THE KINDS OF MICRO-ORGANISMS IN MILK 357 



has been produced, and the further accumulation of acid pro- 

 ceeds rather slowly and is probably due to enzym action, while 

 the growth of bacteria has largely ceased. After forty-eight 

 hours the increase of acid is very slight. 



As pointed out before, during the so-called incubation period 

 of milk there is very little acid formation. It has been suggested 

 that the acid that is actually formed during this period is largely 

 neutralized by some of the calcium held by the casein so that no 

 free acid appears. After the tenth hour the balance of the cal- 

 cium held by the casein is rapidly absorbed and free acid accumu- 

 lated. This accumulation continues up to the point where the 

 acid acts as an inhibitory agent on the further growth of the 

 lactic acid bacteria, while enzyms continue to produce acid to a 

 certain degree. 



Lactic acid is known to exist in three modifications according 

 to its action on polarized Ught, namely: 1, the dextrorotatory; 

 2, the levorotatory; and 3, the racemic or inactive acid. The 

 particular kind of lactic acid produced by different types of lactic 

 acid bacteria has been considered an aid in differentiation of the 

 types, and the subject has, therefore, received considerable at- 

 tention. Each type produces either one of the two active modi- 

 fications, or when one of two types produces the dextro-, the other 

 the levorotatory modification, the racemic variety appears. Con- 

 trary to earlier opinions, it has been shown, chiefly by Harden, 

 that one type of micro-organism always produces the same modi- 

 fication of lactic acid without regard to changes in condition or 

 environment. Bacteria of the Bacillus coli or B. aerogenes type 

 always produce levorotatory acid, while the streptococcus type 

 always produces dextrorotatory acid. This has been shown by 

 Holling and independently by the writer. Pyogenic streptococci 

 have also been shown by the same authors to produce d-acid. 



The kind of lactic acid found in sour milk, therefore, depends 

 upon the predominance of one of the types of lactic acid bacteria, 

 and this again depends upon several conditions. Since Bacillus 

 aerogenes grows more rapidly than Streptococcus lacticus during 

 the initial period of souring, the milk at this stage contains chiefly 

 1-acid. Later, when streptococci grow more rapidly than Bacillus 

 aerogenes owing to the accumulation of acid, d-acid appears. 

 The consequence is that racemic acid is formed through the 

 combination of the two acids, but one or the other of the two 

 active modifications of lactic acid may predominate. 



The kind of lactic acid produced by the different types of lactic 

 acid bacteria has been determined by several investigators, but 

 the results of early work did not seem to agree. Giinther and 

 Thierf elder found that "normally" soured milk did not always 



