10 MISC. PUBLICATION 440, U. S. DEPARTMENT OF AGRICULTURE 



exposure to ammonia fumes yellow onions show brown blotches or 

 a uniform brown color over all the exposed surfaces; red onions 

 change to a deep greenish-black or metallic-black color, and white 

 onions are discolored greenish yellow. If onions are exposed to 

 strong fumes for several hours the fleshy, edible portion of the bulbs 

 becomes watery, yellowish green, and worthless. 



Experiments have shown that ammonia injury will take place at 

 a temperature as low as 31.5° F., and that the rate and severity of 

 discoloration are approximately the same as at higher temperatures. 

 The color changes have been found to take place more rapidly in a 

 humid atmosphere or when the onions are slightly moist. Less than 

 1 percent of ammonia in the air will cause marked discoloration if 

 the onions are exposed for 24 hours or longer. With stronger con- 

 centrations the color changes are noticeable almost immediately and 

 large brownish-black blotches are produced within a few minutes. 



A similar type of discoloration in colored onions has been observed 

 in stock that had been covered with manure for protection against 

 freezing. The ammonia fumes from the manure cause almost a 

 complete blackening of the outer dry colored scales but generally do 

 not discolor the fleshy scales. 



(See $7,28,36.) 



FREEZING INJURY OF ONIONS 



The average freezing point of onion bulbs is about 30° F. Although 

 some onions may freeze at a fraction of a degree higher or lower, 

 this temperature ma} 7 be taken as the danger point. Under some con- 

 ditions onions standing on track or in storage may be undercooled 

 to a temperature of 25° or lower for a short time without becoming 

 frozen or otherwise injured. A slight jar will cause undercooled 

 products to freeze immediately; consequently undercooling rarely 

 occurs during transit. 



There is considerable variation between individual onions in their 

 reaction to low temperature. Some bulbs in a lot may freeze quickly 

 when they reach a temperature of 30° F. and show severe injury 

 when they are thawed out. Others may not freeze or if they are 

 frozen they may thaw out without injury. 



Onions affected with freezing injury show water-soaked, grayish- 

 yellow colored fleshy scales when cut (pi. 3, C). In slight freezing 

 the outer fleshy scales alone are affected, but when the bulbs are 

 exposed to low temperatures for a prolonged period the inner scales 

 may also become water-soaked and discolored. Usually the entire 

 scale is injured all the way around the bulb, but the neighboring 

 scales inside ar>d outside may or may not show injury. Irregularly 

 shaped opaque areas occur in man} 7 of the watery transparent scales. 



Garlic bulbs do not freeze until a temperature of 25.4° F. is reached, 

 The frozen tissues are discolored and water-soaked in the same way as 

 onions. 



In cases of slight freezing injury of onions and garlic there may 

 be little loss if the bulbs are spread out so that the injured scales can 

 dry thoroughly. Experimental evidence indicates that frozen onions 

 will show lrss injury and remain in a better general condition if 

 thawed out at 40° F. rather than at a higher temperature, 



(See 19,57, 68.) 



