I50 BACTERIA IN MILK 
Confining our attention for the present to milk from healthy 
animals, we notice that, if we could keep bacteria out of the milk, 
none of the ordinary changes, not even the souring which is so 
nearly universal in normal milk, would take place. Indeed, milk 
which is free from bacteria will remain visibly unchanged for an 
indefinite time. It is not, however, absolutely free from subse- 
quent chemical changes, since there is present in the milk an 
enzyme which produces slow changes. This enzyme, called 
galactase, is secreted by the milk gland with the milk, and may 
thus be said to be part of the milk. It can slowly convert the 
casein of the milk into soluble proteids. Its action is very slow, 
however, and seemingly of no significance, except in the ripening 
of cheese. At all events, none of the ordinary fermentations 
appearing in milk are attributed to this galactase or to any other 
part of the milk itself, but are all due to microdrganisms. We 
may, therefore, take as a starting-point these two highly impor- 
tant facts: (1) Milk from healthy cows will, if it could be kept free 
from bacteria, show none of the ordinary milk fermentations. 
(2) All of these fermentations are due to microérganisms that get 
into the milk after the milk is secreted from the mammary gland. 
SOURCES OF MILK BACTERIA 
By the time it has been drawn from the cow, received in the 
milk-pail, and removed from the cow stall, it may contain bacteria 
to the extent of many thousands perc.c. The number of bacteria 
in freshly drawn milk varies greatly with the conditions existing in 
the dairy. There may be only a few hundreds in each c.c., or, 
under exceptional conditions, a smaller number still; but it is 
much more likely that the milk, by the time it has been removed 
from the stall, contains a few thousand bacteria per c.c. 
Since all the troublesome changes which occur in milk and 
make it such a difficult product to handle, are due to the action of 
bacteria upon the milk, it is to the interest of the dairyman, the 
milk distributor, and the consumer to have as few bacteria as 
