LACTIC ACID FERMENTATION. 27 



upon abandoning the use of the name Bad. acidi ladici lyeich- 

 mann on the ground of the identity of the organism with 

 Streptococcus lacticus Kruse. The discussion illustrates the 

 difficulties of bacterial classification and has certainly empha- 

 sized the fact of the common occurrence in milk of strep- 

 tococci and other forms almost indistinguishable from them. 

 Members of the B . lactis aerogenes and B . coli communis (37) 

 groups are capable of causing souring of milk, but require a 

 temperature near that of the animal body in order to flourish 

 and become a factor in the souring process. The fact that one 

 species of lactic acid producing organism may predominate in 

 milk in which the fermentation is well advanced, is surely an 

 evidence that the species in question is best fitted to survive. 

 In the earlier stages of fermentation a: greater number of species 

 participate, but gradually succumb in the competition on 

 account of intolerance to acid, other bacterial products, or 

 other environmental conditions. It has been shown that a cer- 

 tain non-acid producing organism, by some means, creates con- 

 ditions more favorable to Bad. acidi lactici, and contributes to 

 the more rapid souring of milk than would occur without its 

 presence (25, 26). The growth of lactic acid organisms in 

 souring milk is not unlimited, for eventually their multipli- 

 cation is checked by the product of their own growth — lactic 

 acid. 



The lactic acid organism owes its pre-eminence in milk to its 

 thorough adaptation to the environment in competition with 

 other organisms. In fresh milk it forms a small percentage of 

 the bacteria present. At 10°C. (50°F.) it multiplies slowly, 

 gaining no advantage over competitors, but at 20°C. (68°F.) 

 it predominates. At this latter temperature milk is curdled in 

 forty hours. 



The lactic acid fermentation is deleterious in the sense that 

 the result of its progress terminates the period during which 

 milk is a salable food product. When milk for domestic pur- 

 poses sours sooner than usual, the blame is laid upon the dealer. 

 This fermentation produces manifestations in the taste and ap- 

 pearance of the milk easily recognized by the consumer and 

 in the popular mind is an index of the keeping quality of the 



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