234 ANIMAL FATS AND PHOSPHATIDES. 



only after vigorous and prolonged shaking, but the emulsion is not pet- 

 sistent. The presence of some soap causes a very fine and permanent 

 emulsion to form easily. Fat produces spots on paper which do not 

 disappear; it is not volatile; it boils at about 300 C. with partial decom- 

 position, and burns with a luminous and smoky flame. The fatty acids 

 have most of the above-mentioned properties in common with the neutral 

 fats, but differ from them in being soluble in alcohol-ether, in having 

 an acid reaction, and by not giving the acrolein test. The neutral fats 

 generate a strong irritating vapor of acrolein, due to the decomposition 

 of glycerin, C 3 H 5 (OH)3 2H 2 O = C 2 H 3 .CHO, when heated alone, or 

 more easily when heated with potassium bisulphate or with other dehy- 

 drating substances. 



The neutral fats may be split by the addition of the constituents of 

 water according to the following equation: 



This splitting may be produced by the pancreatic enzyme and other 

 enzymes occurring in the animal and vegetable kingdoms, for example, 

 the castor lipase. The reverse action, namely, the synthesis of fatty acid 

 esters, can be brought about by enzymes, such as pancreatic lipase (see 

 page 60). The cleavage of the neutral fats can also be accomplished 

 by superheated steam or by dilute acids. We most frequently decompose 

 the neutral fats by boiling them with not too concentrated caustic alkali, 

 or, still better (in biochemical researches), with an alcoholic potash solu- 

 tion or with sodium alcoholate. By this procedure, which is called sapon- 

 ification, the alkali salts of the fatty acids (soaps) are formed. If the 

 saponification is made with lead oxide, then lead plaster, the lead salt of 

 the fatty acids is produced. By saponification is to be understood not 

 only the cleavage of neutral fats by alkalies, but also the splitting of neutral 

 fats into fatty acids and glycerin in general. 



On keeping fats for. a long time in contact with air they undergo a 

 change, becoming yellow in color and acid in reaction, and they develop 

 an unpleasant odor and taste, becoming rancid. In this change a part 

 of the fat is split into fatty acids and glycerin, and then an oxidation 

 of the free fatty acids takes place, producing volatile bodies of an 

 unpleasant odor. 



The three most important fats of the animal kingdom are stearin, 

 palmitin, and olein. 



CH 2 .O.Ci 8 H 35 



Stearin, or tristearin, CsTHnoOe^CH.O.CisHasO, occurs especially in 



CH 2 .O.Ci 8 H 35 O 



the solid varieties of tallow but also in the vegetable fats. Stearic acid, 

 CisH 3 6O 2 , is found in the free state in decomposed pus, in the expectora- 



