The fat in milk is in small emulsified droplets within a thin albuminous sheath. When allowed to stand in a cool place it rises to the top.
Besides casein, there is a certain amount of albumen in milk,—about one-seventh of the total amount. This is called lactalbumin.
A part of the digestion of the casein is performed by pepsin in the stomach and a part by the trypsin of the pancreatic juice.
Digestion of Milk. The larger part of the digestion of the milk sugar is performed by the pancreatic juice; yet it is partly acted upon by the saliva. There is little chance for the saliva to act upon the milk sugar in the mouth, however, as very little saliva is mixed with the milk. This constitutes another objection to the diet of all milk, and is an argument in favor of drinking milk slowly and holding it in the mouth until it is mixed with saliva. It is one reason, also, why children should be given bread broken in the milk, instead of a piece of bread and a glass of milk. By swallowing the milk slowly, the curds formed in the stomach are smaller and the milk is more thoroughly digested.
When the fat (cream) is removed milk digests more readily, so that in case of delicate stomachs skimmed milk, clabbered milk or buttermilk are often prescribed instead of sweet milk. Boiled milk is also more easily digested by some because of the lactalbumin which is separated and rises to the top in a crinky skum. The casein is also more readily digested in boiled milk, forming in small flakes in the stomach instead of in curds.
When one takes from two to three glasses of milk at a meal, less solid food is needed, because the required nutriment is partially supplied with the milk. One reason why milk seemingly disagrees with many people, is because they lose sight of the fact that milk is an actual food, as well as a beverage and they eat the same quantity of food in addition to the milk that they eat if drinking water. This is the reason that milk seems to make some people bilious and causes constipation. It is due to too much food rather than to any quality in the milk.
Constipation may be occasioned by drinking milk rapidly so that large curds are formed by the acids in the stomach, rendering it difficult of digestion. The constipating effect will be overcome by lessening the quantity of food and by the addition of limewater to the milk.
To prepare limewater put a heaping teaspoon of slaked lime into a quart of boiled or distilled water; put into a corked bottle and shake thoroughly two or three times during the first hour. Then allow the lime to settle, and after twenty-four hours pour or siphon off the clear fluid.
Barley water or oatmeal water added to milk also prevent the formation of curds.
In young babes the milk is curdled, or the casein separated from the water and sugar, not by hydrochloric acid, but by a ferment in the gastric juice, known as rennin. It is the rennin, or rennet, from the stomachs of young calves and young pigs, which is used to coagulate the casein in cheese factories.