XIV.] THE COMPOSITION OF BUTTER FAT 271 



and more thorough creaming of the milk from Jersey 

 cows. When the milk fat is separated and examined it 

 proves to be a complicated substance, consisting, like all 

 fats, of a combination of glycerin with certain so-called 

 fatty acids. In butter fats there are a large number of 

 these fatty acids present, differing from one another in 

 their physical constituents, some members being volatile 

 and strongly smelling liquids, while others are solid 

 substances, possessing little smell or taste. We can 

 obtain an idea of these acids of butter fat by taking 

 about a gramme of the clarified fat and warming it with 

 20 C.C. of a solution of potash in alcohol. If the fat is 

 stirred up it will eventually dissolve, the glycerin being 

 set free, while the acids combine with the potash to form 

 a soap. The mixture is now dried up to get rid of the 

 alcohol, and treated with dilute sulphuric acid, which 

 decomposes the soap and sets free the fatty acids. 

 These substances will then be found to possess a very 

 powerful smell of rancid butter, the smell being due to 

 butyric and one or two of the other volatile acids. As 

 the liquid cools the non-volatile acids, which are also 

 insoluble in water, will solidify to a cake on the surface 

 of the liquid. The flavour of butter is due to the 

 liberation of a very small trace of some of these volatile 

 fatty acids through the action of bacteria upon the 

 butter fat. If too much is liberated the smell and taste 

 become too pronounced, and we call the butter rancid. 

 The composition of the butter fat itself is not constant, 

 but varies with the season of the year, the nature of the 

 food, and the period of lactation. Hence follows a 

 variability in the composition of pure butter fat which 

 constitutes the chief difficulty in detecting adulterations 

 of butter, for most of the methods of analysing butter 

 depend upon determinations of the proportion of volatile 

 and insoluble fatty acids. 



