436 



THE ACIDITY OF MILK 



By H. G. Chapman, M.D., B.S., Demonstrator of Physiology 

 in the University of Sydney. 



(From the Physiological Laboratory of the University of Sydney ). 



The reaction of milk is commonly stated in text-books to be 

 alkaline or neutral. This statement is made with regard to the 

 behaviour of litmus indicator towards cow's milk and for milk 

 examined in Europe and America. In the more recent accounts,* 

 however, authors note the amphoteric nature of the reaction 

 By this term attention is drawn to the fact that milk behaves as 

 an alkaline fluid towards indicators sensitive to basic substances 

 and as an acid fluid towards those sensitive to acid substances. 

 Red litmus paper is turned blue, and blue litmus paper red. 



This amphoteric reaction depends upon the dissolved consti- 

 tuents of the milk. It is principally related to the amounts of 

 dihydrogen and monohydrogen phosphate present in solution. 



When acids or alkalies are added to a solution of phosphates 

 an acid or alkaline reaction does not develop at once, and free 

 acid and free alkali are not present in the liquid until a certain 

 quantity of acid or alkali has been added. If, for example, 

 hydrochloric acid is added to a solution of mixed phosphates, 

 there is no free hydrochloric acid present at first, but the acid 



* Raudnitz, Eigeb. f. Physiol. 1903, Abt.l, S.300. 



