CH.XIV BACTERIAL POLLUTION OF MILK 229 



Estimation of the total number of bacteria. 



,, number of B. coli and allied organisms. 



,, ,, ,, streptococci. 



B. enteritidis sporogenes spores. 



Procedures such as the estimation of the degree of acidity and 

 the amount of sediment, which indirectly indicate the bacterial 

 contamination, have been dealt with in Chapters XI. and XII. 

 The advantages and limitations of these four procedures will 

 be briefly considered. 



ESTIMATION OF THE NUMBER OF BACTERIA 



Prejudicial contamination of milk is essentially one with 

 bacteria-holding substances, and milk is dangerous because of 

 the bacteria in it. The estimation of the number of bacteria in 

 milk should serve, therefore, as a reliable index of the degree 

 of pollution to which it has been subjected, and as a measure of 

 the undesirable condition to which it has attained. On the 

 other hand, numerous objections may be raised to this estima- 

 tion, which must be considered. 



(a) Actual estimations vary greatly with the details of 

 the laboratory procedure adopted. To estimate the total 

 number of bacteria is, with present methods, an absolute im- 

 possibility. The bacterial count which is obtained is merely 

 that number of organisms which will develop upon the par- 

 ticular medium used within the arbitrary time selected and at 

 the particular temperature employed. Variation in any of the 

 factors will cause variation in the resulting bacterial count. 



So varied are the kinds of bacteria in milk, and so sensitive 

 are many of them to the action of temperature, chemical re- 

 action, and nutritive material, that the differences met with, 

 due to variations in the methods employed, are not slight 

 and unimportant but very marked. They are very much 

 greater than the differences met with in water enumerations 

 obtained by different methods. Indeed, these laboratory 

 details are so important that very little practical and no 

 scientific utility is attained by comparing the number of 

 bacteria in milk in different places when the exact enumeration 

 procedures are different or not recorded. 



To say that milk shall contain not more than so many 



