MILK 97 



should be carried out in a narrow beaker with a control tube 

 by the side. The result is conveniently expressed in terms of 

 degrees of acidity, each c.c. of -^ alkali required for 100 c.c. of 

 milk being I degree of acidity. Thus, if 50 c.c. of milk are taken 

 and 9 c.c. of alkali are required, then 100 c.c. would require 18 c.c., 

 and the milk has 18 degrees of acidity. Each c.c. of -^ alkali is 

 equivalent to O'OO9 gramme of lactic acid. 



Comparison of methods. 



The above five procedures are made use of by different 

 workers to estimate and measure the general bacterial contami- 

 nation of milk and their relative value for this purpose must be 

 considered. 



Since prejudicial milk contamination is essentially a bac- 

 teriological question it might be supposed that the estimation of 

 the number of bacteria in milk would be a very reliable index of 

 the degree of pollution to which it had been subjected and a 

 measure of the undesirable condition to which it had attained. 

 There are, however, many objections to this estimation for this 

 purpose, the following being the most important. 



(a) The number of bacteria enumerated varies greatly with 

 the exact laboratory procedures adopted. Unless these are 

 rigidly defined, widely different results will be obtained with the 

 same milk sample. The kinds of bacteria in milk are so varied 

 and so sensitive to slight variations of environment that quite 

 considerable differences are readily produced. 



(b} This estimation makes no distinction between bacterial 

 contamination before and after the milk leaves the cow. Since, 

 as shown above, bacteria are added from the teats and milk cistern 

 ordinary bacterial counts cannot be taken as measuring purely 

 external contamination. 



(c) While under rigidly defined conditions bacterial counts 

 are useful for samples collected at the byre this estimation is 

 useless to gauge the extent of outside pollution for ordinary 

 samples of market milk. The initial number of bacteria may be 

 masked by the great multiplication which has taken place sub- 

 sequently to milking. It is not in any way possible to gauge 



s. w. 7 



