GENERAL CHEMISTRY OF MILK 79 



observations. When butter is heated it melts almost uniformly, 

 but if it were a simple mixture the glycerids of low melting-point 

 would liquefy before the others. Tributyrin is contained in but- 

 ter-fat in quantities of 1.5 to 5 per cent., averaging, according to 

 Richmond, 3.85 per cent. Being soluble in water, tributyrin 

 would impart its characteristic bitter taste to the milk and milk- 

 fat, while, as a matter of fact, there is normally no bitter taste in 

 either. Furthermore, when tributyrin is mixed with butter it can 

 be extracted with alcohol, while pure milk-fat does not yield 

 butyrin to alcohol. Although milk-fat contains small amounts of 

 glycerids of volatile acids, there is no distillate if it is heated to 

 275 C. Volatile acids, therefore, must be present in combined 

 form, so that they are not liberated by heating to 275 C. Finally, 

 an artificial mixture of the glycerids which have been isolated 

 from milk-fat has a higher melting-point than the natural product. 

 Butter cannot be reproduced by a mixture of its chemical com- 

 pounds. Moreover, the molecular weight of such a mixture is 

 779.6, against 760.3 of butter-fat. 



It must be admitted, however, that the facts cited do not prove 

 that milk-fat is necessarily a single compound. They prove only 

 that the component parts are of complex nature. This possi- 

 bility is supported by the fact that melted butter does not solidify 

 uniformly, although this might also be explained by decomposi- 

 tion due to heating. 



We frequently read of volatile and non-volatile fats as con- 

 stituents of butter-fat. This is not strictly correct, of course. 

 There are fats which are glycerids of volatile acids and fats which 

 are glycerids of non-volatile acids. The volatile acids can be dis- 

 tilled with steam at normal atmospheric pressure and are more 

 or less soluble in hot water. Acids with high molecular weight 

 are less soluble in hot water than those with low molecular weight. 



The glycerids of volatile fatty acids constitute 17.3 per cent, 

 of milk-fat, according to Richmond: 8.5 per cent, according to 

 Van Slyke, while the glycerids of non-volatile acids make up the 

 balance of 82.7 or 91.5 per cent. The quantities of the different 

 glycerids are given by Richmond as follows: 



PERCENTAGE OF GLYCERIDS OF FATTY ACIDS IN MILK-FAT 



Percent. Percent. ' Percent. 



Butyrin 3.85 yielding 3.43 fatty acid and 1 . 17 glycerin 



