54 A TEXTBOOK OF PHYSIOLOGY 



contact with air (oxygen is taken up and rancid oil produced), the 

 action of fat-splitting enzymes or lipases. 

 The reaction can be represented thus : 



C 3 H 5 (OC 15 H 31 CO) 3 + 31^0 - C 3 H 5 (OH) 3 



Palmitin Glycerine Palmitic acid 



In true saponification the fatty acid formed combines with the 

 alkali present to form a soap : 



C 15 H 31 COOH +NaHO = C 15 H 31 COONa + K.O 



Soap 



Fats from different species of animals vary in composition, and 

 fats from different parts of the same animal have varying composition 

 depending upon the combination of individual fats which form the 

 mixed fat of any part. From the liver, for example, fatty acids, 

 more unsaturated than oleic and belonging to the linolic and linolenic 

 series, have been isolated. Milk fat is characterized by its yield of 

 butyric, caproic, caprylic, and other volatile fatty acids. From 

 kidney fat an acid known as carnaubic acid has been isolated. 



Stearin, CH^C^II^O 

 CHOC ]8 H 35 



occurs generally in animal fat, but is found also in certain vegetable 

 fats. It is the hardest and most insoluble of the three common 

 fats. Its melting-point when pure is 55 C., but as obtained from 

 the tissues about 63 C. 



Stearic acid, the acid of stearin, crystallizes in long rhombic plates, 

 and crystals of it may sometimes be seen in a melted rancid fat after 

 it has solidified. 



Palmitin, 



CHOC 16 H 31 

 CH 2 OC lfi H 3 jO 



occurs largely in human fat. Its melting-point is about 50 C. 

 Palmitic acid crystallizes as fine needles. 



Olein, CH 2 OC 18 H 33 

 CHOC 18 H 33 

 CH 2 OC 1 gH 33 O 



is very widespread, being found in most animal and vegetable 

 fats. At ordinary temperature it is liquid, solidifying at -6 C., 

 and acts as a solvent for the stearin and palmitin of ordinary 

 fat. The melting-point of a fat, its iodine value, and staining 

 properties with osmic acid, depend upon the amount of oleic acid 

 present. 



The fatty acids may be obtained from neutral fats by treating 

 with dilute mineral acid, when the fatty acid will collect as an oily 



