282 III. CHEMISTRY OF NEUTRAL FATS 



chemical action of light on a compound which is present in the oil or which 

 is produced by compounds which give rise to peroxides. *^° Less rancidity 

 develops when the oil is stored in the dark or in the light at wave lengths 

 of 4900 to 5800 A., than when it is kept in ordinary light, even though the 

 peroxide values may be approximately the same in both cases. 



Certain metals and their oxides greatly hasten the onset of rancidity by 

 what is believed to be a catalytic action; for example, lard stored in contact 

 with strips of copper or lead was found to become rancid in 5 or 6 days (as 

 determined by the Kreis test). On the other hand, when the lard was 

 stored in the absence of the metals, a positive organoleptic test for rancidity 

 was not given until the 27th day.^^^ It is suggested that the action of 

 metals may be catalytic, in that they form intermediate compounds with 

 the fats, or because they have a directive effect on the fat molecule which 

 causes the unsaturated portions to be directed toward the surfaces, where 

 they become more readily oxidized.^** 



There seems to be httle doubt that the ethylenic linkages are the primary 

 site of attack in the development of oxidative rancidity. Oleic acid is 

 knowTi to Jorm[an ozonide on'treatment with ozone, which might well rep- 

 resent an intermediate. This reaction is illustrated below. When 



H H 



CH3{CH2)7 CH:CH(CH2)7C00H — - — ► CH3 iCHzh C-Q-ClCHs)? COOH 



Oleic acid Oleic acid ozonide 



ozonides are treated with water, they readily split at the oxygen linkage. 

 In the case of oleic acid ozonide, the end products are hydrogen peroxide, 

 azelaic acid semialdehyde, and pelargonic aldehyde. The latter compound 

 is further oxidized to pelargonic acid. 



CH3(CH2)7C-0-C(CH2)7 COOH " 



1 I 







Oleic acid ozonide 



CH3(CH2)7C-H + H-C(CH2)7C00H + HaOa 



Pelargonic Azelaic acid 



aldehyde semialdehyde 



CH3{CH2)7C-H + He 02 ►CH3(CH^7C-OH+H20 



Pelargonic Pelargonic 



aldehyde acid 



Kerr and Sorber*^^ have showTi that, in the development of rancidity, 

 after the early formation of free acid, a drop in iodine number occurs, an 



^30 M. R. Coe and J. A. LeClerc, Ind. Eng. Chem., 26, 245-248 (1934). 

 "1 R. H. Kerr and D. G. Sorber, Ind. Eng. Chem., 16, 383-385 (1923). 



