26 THE FAT OF MILK. 



Rancidity Products of Decomposition. But little is known 

 of the real nature of the changes which take place when butter 

 becomes rancid. The following statement will give an idea of 

 the probable nature of the changes : 



The first action seems to be hydrolysis of the fat, splitting it 

 up into fatty acids and glycerol ; the latter, perhaps, is not 

 liberated as such, but is oxidised, yielding aldehydes and acids 

 soluble in water, but not so volatile as the soluble acids of butter ; 

 the volatile acids are liberated, and the smell of these can be 

 detected in rancid butter. The odoriferous principle is destroyed. 

 The unsaturated fatty acids are oxidised to form hydroxy acids, 

 which are perhaps slightly soluble in water, but not volatile ; 

 the total capacity for combining with bromine is reduced by 

 this cause. It is probable that the fatty acids of the series, 

 C W H 2;Z+1 COOH, are but slightly affected. 



The effect of rancidity is 



(1) To diminish the glycerol produced on saponification. 



(2) To increase the soluble acids. 



(3) To increase slightly the volatile acids. 



(4) To decrease the insoluble acids. 



(5) To increase the total molecular proportion of acids. 



(6) To increase greatly the free acids. 



(7) To diminish the unsaturated acids by 



(8) The formation of hydroxy acids. 



If the change takes place in the presence of water, some of the 

 products are soluble therein ; hence the fat separated from the 

 water will not have the same characters as fat which has become 

 rancid alone. If freely exposed to the air, some of the products 

 may be volatile. 



