CHAPTEE XII 



CHEMICAL TESTS TO ESTIMATE THE PURITY AND EEESHNESS 



OF MILK 



Acidity EsTurATiONS 



Statements are conflicting as to the reaction of quite fresh 

 cows' milk. This is partly to be explained by the fact that 

 the reaction largely depends upon phosphates, and these vary 

 in their reaction towards different indicators. Using phenol- 

 phthalein as the indicator, the writer has examined the 

 acidity of a large number of fresh milk samples, both of mixed 

 milk and milk directly obtained from individual cows. In 

 every instance with this indicator the reaction has been acid. 

 Swithenbank and Newman^ record that 92'3 per cent of a 

 herd of 2 6 healthy cows gave acid milk, using litmus paper as 

 the indicator. Courant found that fresh milk reacted alkaline 

 to lacmoid and acid to phenol-phthalein. 



It may be accepted that fresh cows' milk is, in general, 

 acid to phenol-phthalein and litmus. The acidity is due in 

 small part to carbonic acid, but mainly to mono- and di-basic 

 phosphates. According to Courant, it is in part also due to 

 dicalcium caseinogenate. 



Milk when kept becomes more acid, due to the production 

 of lactic acid from the lactose of the milk by the action of 

 bacteria. The lactose is not directly fermented, but has to be 

 first converted into glucose and galactose. In the souring of 

 milk as it takes place under ordinary conditions, the acidity is 

 not entirely due to lactic acid ; small quantities of acetic, 

 butyric, and succinic acids are produced. Only when a pure 



1 'Text-book, 1903, p. 154. 



202 



