SECRETION. 339 



Salts (chiefly potassium, 

 sodium, and calcium, 

 chlorides and phos- 

 phates), 



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 The curdling of milk is due to the coagulation 

 of the casein which is kept in solution under normal conditions by the 

 alkaline calcium phosphate. On the addition of an acid, such as acetic, 

 the casein is precipitated. This occurs, too, if it be allowed to stand for 

 some time, its reaction becomes acid: in popular language it "turns 

 sour/' The change appears to be due to the conversion of the milk-sugar 

 into lactic acid, by means] of a special micro-organism, Bacterium lactis; 

 this causes the precipitation (curdling) of the casein: the curd contains 

 the fat globules: the remaining fluid (whey) consists of water holding 

 in solution albumen, milk-sugar, and certain salts. The same effect is 

 produced in the manufacture of cheese, which is really casein coagulated 

 by the agency of rennet (p. 254). When milk is boiled, the scum which 

 forms consists chiefly of serum-albumin. 



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

 pancreatic, 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 carbo- 

 nates of potassium, sodium, and calcium. 



Traces of iron, fluorine, and silica are also found, and the gases, car- 

 bonic acid, oxygen, and nitrogen. 



