RANCIDITY OF BUTTER. 213 



great an exposure* to the air, the butter may be retained for 

 many months without any appreciable deterioration. The 

 changes which are here concerned are profound and very 

 complex. The butter becomes somewhat acid from the de- 

 velopment of both volatile and non-volatile acids, the best 

 known of these products being butyric acid, although others 

 are found. The butter also has in the end a tendency to 

 become somewhat tallowy; but the most familiar change is 

 the development of rancidity. A very large amount of study 

 has been devoted to determining the nature and the cause of 

 the rancidity of butter, which is the most noticeable charac- 

 teristic of old spoiled butter. While as yet the subject has 

 not been wholly explained, the general phenomena associated 

 with rancidity appear to be somewhat as follo.ws : 



Rancidity of Butter. The rancidity of butter is due to a 

 complicated series of chemical changes. The butter fats are 

 split up chemically, and there is formed a considerable quan- 

 tity of volatile acids, fixed acids and ethers. The develop- 

 ment of butyric acid is a universal phenomenon and has been 

 frequently regarded as the distinctive characteristic of ran 

 cidity. Indeed, in the earlier studies on the subject the ran- 

 cidity has been measured by the amount of butyric acid 

 developed. But it has become evident that, besides the for- 

 mation of butyric acid, there appears in the butter a peculiar 

 penetrating, unpleasant flavor, and that this rancid taste and 

 smell is not parallel to the production of butyric acid, or, 

 indeed, to the production of acids in general. No chemical 

 tests can yet give an accurate measure of rancidity, because 

 this phenomenon is really one that appeals to the sense of 

 taste, and is not detected by any chemical analysis. The 

 development of rancidity in the butter is dependent, to a large 

 extent, upon the presence of oxygen, for if the butter is kept 

 out of contact with the air the rancidity may be largely pre- 

 vented, and in the change that takes place in any sample of 

 butter the rancidity is found to progress from the surface 



