360 MILK 



aerogenes group are present in relatively small numbers and are 

 easily outgrown by Streptococcus lacticus. 



Furthermore, it should be remembered that observations have 

 been reported showing that some micro-organisms may assimilate 

 one of the active modifications of lactic acid in preference to the 

 other one, so that the latter remains intact. D-acid seems to be 

 utilized more frequently than 1-acid, and this agrees with Nef's 

 theory that d-acid is more readily dissociated than 1-acid. And 

 finally, we know that when bacteria disintegrate, enzyms are hb- 

 erated, which for a time continue the fermentative process car- 

 ried out during the life of the cell. 



The conditions become still more complicated by virtue of the 

 fact that Bacillus aerogenes and B. coli produce acids other than 

 lactic acid in considerable quantity, while Streptococcus lacticus 

 produces practically pure d-acid. It is, therefore, possible to have 

 1-acid in excess of d-acid only when enormous numbers of the 

 coli-aerogenes group are present, and this occurs chiefly when 

 milk contains excessive amounts of filth or when milk has been' 

 kept at temperatures high enough to favor the growth of Bacillus i 

 aerogenes. 



The presence of d-acid, 1-acid, or r-acid can be readily ex- 

 plained then when we know which one of the two chief types of 

 lactic acid bacteria is predominant. Bacillus aerogenes always 

 produces 1-acid, and when it is present in larger numbers than 

 Streptococcus lacticus, or when the temperature is such as to favor 

 its growth, 1-acid will be formed. But Streptococcus lacticus 

 produces d-acid at the same time and, therefore, part of the 1-acid 

 is neutralized and r-acid appears. In initial stages of souring, 

 therefore, 1-acid will usually be the chief modification, except per- 

 haps in exceptionally clean milk. As the fermentative process 

 proceeds, racemic acid takes the place of inactive acid, and finally 

 d-acid predominates. In loppered milk usually inactive acid is 

 present with d-acid in excess. At high temperature more 1-acid 

 is formed and more rapidly than at low temperature, and, there- 

 fore, more r-acid with less d-acid results. 



The character of the coagulum produced in souring milk by 

 different types of lactic acid bacteria is by no means of the same 

 nature. Streptococcus lacticus forms a smooth jelly, with the 

 separation of only a small quantity of whey. Even after several 

 days the amount of whey that becomes visible is small. Bacillus 

 aerogenes, on the other hand, produces a coagulum that contracts 

 and leaves considerable whey, and the coagulated casein is broken 

 up by gas-bubbles. However, Bacillus coli or B. aerogenes in 

 pure culture does not form a coagulum rapidly, so that much gas 

 escapes before the coagulum is solid enough to retain large amounts. 



