BACTERIA OF MILK 51 



extent is not attended with any appreciable injurious 

 effects, their presence in large numbers is not desirable 

 because it may be accompanied by harmful results. 



There are numerous species of these organisms. For 

 facility of study as well as for practical purposes, it is 

 convenient to group them according to the changes which 

 they bring about in milk. Although some of the species 

 which ferment lactose produce both acids and gases, and 

 although a part of those which act principally upon the 

 lactose also operate upon the casein in a lesser degree 

 and vice versa, nevertheless by grouping the different 

 species according to their dominant effect a very clear 

 conception is obtained of the important changes produced 

 in milk by the organisms of each group. Following this 

 plan, the numerous species of common milk bacteria may 

 be classified in the following groups: 



1. Acid-forming Bacteria. These organisms split 

 up the lactose in milk and form acids. The milk first 

 acquires a sour odor and taste and later curdles. This 

 is the most quickly apparent change which occurs in 

 milk. The acids combine with the calcium of the calcium 

 caseinate, and the casein, being thus set free, is precipi- 

 tated in the form of a smooth, white jelly-like curd, which 

 may contain a few gas bubbles or furrows made by 

 ascending bubbles. In the beginning, the curd is dry and 

 is equal in size to the original volume of the milk, but 

 later on it contracts and expresses a fluid or serum which 

 holds in solution certain of the milk constituents. 



The time required for milk to sour and curdle depends 

 upon the number and kind of acid-forming bacteria it 

 contains and the temperature at which it is kept. On 

 the average, about 0.45 per cent, acidity is necessary to 

 bring about curdling. The acid-forming bacteria con- 



