xiv BACTERIAL POLLUTION OF MILK 233 



(e) The bacteria added to the milk subsequent to the 

 initial pollution at the byre. 



It may be said that the various investigations carried out 

 show that temperature is the most important of these five 

 factors, while, at the same time, it is the one most subject to 

 variation and fluctuation. Temperature being a more potent 

 factor than initial numbers, when the former is unknown or 

 variable it is obviously impossible to measure the initial milk 

 pollution by an examination of ordinary milk samples. 



It may be argued that the B. coli are in themselves 

 harmful, and, if numerous, the milk on that ground is bad, 

 whether the initial pollution was large or not. This view 

 cannot be accepted, the importance of the B. coli estimation 

 depending solely upon its value as an indicator of pollution. 



The problem may be looked at in another way. Maximum 

 limits may be taken and maximum standards, not to be 

 exceeded, framed. Using algebraic notation, we may state 

 the proposition that a milk with an initial coli content of not 

 greater than x, kept for not more than y hours at a tem- 

 perature not greater than t, will show a coli content always 

 below z. 



Leaving aside bacterial pollution subsequent to that at 

 the byre, if a coli content greater than z is obtained with a 

 vended milk sample, it means that either the initial pollution 

 exceeded x, the temperature was above t, or the period since 

 milking was greater than y hours. An increase beyond the 

 limit for any one of these factors is sufficient to cause z 

 to be exceeded. If x, y, and t are all allowed to be vari- 

 able, it is obviously hopeless to arrive at reliable deductions 

 from z enumerations. These variations cannot be denned, but 

 we can define their maxima. 



It is a reasonable proposition that unless milk is kept at a 

 temperature below that at which B. coli multiplies, all milk 

 should be sold within 24 hours. Granted this, y may be 

 taken as 24 hours as a maximum time. 



The temperature of milk, unless artificial cooling is em- 

 ployed, varies with the air temperature, so that in summer it 

 is higher than in winter, and multiplication of B. coli rapidly 

 takes place. 15 C. (59 F.) is a warm temperature for milk, 

 and if milk is initially cooled very ordinary precautions should 



