332 HAND-BOOK OF PHYSIOLOGY. 



TABLE OF THE CHEMICAL COMPOSITION OF MILK. 



Human. Cows. 



Water 890 ... 858 



Solids 110 142 



1000 1000 

 Proteids, including Casein and 



Serum-Albumin . . 35 . .68 



Fats or Butter . . . .25 . . .38 



Sugar (with extractives) . 48 . .30 



Salts 2 6 



110 142 



When milk is allowed to stand, the fat globules, being the lightest 

 portion, rise to the top, forming cream. If a little acetic acid be added 

 to a drop of milk under the microscope, the albuminous film coating the 

 oil drops is dissolved, and they run together into larger drops. The same 

 result is produced by the process of churning, the effect of which is to 

 break up the albuminous coating of the oil drops: they then coalesce to 

 form butter. 



Curdling of Milk. If milk be allowed to stand for some time, its 

 reaction becomes acid: in popular language it "turns sour/' This change 

 appears to be due to the conversion of the milk-sugar into lactic acid, 

 which causes the precipitation of the casein (curdling): the curd con- 

 tains the fat globules: the remaining fluid (whey) consists of water hold- 

 ing in solution albumin, milk-sugar and certain salts. The same effect 

 is produced in the manufacture of cheese, which is really casein coagu- 

 lated by the agency of rennet (p. 248). When milk is boiled, a scum of 

 serum-albumin forms on the surface. 



Curdling Ferments. The effect of the ferments of the gastric, pan- 

 creatic, and intestinal juices in curdling milk (curdling ferments) has 

 already been mentioned in the Chapter on Digestion. 



The salts of milk are chlorides, sulphates, phosphates, and carbonates 

 of potassium, sodium, calcium. 



