Preservation of Milk. 121 



one edge two or three drops, of cream at intervals of an 

 inch or so. Then incline piece of glass at such an angle 

 as to cause the cream to flow down surface of glass. The 

 cream, having the heavier body or viscosity, will move more 

 slowly. If several samples of each cream are taken, then 

 the aggregate lengths of the different cream paths may be 

 taken, thereby eliminating slight differences due to condi- 

 tion of glass. 



Pasteurizing details. While the pasteurizing process is 

 exceedingly simple, yet, in order to secure the best results, 

 certain conditions must be rigidly observed in the treat- 

 ment before and after the heating process. 



It is important to select the best possible milk for pas- 

 teurizing, for if the milk has not been milked under clean 

 conditions, it is likely to be rich in the spore-bearing bac- 

 teria. Old milk, or milk that has not been kept at a low 

 temperature, is much richer in germ-life than perfectl} r 

 fresh or thoroughly chilled milk. 



The true standard for selecting milk for pasteurization 

 should be to determine the actual number of bacterial 

 spores that are able to resist the heating process, but this 

 method is impracticable under commercial conditions. 



The following method, while only approximate in its re- 

 sults, will be found helpful: Assuming that the age or 

 treatment of the milk bears a certain relation to the pres- 

 ence of spores, and that the acid increases in a general way 

 with an increase in age or temperature, the amount of acid 

 present may be taken as an approximate index of the suit- 

 ability of the milk for pasteurizing purposes. Biological 

 tests were carried out in the author's laboratory * on milks 

 having a high and low acid content, and it was shown that 



i Siiockley, Thesis, Univ. of Wis., 1896. 



