AGRICULTURAL BACTERIOLOGY. 



trouble must remember that, under these conditions he is no 

 longer obtaining pure milk, but milk which is probably filled 

 with bacteria, some of which are the cause of the diseased 

 conditions of the udder, and may also be the source of some 

 kind of disease to man. Udder diseases are thus a most serious 

 menace to the purity of the milk supply. 



Confining our attention for the present to normal milk, we 

 notice next that, if the milk can be kept from being contami- 

 nated with bacteria, the ordinary and almost universal phe- 

 nomenon of souring will not occur. Indeed, milk free from 

 bacteria will remain for an indefinite period without visible 

 change. It is a fact, recently proved, that there are some 

 chemical changes which will slowly occur in such milk. In 

 normal milk when secreted from the cow there is present one 

 of the chemical ferments (enzymes) referred to in an earlier 

 chapter. This enzyme is produced by the cow and appears to 

 be a normal constituent of milk. It is called galactasc and 

 has the power of slowly converting the insoluble casein into 

 soluble proteids. This " digestion " is a somewhat slow proc- 

 ess and is a phenomenon of no significance in the handling of 

 milk or in butter-making. In the problem of cheese-making 

 the galactase is a factor of more importance. But in the 

 handling of milk other phenomena, due to bacteria, are so much 

 more prominent as to conceal entirely the action of the galac- 

 tase. We may therefore take for our starting point the highly 

 important facts that, (i) so far as concerns healthy milk, it will 

 remain unaltered if free from bacteria, and that, (2) all of the 

 ordinary changes occurring in milk are due to the action of 

 bacteria which get in subsequent to the time when the milk is 

 secreted in the mammary gland. 



SOURCES OF BACTERIA IN MILK. 



Bacteria are sure to find their way into the milk. Recog- 

 nizing the sterile condition of the freshly secreted milk we are 



