CHAPTER IV 



BACTERIA OF MILK 



UNLESS 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 



50 



