TAINTS AND DEFECTS OF BUTTER 351 



In rancid butter the following substances are usually 

 found, few or none of which are present in fresh 

 butter : 



1 . Free fatty acids ; 



2. Certain oxy-acids ; 



3. Aldehydes and ketones ; 



4. Esters or ethereal salts, especially of butyric and 



volatile fatty acids. 



The amount of each varies considerably with the age 

 of the sample and the extent of the action of the various 

 agents to which these products are due. 



Duclaux and several of the earlier investigators attri- 

 buted the peculiar rancid taste and odour to free butyric 

 acid or some allied fatty acid, but that is not the case. 



Very frequently it is found that high acid content 

 goes along with strong rancid conditions ; but stored 

 butter may become strongly acid and possess an objec- 

 tionable flavour sufficient to render it uneatable without 

 being rancid ; on the other hand, rancid butter may 

 contain little or no free acid. 



The free fatty acids arise through the splitting of the 

 fats, glycerine being set free at the same time. Some 

 of the glycerine undergoes oxidation, and aldehydes and 

 ketones with rank taste and evil smell are produced. 



Moreover, oxygen is absorbed by unsaturated com- 

 pounds, such as oleic acid. 



None of these compounds, however, give the strictly 

 rancid flavour. 



According to Amthor and Reinmann and others, the 

 characteristic rancid odour and flavour is that of the esters 

 of the volatile acids, and especially of ethyl-butyrate, the 

 ethyl ester of butyric acid. 



Amthor believed that the ethyl group of the butyric 



