LACTIC ACID FERMENTATION 199 



a warm place, they multiply very rapidly and usually over- 

 grow other kinds of bacteria which may be present. Sugar 

 is rapidly transformed into lactic acid. The total number 

 of the lactic acid bacteria developing will usually run into 

 the hundreds of millions per cubic centimeter. Eventually 

 sufficient lactic acid develops to curdle the milk and 

 effectually stops the growth of the bacteria. Streptococcus 

 lacticus is usually inhibited first. Certain species of lacto- 

 bacilli may go on growing, producing larger quantities of 

 lactic acid, after the inhibition of the streptococci. 



When members of the genus Bacterium, particularly 

 Bacterium aerogenes, are present in sufficient quantities to 

 modify the course of fermentation, the curd developed, as 

 noted above, is usually broken and tends to shrink. 



Soured Milk Beverages. An artificial buttermilk (fre- 

 quently sold at soda fountains) is prepared by inoculating 

 milk with a previously soured sample of milk or with a pure 

 culture of lactic acid bacteria. In most cases the resultant 

 souring is produced by an organism of the Streptococcus 

 lacticus type. When the milk has curdled it is beaten or 

 churned, so that the casein becomes finely divided. In some 

 respects it is preferable to the true buttermilk as a beverage 

 because of the greater uniformity in flavor and texture 

 secured. 



Milk sherbets (as lacto) are frequently prepared from 

 milk soured in this manner, flavored with fruit juices, and 

 frozen. 



In some cases, the organisms used for souring the milk 

 belong to the genus Lactobacillus, usually Lactobacillus 

 bulgaricus. As noted above, milk soured by this organism 

 is somewhat more slimy and has a smoother curd than milk 

 soured with Streptococcus lacticus or related organisms. 

 The vogue of "the so-called Bulgarian soured milk owes its 

 origin to the work of Metschnikoff. This observer came to 

 the conclusion that the consumption of soured milk bev- 



