CHEMICAL PROPERTIES OF FATS AND OILS 285 



Tracy et aU^^ have suggested that the rancidity can be predicted from 

 1 he redox potentiah When this vakie exceeds 0.300 volt in milk, rancidity 

 is apt to develop. Milk was found to be less stable in the winter; this was 

 interpreted as indicating that the lower bacterial growth allowed more 

 oxygen to be available for oxidation. We now know that such a phenome- 

 non can probably best be explained by a lower content of antioxidants 

 in the milk when the cows are no longer given green feed. Oxygen ab- 

 sorption has also been used to estimate rancidity, but it is of more use in 

 the determination of incipient rancidity or stability of fats. Recent re- 

 views on the subject of rancidity include the comprehensive report of 

 Lea,''53 and those of Beadle,^^* Mattill,^^^ Olcott,*^^ VibranSj^^^ Olcott and 

 Mattill,*'^ T^undberg,'*^^ Riemenschneider and Ault,*^^ and Quackenbush.'*^" 



c. Tests for Incipient Rancidity. Although oxidative rancidity is 

 largely dependent upon the presence of unsaturated fats, there is a wide 

 variation in the speed at which rancidity develops in fats possessing an 

 equal degree of unsaturation. For example, sweet almond and sesame oils 

 will remain fresh for much longer periods than fats with a similar or lower 

 iodine number.'*" Wheat germ oil is extremely stable, *^^ in spite of its 

 high degree of unsaturation. We owe to Mattill,*^^ the idea that such 

 stability is to be attributed to antioxidants naturally occurring in the fats. 

 As proof of this concept, it was sho^vn that the addition of wheat germ oil 

 to diets containing lard prevented the early development of rancidity. '^^^ 

 It thus becomes apparent that it is considerably more important to 

 determine the stability of a given fat, i.e., its ability to avoid becoming 

 rancid, than to ascertain to what extent such a reaction has already 

 occurred. 



One method for ascertaining the resistance of a fat to oxidation is based 

 upon the amount of peroxides it contains. This value can readily be 

 estimated by the determination of the iodine liberated from an acidified 

 solution of potassium iodide by the so-called "peroxide oxygen" of the fats. 

 Many different procedures have been suggested, all of which are based on 



«2 p. H. Tracy, R. J. Ramsey, and H. A. Ruehe, Univ. III. Agri. Exp. Sta., 27, Bull. 

 No. 389, 579-595 (1933). 



^^3 C. H. Lea, Rancidity in Edible Fats, Food Investigation Special Report No. 46, 

 H. M. Stationery Office, London, 1938; Chemical Pub. Co., New York, 1939. 



«" B. W. Beadle, Oil & Soap, 23, 33-35 (1946). 



^ H. A. Mattill, Oil & Soap, 18, 73-76 (1941). 



«6 H. S. Olcott, Oil & Soap, 18, 77-80 (1941). 



«^ F. C. Vibrans, Oil & Soap, 18, 109-112 (1941). 



** W. O. Lundberg, A Survey of Present Knowledge, Researches and Practices in the 

 United States Concerning the Stabilization of Fats, Publication No. 20, Hormel Institute, 

 University of Minnesota, 1947. 



«9 R. W. Riemenschneider and W. C. Ault, Food Ind., 16, 892-894, 936-939(1944). 



*«» F. W. Quackenbush, Oil & Soap, 22, 336-337 (1945). 



«i L. W. Elder, Jr., Oil & Soap, 18, 38-42 (1941). 



«2 H. A. Mattill, J. Am. Med. Assn., 89, 1505-1508 (1927). 



«3 L. T. Anderegg and V. E. Nelson, Ind. Eng. Chem., 18, 620-622 (1926). 



