254 BACTERIA IN MILK AND THEIR SOURCES 



contamination which occurs at the farm. These are often 

 returned to the farmer without being cleaned properly, 

 often with small amounts of sour milk in them, the 

 latter sometimes containing as many as 300,000,000 

 per cubic centimetre. Even where steam had been used 

 to clean the cans before being returned washings with 

 100 c.c. of sterile water were proved to contain from 

 400 to 1 800 bacteria per c.c. Both Stocking and Orr 

 have shown also that the use of milking machines 

 increases the bacterial contamination very considerably : 

 this is due to the difficulty of thoroughly cleaning the 

 connecting tubes and also to the sucking in of dust laden 

 air when the cusps fall off the teats of the animals. 



The water to which the cows have access, and which 

 is used for washing and rinsing the cans, churns, and 

 other vessels in the cowshed and dairy, often contain 

 bacteria, which find their way into the milk. 



The cow, the milker, the air of the cowshed, flies, 

 water supply, and imperfectly cleaned farm dairy utensils, 

 are the main sources from which milk becomes con- 

 taminated with bacteria on the farm, and, without doubt, 

 it is here that the chief troublesome forms become 

 associated with it. 



As already indicated, the number found in any par- 

 ticular sample will depend upon the amount of care taken 

 to remove the causes which lead to the introduction of 

 bacteria into it. Where dirty conditions prevail the 

 milk may contain vast numbers of organisms ; where 

 careful methods are adopted the bacterial content may 

 be greatly reduced. Freudenreich, Knopf, Orr, and others 

 found an average of 50,000 to 80,000 bacteria per c.c. 

 of milk collected in the cowshed soon after milking. In 

 some few instances the number was as low as 5000, and 



