I20 MODERN METHODS OF TESTING MILK 



Variation in specific gravity of milk. — Since the 

 specitic gravil}' uf milk largely depends upon the 

 amount of solids in it heavier than water, the specific 

 gravity should vary, since we know that the amount 

 of solids in milk varies considerably. And so we find 

 the specific gravity of some milks below 1.030 and of 

 some others above 1.035; ^^^t most normal milks from 

 herds of cows liave specific gravities lying between 

 1.030 and -1.034. 



The solids of milk heavier than water are casein, 

 albumin and milk-sugar. They constitute the solids- 

 not-fat of milk and have a specific gravity of about 

 1.500. 



Effect of milk-fat on specific gravity of milk. — 

 ]\Iilk-fat is lighter than water, its specific gravity be- 

 ing about 0.900 compared with that of water as i.ooo. 

 Therefore, an increase of fat in milk, relative to the 

 other solids, lowers the specific gravity of milk. Thus, 

 by adding cream to normal milk, we can r^ike its 

 specific gravity lower than that of normal milk. On 

 the other hand, by removing fat from milk, we in- 

 crease the specific gravity, because we remove what 

 is lighter, and leave what is heavier, than water. 



Effect of adding water and other substances to 

 milk. — Water being lighter than milk, the specific 

 gravity of milk is lowered by addition of water. There- 

 fore, it is easily possible by removing cream from nor- 

 mal milk to increase the specific gravity and then, by 

 adding water, to decrease the specific gravity again 

 to that of normal milk. The addition of sugar, salt or 

 any similar substance to milk increases the specific 

 g-ravitv. Since water has been the most common adul- 



