BACTERIA IN BUTTER-MAKING. 



a few days ; but if it is kept cool and away from the light it 

 may remain sweet and good for many months. If, however, 

 it is not kept very cold, further changes soon begin to ap- 

 pear which slowly progress and eventually ruin the butter. 

 The most noticeable feature is the appearance of rancidity. 

 This change is accompanied by the development of butyric 

 acid and frequently by a considerable change in the consist- 

 ency of the butter. It finally becomes strongly rancid and 

 tallowy, and totally ruined for use. 



The cause of all these changes is still under dispute. The 

 loss of the original fresh aroma is probably due simply to the 

 volatilization of the products which give the aroma, a very 

 natural phenomenon inasmuch as the aroma is necessarily due 

 to volatile products. The later changes are more uncertain. 

 We have already noticed that certain extremely common bac- 

 teria found in milk and butter have the power of producing 

 butyric acid when growing in the absence of oxygen, and it 

 would seem almost certain that they would produce this effect 

 in the butter and thus explain the development of the butyric 

 acid. This has indeed been generally assumed as the chief 

 cause of the acid. Indeed, many varieties of butyric acid bac- 

 teria have been found by different bacteriologists and have been 

 assigned a share in this universal change occurring in butter. 

 There can be little doubt that they do play an important part 

 in the process. 



But the development of rancidity is a different matter. It 

 is true that the rancidity commonly appears parallel with the 

 butyric acid, but it has been shown that they are totally dif- 

 ferent and probably unrelated phenomena. The amount of 

 acid is no measure of rancidity. Butter may, under certain 

 conditions, develop butyric acid without the appearance of any 

 rancidity. The rancid flavor is due to some distinct phenom- 

 enon. Whether it is to be attributed to bacteria or to chem- 

 ical ferments is uncertain. If butter is made from sterilized 



