EXPERIMENT STATION BULLETINS. 



671 



because it measures the number of bacteria rather than the by-products 

 of their growth; and because of its simphcity and practicable advantages." 

 The brom-thymol blue test mentioned above can be compared in no way 

 with the pH method of testing milk given in this and previous papers for 

 the following rea.sons : 



(1) The milk-broth-indicator mixture is incubated for only 2 hours in- 

 stead of the 8 hours necessary in our method. Changes in color which are 

 used as an index of the condition of the milk seldom develop before the third 

 hour. 



(2) The readings were not made by comparing unknowTis with standards 

 of known pH. 



As it was impossible to make microscopic examination of the samples of 

 milk tabulated in Table VII at the time the other tests were made the data 

 presented in Table VIII were collected to show the relation between the 

 actual keeping quality of the milk and the microscopic determination of 

 individual bacteria and groups of bacteria. 



Table VIII 



A comparison of the keeping quality of milk and the microscopic deter- 

 mination of individual bacteria and groups of bacteria. 



The microscopic counts of the sixty-one samples tabulated above show 

 considerable error if used to predict the keeping quality of the samples of 

 milk which they represent. 



SUMMARY AND CONCLUSIONS 



The technique for determining the keeping quality of milk by the pH 

 method is presented. 



Apparatus for simphf jdng the method which have been devised at this lab- 

 oratory are described. 



In the past a number of investigators have come forward with new meth- 

 ods for testing milk. The tendency has been to try to make new methods 

 check as closely as possible with the bacterial count, and several good 

 methods for the determination of keeping quality have been thrown into 

 the discard because of their failure to check with the counts. The writer 

 makes no claims that the pH method checks closely with the bacterial 

 count, though results show that it does check in a general way. 



