154 CITY MILK SUPPLY 



TABLE 42. EFFECT ON THE BACTERIAL COUNT OF STRAINING MILK THROUGH 



CHEESE CLOTH (STOCKING) 



Five other samples were filtered in the same way and the keeping 

 power of the filtered and unfiltered portions compared, with the result 

 that it was found that the keeping quality of none of the milk was im- 

 proved by filtering and that of some was impaired. The use of strainer 

 cloths on the farm probably has a bad effect, for they are commonly used 

 again and again without being boiled out and dried quickly every time 

 they are used; hence they seed the milk with bacteria. Moreover, when 

 the filters clog, the milkers are prone to poke them and so contaminate 

 the milk with their fingers. 



Influence of the Cream Separator on the Bacterial Count of Cream 

 and Skim-milk. Milk can be skimmed and clarified by centrifugal 

 force. On the farm, cream separators are used for skimming the milk 

 and also to some extent for clarifying it. In city milk plants clarification 

 is accomplished by specially designed centrifugal clarifiers. 



By the use of cream separators the bacterial count of the milk is 

 increased in two ways, namely : by the actual addition of more germs to 

 the milk, by contact with the insterile parts of the separator through 

 which it passes; and second, by the breaking up of bacterial clumps 

 and chains as the milk passes through the machine. Of these two, the 

 former is the more important when the cleaning of the separator is ha- 

 bitually slighted. The breaking up of the clusters and chains most notice- 

 ably increases the number of bacteria when the milk being separated or 

 clarified has a high count. In the passage of the milk through the sepa- 

 rator, part of the germs go with the cream, part with the skimmed milk, 

 and part are thrown out with the slime. Apparently the percentage of 

 bacteria that goes to each varies, for conflicting results have been ob- 

 tained by different experimenters. Most find that after separation 

 both the skim-milk and the cream contain more bacteria than did the 

 original milk, and that the cream contains more than the skim-milk but 

 that both contain fewer than the slime. Possibly a distribution after 

 separation of 25 per cent, of the bacteria in the skim-milk, 28 per cent, 

 in the cream, and 47 per cent, in the slime, is an approximation of what 

 may be expected but no fixed ratio can be predicted. Heinemann, 

 Luckhart and Hicks found fewer bacteria in the separated cream and 



