152 SCIENCE AND PRACTICE OF CHEESE-MAKING 



Whether this suggested distribution of com- 

 pounds among the salts of milk is near the truth 

 or not, it emphasizes the fact that the matter is 

 one of no little complication. Some investigators 

 believe that the calcium phosphate exists entirely as 

 tri-calcium phosphate ; others, as the di-calcium com- 

 pound, probably on the basis of better evidence. The 

 presence of soluble acid phosphate and, probably, of 

 acid citrate also, accounts for a part of the acidity of 

 fresh milk. 



When milk is heated, the amount of soluble calcium 

 salts is decreased as the result of being changed to 

 insoluble forms. 



The presence of soluble calcium salts in milk is 

 essential to the coagulation of milk by rennet-extract 

 (p. 306). 



Acidity of milk. In this connection, we will call 

 attention to the acidity of fresh, normal milk. Milk 

 in which lactic acid has had no chance to develop has 

 the power of neutralizing alkalis and in that respect 

 behaves like a solution containing acid. 



The acidity of fresh milk varies with a number of 

 conditions, such as (i) the milk of the same ani- 

 mal at different times, (2) the milk of different 

 cows, and (3) with the stage of lactation, being 

 highest at the beginning of lactation and gradually 

 decreasing with the advance of the lactation period. 

 The acidity of fresh normal milk is caused by no 

 one substance, but is due to the presence of (i) 

 acid phosphates, (2) citrates and (3) casein. It 

 has been found to vary widely, from below 4 to over 

 10, expressed as cubic centimeters of one-tenth 

 normal alkali, but in most cases it is between 7 

 and 9. 



