The cream is the carbonaceous part, and forms ten or twelve per cent of the whole. It consists of very small globules of fat, or butter, invisible to the naked eye, surrounded by a fine membrane of caseine or albuminous matter. These are dispersed in the milk at first; but as they are lighter than the other parts, they rise to the top of the milk in the form of cream. When the cream is churned, the membrane of each globule is ruptured, causing the butter to cohere in a separate mass. If all this caseine, or albuminous matter, be not removed, the butter soon becomes rancid. The butter has more margarine than oleine, and is therefore hard and firm instead of fluid. It contains butyric acid, which gives it its peculiar flavor.
The curd is the albuminous part of the milk, which separates in a solid form whenever milk sours or curdles. This separation is occasioned by the action of the oxygen in the air upon the nitrogen in the caseine, causing a portion of the caseine to ferment. This ferment acts upon the sugar of the milk, and converts it into lactic acid; and this acid acts as any other acid would, and causes the sourness of the milk. The caseine is insoluble in water, but in the milk it is combined with soda, and this compound is soluble. So, when the milk sours, and a sufficient quantity of acid is formed, it seizes upon the soda, takes it away from the caseine, and forms lactate of soda. The caseine, being thus set free, shrinks, and gathers into an insoluble curdy mass. This separation is spontaneous; but it is often caused artificially, by the use of an acid, - usually rennet, which is the lining membrane of a calf's stomach. The curd thus separated is pressed and prepared in various forms of cheese. It contains the nutritious elements of the milk in a condensed, but somewhat indigestible form. Cheese is more digestible when made from fresh than from skimmed milk.
The whey is the water of the milk that separates from the curd In souring, and contains dissolved in it the sugar of milk and the saline and mineral ingredients. It is much better to use milk in its natural state than to use the butter or cheese obtained from it. It is the cheapest form in which animal food can be obtained, and should be used freely, especially by the young or weak.
Condensed milk, when well prepared, is convenient and valuable as an article of diet, if fresh milk cannot be obtained. It can only be made from pure milk, and is therefore perfectly harmless.