SAPONIFIABLE LIPIDS 



73 



Chemically, the drying oils are characterized by having a high proportion of polyunsatu- 

 rated acids such as linolenic. The edible oils are characterized rather by having acids 

 such as oleic and palmitoleic. The oxidation of unsaturated fatty acids begins with the 

 attack of oxygen on an allylic carbon atom to form a hydroperoxide: 



-CHoCH = CH - 2? ^ - CHCH = CH - 



I 

 OOH 



The hydroperoxide then undergoes secondary reactions to produce epoxides, glycols, and 

 split products such as aldehydes and shorter chain carboxylic acids. It is these secondary 

 products which are responsible for the rancid taste of oxidized fats and oils. 



In plants fats and oils constitute important food storage materials, but they consti- 

 tute a negligible fraction of the total lipids in such actively metabolizing organs as leaves 

 (1). Their metabolic breakdown yields more energy per gram than that of any other stor- 

 age material, and their insolubility in water avoids the osmotic problems associated with 

 maintaining a high concentration of water soluble material in cells. The majority of en- 

 ergy yielded by fat breakdown is probably produced by conversion of the fat to acetyl-CoA 

 and oxidation of this through the glyoxylate and citric acid cycles (Chapter 3). 



Table 3 gives the fatty acid composition of some common fats and oils. Extensive 

 surveys by Hilditch (cf. general references) and others have indicated a close relation 

 between plant families and their seed glycerides when fatty acid components are tabulated 

 quantitatively and compared to botanical classification. 



OTHER FATTY ACID ESTERS 



Besides the triglycerides, other simple esters of long-chain fatty acids are com- 

 monly found in plants. Whereas the triglycerides usually function as food storage com- 

 ponents, the other fatty acid esters seem to be more concerned in protective coatings on 

 leaves, fruits, stems, etc. They are chemical constituents of the substances known bot- 

 anically as wax, cutin, cork, etc. , although each of these substances, contains other 

 types of compounds as well (cf. Chaps. 6, 8). Rarely, seeds are found which contain 

 high-molecular weight esters used as food reserves, and some triglycerides resemble 

 waxes in being found as coatings on fruit. 



The most familiar ester components of plant waxes contain long chain alcohols com- 

 bined with the fatty acids. The acids found in such waxes are generally longer chain com- 

 pounds than the acids of triglycerides. The C24 - C36 acids are most common. The alco- 

 hols have the same range of chain-lengths, and both the alcohols and acids are usually 

 saturated. Unsaturated alcohols are more common than unsaturated acids. Secondary 

 alcohols and dihydroxy alcohols are also found occasionally. Small amounts of unesteri- 

 fied fatty acids and alcohols may be found in plant waxes. 



As examples of some esters found in common plant waxes carnauba wax contains 

 75% myricyl cerotate; snow brush wax 80% of a mixture of ceryl palmitate and ceryl 

 stearate. 



Other variants on the long chain ester structure are found in certain plant waxes 

 which have hydroxy acids (2). When these are present, esters may form between the car- 

 boxyl group and the hydroxyl group of the same acid or with the hydroxyl group of another 

 acid. In the first case lactones are formed. An example is aparajitin from the leaf wax 

 of Clitoria ternatea, which is also interesting for its branched chain structure: 



