234 MILK AND THE PUBLIC HEALTH CHAP. 



prevent it being exceeded. To take t as 15 C. is to assume 

 that the milk may be kept at that temperature for the whole 

 period, and is obviously too generous a limit, but may be 

 taken as an extreme maximum. 



A reasonable limit for initial pollution is that there should 

 not be more than one B. coli or allied organism per c.c., so that 

 x may be taken as one B. coli per c.c. 



The problem can now be re-stated. To arrive at the 

 maximum pollution allowable in vended milk we have to 

 ascertain what will be the final coli content (z) in a milk 

 sample initially containing not more than one B. coli per c.c., 

 kept for 24 hours at a temperature of 15 C. The figure z so 

 obtained can be reasonably accepted as the maximum number 

 of B. coli to be allowed in vended milk at any time of the 

 year. 



Like all biological problems, in contrast to chemical, such 

 determinations can never be precise and unvarying, but within 

 limits they are very valuable. Two important factors leading 

 to variation in actual practice have to be considered. 



1. The above considerations take no account of pollution 

 in transit or in the shops of milk purveyors. It may reason- 

 ably be contended that no B. coli should be admitted to the 

 milk in transit. Even under the present-day unsatisfactory 

 conditions it should be comparatively easy to prevent any 

 considerable B. coli addition during transit or while being 

 purveyed. This factor does not seriously affect the above 

 contention. 



2. The influence of the number and kinds of the other 

 bacilli present cannot be neglected. They considerably in- 

 fluence the multiplication rate of B. coli in milk. The 

 significance of this factor can only be determined by a wide 

 series of experiments. 



The writer has carried out a series of such experiments. 

 The chief results and conclusions obtained are recorded in 

 Chapter IV. Applying these results to the practical question 

 of B. coli standards for vended milk, and taking the maximum 

 limits laid down above, it would seem that if the initial number 

 of B. coli and allied lactose fermenters is not more than one 

 per c.c. the vended milk should not contain, as a rule, more 

 than 100, and certainly not more than 1000 B. coli and allied 



