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- 
