84 GENERAL BIOCHEMISTRY 



powders and cosnictics. In paints, lead, manganese, and cobalt soaps 

 hasten the process of tlrying. 



Fats containing unsaturated latty acids can be hydrogenated to 

 produce a fat containing only saturated glycerides. Thus, liquid fats 

 can be converted to solids having appreciably higher melting points. 

 It is also possible to partially hydrogenate a liquid fat to form a 

 product having properties intermediate between those of the liquid 

 and the solid fat. Commercial use is made of such controlled hydro- 

 genation in the production of shortenings and margarines from 

 vegetable fats. 



Rancidity 



Fats exposed to light, air, heat, or moisture for an extended period 

 of time slowly develop obnoxious flavors and odors and are said to be 

 rancid. There are two principal types of fat spoilage, namely, hy- 

 drolytic and oxidative rancidity. 



Hydrolytic rancidity is the result of the hydrolysis of fats with the 

 liberation of small amounts of one or more volatile fatty acids (butyric 

 through capric). Dairy products are particularly affected by this type 

 of spoilage because of the rather large amounts of the low-molecular- 

 weight acids found in the glycerides of milk and milk products. The 

 liberation of a trace of butyric acid, for example, will affect the odor 

 and taste of dairy products to the point where sales value is appre- 

 ciably lowered. 



All fats undergo partial hydrolysis when exposed to moisture for an 

 extended period of time. However, the majority of them do not 

 become rancid, owing to the fact that they do not contain volatile 

 fatty acids and therefore do not develop the obnoxious odors or flavors 

 associated with rancidity. 



From a fat-economy standpoint, oxidative rancidity is the more 

 important form of spoilage. In this type, the unsaturated fatty acid 

 fragments of the glycerides are oxidized at their double bonds with 

 the ultimate production of aldehydes, ketones, and acids with fewer 

 carbon atoms in the molecule. The objectionable odor and taste of an 

 oxidized fat are due to the presence of these decomposition products. 

 A saturated fat (that is, one without any unsaturated acids in the 

 glycerides) is almost completely resistant to oxidation and therefore 

 does not exhibit oxidative rancidity. 



It has been known for a long time that different unsaturated fats, 

 when exposed to an atmosphere of pure oxygen, do not become rancid 

 in the same length of time. Indeed, certain unsaturated fats are quite 

 resistant to oxidation. However, when this initial resistance is over- 



