CHAPTEE VII. 

 TESTING THE ACIDITY OF fllLK AND CREAfl. 



125. Cause of acidity in milk. Even directly after milk 

 is drawn from the udder it will be found to have an acid 

 reaction, when phenol phtalein is used as an indicator. 1 

 The acidity in 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 therefore found. 

 When the milk is received at the factory it will rarely 

 test less than .10 per cent, of acid, calculated as lactic 

 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 contains .30 to .35 per 

 cent, of acid. 



126. 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 the same milk at dif- 

 ferent times have shown that there is a direct relation 



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

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



