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 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 1 BULLETIN No. 916 
 
 Contribution from the Bareaa of Plant Industry 
 WM. A. TAYLOR, Chief 
 
 4\-^^W'(U 
 
 ji\^?^^§*Lnu 
 
 Washington, D. C. 
 
 PROFESSIONAL PAPER 
 
 June 13, 1921 
 
 FREEZING INJURY TO POTATOES WHEN 
 UNDERCOOLED.^ 
 
 By R. C.^ Wright, Physiologist, and George F. Taylor, Chemical Lahoratoriany 
 Office of Horticultural and Pomological Investigations. 
 
 CONTENTS. 
 
 Page. 
 
 Scope of the investigations 1 
 
 Physiology of the freezing process 2 
 
 Plan of the work 4 
 
 Page. 
 Inoculation of uudeicooled potatoes- 7 
 
 Summary 14 
 
 Literature cited 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 for a short time. 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. 
 
 1 This bulletin gives the results of a portion of the work carried on under the project 
 " Factors affecting the storage life of vegetables." 
 11310°— 21 
 
 %(l^ mJ/' 
 
2 BULLETIN 916, U. S. DEPARTMENT OE 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 i^otato 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. Goppert (3) and Sachs (6) both observed the presence 
 of ice within the cells and the intercellular spaces of pla-nt tissue. 
 However, Sachs found crystals of ice usually present in the inter- 
 cellular sjDaces. 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 (T) 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 ci ted " at the end of this 
 bulletin. -■ — •"-"^ — ' " ^ 
 
 LlSRAI^V'Cf CONGRESS 
 
 •V^' 
 
 JUN2lt9^1 
 
 DOCUMENTS DIVISION 
 
<V' 
 
 FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 
 
 It was shown that potatoes could be imdercooled 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 b}^ 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 reacliing 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- 
 
 3 It is consideaed 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 disttirbed 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, Eural New Yorker, 
 
FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 
 
 Russet Eiiral, 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 stora,ge, 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. 
 
 Variety. 
 
 Freezing 
 point. 
 
 Variety. 
 
 Freezing 
 point. 
 
 Triumph 
 
 "F. 
 
 29.04 
 29.66 
 29.33 
 29.64 
 
 Rura 1 New Yorker 
 
 °F. 
 28.70 
 
 Irish Cobbler 
 
 Russet Rural 
 
 28.32 
 
 Spaulding No. 4 
 
 Green Mountain 
 
 28.50 
 
 American Giant 
 
 
 
 
 
 Tables II and III show the results obtained from these tests at 28° 
 and 25° F. The data presented in Table II show that potatoes did 
 not freeze to any serious extent when exposed to 28° F. 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. 
 Onl}^ two varieties showed injury. These included two specimens of 
 Irish Cobbler and one specimen of the American Giant. In the third 
 experiment two specimens of each variety were held 48 hours, one 
 Rul-al 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 TO hours. 
 
BULLETIN 916, U. S. DEPAKTMENT 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 Eural 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. 
 
 Table II. — Freezing injury to seven varieties of potatoes held at 28° F. for 
 
 different lengths of time. 
 
 Variety. 
 
 Triumph 
 
 Irish Cobbler 
 
 Spaulding No. 4 
 
 American Giant 
 
 Rural New Yorker. 
 
 Russet Rural 
 
 Green Mountain . . . 
 
 Total 
 
 Number of experiment and duration of exposure. 
 
 No. 1,7 
 hours. 
 
 No. 2, 24 
 hours. 
 
 No. 3, 48 
 hours. 
 
 No. 4, 48 
 hours. 
 
 No. 5, 70 
 hours. 
 
 No. 6, 96 
 hours. 
 
 Num- 
 ber of 
 
 Num- 
 ber 
 
 Num- 
 ber of 
 
 Num- 
 ber 
 
 Num- 
 ber of 
 
 Num- 
 ber 
 
 Quan- 
 tity of 
 
 Num- 
 ber 
 
 Num- 
 ber of 
 
 Num- 
 ber 
 
 Num- 
 ber of 
 
 Num- 
 ber 
 
 speci- 
 mens. 
 
 in- 
 jured. 
 
 speci- 
 mens. 
 
 in- 
 jured. 
 
 speci- 
 mens. 
 
 in- 
 jured. 
 
 speci- 
 mens. 
 
 in- 
 jured. 
 
 speci- 
 mens. 
 
 in- 
 jured. 
 
 speci- 
 mens. 
 
 in- 
 jured. 
 
 2 
 2 
 2 
 
 2 
 
 2 
 2 
 2 
 
 
 
 
 1 
 
 
 
 
 20 
 20 
 20 
 20 
 20 
 20 
 20 
 
 
 2 
 
 1 
 
 
 
 
 2 
 2 
 2 
 2 
 2 
 2 
 2 
 
 
 
 
 (; 
 1 
 
 
 
 
 Bush, 
 h 
 
 
 
 ^^ 
 
 5 
 6 
 5 
 4 
 1 
 2 
 ] 
 
 6 
 6 
 6 
 6 
 6 
 6 
 6 
 
 2 
 2 
 
 1 
 
 2 
 h 
 
 
 
 
 
 
 
 2 
 
 
 1 
 
 
 
 
 
 
 14 
 
 1 
 
 140 
 
 3 
 
 14 
 
 1 
 
 2 
 
 
 
 77 
 
 24 
 
 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, folloAved 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. 
 
FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 
 
 Table III. — Freezing injury to seven varieties of potatoes held at 25° F. for 
 
 different lengths of time. 
 
 Variety. 
 
 Triumph 
 
 Irish Cobbler 
 
 SpauldingNo.4... 
 American Giant.. . 
 Rural New Yorker 
 
 Russet Rural 
 
 Green Mountain . . 
 
 Total 
 
 Niunber of experiment and duration of exposure. 
 
 No. 1, 5 hours. 
 
 Number 
 of speci- 
 mens. 
 
 14 
 
 Number 
 injured 
 
 No. 2, 19 hours. No. 3, 24 hours 
 
 Number 
 of speci- 
 mens. 
 
 42 
 
 Number 
 injured. 
 
 Niunber 
 of speci- 
 mens. 
 
 77 
 
 No. 4, 43 hours. 
 
 Nimiber 
 injured 
 
 36 
 
 Number 
 of speci- 
 mens. 
 
 Number 
 injured. 
 
 21 
 
 o2 
 1 
 3 
 
 
 b3 
 1 
 
 11 
 
 a 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 OE 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 lot 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 5 J hours. Then each 
 specimen in one lot was dropped from a distance of 4 feet to the 
 concrete floor. After IT 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 
 Irish 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 all frost injured, as was 
 shown by extensive blackened vascular areas of the blotch type. 
 (PI. I.) None of the potatoes 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. 
 
Bui. 916, U. S. Dept. of Agriculture. 
 
 Plate I, 
 
 >s. 
 
 Freezing Injury to Potatoes Artificially Produced. 
 
 The potatoes were undercooled at 25° F. for 24 hours, then inoculated by dropping 
 6 inches. In one hour after inoculation they were removed to a warm room. 
 One specimen (Irish Cobbler, fig. 1) shows only sUght freezing injury of the ring 
 type. The other specimen (American Giant, fig. 2) shows severe injurj' of the 
 ring and blotch typo. 
 
rREEZIN"G INJURY TO POTATOES WHEN UNDERCOOLED. 
 
 9 
 
 EXPERIMENT NO. 4. 
 
 Duplicate lots of two potatoes of each A^ariety Avere held at 28° F. 
 for 24 hours, after which all of one lot were dropped 1 foot. After 
 another 24 hours all were remoA^ed. Examination showed no bruis- 
 ing; hoAveA^er, most of the potatoes AAere frozen. The results are 
 shoAA^n in Table IV. 
 
 Table IY. — Freezinfj injury to potatoes of seven different varieties undercooled 
 
 at 28° F. and dropped 1 foot. 
 
 Variety. 
 
 Dropped. 
 
 Injured. Uninjured. 
 
 Triumph 2 
 
 Irish Cobbler, 2 
 
 Spaulding No. 4 
 
 American Giant 2 
 
 Rural New Yorker 2 
 
 Russet Rural 2 
 
 Green Mountain 
 
 Total 10 
 
 Check. 
 
 Injured. Uninjured. 
 
 13 
 
 EXPERIMENT NO. 5. 
 
 A total of 24 potatoes of the Eural Xew Yorker A^ariety were placed 
 at 28° F. After 24 hours 12 were dropped 12 inches. In no case was 
 a potato bruised or eA^en the skin broken. In 4 hours both lots were 
 remoA^ed. Examination showed all of the lot that was dropped to 
 be injured by freezing. None of the check lot Avas 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. FiA^e 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 remoA^ed. Examina- 
 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 shoAvn in Table Y. 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 i^urposely selected and weighed, so as to haA'e 
 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 ]3otato 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 Y. 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 ordinar}^ 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 undereooled 
 at 28° F. and inoculated in different ways. 
 
 Variety. 
 
 Dropped 
 2 inches. 
 
 Dropped 
 4 inches. 
 
 Dropped 
 6 inches. 
 
 Struck with 
 pencil. 
 
 Check. 
 
 In- 
 jured. 
 
 Unin- 
 jured. 
 
 In- 
 jured. 
 
 Unin- 
 jured. 
 
 In- 
 jured. 
 
 Unin- 
 jured. 
 
 In- 
 jured. 
 
 Unin- 
 jured. 
 
 In- 
 jured. 
 
 Unin- 
 jured. 
 
 Triumpli 
 
 3 
 3 
 1 
 3 
 3 
 3 
 3 
 
 
 
 2 
 
 
 
 
 
 3 
 3 
 3 
 3 
 3 
 3 
 2 
 
 
 
 
 
 
 
 
 1 
 
 3 
 3 
 2 
 3 
 3 
 2 
 2 
 
 
 
 1 
 
 
 
 1 
 1 
 
 1 
 
 1 
 1 
 2 
 2 
 2 
 2 
 
 1 
 
 1 
 1 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 1 
 
 3 
 
 Irish Cobbler 
 
 Spaulding No. 4 
 
 American Giant 
 
 Rural New Yorker. . 
 
 Russet Rural 
 
 Green Mountain 
 
 3 
 3 
 2 
 3 
 3 
 2 
 
 Total 
 
 19 
 
 2 
 
 20 
 
 1 
 
 18 
 
 3 
 
 11 
 
 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. The specimens 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. 
 
FREEZmG 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 
 
 ^^ /5 JO 4S 60 rs &0 /OS" /£V A3S /50 /65- /60 /9S 2/0 2eS 3^i7 eSS 2/^0 ^SS 30C 
 
 Fig. 1. — Temperatures at the center and near the surface of a Triumph potato as it 
 undercooled to between 26° and 27° P. 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 
 slaown. 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 di:fference 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° F. 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 «>ame, 28.7° F. This represents the freezing point. 
 
12 
 
 BULLETIN 916, U. S. DEPARTMENT OF AGPJCULTURE. 
 
 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. Wlien dropped 6, 4, and 2 
 inches once, no freezing injur}^ resulted ; but when drop]3ed 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. — Fi^eezing injury to potatoes of seven different varieties Jield at 
 28° F. for 24 hours and then wet ivhile undercooled. 
 
 Variety. 
 
 Wet. 
 
 Check. 
 
 Injured. 
 
 Uninjured. 
 
 Injured. 
 
 Uninjured. 
 
 Triumph 
 
 2 
 2 
 
 1 
 2 
 2 
 1 
 
 
 2 
 1 
 
 
 1 
 
 
 
 
 
 1 
 
 
 
 2 
 
 Irish Cobbler 
 
 2 
 
 Spaulding No. 4 
 
 2 
 
 American Giant 
 
 2 
 
 Rural New Yorker 
 
 1 
 
 Russet Rural 
 
 2 
 
 Green Mountain 
 
 2 
 
 
 
 Total 
 
 10 
 
 4 
 
 1 
 
 13 
 
 
 
 a Faint blotch. 
 
FREEZmG 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 VII. 
 
 Table YU.— Freezing injury to potatoes of four different varieties held at 28° F. 
 for 24 hours and then rolled 30 feet in a Mg while undercooled. 
 
 Variety. 
 
 Triiimph 
 
 Spaulding No. 4, 
 American Giant 
 Russet Rural.... 
 
 Rolled. 
 
 Per cent 
 injured. 
 10 
 
 
 20 
 50 
 
 Check. 
 
 Per cent 
 injured. 
 
 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 
 28° F. for 24 hours and then rolled 50 feet in a 
 
 different varieties held at 
 bag while undercooled. 
 
 
 Rolled. 
 
 Check. 
 
 Variety. 
 
 Number 
 injured. 
 
 Number 
 uninjured. 
 
 Number 
 injured. 
 
 Niunber 
 uninjured. 
 
 Triumph 
 
 5 
 
 12 
 
 2 
 12 
 8 
 7 
 
 7 
 
 12 
 10 
 
 4 
 5 
 
 
 
 
 
 
 
 
 
 12 
 
 Irish Cobbler 
 
 12 
 
 Spaulding No. 4 
 
 12 
 
 American Giant 
 
 12 
 
 Rural New Yorker 
 
 12 
 
 Russet Rural 
 
 12 
 
 Green Mountain 
 
 12 
 
 
 
 Total 
 
 46 
 
 38 
 
 
 
 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 
 
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