Butter

Butter consists almost entirely of separated milk fat. Churning causes the fat globules to unite into a solid mass. The cream is first allowed to ripen for some hours. This process gives the characteristic taste and odor, which is due to action of bacteria. When kept, butter tends to turn rancid, owing to the fermentation of a small quantity of casein present. Salt is added largely to prevent this change. Butter is very palatable, and one of the most digestible forms of fat.

Cheese

Cheese is made from full milk, skim milk or cream. It consists of the casein of the milk and more or less of the fat and mineral matters. The flavor is due to the action of enzymes in molds or bacteria. Cheese is a very concentrated nutritious food, and very thoroughly assimilated. It is not usually well borne by invalids and convalescents, however.

Cream

Cream is the fatty layer which forms at the top of milk which is allowed to stand undisturbed. It contains the fat of the milk, water, some protein, carbohydrates and mineral matter. The percentage of fat is exceedingly variable, ranging from about 16 per cent. to 40 per cent. The cream which rises on milk after 24 hours is called gravity cream and contains about 16 per cent. fat. The richer creams are obtained by centrifugalizing the milk. Cream in large quantities is less easily digested than an equal amount of whole milk because of the large amount of fat, but this form of fat is easily digested compared with other food fats, and hence is often ordered by physicians.

Curds

When milk sours, owing to the formation of lactic acid by the action of lactic acid bacteria upon the sugar, or when the enzyme rennet is added to fresh milk at body temperature, and the clot is stirred, the curds separate from the whey.

Curds consist of coagulated casein, which commonly carries with it the fat; gentle heat facilities this separation, but a high temperature renders the curd tough and indigestible; with or without the addition of cream, curds are used as cottage cheese. It is a very cheap, nutritious food.

Junket

If sweet milk is allowed to stand undisturbed after the addition of rennet, the thickened, custard-like mass is called junket. This is a valuable method of using milk, especially for invalids, children, and those who from personal idiosyncrasy cannot drink milk.

Whey

This substance may be either sweet when formed by junket or sour when otherwise produced. It contains most of the lactose, lactalbumin and ash constituents of the milk, but has so little nutritive value, owing to the removal of the milk fat and casein, that it may be regarded as a beverage. See page 185.

Sour Milk

When whole milk sours, with the formation of "curds and whey," the entire product is known as clabber or bonny-clabber. It is wholesome, and nutritious, since it contains all the ingredients naturally present in milk. By some it is used as a beverage. When not so relished, it can be used to good advantage in cooking, adding its nutritive value to any dish in which it is incorporated.