MAMMAKY SECKETION. 619 



the other third remains in suspension in the buttermilk. Churning is 

 best accomplished at about 14° C, with thirty to forty strokes of the 

 churn per minute. 



The average composition of buttermilk may be placed as follows : 

 Fat, 0.50 to 0.75 per cent. ; casein, 3.60 per cent. ; albumen, 3.1 per cent. ; 

 sugar, 0.52 per cent. ; ash, 0.52 per cent. ; water, 91.7 per cent. 



The amount of cream which may be obtained from milk varies very 

 considerably in different animals and under different conditions, and will 

 be subsequently referred to. 



Usually about 80 per cent, of the entire amount of fat contained in the 

 milk passes into the cream, and if the average percentage of fat in milk 

 be placed at 3.5 per cent., the fat in cream would amount to 2.7 per cent. 



When butter becomes rancid the volatile fatty acids are set free and 

 acrolein and formic acid are formed from the glycerin. 



Skimmed milk has a higher specific gravity than unskimmed milk, 

 from the fact that the lighter constituents, that is, the oils, have been 

 largely removed. The specific gravity of skimmed milk may rise to 

 1037, or even higher. Its composition ma}' be placed as follows : — 



Water, . . . ... 89.65 



Fat, . . . ... 0.79 



Casein, . . . 3.01 



Sugar, . . . . . . 5.72 



Salts, . . ... 0.83 



5. The Inorganic Constituents of Milk. — In addition to the albu- 

 minoids, fats, and carbohydrates contained in milk, the mineral constitu- 

 ents necessary for the support of the animal body are also present, 

 their average amount being about as follows : — 



Phosphoric acid, 



Chlorine, 



Calcium oxide, 



Potassium, 



Sodium, 



Magnesium 



28.31 

 10.34 

 27.00 

 17.34 

 10.00 

 4.07 



Ferric oxide, . ... 0.62 



Oxygen, nitrogen, and carbon dioxide are also present ; and since 

 these gases may be entirely removed by pumping, they are, therefore, in a 

 condition of solution in the milk. 



Nitrogen is present in 0.7 volumes per cent. ; oxygen, 0.1 per cent. ; 

 carbon dioxide, 7.7 per cent. 



6. Variations in the Quantity and Composition of Milk. — The 

 variations which occur in the quantity and composition of milk are 

 largely dependent on the quantity and composition of the food; in- 

 sufficient food. leads to a reduction in both the absolute quantity and 

 solid constituents of the milk. In addition to these conditions, to be 



