702 BACTERIA IN AIR, SOIL, WATER, AND MILK 



In consequence, they are able to persist and develop when cultural con- 

 ditions are absolutely unsuited to other bacteria. 



Consequent upon acidification of the milk by lactic-acid formation, 

 there is coagulation of casein. Casein precipitation, however, may 

 also be due to a non-acid coagulation caused by a bacterial ferment. 

 Casein precipitated in this way may be redissolved by a bacterial 

 trypsin or casease, produced by the same or other bacteria, the milk 

 again becoming entirely liquid, transparent, and of a yellowish color. 

 The casein precipitated by lactic-acid formation, however, is never 

 thus redissolved, because the high acidity does not permit the pro- 

 teolytic ferments to act.^ 



Butyric-acid fermentation in milk, a common phenomenon, is also an 

 evidence of bacterial growth. As a rule, it is produced by the anaerobic 

 bacteria, and is a process developing much more slowly than other fer- 

 mentations. A large number of bacteria have been described which 

 are capable of producing such changes, the chemical process by which 

 they are produced being, as yet, not entirely understood. It is probable 

 that the process takes place after hydrolysis of the disaccharid some- 

 what according to the following formula: 



Ga Hj2 Og = C4 Hg O2 + 2 CO2 -1- 2 H2. 



Special bacteria have been described in connection with this form of 

 milk fermentation,^ most of them non-pathogenic. It is unquestionable, 

 however, that many of the well-known pathogenic bacteria, such as 

 Bacillus aerogenes capsulatus, Bacillus cedematis maligni, possess the 

 power of similar butyric-acid formation. While less commonly observed 

 in milk, because milk is rarely kept long enough to permit of the action 

 or development of these enzymes, the butyric-acid fermentation is of 

 importance in connection with butter, where it is one of the causes pro- 

 ducing rancidity. 



Alcoholic fermentation may take place in milk as a result of the ac- 

 tivities of certain yeasts. Upon the occurrence of such fermentations 

 depends the production of kefyr, koumys, and other beverages which 

 have been in common use for many years, especially in the region of the 

 Caucasus. The characteristic quality of these beverages is contrib- 

 uted by the feeble alcoholic fermentation produced by members of the 

 saccharomyces group, but side by side with this process lactic-acid forma- 



> Conn, Exper. Stat. Rep., 1892. 



2 Schattenfroh und Grasherger, Arch. f. Hyg., 37, 1900. 



