EXPERIMENT STATION BULLETINS. 201 



are apparently zooglea of bacteria and yeast plants [ See Zooglea Bui. 

 139. ] used as we employ yeast. 



LACTIC ACID FERMENTATION. 



This is the fermentation with which we have so commonly to deal; it is 

 the ordinary souring of milk. Before the days of bacteriology, it was 

 supposed to be due to a chemical change instituted by the oxygen of the 

 air; it was also recognized that the nitrogenous components of the milk 

 had an important bearing in the development of lactic acid in the milk. 

 Pasteur in his biological explanations of fermentation was able to demon- 

 strate that lactic acid fermentation or the ordinary souring of milk was 

 due to micro-organisms. Lister was the first to isolate a single species 

 which had this special function. Hueppe has given us several species 

 which are well known as Hueppe's lactic acid bacteria. Many lactic acid 

 micro-organisms have been described since Hueppe's, all having certain 

 characteristics in common but differing in some morphological or cultural 

 features. 



Each dairy has its lactic acid bacteria and it would be found that while 

 belonging to the lactic acid group of micro-organisms, they would repre- 

 sent perhaps several species. These lactic acid bacteria become the 

 property of every dairy; once located they grow and increase with the 

 dairy, inhabiting every cranny, dairy utensil and barn ; the entire milk- 

 ing outfit is their home and they are fed at each milking and at the same 

 time multiply rapidly. 



The souring of milk is the conversion of milk sugar into lactic acid by 

 the action of micro-organisms. In this change small quantities of car- 

 bonic acid gas and alcohol are likely to be evolved also; however these 

 are lost sight of in the formation of the acid. The casein of the milk is- 

 cast down by the acid formed and thus gives the curdled appearance of 

 soured milk. After a small per cent of acid is formed, the lactic acid 

 bacteria cease to grow; hence it is that the acidity never reaches a very 

 large per cent, from .5 to .8 per cent. 



The temperature at which this fermentation takes place lies between 

 18^° C. [ 65° F. ] and 35° C. [ 95° F. ] but is perhaps conducted best 

 between 23° C. [75° F.] and 30° C. [85° P.J. The bacteria which pro- 

 duce the fermentation grow very slowly below 15° C. [ 60° F. ] and are 

 readily killed at 68° C. [ 155° F. ]. Bearing in mind the relation of tem- 

 perature to the growth of the lactic acid bacteria, the regulation of this 

 fermentation may be carried out at will. The questions: "Why does 

 milk sour more quickly in hot weather? " or " Why does it sour so sud- 

 denly before a thunderstorm?" find their answers in the consideration 

 of temperature. 



In the process of Pasteurization, the lactic acid bacteria are usually 

 killed ; accordingly, those bacteria which remain in the milk after Pasteur- 

 ization are those bacteria which precipitate the casein or curdle the milk 

 not by the formation of any acid but by the secretion of a ferment; conse- 

 quently, curdled Pasteurized milk has little acidity. 



A lactic acid fermentation product which has been used by the Armen- 

 ians as a drink and the ferment of which they have employed in the 

 making of butter is called "matzoon." Its taste is that of a rich butter- 

 26 



