236 MILK I ITS ORIGIN AND COMPOSITION 



essential that three separate terms with limited conno- 

 tation should be used for them. 



The protein caseinogen is an acid body, capable of* 

 uniting with soda, potash, and lime to form salts, those of 

 the two alkalis being soluble in water. The condition 

 in which it exists in milk is a matter of dispute, but the 

 most reliable evidence seems to support the view that it 

 is combined with calcium as calcium caseinogenate along 

 with a certain amount of calcium phosphate. 



From the fact that caseinogen or its compounds do 

 not pass through a porcelain filter, and can be collected 

 as a slime by centrifuging milk, it would appear that these 

 bodies are present in the form of minute particles sus- 

 pended in the milk serum. 



Calcium caseinogenate can be thrown down as a 

 precipitate by saturating milk with -sodium chloride, 

 magnesium sulphate, or other agents used for " salting 

 out " proteins (p. 86). 



When lactic, acetic, or other acids are added to milk, 

 a reaction takes place, which may be represented 

 thus : 



*. Cv* . Hfw 



Calcium caseinogenate + lactic acid = calcium lactate + caseinogen 



the free caseinogen being a substance insoluble in 

 water, appears as a precipitate which we may call 

 acid-curd. Such a curd, of course, is simpler than the 

 caseinogenate from which it was derived ; in a pure 

 state it contains no calcium, although as usually obtained 

 it has entangled within it some of the milk-serum and 

 its dissolved calcium phosphate, as well as the fat- 

 globules of the milk. 



Rennet contains a coagulating enzyme, rennin, which, 

 when added to warm unboiled milk, precipitates a 



