MARKET DISEASES OF VEGETABLES. 55 



Neither net necrosis nor internal brown spot should be 

 confused with freezing injury, which sometimes produces 

 very similar symptoms, but usually is otherwise distin- 

 guishable. 



These internal discolorations are not decay and do not 

 impair the food value of the tuber, but affected portions 

 usually are rejected in the preparation of potatoes for the 

 table. 



Ref. (49); (50). 



POTATO: FREEZING INJURY. 



Cause : Exposure to low temperatures in the soil or dur- 

 ing harvest, transit, or storage. 



The symptoms of freezing injury are varied and complex 

 and may be general, appearing in all the tissues of a tuber, 

 or local and restricted to sharply limited regions of the 

 tuber. They depend upon the variety, maturity, and indi- 

 viduality of the potato affected, upon the varying predispo- 

 sition of the several tissues of the tuber to freezing injury, 

 upon the temperature to which the tuber was exposed, and 

 upon the rate of fall of the temperature as well as the dura- 

 tion of the exposure. The symptoms also depend upon the 

 interval between the time of injury and examination of the 

 affected tuber, and may or may not depend upon the rate of 

 thawing and the conditions under which this takes place. 



Thoroughly frozen potato tissue no longer possesses the 

 natural crispness or brittleness of the sound potato. Frozen 

 tissue looks dull and does not cut readily nor with snap. 

 This is due to the formation of ice in the tissues. In the 

 freezing process, the cell water passes out of the cells into 

 the intercellular spaces, the spaces between the cells, where 

 it forms ice crystals. The extent to which the cell water 

 leaves the cells depends upon the point to which the tem- 

 perature is lowered, upon the rate at which this takes place, 

 upon the duration of the exposure, and upon the nature of 

 the potato. 



The symptoms presented by thawed potato tissues are 

 very perplexing. Sometimes in case of severe freezing, the 

 tissues become turgid, blister-like, and swollen, and the skin 

 may be discolored. At other times they are quite firm and 

 remain intact and, contrasted with healthy tissue, merely 

 appear dull and colorless. If only a portion of a tuber was 

 severely frozen, usually a purplish band marks the border 

 of the uninjured tissue. When such a tuber is cut, the 

 border between healthy and injured tissue generally is 

 marked by a sharp purplish to dark brown line. However, 

 this line is not always present. 



If no infection sets in, the more or less watery thawed 

 tissue dries down to the consistency of a rather moist mealy 

 mass or a shrunken, dull, grayish, very tough and leathery 

 granular mass which is composed of shrunken cells and 

 starch. Very frequently bluish to black colors develop. In 



