l8o PHYSIOLOGICAL CHEMISTRY 



The three fatty acid radicals entering into the structure of a neutral 

 fat may be the radicals of the same fatty acid or they may consist of 

 the radicals of three different fatty acids. 



By hydrolysis of a neutral fat, i.e., by the addition to the molecule 

 of those elements which are eliminated in the formation of the fat from 

 glycerol and fatty acid, it may be resolved into its component parts, 

 *> glycerol and fatty acid. In the case of palmitin the following 

 would be the reaction: 



Palmitin. Glycerol. Palmitic acid. 



This process is called saponification and may be produced by boiling 

 with alkalis; by the action of steam under pressure; by long-continued 

 contact with air and light; by the action of certain bacteria and by 

 fat-splitting enzymes or lipases, e.g., pancreatic lipase (see page 191). 

 The cells forming the wafts of the intestines evidently possess the pecu- 

 liar property of synthesizing the glycerol and fatty acid thus formed so 

 that after absorption these bodies appear in the blood not in their 

 individual form but as neutral fats. 



The principal animal fats with which we have to deal are stearin, 

 palmitin, olein, and butyrin. Such less important forms as laurin and 

 myristin may occur abundantly in plant organisms. The older system 

 of nomenclature for these fats was to apply the prefix "tri" in each 

 case (e.g., ^-palmitin) since three fatty acid radicals are contained in 

 the neutral fat molecule. 



The fatty acids corresponding to the above-mentioned animal fats 

 are stearic, CH 3 (CH 2 )i 6 COOH; palmitic, CH 3 (CH 2 )i 4 COOH; oleic, 

 CH 3 (CH 2 ) 7 CH = CH(CH 2 ) 7 COOH; and butyric, CH 3 (CH 2 ) 2 COOH. 

 Stearic, palmitic and butyric acids are saturated fatty acids, whereas 

 oleic acid belongs to the class of unsaturated acids. Linoleic acid is 

 also unsaturated. Upon the presence of these unsaturated fatty acids 

 depends the property which certain fats possess of absorbing or combin- 

 ing with iodine. The determination of this so-called "iodine absorption 

 number" is important in the differentiation of fats and oils. Fats 

 containing the unsaturated acids oleic and linoleic may be transformed 

 by "hydrogenation" 1 into the fats containing the corresponding 

 saturated acid (stearic). The oleic acid is changed thus: 



C 17 H 33 COOH+2H-*Ci 7 H 3 5COOH. 



Oleic acid. Stearic acid. 



Fats occur ordinarily as mixtures of several individual fats. For 

 1 Addition of hydrogen to the molecule, producing a"hydrogenatedfat." 



