90 LIPIDES (fats and RELATED SUBSTANCES) 



material is converted into edible fats, great quantities of well known 

 commercial products, e.g., "Crisco" and ''Snowdrift/' are prepared in 

 this manner from peanut, cottonseed, and other oils. These fats are 

 more stable to heat than natural fats, such as lard, and are, therefore, 

 peculiarly well adapted to certain cooking operations, such as deep-fat 

 frying. The natural fats tend to decompose at higher temperatures, 

 owing, it is assumed, to the presence of small amounts of free fatty 

 acid. The more free acid present in a fat, the more readily it is decom- 

 posed by heat. 



Rancidity of fats 



When fats are kept for a long time, they develop objectionable odors 

 and tastes, a condition which is known as rancidity. Many different 

 factors such as heat, light, moisture, air, enzymes, bacteria, and metals 

 are involved in the decomposition of fats. The principal chemical changes 

 are hydrolysis and oxidation. The former is particularly important in 

 the case of butter and other dairy products because the free fatty acids, 

 produced when butter fat is hydrolyzed, include several of the lower, 

 saturated series {e.g., butyric, caproic, etc., see Table 4—5), which have 

 very sharp, unpleasant odors. Rancidity due to oxidation develops par- 

 ticularly in moderately unsaturated fats and oils, a group which includes 

 the bulk of the common food fats. Atmospheric oxygen slowly reacts 

 to produce hydroperoxides, fats or fatty acids having an — OOH group 

 attached to a carbon atom next to a double bond. Once formed, the 

 hydroperoxides serve as catalysts for further oxidation. As a result, 

 lower fatty acids, ketones, peroxides, and other substances are formed, 

 and the glycerol disappears. The unpleasant odors and flavors of these 

 products make the fat rancid. Rancidity is a term which applies to 

 any objectionable odor or taste in fats, no matter how brought about. 



There exist a number of different substances, not themselves fats, 

 which have a remarkable power of slowing down the development of 

 oxidative rancidity. Such substances are called antioxidants. They are 

 present naturally in many fats, which have not been too extensively 

 refined, and have a large influence on the keeping qualities of fats. Ex- 

 amples of antioxidants are crude lecithin, hydroquinone (HOC6H4OH), 

 vitamin C, and the tocopherols (vitamins E, p. 216). Only small 

 amounts, of the order of 1 per cent of the fat, or less, are sufficient to 

 delay the onset of oxidative rancidity for extended periods. Antioxidants 

 appear to function by interfering with the catalytic effect of the hydro- 

 peroxides mentioned above. In so doing they are themselves slowly 

 oxidized, so that their effect eventually wears off. Antioxidants are 

 deliberately added to many food fats to improve their keeping qualities. 

 They are also thought, by many investigators, to play an important 

 biological role in preventing unwanted oxidations from occurring in vivo. 



