THE KINDS OF MICRO-ORGANISMS IN MILK 359 



salt, and some of the latter may remain in solution and be over- 

 looked, unless the remaining solution is tested separately. 



In a series of studies made by the writer it was found that the 

 kind of acid produced in souring milk depends upon the following 

 factors : 



1. The relative number present of Streptococcus lacticus and 

 Bacillus aerogenes. Since Streptococcus lacticus produces d-acid 

 and Bacillus aerogenes 1-acid, we will have racemic acid with ex- 

 cess of d-acid when Streptococcus lacticus predominates and with 

 excess of 1-acid when Bacillus aerogenes predominates. During 

 the initial period of acid fermentation Bacillus aerogenes is usually 

 present in larger numbers than Streptococcus lacticus, and 1-acid 

 is the chief product. Furthermore, during the initial period the 

 proteins, as pointed out before, are not broken up, and food 

 conditions for Streptococcus lacticus are relatively unfavorable. 

 Bacillus aerogenes, therefore, multiplies at a greater rate than 

 Streptococcus lacticus. 



2. The temperature at which the fermentation occurs is of 

 influence. Since Bacillus aerogenes grows more rapidly at rela- 

 tively high temperature — i. e., 30° C. and higher — than Strepto- 

 coccus lacticus, more 1-acid is formed at this temperature than at 

 lower temperature. 



3. The kind of acid present varies according to the length of 

 time the fermentation has lasted. In initial stages of souring 

 Bacillus aerogenes predominates, and consequently 1-acid is found 

 in excess, but later, when the food material is more suitable for 

 Streptococcus lacticus and when the acid has accumulated to 

 a degree to inhibit further growth of Bacillus aerogenes, d-acid 

 is the chief product. The d-acid and 1-acid combine to form 

 r-acid, and during later stages of fermentation d-acid remains in 

 excess. 



The facts are probably not entirely explained by the condi- 

 tions mentioned. Autotransformation of active acid into racemic 

 acid has been observed by Nef, and, furthermore, lactic anyhdrid 

 is formed, according to Jungfleisch and Godchot, when solutions 

 of lactic acid or of lactates are evaporated to concentration. This 

 lactic anhydrid is levorotatory and its presence may bring mis- 

 leading results. 



In milk obtained under unusual conditions of cleanliness, 

 such as certified milk, for example, the number of bacteria is ex- 

 ceedingly small, and consequently Streptococcus lacticus, being 

 more adapted to growth in milk than most other bacteria, rapidly 

 becomes predominant, and consequently mostly d-acid is formed. 

 It follows that the presence of d-acid in sour milk indicates de- 

 sirable conditions for dairy products, since members of the coli- 



