num-} Num-/Num-) Num-|Num-} Quan-|Num-) Num- |Num-} Num- | Num-
| ber of} ber | berof|} ber | berof] ber |tityof| ber | berof} ber | berof} ber
speci-} in- |speci-| in- | speci-| in- | speci-| in- | speci-| in- | speci-| in-
mens. | jured. | mens. | jured. | mens. |jured.| mens. jured.| mens. |jured.| mens. |jured.
Bush
Triumphsi.2 here 2 0 20 0 | aif sb 1 0 11 5 6 z,
Trish Cobbler... ..-. 2 0 20 | ze 2 | On sees calcein 11 6 6 2
Spaulding No. 4.... 2 0) 20 | 0 2 G z CG 11 5 6 0
American Giant.... 2 0 20 1 2 (: 3 0 11 4 6 2
Rural New Yorker. 2 1 20 0 2 iho Se 0 ll 1 | 6 0
Russet Rural...... 2 0 20 0 7 Vick) 3 0 12 6 1
Green Mountain... 2 0 20 | 0 DAO) | clerics --==-- come 5 I a 6 0
| —= —_ i Ss
Rotaley ae 2 14 1). =1404])- 23" an a4 1 2 Oh eles 42 | 7

In no case at 28° F. were the potatoes frozen solid. A superficial
examination would not have revealed any evidence of freezing injury ©
in any of these varieties. It is true that during subsequent storage,
however, these internally injured specimens are the first to break
down, although these potatoes do not show injury so far as their
external appearance is concerned and would be as salable in the
ordinary market as the uninjured ones. After a few days at ordinary
room temperature they are quite unfit for food. While no serious
injury occurred till the potatoes were held at least 70 hours, isolated
cases of freezing injury did occur even when potatoes were held only
7 hours. It is difficult to explain these isolated cases. For some
unknown reason certain individuals seem to bear much less under-
cooling than others of the same variety. Similarly, certain varieties
bear less undercooling than other varieties. There is a certain va-
rietal difference shown here. The Irish Cobblers seem to be the
first to succumb to freezing injury, followed by the Triumph, Ameri-
can Giant, Spaulding No. 4, and Russet Rural varieties. The Rural
New Yorker and Green Mountain varieties are the most resistant.

i

Taste IlIl.—Freezing injury to seven varieties of potatoes held at 25° F. for
different lengths of time.

FREEZING INSURY TO POTATOES WHEN UNDERCOOLED.

Number of experiment and duration of exposure.
No. 1, 5 hours. No. 2,19 hours. | No.3, 24hours. | No. 4, 43 hours.
Variety.
Number] ., Number Number Number e
of speci- puuaet of speci- ee ofspeci- Rae of speci- Red
mens. } ‘| mens. } ‘| mens } *| mens: ‘
Uy ghey bee Aemoeseaosedeace 2 0 6 0 11 5 3 a1
irish Cobblers. esse ee 2 0 6 2 11 6 3 a2
Spaulding No.4: 222 232-2225 -2 2 6 6 0 11 8 3 1
American Giante.cse ss. se sees 2 0 6 2 11 6 3 3
Rural New Yorker........... 2 0 6 1 ial 5 3 0
BUUSSCUAR Uae seems e ae 2 0 6 1 11 2 3 b3
Green Mountain. .2.-.22-..-- 2 0 6 1 11 4 3 1
Lotale: HER eee nes 14 0 42 7 77 36 Pal 11

@ Frozen solid. b Two were frozen solid.

In Table III are shown the results obtained when the different
varieties were held at 25° F. In experiment No. 1 two specimens
of each variety were held 5 hours with no evidence of injury. In
experiment No. 2 six specimens of each variety were stored 19 hours,
with injury to two Irish Cobblers, two American Giants, one Rural
New Yorker, one Russet Rural, and one Green Mountain. It is
worthy of note that the Triumph and Spaulding No. 4 varieties,
which according to Table I have a comparatively high freezing point,
did not show injury in this experiment. In experiment No. 3 eleven
specimens of each were held 24 hours, with serious consequences to
most of the varieties. Injury was found in five Triumphs, six Irish
Cobblers, eight Spaulding No. 4, six American Giants, five Rural New
Yorkers, two Russet Rurals, and four Green Mountains. The total
injury was 47 per cent. In experiment No. 4 three tubers of each
variety were held 43 hours, with injury to one Triumph, two Irish
Cobblers, one Spaulding No. 4, three American Giants, three Russet
Rurals, and one Green Mountain. Tubers of the Rural New Yorker
were not injured; one specimen of Triumph, one of Irish Cobbler,
and two of the Russet Rural varieties were frozen solid.. The total
injury in this experiment was 52 per cent.

Generally speaking, when potatoes of the seven varieties were held
for varying lengths of time at 28° or 25° F. they did not freeze in
any definite order or with relation to their freezing points. At 28° F.
the early-maturing varieties with the higher freezing points possibly
showed more freezing injury.

INOCULATION OF UNDERCOOLED POTATOES.

Attention is called to the fact that in the experiments just de-
scribed the potatoes were purposely held entirely undisturbed, as it
had been found that potatoes when undercooled are liable to freeze

8 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE.

when jarred. It is considered highly important to call the attention
of growers and shippers to this point, since under some circumstances
it would be entirely possible that a lot of potatoes or even other prod-
ucts might be undercooled considerably below the freezing point and
escape injury if they remained undisturbed till their temperature was
above the freezing point. If it should be found necessary to move
such a let in order to protect them from further lowering of the
temperature, this should be very carefully done, having in mind the
danger of freezing as the result of jarring or jolting. These state-
ments seem justified and are supported by the results brought out by
the following experiments, which were conducted to determine the
amount and character of the disturbance necessary to cause freezing
when different varieties of potatoes are held at temperatures between
28° and 25° F. The same seven standard varieties before named were
used in these experiments.

EXPERIMENT NO. 1.

Duplicate lots of potatoes, consisting of a single specimen of each
variety, were held between 26° and 27° F. for 54 hours. Then each
specimen in one lot was dropped from a distance of 4 feet to the
concrete floor. After 17 hours from the time they were dropped,
both lots were removed and held at ordinary room temperature
several hours before they were cut open. In the check lot only the
Trish Cobbler and the American Giant were injured. All specimens
in the treated lot were not only badly bruised, as might be expected
from the rough treatment, but they were ali frost injured, as was
shown by extensive blackened vascular areas of the blotch type.
(Pl. I.) None of the potatces were frozen hard enough to cause
collapse of the tissues.

EXPERIMENT NO. 2.

Duplicate lots of one potato of each variety were held at 28° F. for
22 hours. One lot was then dropped as before and left for three
hours. Both lots were then removed. As before, at the lower tem-
perature all the dropped potatoes were bruised and showed extreme
freezing injury. In the undisturbed check lot only the Irish Cobbler
was injured.

EXPERIMENT NO. 3.

Duplicate lots of one potato of each variety were placed at 28° F.
for 24 hours. Each specimen from one lot was dropped from a dis-
tance of 2 feet, and after another 24 hours all were removed. On
the usual examination it was found that these potatoes were badly
bruised and showed extensive injury, as before, except the American
Giant and Spaulding No. 4 varieties, which showed no frost injury
and were not bruised.

Bul. 916, U. S. Dept. of Agriculture. PLATE lI.

FREEZING INJURY TO POTATOES ARTIFICIALLY PRODUCED.

The potatoes were undercooled at 25° F. for 24 hours, then inoculated by dropping
6inches. In one hour after inoculation they were removed to a warm room.
One specimen (Irish Cobbler, fig. 1) shows only slight freezing injury of the ring
type. The other specimen (American Giant, fig. 2) shows severe injury of the
ring and blotch type.

FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 9
EXPERIMENT NO. 4.

Duplicate lots of two potatoes of each variety were held at 28° F.
for 24 hours, after which all of one lot were dropped 1 foot. After
another 24 hours all were removed. Examination showed no bruis-
ing; however, most of the potatoes were frozen. The results are
shown in Table IV.

Taste 1V.—Freesina injury to potatoes of seven different varieties undercooled
at 28° F. and drepped 1 foot.

| Dropped. Check,
Variety. aan a a

Injured. | Uninjured.| Injured. | Uninjured.
IM Disses so stec ee aes coe eee ee een cence cere ececeeee | 2 0 0 2
ATISH CODD Chan = eee eo ere ce eects Cate ei ae bo ee taee eis 2, 0 0 2
S pauldineEN Onassis cic winelocecincct conse eciceiecs 0 2 0 2
AUTION CUMEGI ACE eto eee ame e resets cietacicnet a wists eS min ieece 2 0 0 2
RVUTAENG WEY OL KCL sacs seersaccsmselsabee tickers sSeetenicessee sy | 2 0 1 1
MEU USSC tm E UI esoie ey ar cde re ici oie Ste ms Sera oops oe Richy eral Maar | 2 0 | 0 2
(GigsGiney OPA b tL et es BOS SEBS a ene eee, ais | 0 2 0 2
AUT RA RSS Ses TO CS raters aE ReR es EE re Man Eee 10 4 1 13

EXPERIMENT NO. 5.

A total of 24 potatoes of the Rural New Yorker variety were placed
at 28° F. After 24 hours 12 were dropped 12 inches. In no case was
a potato bruised or even the skin broken. In 4 hours both lots were
removed. Examination showed all of the lot that was dropped to
be injured by freezing. None of the check lot was injured.

EXPERIMENT NO. 6.

This experiment was conducted to ascertain, if possible, the mini-
mum distance from which an undercooled potato may fall and still
succumb to freezing injury. Five lots of 12 potatoes of the Rural
New Yorker variety were held at 28° F. for 18 hours. Then lots
were dropped 2, 4, and 6 inches, and in one lot each individual was
struck with a pencil. After seven hours all were removed. Hxamina-
tion showed that only the potatoes that were struck with the pencil
were injured.

EXPERIMENT NO. 7.

Lots of three potatoes of each variety were dropped, as in experi-
ment No. 6, with the exception that each potato was dropped six times
instead of once. The results are shown in Table V. Practically the
same amount of freezing injury was produced in all dropped pota-
toes without relation to the length of the fall. The injury produced
was of the blotch type. It may be stated here that the potatoes used
in this experiment were purposely selected and weighed, so as to have
both large and small specimens in each lot, thus varying the force of

10 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE.

the blow caused by the fall. There was apparently no relation be-
tween the weight of the potato and the freezing injury. In addition
to the lots already described in this experiment, an additional lot of
two of each variety was treated by striking each potato sharply with
a pencil once, with the results shown in Table V. It is difficult to
state why it was necessary to drop the potatoes several times from
2,4, and 6 inch heights to produce frost injury, while one sharp blow
with an ordinary pencil results in serious injury. The possible expla-
nation is that the blow from the pencil caused a more violent concus-
sion in a smaller area, from which the whole potato was inoculated.

TABLE V.—Freezing injury to potatoes of seven different varieties undercooled
at 28° F. and inoculated in different ways.

|
Dropped Dropped Dropped Struck with Gheck
2inches. ~ 4inches. 6 inches. pencil. ECs:
Variety. aa
Tn- Unin- | In- Unin- | In- Unin- In- Unin- | In- Unin-
jured. | jured. | jured. | jured. | jured. | jured. | jured. | jured. | jured. | jured.
a eg
rum phases 3 0 3 0 3 0 1 1 0 3
Irish Cobbler....... 3 0 3 0 3 0 1 1 0 3
Spaulding No. 4....- 1 2 3 0 2 1 1 1 0 3
American Giant--.-- 3 ) 3 0 3 0 2 0 1 2
Rural New Yorker... 3 0 3 0 3 0 ale 0 0 3
Russet Rural....-..- 3 0 3 0 2 1 2 0 0 3
Green Mountain... . 3 0 2 1 2 1 2 0 1 2
otal. ...2s.02 19 2 20 1 | 18 3 i 3 2 19

EXPERIMENT NO. 8.

Two lots of one specimen of each variety of potato were held at
28° F. for 24 hours, after which each potato of one lot was repeatedly
bounced on the floor for about 10 seconds by dropping it from a height
of not over 1 inch. After another 24-hour period all were removed.
While none of the bounced specimens showed bruising or mechanical
injury, they were all frost injured. Thespecimens of American Giant
and Green Mountain froze solid, with resultant complete collapse on
thawing. Of the check lot the Spaulding No. 4 and the Green Moun-
tain varieties showed slight freezing injury.

EXPERIMENT NO. 9.

Lots of three potatoes of each variety were held at 25° F. for 18
hours, after which they were dropped once from distances of 6, 4, and
2 inches. After another hour all were removed. Examination
showed that all dropped potatoes were injured, while in the check
lot frost injury resulted to only one each of the Irish Cobbler, Ameri-
can Giant, Russet Rural, and Green Mountain varieties.

Several experiments were conducted to test the rate at which the
temperature of a potato will fall when exposed to a temperature be-
low its freezing point. Figure 1 illustrates a typical set of results.

FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 11

A Triumph potato was suspended in a metal container 18 inches long
by 4 inches in diameter, which was in turn immersed in a brine tank.
Thermoelectric couples were located at the center and near the surface
of the potato; also one was suspended in the air 1 inch from the
potato. Periodical readings were made from the time the temperature
of the potato was near 40° F. The temperature of the atmosphere
surrounding the potato gradually fell from 27.2° to 25.6° F. The
temperature of the potato at the beginning was 40.8° at the surface

oN

ja eee ener [Pee be
eens gees ceeeeeet
Ble | oF haf <2 te ee ae

75 GO M5, 120 (35 [50 /65 180 steal 225 elite ZO SHE SOC
DRO ATE TIME (. MINUTES)

Fic. 1.—Temperatures at the center and near the surface of a Triumph potato as it
undercooled to between 26° and 27° F. At this point it was inoculated by tapping it
sharply with a pencil. As the potato froze, the temperature rose. When all the heat
was liberated, the temperature of the potato reached 28.7° F. and remained several
minutes. The temperature of the air bath to which the potato was exposed is also
shown. Note the accompanying rise in the temperature of the surrounding air as
the potato froze and liberated heat.

and 39.9° F. at the center. The difference in temperature at the
surface and center gradually diminished until at 240 minutes there
was a difference of only 0.4 of a degree. After this the difference
became somewhat greater. In 285 minutes the temperature fell from
40.8° and 39.9° F. at' the surface and center, respectively, to 26.7° and
26.2° I. At this point the potato was tapped sharply with a pencil,
and freezing commenced. The temperature at both points began to
rise as heat was liberated, due to crystallization of the water. In five
minutes the temperature at the center rose to 30.2° F., and at the
surface to 29.5° F. In 10 minutes the temperature at both points was
practically the same, 28.7° F. This represents the freezing point.

12 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE.

The temperature of the air surrounding the potato at the time freez-
ing took place rose from 25.6° to 26.6° F., due to heat liberated by the
potato.

SUMMARY OF RESULTS OBTAINED BY DROPPING UNDERCOOLED POTATOES.

When undercooled at 28° F. and dropped 4 feet and 2 feet, the
potatoes were severely bruised and showed extreme freezing injury
of the vascular type. When dropped 1 foot, no bruising resulted, but
all varieties froze except the Green Mountain and Spaulding No. 4.
The type of injury was a faint blotch. When dropped 6, 4, and 2
inches once, no freezing injury resulted; but when dropped six times
from these heights, severe injury followed. When bounced several
times by dropping 1 inch, severe injury followed; in some cases the
potatoes were frozen solid. When undercooled at 25° F., dropping
once from 6, 4, and 2 inches caused injury.

Some further experiments were undertaken to determine what
other forms of disturbances will inoculate undercooled potatoes.
Duplicate lots of 12 specimens each of the Rural] New Yorker variety
were placed at 25° and 28° F. for 24 hours. At each temperature
one lot was placed in the direct draft from an electric fan, while the
other lot was protected from the draft to act as a check. At 25° F.
10 of the 12 potatoes in the draft showed injury; also 3 of the
check lot were injured. At 28° F. no injury was found in either lot.
A possible explanation for the fact that the potatoes froze when
exposed at 25° F. to a strong current of air is that they undercooled
much more rapidly than the lot not in the draft. |

Two lots of six specimens each of Rural New Yorker were held at
28° F. for 24 hours, then wet and removed after another 24 hours.
All of the lot that were wet were injured, while three of the lot that
were not wet also were injured. In the next experiment two lots
consisting of two of each variety were held at 28° F. for 24 hours;
they were then wet and held for another 24 hours. The results ob-
tained on examination are shown in Table VI.

TABLE VI.—Freezing injury to potatoes of seven different varieties held at
28° F. for 24 hours and then wet while undercooled.

Wet. Check.
Variety.

Injured. | Uninjured.| Injured. | Uninjured.

ADVAN TAY Of Ole Cee Saas ee Re aaNet eas tet 2 (2) 0 2
TrISHMCODDICT eee ae So Beles | AGAR eae Ne NS bee ae 2 0 0 2
Spaulding No Awe eyes wie eh ag ee Le a een mike ea 0 2 0 2
PANTING Tal GevTUR Gr ean eye ec esis et cele a cosh gen VL 1 1 0 2
RATING We YCOUIKCTy pee eae ican ae La I eo ae BOG See 2 0 1 1
ARUUASSE Gee UAT lie ea eee aera ee wes Oe eter Raa Crete ae a eta 2 0 0 2
Greens Mo uma Naw Oe Re Ne AN eee eee el i | 1 0 2)
Total eR ee RMI ans caesar Slee rhe oy 10 | 4 | 1 13

@ Faint biotch.

f

FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 13

The injury to these potatoes was not serious. All affected ones
showed only a faint blotching. In the next experiment a mixed lot
of 12 potatoes was held at 28° F. for 72 hours while supporting a
100-pound bag of sand. No evidence of injury was apparent.

An attempt was made to show that potatoes when undercooled are
liable to be injured by ordinary handling. Duplicate half-bushel
lots of potatoes representing four varieties, viz, Triumph, Russet
Rural, Spaulding No. 4, and American Giant, were put together in
four bags and held at 28° F’. for 24 hours. Then one lot of two bags
was rolled across the floor for a distance of about 30 feet. After
24 hours all were removed. Examination showed the results pre-
sented in Table VIT.

TABLE VII.—Freezing injury to potatoes of four different varieties held at 28° F.
for 24 hours and then rolled 30 feet in a bag while undercooled.

Variety. Rolled. | Check.

Per cent Per cent
injured. injured.
10

ALT ARD TT AO) Es NS Slee aS ae EE ane Ne ES Pe 0
Sa uldins Noy 4 eae: Sees a aE ace es coat e sees tir Moe 2 See 0 0
ASAT ERUGRT OE ME TAB SS oh OO as Se re Oey CO ae CIES Can ar A SE ate 20 0
RV TISSCE MUU a st eres ee ca hs a eee Nirah ity dee eee Deg Mine are he eS YC 50 0

A similar experiment was carried out in which two lots of 12 of
each of the seven varieties were placed in two bags and held at 28° F.
for 24 hours. One was then rolled about 50 feet. After another
period of 24 hours all were removed and held for examination. The
results obtained are shown in Table VIII.

TABLE VIII.—Freezing injury to potatoes of seven different varieties heid at
28° F. for 24 hours and then rolled 50 feet in a bag while undercooled.

Rolled. Check.
Variety.
Number} Number | Number | Number
injured. | uninjured. | injured. | uninjured.
USOT O DEE De one hae pam ann ee ae hie Ge eee OP ae 5 7 0 12
BETIS COD DICE eae ee eee ee ee TAR RATE en 12 0 0 12
SEAL GIN GUN Ona peas oo ceil) Seen Diag. eae o Sedee i Aly 0 12 0 12
PAMITICTICAIN Gants! er occ ais Race Oe ee ees 2 10 0 12
FEV AIEN CW Y(OD KOT © pon joes Mae cai hierar rane hee ahaa eg Coes at 12 0 0 12
ECIISSO UEC EIN lee ries ohn pe pen eatery tae Saye 8 4 0 12
PiTceneMOUNntAIN= cere oes oe oe See sens ocean mcmle 7 5 0 12
ROY He 1) [aiagh SIRE oie re nn ER RR IR eva ns Sa 46 38 0 84

A distinct varietal difference is shown here. In neither experiment
did the Spaulding No. 4 variety show injury. The American Giant
was also less susceptible than the rest of the varieties. The Irish
Cobbler, Rural New Yorker, and Russet Rural showed the greatest
percentage of injury. In these experiments the potatoes did not
receive the amount of jolting or rough handling that they might be

14 BRUIT ETIN TT, S. DEPARTMENT OF AGRICULTURE.

subjected to under commercial conditions. It is unfortunate that
lack of material prevented further tests on a commercial basis. How-
ever, enough evidence was obtained to justify the publication of the
results already obtained and to call the attention of growers and
shippers of potatoes to the facts here presented.

SUMMARY.

(1) Potatoes freeze more quickly when exposed to a rapidly dimin-
ishing temperature than when the temperature diminishes slowly.

(2) Potatoes can be undercooled several degrees below their true
freezing point and then warmed again above the freezing point with-
out freezing injury, provided no ice formation takes place within
the tissue.

(3) When undercooled, jarring resulting from rough handling or
incidental to hauling is lable to cause potatoes to freeze.

(4) When undercooled in a temperature of 28° F., dropping from a
height of 2 feet caused potatoes to bruise badly and to show exten-
sive freezing injury. When dropped 1 foot they did not bruise, but
showed frost injury. When dropped 6, 4, or 2 inches once no injury
resulted, but when dropped six times frost injury followed. When
wet, injury also resulted. Potatoes supporting a 100-pound weight
were not injured.

(5) When undercooled at 28° F. and rolled across the floor for a
distance of about 50 feet in bags freezing injury resulted.

(6) When undercooled at 25° F. and dropped 6, 4, and 2 inches
once frost injury was apparent. |

(7) After freezing commences it is progressive. The amount of
injury caused within a stated time seems to depend upon the sur-
rounding temperature.

(8) Immediately after inoculation the temperature of the potato
rises to its true freezing point and remains for a varying length of
time, depending upon the surrounding temperature.

(9) Some varieties apparently are inoculated more easily than
others, even though their freezing point is higher.

1G epee 2 eels 1), FT ey Ailfins

. e LITERATURE CITED.
(1) Apps, C.
1894. The influence of cold on plants. A résumé. In Exp. Sta. Record,
v. 6, no. 9, p. 777-781.
(2) APPLEMAN, CHARLES O.
1912. Changes in Irish potatoes during storage. Md. Agr. Exp. Sta.
Bul. 167, p. 327-834.
(3) GOPPERT, H. R.
1830. Ueber die Wirme-Entwickelung in den Pflanzen, deren Gefrieren
und die Schutzmittel gegen dasselbe. xiv, 272 p. Breslau.
(4) Jonss, L. R., Mitier, M., and Battey, BH.
1919. Frost necrosis of potato tubers. Wis. Agr. Exp. Sta. Research
Bul. 46, 46 p., illus. Literature cited, p. 45-46.
(5) Mtxier, Herman, Thurgau.
1880-86. Ueber das Gefrieren und Erfrieren der Pflanzen. In Landw.
Jahrb., Bd. 9, p. 183-189, 1880; Bd. 15, p. 453-610, 1886.
/ (6) SacHs, JULIUS.
1860. Krystallbildungen bei dem Gefrieren und Veriinderungen der
Zellhiute bei dem Aufthauen saftiger Pflanzentheile. Jn Ber.
Verhandl. Siichs. Gesell. Wiss. Leipzig, Math.-Phys. Cl, Bd.
12, p. 1-50.
(7) VAUGHAN, R. E., and MILLEr, M.
1919. Freezing injuries to potato tubers. Wis. Agr. Col. Ext. Serv.
Cire. 120, 4 p., 3 fig.
(8) Watch potatoes in storage and transit. In Potato Mag., v. 2, no. 7, p. 16.
1920.
(9) WricHt, R. C., and Harvey, R. B.
1921. The freezing point of potatoes as determined by the thermo-
electric method. U.S. Dept. Agr. Bul. 895, 7 p., 1 fig.

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