Fermentations of Milk. 93 



fectly sweet milk, producing a curd that looks like that 

 formed in the acid fermentation of milk. The cause of 

 these sweet curdling milks, which appear from time to 

 time, is due to the introduction of certain bacteria which 

 have the power of secreting an enzyme resembling that 

 found in rennet. In such cases the milks curdle prema- 

 turely especially when warmed. The curd may gradu- 

 ally disappear, for the bacteria also produce another 

 enzyme that digests the curd, and thus renders it soluble. 

 When this advanced phase becomes evident, it is often 

 called the digestive fermentation of milk. This change 

 is produced largely by putrefactive bacteria of various 

 kinds that find their way into milk with dust and dirt. 

 Many of them are spore formers; hence, are not killed 

 when milk is heated, as in pasteurization, while the acid- 

 formers are destroyed. Pasteurized milk is thus likely 

 to undergo the sweet-curdling fermentation, if it is kept 

 for any length of time. Raw milk rarely undergoes this 

 type of decomposition, since the rennet-forming bacteria 

 under ordinary conditions are unable to develop in com- 

 petition with the acid-forming bacteria. 



Butyric acid fermentation of milk. A fermentation 

 that is much less frequently noted than the two pre- 

 viously discussed is known as the butyric fermentation, 

 since butyric acid is the principal by-product. The 

 causal bacteria cannot compete with the ordinary acid- 

 forming bacteria in raw milk ; hence it is most frequently 

 noted in pasteurized milk, since the organisms produce 

 spores and are not killed by the heating. Pasteurized 

 milk under the action of the butyric acid bacteria under- 

 goes a gassy fermentation, developing a pronounced acid- 

 ity and the disagreeable odor of butyric acid, which re- 

 sembles that of rancid butter. The butyric acid bacteria 



