300 III. CHEMISTRY OF NEUTRAL FATS 



carotenoids and chlorophyll are the most common natural products which 

 act on fats. Carotene, xanthophyll, and lycopene have all been shown to 

 lower the induction period of lard or mixtures of lard and cod liver 

 Qjj 484,485,488,500 Howcvcr, uuder the usual conditions, the carotenoids do 

 not have an opportunity to exert their prooxidant effect, since this reac- 

 tion is prevented by the inhibitols also present. Thus, the xanthophyll in 

 butter or chicken fat has only slight prooxidant activity.^^" In fact, New- 

 ton^^^ was formerly of the opinion that carotene is an antioxidant, since the 

 fatty extracts of alfalfa, paprika, and palm oil act as stabilizers for lard. 

 However, it is apparent that this effect cannot be attributed to the caro- 

 tene, since it is enhanced when the carotene is bleached. ^^^ That some 

 specificity in activity is exhibited is shoAvn by the fact that carotene and 

 xanthophyll are prooxidants for oleic and linoleic acids, but are inactive 

 toward linseed and olive oils.^^^ 



Chlorophyll is especially effective in augmenting the oxidation of oils 

 when they are exposed to light. ^*° The induction period of the oil is mark- 

 edly lowered in the presence of chlorophyll, and rancidity develops with a 

 very low peroxide value. ^^^ The prooxidant activity of chlorophyll is in- 

 creased by purification. ^^^ The amount of this pigment is appreciable in 

 crude oils, but it is present only in traces in refined oils.^^^ However, the 

 process of refining usually removes a considerable amount of inhibitols, so 

 that the net result of purification is a product which is less stable, in spite 

 of the fact that it also contains less prooxidant. However, chlorophyll 

 does not lower the induction period of lard when there is only ordinary in- 

 door illumination.*^^ 



Although the concentration of metals in oils is extremely minute, it may 

 be sufficient to promote a more rapid oxidation of the fat. Copper and 

 iron salts are the mdst potent prooxidants.'*^^ The antioxidant effect which 

 has been demonstrated for potassium cyanide^^* occurs only when cobalt 

 oleate is present; cyanide is therefore not a true antioxidant but only an 

 antagonist to the prooxidant or an anti-prooxidant. In the absence of the 

 metallic catalysts, cyanide is entirely ineffective in preventing oxidation. ^^^ 



(5) Drying and Hardening Properties 



The unsaturated linkages of fat are responsible not only for the develop- 

 ment of oxidative rancidity but also for the formation of hard insoluble 

 films. This reaction, which is of great importance in the paint and varnish 

 industry, involves the formation of polymers following the absorption of 



"0 K. Tiiufel and R. Muller, Biochem. Z., 304, 137-146 (1940). 

 6" R. C. Newton, Oil & Soap, 9, 247-252 (1932). 

 «" W. Franke, Z. physiol. Chem., 212, 234-255 (1932). 



"3 W. G. Bickford, S. Anderson, and K. S. Markley, Oil & Soap, 17, 138-143, 252-256 

 (1940). 



