284 III. CHEMISTRY OF NEUTRAL FATS 



such acids contribute largely to the smell of rancid butter. The higher 

 fatty acids, such as palmitic and stearic, have little odor. Thus, the 

 "off" flavor of fats cannot originate from such acids. 



(c) Ketonic Rancidity. This type is most frequently encountered as a 

 result of the action of such fungi as Aspergillus niger and the blue-green 

 mold, Penicillium glaucum, on fats such as coconut oil. Water and some 

 source of nitrogenous matter seem to be necessary for the ketone formation 

 to occur. The "tallowy" odor present in some cases of rancidity may be 

 the result of aldehydes and ketones formed when oxidized unsaturated 

 fatty acids are split. The products formed from oleic acid give the most 

 intense tallowy odor, while the oxidation of linoleic acid causes a much 

 lower amount of "off" odors. Only a slight odor results from the oxida- 

 tion of linolenic acid. 



The products of rancidity include free fatty acids, aldehydes, ketones, 

 and peroxides, as well as some alcohols.*^' Methyl amyl, methyl nonyl, 

 and methyl heptyl ketones have been demonstrated,^^* while it is suggested 

 that methyl alkyl ketone is also found in rancid fats.**^ 



b. Organoleptic Tests. The most widely accepted test used as a cri- 

 terion of rancidity is the Kreis test.**® Triebold**^ has shown that the in- 

 tensity of the color developed in this procedure as measured spectrophoto- 

 metrically and as determined by the oxygen absorption method is propor- 

 tional to the degree of rancidity. 



The presence of epihydrin aldehyde, CH2 — CH — CHO, is necessary for a 

 positive Kreis test. The products of ketonic rancidity do not give a posi- 

 tive response. Kerr**^ has reported further investigations on the use of 

 the Kreis test. 



Almost any test for aldehydes, as well as for peroxides, can likewise serve 

 as a test for rancidity. Thus, the reduction of methylene blue**^ and the 

 Schiff test*^" have been proposed for this purpose. Issoglio*^^ used as a 

 test the amount of potassium permanganate that was required to oxidize 

 the water-soluble aldehydes extracted from the fat by shaking with dis- 

 tilled water. 



^" M. J. Cumniings and H. A. Mattill, J. Nutrition, 3, 421-432 (1930-1931). 



«^ W. N. Stokoe, Biochem. J., 22, 80-93 (1928). 



"5 H. E. Fierz-David, Z. anqew. Chem., 38, 6-8 (1925). 



"6 M. Winckel, Z. Untersuch. Nahr. Genussm., 9, 90-96 (1905). M. Winckel and H. 

 Kreis, ibid., 9, 607 (1905). 



4^7 H. O. Triebold, Cereal Chem., 8, 518-532 (1931). 



4^8 R. H. Kerr, Ind. Eng. Chem., 10, 471-475 (1918). 



^« H. D. Royce, Soap, 7, No. 9, 25-27, 38 (1931); Chem. Abst., 26, 1145 (1932). 



«o H. S. Bailey and H. C. Ebert, Cotton Oil Press, 7, No. 8, 35 (1923). 



«i G. Issoglio, Ann. chim. applicata, 6, 1-18 (1916) ; Atti accad. set., Torino, 51, 582-605 

 (1916). 



