CHAPTER IV 



THE GROWTH OF BACTERIA IN MILK 



THE original bacterial contamination of milk accounts for 

 the bacteria that may be found in fresh milk, but gives no idea 

 as to what may be found at any subsequent time. The rapid 

 growth of those which get in and the great differences in rapid- 

 ity of growth of different species are such that the bacterial 

 content of any sample is undergoing constant changes. To un- 

 derstand dairy problems we must study the growth as well as 

 the source of milk bacteria. 



Bacteria in Normal Milk. The number of bacteria in fresh 

 milk varies widely. It may be as low as 100 per c.c. in some 

 of the best dairies, or it may be as high as half a million, or 

 even higher, in the poorer type of dairy. Absolutely no average 

 can be given that has any meaning, for, as will be pointed out 

 in a later chapter, the number will depend upon the methods used 

 in the dairy. The kinds of bacteria present in different samples 

 of fresh milk will also vary widely. If, however, we take the 

 milk of an average dairy kept in fairly good condition, we 

 should find the bacteria in fresh milk somewhat as follows: 



There will be very few of the Bad. lactis acidi type. These 

 are frequently so few that they cannot be detected by the com- 

 mon methods of analysis. There may be one or two per cent., 

 or sometimes more, but they are always in small numbers, 

 rarely as high as 10 per cent. Their number is largely dependent 

 upon the cleanness of the milk vessels. Gas-producing acid 

 organisms are also liable to be present; but these, too, are few 

 in numbers in good milk, and their presence in any considerable 

 numbers is unfortunate, because they are liable later to produce 



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