FATS, PHOSPHATIDS, AND ALLIED SUBSTANCES 65 



to break up into minute droplets and enter the lymph drop. 



Physiologically the formation of emulsions is of great im- 

 portance. In digestion in the stomach only emulsified fats are 

 attacked to any extent. In the intestine, fats of the food are 

 emulsified by the pancreatic juice, a process which is extremely 

 important for their proper digestion and absorption. The 

 mechanism of emulsion formation has been the subject of much 

 study. Probably different emulsifying agents act in different 

 ways, or a single substance may act in more than one way. 

 It is believed that the soap, albumin, etc., collects around the 

 tiny fat droplets and serves to insulate them and thus lessen 

 the tendency to run together. The lowering of the surface 

 tension is also a factor in the production of emulsions, and 

 possibly also electrical forces tending to repel the similarly 

 charged particles of fat. Milk is an example of a fairly perma- 

 nent emulsion. The fat droplets are suspended in a liquid 

 which contains protein. On standing, a considerable portion 

 of the milk fat finally will float to the surface. When removed 

 from the skimmed milk beneath, this is known as cream. On 

 churning, the emulsified fat runs together and butter is formed. 



Saponification. If a fat is boiled with an alkali, or an acid, 

 it is split into fatty acids and glycerine. This process is known 

 as saponification. If an alkali is used, the fatty acids react 

 with the alkali to form salts. These salts of the higher fatty 

 acids have a slippery feeling, and their solutions foam on being 

 shaken. They are called soaps. The accompanying equation 

 illustrates the process: 







CH 2 C. C 17 H S , 

 O 



II 

 C . (VHg,, 



O 

 CH 2 C . C 17 H 35 CH 2 OH 



CHO C . (VtL, 3 NaOH - CHOH + 3C 17 H ,COONa 



O = 



