CHAPTER VII. 

 TESTING THE ACIDITY OF MILK AND CREAM. 



128. Cause of acidity in milk. Even directly after 

 milk is drawn from the udder it will be found to have 

 an acid reaction, when phenolphtalein is used as an in- 

 dicator. 1 The acidity of fresh milk is not due to the 

 presence of free organic acids in the milk, like lactic 

 or citric acid, but to acid phosphates, and possibly also 

 in part to free carbonic acid gas in the milk or to the 

 acid reaction of casein. Even in case of so-called sweet 

 milk, nearly fresh from the cow, a certain amount of 

 acidity, viz., on the average about .07 per cent., is there- 

 fore found. When the milk is received at the factory 

 it will rarely test less than .10 per cent, of acid, calcu- 

 lated as lactid acid; some patrons bring milk day after 

 day that does not test over .15 per cent, of acid; that 

 of others tests from .20 to .25 per cent., and some lots, 

 although very rarely, will test as high as .3 of one per 

 cent, of acid. It has been found that milk will not 

 usually smell or taste sour or "turned," until it con- 

 tains .30 to .35 per cent, of acid. 



129. The acidity in excess of that found normally in 

 milk as drawn from the udder, is due to other causes 

 than those described. Bacteriological examinations of 

 milk from different sources and of milk of the same 

 origin at different times have shown that there is, roughly 

 speaking, a direct relation between the bacteria found 



1 Freshly drawn milk shows an amphoteric reaction to litmus, i. e., 

 it colors blue litmus paper red, and red litmus paper faintly blue. 



