54 Agricultural Bacteriology. 



a perfectly normal occurrence. Indeed any milk that 

 does not sour when kept at ordinary temperatures for 

 some time is regarded with suspicion and is thought to 

 have been treated with preservatives. The souring of 

 aiilk is not an inherent property, but is due to certain 

 kinds of bacteria that gain entrance to it after its with- 

 drawal from the cow. These bacteria are found uni- 

 versally distributed and unless precautions are taken 

 to exclude them, they are certain to enter the milk. 



The bacteria causing the souring of milk use a part 

 of the milk sugar as food, forming from it lactic acid. 

 This substance is not a solid like the sugar but a syrup- 

 like liquid, having no color or odor and a sour taste. 

 The bacteria producing the change are known as the 

 lactic acid bacteria. The acid they form gives the fer- 

 mented milk its. sour taste and causes it to curdle. 

 Milk that contains about 0.3 per cent of acid tastes 

 sour and curdles when heated. As the bacteria con- 

 tinue to develop the acidity increases. At 0.6 per cent 

 the milk curdles at ordinary temperatures. When the 

 acidity reaches about 1 per cent, bacterial growth 

 ceases and no more acid is formed. The bacteria like 

 all other forms of life are injured by their own by-pro- 

 ducts unless these are removed. In milk the acid is 

 formed in such quantities as to cause ultimately the 

 death of the cells. The bacteria in the milk can not get 

 away from the acid they have produced, hence growth 

 ceases, although plenty of food remains. If all of the 

 sugar of the milk were changed to acid, the acidity would 

 be 4 per cent, instead of 1 per cent. 



Kinds of lactic acid bacteria. The lactic acid bac- 

 teria come from various sources. One type is often 

 found in the dust coming from the skin of the animal 



