354 MILK 



broken-down cells. Sometimes the lactic acid bacteria increase 

 in number, then the number decreases, only to increase again to 

 a second maximum, which may be greater or smaller than the 

 first maximum. Such fluctuations have been referred to in an- 

 other connection (see page 266). Under the action of different 

 types of bacteria the available food-supply is constantly changing. 



The relation of numbers of bacteria to the amount of acid 

 formed is illustrated in the chart (Fig. 167). 



The chart shows that in sterile milk inoculated with large 

 numbers of Streptococcus lacticus coagulation is slower and acid 

 formation smaller than in raw milk, other conditions being equal. 

 It is furthermore clear from this chart that coagulation is not 

 directly dependent upon the presence of a certain amount of acid 

 or upon the number of lactic acid bacteria. This absence of a 

 definite relation between coagulation, on the one hand, and acid 

 and number of bacteria, on the other hand, may be due to the 

 kinds of associate bacteria present, the kind of acid formed, and 

 the activity of enzyms produced by bacteria. 



The quantity of peptone present in the culture-medium has a 

 pronounced influence on the amount of acid formed by lactic acid 

 bacteria. As stated before, Jensen has shown that the lactic 

 acid bacteria of the Streptococcus lacticus type attack peptones 

 and caseones with greater ease than complex proteins. Koestler 

 obtained similar results, and determined that the amount of acid 

 increased as the percentage of peptone was increased. This 

 author claims that part of the acid is produced from the peptone 

 and not solely from the carbohydrate. Rahn, on the other hand, 

 thinks that the peptone simply favors greater growth of the bac- 

 teria, so that more acid is produced. He determined the exact 

 amount of acid produced by a single cell of Streptococcus lacticus, 

 and found a larger amount of acid to be formed when the quantity 

 of peptone was increased, but that each cell produced the same 

 amount no matter what the peptone content was. 



Koestler studied the influence of oxygen pressure on the 

 amount of acid formed by Streptococcus lacticus, and found that 

 in deep layers of the medium, when air had but a small relative 

 surface contact, the amount of acid formed was greater than in 

 shallow layers. 



An interesting study of the acid formation and the effects 

 resulting from the acid formation in milk under the influence of 

 Bacillus acidi lactici (B. aerogenes) and Bacterium lactis acidi 

 (Streptococcus lacticus) was published by Van Slyke and Bos- 

 worth. Milk was inoculated with both these organisms and al- 

 lowed to ferment for sixty hours, and longer. The authors found 

 that 22 per cent, of the milk-sugar had been decomposed and that 



