RELATION OF BACTERIA TO DAIRY INDUSTRY. 71 



the milk. After this acid is produced in small 

 quantity its presence proves deleterious to the 

 growth of the bacteria, and further bacterial 

 growth is checked. After souring, therefore, the 

 milk for some time does not ordinarily undergo 

 any further changes. 



Milk souring has been commonly regarded as 

 a single phenomenon, alike in all cases. When it 

 was first studied by bacteriologists it was thought 

 to be due in all cases to a single species of micro- 

 organism which was discovered to 

 be commonly present and named 

 Bacillus acidi lactici (Fig. 19). This 

 bacterium has certainly the power 

 of souring milk rapidly, and is 

 found to be very common in dai- 

 ries in Europe. As soon as bacte- FlG 

 riologists turned their attention dcidiiactici,^ 

 more closely to the subject it was common cause 

 found that the spontaneous sour- 

 ing of milk was not always caused by the same 

 species of bacterium. Instead of finding this Ba- 

 cillus acidi lactici always present, they found that 

 quite a number of different species of bacteria 

 have the power of souring milk, and are found in 

 different specimens of soured milk. The number 

 of species of bacteria which have been found to 

 sour milk has increased until something over a 

 hundred are known to have this power. These 

 different species do not affect the milk in the 

 same way. All produce some acid, but they 

 differ in the kind and the amount of acid, and 

 especially in the other changes which are effected 

 at the same time that the milk is soured, so that 

 the resulting soured milk is quite variable. In 

 spite of this variety, however, the most recent 



