Milk Fermentations. 55 



The various fermentations may be grouped according 

 to the substances in the milk upon which they chiefly 

 act, such as those affecting milk-sugar, or casein. This 

 arrangement is followed in this connection, but it should 

 be borne in mind that many organisms act upon several 

 of the milk constituents, and hence, it is difficult to 

 correctly classify the same. 



Another method of classification is based upon the 

 products manufactured during the fermentation, but even 

 here confusion is liable to enter to some extent, because 

 in many cases there are several distinct substances formed. 



Milk is such a complex substance that the changes pro- 

 duced by a single germ are often so numerous that the 

 processes can not be separated in their reactions. It must 

 be remembered then, in referring to the different types 

 of fermentations, that perhaps a distinct by-product 

 is being formed, but it is more than probable that there 

 are a series of changes, in which the most marked decom- 

 position process is alone taken into consideration. For 

 example, there is a fermentation classed under the head 

 of the butyric changes, a decomposition process in which 

 butyric acid is the chief product formed, but this may be 

 associated with an alkaline condition of the milk and the 

 production of a bitter substance in the same. Thus, the 

 subdivision followed here will of necessity be imperfect 

 and occasional instances will be noted where some changes 

 in milk might well be. described under several heads. 



Some of these fermentation changes occur so con- 

 stantly that they are to be regarded as normal or natural 

 in their character. Normal, however, only in the sense 

 that the bacteria that cause them are so widely distributed 

 that the change is inevitable and not that the milk itself 

 would undergo any of these changes if it were not for 

 the presence of the different germs that bring them 



