100 Dairy Bacteriology. 



For high grade pasteurizing, it is advisable to purify the 

 milk by passing it through a separator, unless the raw 

 product is secured under conditions that exclude the ad- 

 mission of dirt. 



The age of the milk and the conditions under which it 

 has been kept should also be carefully noted. Old milk 

 is almost always richer, not only in bacterial germs but 

 in the latent spore forms as well; so the fresher the milk 

 the fewer bacteria it will have, and therefore the pasteur- 

 izing process will be the more complete. 



101. Selecting" milk by acid test. The true stand- 

 ard for selecting milk for pasteurization should be to de- 

 termine the actual number of bacterial spores that are 

 able to resist the heating process, but this method is im- 

 practicable under commercial conditions. 



The following method while only approximate in its 

 results 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 suitability of the milk for pasteurizing purposes. 

 Biological tests were carried out in the author's labora- 

 tory 1 on milks having a high and low acid content, and 

 it was shown that the milk with the least acid as a rule 

 was the freest from spore-bearing bacteria. 



This acid determination can be made at the weigh-can 

 by employing the Farrington alkaline tablet which is 

 used in cream-ripening. Where milk is pasteurized under 

 general creamery conditions, none should be used con- 

 taining more than 0.2% acidity. If only perfectly fresh 

 milk is used the amount of acid will generally be about 

 0.15% with phenolphthalein as indicator. 



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



