MARKET DISEASES OF FRUITS AND VEGETABLES \) 



BREAK-DOWN OF ONIONS 



This trouble is encountered in onions from various regions. It is 

 often so important in storage stock that many refer to the injury 

 as "storage break-down," but investigations have shown that it also 

 occurs in onions taken directly from the field. 



This break-down is characterized by the grayish water-soaked ap- 

 pearance of the outer fleshy scales of the bulbs, and is similar to 

 some cases of freezing injury. In fact the symptoms are so alike 

 that an accurate diagnosis of these two troubles is often not possible 

 until a complete history of the stock is known. However, in physio- 

 logical break-down the outer two or three fleshy scales are usually 

 the only ones affected ; the water-soaked tissues may not extend com- 

 pletely around the bulb; and they do not contain the opaque areas 

 often found in frozen onions. 



The cause of this type of break-down is not known. Apparently 

 certain growing conditions favor the development of this injury in 

 the field and later in storage. Some experiments have indicated 

 that break-down is more likely to occur in storage onions at high 

 humidities and temperatures of 40° to 50° F. than at lower humidi- 

 ties and temperatures near 32°. A relative humidity of about 64 

 percent is probably best for most onions, because it prevents root 

 growth that is likely to occur at higher humidities. 



(See S?\ 58.) 



CHEMICAL INJURY OF ONIONS 



One of the most common types of discoloration of onions caused 

 by chemicals is illustrated by the "scorched spot," "alkali spot," and 

 "bag print" blemishes sometimes found on colored onions that have 

 been in contact with moist bags or in damp storage for some time. 

 Small oval spots varying in size from y 2 to 1 inch in diameter 

 are produced on the sides of the onions that come in contact with 

 the fabric (pi. 5, B). Sometimes the spots are uniformly dark-brown 

 or black, as if the onions had been scorched, and sometimes the mesh 

 of the bag is distinctly printed in dark, brownish-black on the onion 

 scales (pi. 5, C). This discoloration is caused by a chemical in the 

 fabric of the bag that produces an alkaline reaction with the color 

 pigments of those scales in contact with the fabric. If the bag 

 is moist, the discolored areas are uniformly brown or black; if it 

 is comparatively dry the diffusion of the chemical is slight, and the 

 color change takes place so slowly that the imprints of the individual 

 meshes of the fabric become prominent. With a discoloration of 

 this type, however, no injury to the underlying fleshy scales that 

 would impair the eating qualities of the onion has ever been observed. 



Not all onion bags give this chemical reaction; consequently such 

 blemishes are likely to be more or less sporadic and generally are 

 not serious. 



A more uniform and severe type of discoloration caused by a 

 chemical is occasionally found in onions held in cold storage 

 (pi. 5, J.). This injury is caused by the accidental exposure of the 

 stock to ammonia fumes from the storage plant. The fumes cause 

 an alkaline reaction with the color pigments in the outer scales of 

 the onions and often produce such pronounced discoloration that 

 the marketability of the stock is greatly reduced. As a result of 



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