FERMENTED MILK 489 



— the usual practice — the acidity is sufficient to coagulate 

 the casein in the cream. In the churning process this curd is 

 broken up into very fine particles. These curd particles settle 

 very slowly, and if the buttermilk is agitated occasionally it 

 will retain its milky appearance. When the buttermilk is 

 allowed to stand undisturbed for several hours, the curd par- 

 ticles sink to the bottom, leaving an opalescent whey at the 

 top. At the present time a large part of the so-called "butter- 

 milk" sold in cities is not buttermilk, properly speaking, since 

 it is not made by churning cream, but is simply soured skimmed 

 milk which has been churned or stirred in order to break up the 

 curd. The same product is sold also under the name of " ripened 

 milk." 



The souring of milk or cream is brought about by the ac- 

 tivity of certain bacteria which form lactic acid by decompos- 

 ing the milk-sugar (lactose). The ability to form acid from 

 lactose and other sugars is possessed by many kinds of bac- 

 teria, but is so characteristic of a certain group that they are 

 commonly spoken of as the lactic acid bacteria. These bac- 

 teria have been described as distinct species or varieties under 

 many names. Among them may be mentioned Bacterium 

 guntheri, Bacillus acidi lactici, and Streptococcus lacticus. In 

 spite of the confusion in nomenclature it is evident that the 

 term "lactic acid bacteria" includes a fairly well-defined group 

 of closely related varieties possessing in common several defi- 

 nite characters. Variations from the type in minor characters 

 produce an almost infinite number of varieties. These varia- 

 tions may be in the ability to ferment different sugars, in the 

 tendency to grow in chains, in the kind of flavor formed in 

 milk, in the intensity of acid formation, and in the ability to 

 produce pathological conditions in animals. 



In many creameries the cream is allowed to sour spontane- 

 ously. In this case many bacteria other than the true lactic 

 bacteria will take part in the acid formation, and in addition 



