2l6 PRACTICAL DAIRY BACTERIOLOGY 



in old butter, and one of the easiest methods by which a culture 

 of this organism can be -obtained is to make a common gelatin 

 plate from a sample of old butter. From the facts just stated 

 it is evident that bacteria will have less effect upon the butter 

 after it is made than upon milk or cheese. 



RANCIDITY OF BUTTER 



It is well known that butter will not keep indefinitely. In a 

 few days the fresh aroma of a finely ripened butter disappears, 

 due probably to the dissipation of the volatile gases; but from 

 this time on for many weeks the butter may remain in prac- 

 tically the same condition. If it is kept cool, it may be months 

 before there is any further deterioration. But later there is an 

 inevitable appearance of rancidity, and the butter slowly be- 

 comes stronger and finally unfit for use. The cause of the devel- 

 opment of this rancidity has been difficult to determine, ap- 

 parently because a variety of factors contribute to it. 1 It is, 

 in part, probably due to chemical fermentation, produced by 

 enzymes from the milk, and in part to the growth of bacteria. 

 The rancidity is much more likely to occur if the butter is ex- 

 posed to the light, and develops more readily at warm than cold 

 temperatures. At temperatures below freezing rancidity does 

 not occur. If butter is, therefore, kept cool and in large masses, 

 it may be held for a long time without the appearance of any 

 very noticeable strong flavor. In the end, however, the rancidity 

 is practically sure to appear. To what extent bacteria are con- 

 cerned in this change we do not know yet, although most in- 

 vestigators have concluded that they are prominently concerned 

 in the phenomenon. There are some species of bacteria known, 

 the butyric acid organisms, which have the power of producing 

 butyric acid. Oidium lactis, B. butyricus, B. fluorescent, Butter- 

 sdure bacillus of Grassberger and other species have been found 



* M'Kay and Larson. Bui. 71, Iowa Exper. Sta., 1903. 

 Rogers. Bui. 57, Bu. An. Ind., 1904. 



