90 Dairy Bacteriology. 



ous to the health of the consumer. Milk that has been 

 fermented by the desirable kinds of acid-forming bac- 

 teria is not harmful. It is consumed in a variety of 

 forms (buttermilk, cottage cheese) as a common article 

 of food and its use is rapidly increasing. The prepara- 

 tion of the pure culture buttermilks or artificially 

 soured milks that are now so frequently recommended 

 for digestive troubles rests upon an acid fermentation 

 of this type. 



Undesirable acid-forming bacteria. Other types of 

 bacteria capable of forming substances that impart to 

 milk an offensive odor and a disagreeable taste not in- 

 frequently appear instead of the desirable group. In- 

 stead of producing from the sugar of milk large quan- 

 tities of lactic acid, these types generate other acids, 

 such as acetic and formic, which impart a sharp taste 

 to the milk. Besides the acids the bacteria of this 

 group form gases from the sugar of the milk. Some 

 produce small amounts of gas ; others so much that the 

 curd will be spongy and will float on the surface of the 

 whey. The fermentation caused by them is often 

 called a "gassy fermentation" and is dreaded by butter 

 and cheese makers since the gas is indicative of bad 

 flavors that will appear in the product. Gas may also 

 be produced in other types of fermentations to be dis- 

 cussed later. 



This class of bacteria enters the milk with the dust, 

 dirt, and manure, in which materials they are especially 

 abundant. No spores are formed; hence they are easily 

 killed by heating the milk. They grow both in the pres- 

 ence and in the absence of free oxygen. High temper- 



