SCIENCE- GOSSIP. 



THE NATURE OF ANIMAL FAT. 



By C. Ainsworth Mitchell, B.A. (Oxon.) 



"TTTE owe to the distinguished French chemist 

 ' " Chevreul our first insight into the chemistry 

 of fats and oils. His classical work " Recherches 

 sur les Corps Gras," published in 1823, was supple- 

 mented, and to some extent corrected, in numerous 

 investigations — notably by Liebig and his pupils, 

 and more recently by von Hiibl, Hazura, Hehner 

 and many others. Notwithstanding all this re- 

 search, however, the subject remains obscure in 

 many respects, and presents numerous problems 

 which will only be solved when more accurate 

 methods of analysis have been devised. 



What is popularly known as the " fat " of an 

 animal is really part of the connective tissue con- 

 taining cells, in which the original protoplasm has 

 been gradually displaced by the true fat leaving 

 the, cell walls intact. These cells are usually 

 grouped together and supported by the fibrous 



iii 



Fig. 1. Crystals from Mixed Lard and Beef Fat. 



substance of the connective tissue, as is shown in 

 figs. 3 and 4. 



In order to separate the fat from the tissue the 

 cell walls must be broken down, and in manu- 

 facturing processes, such as the rendering of lard, 

 this is done by means of heat. 



The isolated fat of most land animals consists 

 essentially of compounds of various acids, known 

 as fatty acids, with glycerin, these compounds 

 being termed " glycerides." 



If we boil a fat with a strong solution of soda or 

 potash, decomposition takes place, the fatty acids 

 combining with the alkali to form soap, whilst the 

 glycerin is set free. On now treating the soap 

 solution with hydrochloric acid the soap is decom- 

 posed, and a mixture of different fatty acids rises 

 to the surface of the liquid. 

 May 1901.— No. 84, Vol. VII. 



The nature of these fatty acids varies with 

 the particular fat, but the mixture usually con- 

 sists of palmitic, stearic, and oleic acids, with, 

 at all events, in some cases linolic and linolenic 

 acids. 



Palmitic acid derives its name from being a 

 chief constituent of palm oil. It is a white, soft, 

 soapy solid, practically without taste or odour. 



Stearic acid, from o-reop, otherwise tallow, is, as its 

 name suggests, an important constituent of tallow. 

 In its general properties it resembles palmitic 

 acid, but melts at a higher temperature, and is less 

 soluble in alcohol and other solvents. A mixture 

 of palmitic and stearic acids recalls the behaviour 

 of an alloy of different metals, the mixture having 

 a lower melting point than either of its constituents. 

 Until recently there was no exact method of 

 quantitatively separating these two acids. 



Fig. 2. Crystals from Beef Pat. 



Oleic acid, from oleum, otherwise oil, is a colour- 

 less liquid when pure. It also differs from the 

 preceding acids in being unsaturated. Thus, on 

 treatment with a solution of iodine it absorbs a 

 large proportion of that element, yielding a definite 

 compound. 



Linolic acid and linolenic acid derived their 

 name from linseed-oil — oleum liniwm—ia which 

 they were first discovered. They resemble oleic 

 acid, but are still more unsaturated — linolenic 

 acid much more so than linolic acid. The drying 

 properties of linseed-oil are attributed to linolenic 

 acid. 



The manner in which the glycerin is combined 

 with these various acids is not known with cer- 

 tainty. Glycerin has three available groups that 

 enter into combination, so that there are numerous 

 1 Published April 25tb, 19U1. 



