l<64 MASS. EXPERIMENT STATION BULLETIN 260 



mustard oil would result, through decomposition, in the discoloration of 

 the can to some degree. 



Tests indicated that the metallic film formed on the sides of the can 

 was a sulfide of tin rather than of iron. 



Black deposits occurring on the head space end of some enameled cans 

 were removed and tested for their metallic constituents. Both iron and 

 zinc were found present. Zinc oxide is a constituent of the enamel coat- 

 ing. The amount of iron obtained indicated other forms besides^ the 

 sulfide, presumably oxides. 



D is color ation of the Onion. In all cases canned onions were found to 

 be normal both in firmness and color when removed from the can, while 

 the onion liquor was of a pale green color. Exposure to the air or to 

 the influence of oxidizing agents resulted in the development of a dark 

 olive green color of tissue and to a dark green to brown coloration of 

 the liquor, regardless of whether the product was processed in plain tin 

 or enameled cans. Onions processed in glass, on the other hand, when 

 exposed to the same oxidizing conditions, discolored slowly, changing in 

 color to a light brown only upon long standing. This pronounced differ- 

 ence in color between onions processed in glass and those canned in tin 

 was believed to be due to the influence of soluble iron. 



Examination of the liquor from both the plain and enameled cans 

 proved! the presence of considerable ferrous iron in solution. Much more 

 iron was recovered from the liquor of the plain can than from the 

 enameled can. 



A flavonol pigment, probably quercetin, was proved to be present in 

 the onion bulb. It was found distributed throughout the fleshy inner 

 tissues as well as in the dried outer scales. Experiments carried out with 

 commercial quercetin showed that ferrous ions failed to produce any 

 colored compound, whereas ferric ions resulted in the formation of an 

 olive green to reddish brown ferric compound and also reddish brown 

 solutions depending upon the proportion of iron to pigment. 



These investigations indicate that the discolored film produced upon the 

 sides of the plain can in the processing of the onion is due to the format 

 tion of sulfide of tin, the source of the sulfur being the sulfur compounds 

 of specific proteins; while the black deposit occurring on the head space 

 end of some enameled' cans was largely iron, part as a sulfide and part as 

 oxide. The discoloration of the onion itself is due probably to the pres'- 

 ence of iron and quercetin or a similar pigment in solution. The de- 

 velopment of the discoloration is believed to involve the following steps: 

 First, metallic iron from the can is rendered soluble by the action of oxy- 

 gen contained within the head space of the can and also by the solvent in- 

 fluence of the acids and salts of the onion; secondly, the ferrous iron thus 

 formed reacts with the pigment forming the ferrous compound of the 

 pigment; and finally, this ferrous iron, when exposed to the atmosphere, 

 oxidizes and forms an olive green ferric derivative of flavonol- 



