86 Dairy Bacteriology. 



Types of acid-forming bacteria. When substances 

 undergo decomposition, it is a common belief that com- 

 pounds offensive to the odor and taste are formed ; but 

 such is not necessarily the case. The products of the 

 decomposition may be as agreeable and as harmless as 

 the compounds decomposed. Whether the decomposi- 

 tion products of any substance are offensive or not is 

 dependent on the kinds of micro-organisms acting on 

 it. There are forms of acid-producing bacteria that 

 change milk in odor, taste, and appearance, yet the sour 

 milk is not offensive in any sense of the word. Other 

 bacteria also sour the milk, but produce offensive odors 

 and a disagreeable taste. Thus, the acid-forming bac- 

 teria may be divided into two main groups, which may 

 be designated as desirable and undesirable. This divi- 

 sion is of importance to the butter and cheese maker 

 and to the consumer of milk. 



Desirable acid-forming bacteria. If milk is pro- 

 duced under clean conditions, it is not likely to have a 

 disagreeable odor or taste at any time, even when it is 

 sour; rather the taste is agreeable like that of good but- 

 ter milk. The curd is perfectly .homogeneous, showing 

 no holes or rents, due to the development of gas, and 

 there is but little tendency for the whey to be ex- 

 pressed from the curd. This type of fermentation is 

 largely produced by the group of bacteria to which has 

 been given the name, Bacillus lactis acidi. 



The main by-product of this group of bacteria is lac- 

 tic acid ; small amounts of acetic acid and alcohol, with 

 traces of other compounds, are also formed. The agree- 

 able odor and to some extent the flavor of milk fer- 

 mented by these bacteria is due to other by-products 

 than lactic acid, for this has no odor and only a sour 



