MARKET DISEASES OF VEGETABLES. 57 



condition also is frequently referred to as chilling injury. 

 When potatoes that have become sweet following exposure 

 to low temperatures are kept at normal temperatures the 

 sweet taste disappears. This, is due to the fact that the 

 increased respiration due to rise in temperature oxidizes the 

 excess of sugars which accumulated at temperatures of 29 

 to 34 F. At these tempertures the rate of respiration is cut 

 down more than the rate of digestion of starch into sugar 

 and consequently the sugars accumulate. 



It is very difficult at times to differentiate immature 

 tubers from those showing the less severe types of freezing 

 injury. In both cases the tissues may have a greenish yellow 

 color, and may be flabby and bitter in taste. Immature 

 tubers are also bruised easily by impact or pressure, which 

 causes regions in the tuber to resemble chilled tissue so 

 closely that a diagnosis is excedingly difficult. It also is 

 exceedingly difficult to differentiate between thawed, dried- 

 out tissues and bruised tissues. 



It seems in fact that the presence of ice in the tissues is 

 the only positive proof of freezing injury. All other char- 

 acteristics enumerated as symptoms of freezing and chilling 

 injury can also be brought about by other causes. 



Slightly affected stock without extensive, discolored, 

 killed regions is suitable for food even though it is not as 

 palatable as the uninjured. It shrinks excessively in transit 

 and storage. 



Guarding against the exposure of potatoes to tempera- 

 tures under 32 F. will prevent this type of injury. Losses 

 during transit may be avoided by careful sorting to elimi- 

 nate field frozen tubers previous to shipment, and the use 

 of all proper precautions to prevent freezing injury during 

 hauling, loading, and rail shipment. This involves proper 

 lining and preheating of the car, installation of false floor- 

 ing and ends, and an adequate heating apparatus regulated 

 during transit so as to avoid both freezing and black heart 

 injury. 



Ref. (33a). 



POTATO: BLACK HEART. 



Cause : High temperature or insuffcient aeration. 



The symptoms of black heart vary, depending upon 

 whether the potatoes are exposed to high temperatures with 

 an adequate air supply, or to high or normal temperatures 

 with an insufficient air supply. In the former case, no ex- 

 ternal symptoms develop ; in the latter, both external and 

 internal symptoms appear. 



The external symptoms of black heart are moist areas on 

 the surface which may be purplish at first but turn brown 

 and black in a short time. The internal symptoms are a 

 dark gray to purplish discoloration which later becomes 

 jet-black. The discolored areas are usually sharply set off 

 from the healthy tissue. 



