CHKMICAL PROPERTIES OF FATS AXD OILS 281 



teresterification. Thus, while untreated palm oil does not have a good 

 plastic range for use as a shortening, it is markedly improved after directed 

 interesterification. Tallow, also, can be improved for use in shortenings 

 by ester-ester interchange. Coconut oils and others of this class become 

 harder on directed interesterification, which renders them more satisfactory 

 for confectioners' butters. The same type of procedure can be applied to 

 fish oils, for removing the saturated acids. The liquid portion remaining 

 after the removal of the saturated triglycerides is a proportionally better 

 drying oil than the original fish oil, and has a higher iodine number. 



c. Acidolysis. Acidolysis involves a change quite similar to alcoholysis. 

 In the former case, when the ester is mixed with an excess of an acid. A, 

 a new ester will originate, containing the original alcohol group combined 

 with the added acid, A. In the second type of reaction, the same general 

 change occurs, but it is concerned with the alcohol group. 



Low molecular weight acids tend to replace the higher weight acids in 

 the esters. Thus, when coconut oil is treated with acetic acid, mixtures of 

 myristodiacetin and laurodiacetin result. However, the higher fatty acids 

 may, in some cases, replace the lower ones as is evidenced by the fact that 

 the fatty acids of cottonseed or palm oils can replace those of coconut oil 

 when the former are reacted with the latter at 260-300 °C., even without a 

 catalyst."" 



(4) Rancidity 



Like other organic materials, fats are subject to deterioration. The 

 chief form of degradation in the case of fats is referred to as rancidity. 

 The reactions involved in producing rancidity may be hydrolytic or oxida- 

 tive in character, and may originate from enzymic activity or from spon- 

 taneous atmospheric oxidation. The w4de variety of changes which are 

 grouped in the category as constituting "off" flavors or odors is indicated by 

 the large number of terms which are used to describe various types of 

 rancidity. Some of these are rancid, pungent, tallowy, soapy, oily, ester- 

 like (in ketonic rancidity), metallic, mustj^ fishy, bitter, cardboard, and 

 burnt-^^s 



a. Types of Rancidity, (a) Oxidative Rancidity. This is the commonest 

 variety and the type which is usually referred to by this term. The oxida- 

 tion takes place at the unsaturated linkage. Oxygen is required for this 

 reaction, and the rate of development is increased by light, moisture, and 

 heat. Greenbank and Holm"^^ report that light of wave length, 3600 A., 

 has the greatest effect on the oxidation of cottonseed oil. The oxidative 

 rancidity of corn and cottonseed oils is believed to be due to the photo- 



^2' G. Barskv (to Wecoline Products), U. S. Patent No. 2,182,332 (Dec. 5, 1939). 



^=8 H. S. Olcott and H. A. Mattill, Chem. Rers., 29, 257-268 (1941). 



^29 G. R. Greenbank and G. E. Holm, Ind. Eng. Chem., 25, 167-108 (1933). 



