CHEMISTRY OF COWS' MILK 13 



MILK-SUGAR 



Milk-sugar, also called lactose, is present in cows' 

 milk in solution. In general composition, it resembles 

 ordinary sugar, but it is less sweet and less soluble 

 in water. The amount of sugar in milk varies from 

 below 4 to over 6 per cent, and averages about 5 per 

 cent. Its importance in dairy work, especially in con- 

 nection with the manufacture of butter and cheese, 

 comes from the ease with which it is converted into 

 lactic acid by certain forms of bacteria. In the ordi- 

 nary souring of milk, the amount of milk-sugar de- 

 creases somewhat more than one-fourth and there is 

 formed as a maximum about 0.9 per cent, of lactic 

 acid. More acid may be formed after some time. 

 Hence, sour milk, when two or three days old, con- 

 tains only 3.5 to 4 per cent, of milk-sugar. The sugar 

 of milk passes largely into the whey in cheese-making 

 and forms over 70 per cent, of the solids in whey. 

 The milk-sugar of commerce is usually prepared by 

 evaporating whey and purifying the impure product 

 first obtained. 



THE SALTS OF MILK 



The salts of milk, commonly included under the 

 term "ash," are present in only small amounts, 0.7 

 per cent, on the average ; but they have important 

 relations to milk and its products. Our knowledge of 

 these compounds is very incomplete. The salts of milk 

 are commonly spoken of as the ash or mineral constitu- 

 ents. This conception is somewhat misleading, be- 

 cause the materials appearing in the ash of milk are, 

 to some considerable extent, combined in organic com- 

 pounds, instead of existing in the milk as separate 



