CHAPTER I 



GENERAL CHARACTERISTICS AND 

 CLASSIFICATION OF LIPIDS 



1. Introduction 



Fats have been recognized as a separate category of foodstuffs since 

 prehistoric times. According to Markley,^ the use of fats as foods is prob- 

 ably instinctive ; the apphcation of fats, and also of waxes, as illuminants, 

 in cosmetics, in medicinals, and as lubricants, dates back to before our 

 earliest records of civilized man. 



The understanding of the chemical nature of one class of lipids, namely 

 the fats, also predates our knowledge of the other foodstuffs. Although 

 proteins were recognized as a definite class of substances from the time 

 Mulder- coined the term -protein in 1839, the determination of their struc- 

 tural relationship to the amino acids had to await the classical work of 

 Kossel and especially of Emil Fischer, late in the nineteenth and early in 

 the twentieth century. The chemical nature of the carbohydrate molecule 

 was also obscure until the classical research of Emil Fischer and his co- 

 workers. In contrast to these findings, Chevreul,' as early as 1823, was 

 able to establish the fact that the common animal and vegetable fats are 

 combinations of the alcohol, glycerol, with the higher fatty acids. In fact 

 Schule had obtained glycerol in 1779 by the saponification of olive oil with 

 litharge, but he did not recognize how this fact was related to the structure 

 of fat. 



In spite of the advantage which fat gained from its early discovery and 

 the recognition of its structure, few chemists specialized in the study of fats 

 during the nineteenth century. Methods of analysis and of purification 

 which were applicable to such water-soluble compounds as carbohydrates 

 and proteins, and especially to their hydrolysis products, could not be 

 applied to any great extent to the fats. Fats did not crystallize, and hence 

 they could not readily be purified. Moreover, in nature they occurred as 

 mixtures in which the individual components had a simple structure when 



' K. S. Alarkley, Fatty Acids, Interscience, New York, 1947. 



2 G. J. Mulder, /. prakt. Chem., 16, 129-152 (1839). 



^ M. E. Chevreul, Recherches chimique sur les corps gras d'origine animale, Levrault, 

 Paris, 1823. Cited by J. W. Lawrie, Glycerol and the Gli/cols, Reinhold, New York, 1928, 

 pp. 17-18. 



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