MILK AS A FOOD 41 



Milk-sugar. There is a special form of sugar in milk 

 known as lactose. Lactose is peculiar to milk; that is, it is 

 found nowhere else in nature. It does not exist in the 

 blood or in any of the internal organs of the body. Sugar 

 exists in the blood in the form of dextrose, and this must be 

 transformed into lactose by the chemical activity of the 

 secreting epithelial cells in the mammary gland. 



Lactose, or milk sugar, belongs to the group of foods 

 known as carbohydrates. These substances serve as fuel 

 to yield force and energy in the form of heat and muscular 

 power. They do not build up new tissue in the same way 

 that the proteins or albuminous substances do. 



When milk sugar is heated above the boiling-point of 

 water, it changes to a brownish color; that is, the lactose is 

 partially charred and a substance resembling caramel is 

 formed. It is due to this fact that, when milk is boiled for 

 a long time or heated to high temperatures, it turns a 

 brownish color. 



There is much less milk sugar in cow's milk than in hu- 

 man milk. Cow's milk contains on the average ordy about 

 4.5 per cent of lactose: woman's milk, about 7.0 per cent. 

 This is why it is customary to add sugar to cow's milk used 

 in infant feeding. The percentage of lactose is very con- 

 stant in the milk of any particular species of animals at all 

 times throughout lactation. It is therefore rarely neces- 

 sary in cKnical laboratories to make a special analysis to 

 determine the amount of lactose. For public health pur- 

 poses the percentage of lactose often furnishes a better 

 guide to the adulteration of milk than the percentage of 

 fat. 



The milk sugar interests us particularly for the reason 

 that it is readily acted upon by the bacteria which grow in 

 milk. It is split up or fermented by these micro-organisms 

 into lactic acid. The lactic acid thus formed causes the 

 milk to turn sour and curdle, by throwing the casein out of 

 solution. 



