Milk is easily digested by most persons. It is usually spoken of as a liquid food, but when it reaches the stomach it is converted at once into a finely divided solid. Its casein is curdled and precipitated in flocculent particles by rennin, the milk-curdling ferment of the stomach. The gastric acids coagulate it also. When this process goes on with unusual rapidity, instead of flocculent particles of casein there may be formed large masses of cheese-like material that will prove slow of digestion. This often occurs in the stomachs of both infants and adults whose digestion is enfeebled. Cow's milk curdles in larger particles than human milk, and is therefore more difficult of digestion. After milk is curdled in the stomach its components are digested as are other foods.

It is often important to prevent the formation of large curds in the stomach. Adults who feel uncomfortable after drinking milk can frequently be taught to like it and to dispose of it readily. It must be given at first in small doses - one or two tablespoon-fuls every quarter or half hour - and gradually in larger quantities and at longer intervals. Patients should be instructed to drink milk slowly, sipping or 'chewing' it; often hot milk will be found better than cold milk for this purpose, and those who become accustomed to the flavor of heated milk usually prefer it. Giving it in small quantities prevents it from forming large cheese-like masses in the stomach; and the same purpose is accomplished by slow sipping, and thorough mixing with the saliva. Lime-water is frequently added to milk to prevent it from coagulating into large chunks of casein. Usually two or three tablespoonfuls are added to a glass of milk. Lime-water partly neutralizes the acids in the stomach and prevents them from still further contracting or coagulating the milk curds. Other antacids are useful for this purpose. Sodium citrate is added to milk to prevent coagulation which it does by converting the soluble lime salts into insoluble citrate. For this purpose, add to milk, one-fifth of its bulk of a 25 per cent, solution of sodium citrate. It is scarcely detectable by taste. Aerated waters, such as Vichy and Selters, when added to milk increase its palatability for many persons and combat its doting, in part by diluting it and in part by making the clot friable. Dilution of the milk also insures dilution of the acids in the stomach. Barley-water and flour ball are used to prevent the formation of large, firm clots in the stomach. They do this by mingling with the particles of casein the mucilaginous particles of starch, which prevent the clot from shrinking into a tough mass.

Boiled milk does not clot so firmly as raw milk out of the stomach, but in the latter it does. Boiling renders some of the lime-salts less soluble, but when they are introduced into the stomach, they are redissolved by its acid.

It is interesting to note the time that milk remains in the stomach after it is taken. This is determined by drinking a measured quantity and by washing the residue out of the stomach at different times. It has been found1 that 602 c.c. of raw milk disappeared from the stomach in three and one-half hours. 602 c.c. of skimmed milk disappeared from the stomach in three and one-half hours. 602 c.c. of sour milk (buttermilk) disappeared in three hours. 602 c.c. of boiled milk disappeared in four hours.

However, the correctness of these figures is disputed by all good observers and it is still undecided that there is a difference in the digest-abilility of boiled and raw milk.

Absorption

Milk is not completely absorbed from the intestine. In adults it leaves a considerable residue. In infants and children it is much more completely taken up. In childhood the residue is about 4 per cent., as compared to 10 per cent, in adults, and is still less in infants. If two liters of milk are the only food taken by an adult, the residue will be from 5 to 8 per cent.; if three liters are taken, from 10 to 11. 16 per cent, will remain unabsorbed. When milk is taken with other food, a similar residue is left; for instance, on an exclusive milk diet 92.1 per cent, of protein and 86.3 per cent, of carbohydrate will be digested and absorbed, and upon a bread and milk diet 97.1 per cent, of protein and 98.7 per cent, of carbohydrate will be digested and absorbed.

1 Hutchison, "Food and the Principles of Dietetics," p. 122, London, 1911.