THE MILK IN ITS RELATION TO CHEESE 17 



22. Acid fermentation of milk. By far the most 

 common and important fermentation taking place in 

 milk is due to the action of the lactic acid-forming bacteria 

 on the milk-sugar or lactose. The bacteria that bring 

 about this fermentation may be divided into several 

 groups on the basis of their morphology, proteolytic 

 activity, gas production, temperature adaptation and 



FIG. 2. Effect of different fermentations of milk: U, Curd pitted with 

 gas holes ; G and 0, gassy curds which float ; K, smooth, solid 

 desirable curd. 



production of substances other than lactic acid. The 

 larger number of organisms producing lactic acid in 

 milk also produce other organic acids in greater or less 

 abundance. Inasmuch as lactic acid is the principal sub- 

 stance produced, they are called lactic acid organisms. 

 This group contains different kinds of organisms which 

 may be subdivided into small groups as follows : 



(a) Bacterium lactis-acidi group. 



(b) Bacterium colon-aerogenes group. 



