FREEZING INJURY OF FRUITS AND VEGETABLES 21 



of ice. When ice occurs in only part of the tuber, its presence can 

 be detected at times by a crunching sound if, while held to the ear, 

 the tuber is pressed between the fingers. 



Tubers that are frozen solid collapse promptly upon thawing, 

 becoming soft and watery. Their wet surfaces make detection and 

 sorting out easy. The presence of moisture on a tuber is not neces- 

 sarily a sign of freezing injury, however, since cold tubers when 

 brought into moist air with a temperature higher than their own 

 very readily condense water from the air on their surfaces. Some- 

 times if only a part of a tuber is exposed to freezing temperature 

 that portion is frozen to death and upon thawing becomes turgid 

 and blisterlike and the skin frequently discolors. If tissues that 

 have been frozen to death are cut shortly after thawing, they may 

 appear dull and colorless at first. Upon exposure to the air they 

 frequently pass promptly through pink and red stages and ulti- 

 mately become brown, gray, or inky black. The rate of develop- 

 ment of these colors depends upon the temperature of the air. At 

 temperatures below 75° F. several hours may be required, while 

 only V2 to 1 hour may suffice at higher temperatures. The occur- 

 rence of these color changes is also determined by the extent to 

 which air reaches the affected tissues. 



When extensive areas of tissues are killed by freezing they 

 usually become infected with bacteria, which cause foul-smelling, 

 slimy, or sticky rots if thawing takes place in a warm, humid 

 atmosphere. They dry down to a mealy or tough, leathery, granu- 

 lar, chalky mass of starch and tissue remnants if they thaw in cold 

 or dry air. Tubers that have only one side frozen frequently have 

 the killed portion sharply set off from the unaffected area by a 

 purplish or ultimately brown line of corky tissues. Often infection 

 by species of Fusarium sets in before the unaffected cells are 

 sealed off by the corky layer. 



From experience with potatoes in the field, in storage, and in 

 transit and from experiments it is generally known that when a 

 lot of tubers has been exposed to low temperatures for a long 

 enough time for some to freeze solid, there frequently are others 

 which do not freeze at all and still others which seem to be unaf- 

 fected so far as external appearances indicate, but when cut they 

 may show the various types of symptoms known as freezing or 

 frost necrosis. 



The mildest type of freezing necrosis is one marked by tissues 

 that are drier than the surrounding ones and that have a grayish^ 

 white tint. When the injured tissues are cut soon after thawing, 

 they may turn red in a short time and finally become brown. In 

 uncut tubers, even after a long time, the only sign of injury is a 

 grayish, dull, dry, collapsed appearance of some of the tissues. 



Generally, however, freezing necrosis is marked by decided dis- 

 colorations. These may not be apparent immediately after thaw- 

 ing, but they usually set in after 5 or 6 hours. Usually the color 

 changes from dull white to pink or brick red and finally to gray, 

 brown, or black. 



There are several types of discoloration. One, the ring type, is 

 limited to the vascular (water-conducting) ring and immediately 

 adjoining tissues and is characterized by pronounced blackening. 



