MICRO-ORGANISMS IN MILK 265 



The period is of short duration, lasting usually not beyond twelve 

 hours. The cleaner the milk, the longer is the duration of this 

 period, because in clean milk the number of bacteria is relatively 

 small, and multiplication, therefore, does not become evident for 

 some time. During the incubation period all those bacteria that 

 are able to grow in milk increase slowly in numbers and their 

 products neutralize each other to some extent. Some types are 

 restrained by the so-called germicidal property of fresh milk, 

 while others — among them the lactic acid bacteria — are not influ- 

 enced. However, in spite of growth of lactic acid bacteria, the 

 acidity apparently does not increase for several hours after milk- 

 ing. This phenomenon may be due to at least four causes, 

 namely: 



1. Carbon dioxid escapes, so that acid formation in small 

 quantity is obscured. 



2. Some of the acid produced by lactic acid bacteria combines 

 with calcium and phosphates, so that these are precipitated, and 

 the increase in acidity is not recorded by the usual chemical 

 indicators. 



3. The increase in acid is slow at best, so that titration figures 

 may be within the limits of experimental error. 



4. Proteolytic bacteria produce small amounts of alkaline 

 protein decomposition products which may partially neutralize 

 the acid formed. 



During the second phase the increase of acid is distinct, due 

 to greater multiphcation of lactic acid bacteria. Proteins have 

 been partly broken down by proteolytic bacteria and more suitable 

 food conditions are created thereby for lactic acid bacteria. As 

 acid increases proteolysers are restrained, while Bacillus coli, 

 B. aerogenes, and Streptococcus lacticus multiply readily. More 

 or less gas is formed, the amount depending upon the relative 

 number of bacteria of the Bacillus coli group present and upon the 

 temperature. When the temperature is above 30° C. and ap- 

 proaches 37° C. gas formation is marked. 



The third phase finds enough acid produced to restrain pro- 

 teolytic bacteria entirely, and bacteria of the Bacillus coli group 

 grow less and less rapidly up to a point where they cease mul- 

 tiplying entirely. Streptococcus lacticus, however, continues to 

 multiply, as it is able to resist a larger amount of acid than the 

 Bacillus coK group. But soon the acid accumulates to a degree 

 which inhibits further growth of Streptococcus lacticus, and the 

 maximum of acid has been reached. 



Now the fourth phase commences. Molds and yeasts which 

 prefer an acid medium begin to multiply. The former attack 

 chiefly the proteins, while the latter ferment the milk-sugar which 



