CHAPTER IV 
BACTERIA OF MILK 
Untess drawn under special conditions, which are 
not obtainable in dairy practice, milk always contains 
bacteria. Some of them come from the udder; others are 
derived from the cow’s skin, the dust of the fodder and 
litter, the milk vessels and utensils, the person and cloth- 
ing of the milker, etc., and enter the milk during the 
process of milking and in the subsequent handling of 
the milk. They are, under normal conditions, non-patho- 
genic organisms, and, since they are always present in 
milk, are called the common milk bacteria. Under cer- 
tain conditions, which are discussed in another chapter, 
milk contains also pathogenic bacteria. 
COMMON MILK BACTERIA 
Some of the non-pathogenic bacteria do not bring 
about any perceptible change in milk. Many of them, 
however, produce marked alterations, and it is because 
‘of their presence that milk is so extremely perishable or 
unstable. In growing in milk, these organisms split up 
certain constituents, notably the lactose and casein, into 
various products, some of which are capable of exerting 
an injurious effect upon persons drinking the milk, par- 
ticularly children and invalid adults. Certain of these 
changes, including the more harmful kinds, may be con- 
siderably advanced before they are indicated by any alter- 
ation in the appearance, odor, or taste of the milk. There- 
fore, while the common milk bacteria are in themselves 
harmless, and while their growth in milk to a limited 
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