THE KINDS OF MICRO-ORGANISMS IN MILK 353 



the same in all cases. It appears to be due to products manu- 

 factured by the associate micro-organisms, sometimes stable to 

 heat, sometimes unstable; sometimes under alkaline conditions, 

 sometimes under acid conditions; sometimes with apparent diges- 

 tion, sometimes with no apparent digestion. The period at the 

 beginning of lactic fermentation during which no lactic acid forma- 

 tion can be determined, and during which the number of bacteria 

 is continually increasing, may be greatly shortened by vigorous 

 associate bacteria influencing the lactic micro-organisms. Usually 

 the associate micro-organisms disappear, with the formation of 

 appreciable amounts of lactic acid; yet the associate micro-organ- 

 isms may continue to persist, causing jibnormal lactic fermenta- 

 tion." The work of Marshall shows clearly that the amount of 

 acid formed during lactic fermentation depends to some degree 

 upon the presence of associate bacteria which furnish food for 

 more or less rapid multiplication of the lactic acid bacteria and, 

 since there is no regularity about the number or kind of associate 

 organisms, the souring process must be subject to variations. 

 Koestler also found that the presence of bacteria of the hay 

 bacillus group favors acid formation by using up the oxygen 

 and partially breaking up the proteins, rendering them more avail- 

 able for nutrition of lactic acid bacteria. 



It is obvious that the relative number of lactic acid bacteria 

 of one group as related to the number present of another group 

 may influence the result. When the coli-aerogenes group is well 

 represented their products will be produced in larger measure 

 than when present in small numbers. The temperature at which 

 the souring process occurs also influences the result. High tem- 

 peratures — i. e., above 30° C. — favors growth of the coli-aerogenes 

 group, while streptococci multiply as readily at lower tempera- 

 tures. At lower temperature, therefore, lactic acid will be pro- 

 duced in larger quantity than at higher temperature, and gas 

 will be produced more readily at the higher temperature. 



Since the conditions that govern the production of acid in 

 milk vary to a marked degree, we cannot expect to find a definite 

 relation between the number of lactic acid bacteria in milk and 

 the amount of acid formed, or the time required to produce com- 

 plete coagulation. Variety of types, temperature, and, as Mar- 

 shall has shown, the presence of other bacteria influence the 

 result. The number of lactic acid bacteria increases to a given 

 point, then decreases. At high temperature the maximum is 

 reached in a shorter time than at lower temperature. After the 

 maximum number has been reached the bacteria decrease rapidly, 

 but acid formation continues for some time. This phenomenon 

 is due to the liberation of lactic-acid-forming enzyms from the 



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