A fluid secreted by the females of the class mammalia. Although the proportions of its constituents vary in different animals, its general properties are the same in all. When this fluid is allowed to stand for some time, it undergoes spontaneous changes, and is resolved into its component parts: a thick yellowish substance collects on the surface, which is cream, while the milk beneath becomes thinner than before, and is of a pale bluish colour. When cream is kept for some days without being disturbed, it gradually becomes thicker, till at last it acquires the consistence of cheese; so that one method of making cream cheese is merely by putting cream into a linen bag, and leaving it there till it becomes solid. When cream is shaken, it is resolved into its component parts. The process by which this is accomplished is called churning, whence two products result, - butter and butter-milk. In the making of butter, cream is allowed to stand for some time, during which an acid is generated. It is then put into a churn and agitated, by which the butter is gradually separated; what is left (the butter-milk) has a sour taste, but not so much as that of the cream before churning.
Butter is also sometimes made from cream which has not become sour, but the process is much more tedious, owing to the want of acid to favour the separation. Butter is merely an animal oil, solid at a natural heat, but held in solution, in milk, by some of the other substances; thus obtained, however, the butter is not pure, and requires much washing in water to free it from its impurities, and, by the subsequent addition of salt, it may be kept good a long time. Milk from which butter has been taken undergoes spontaneous changes; it becomes much sourer, and assumes a gelatinous form. When heated, the fermentation of this coagulum is hastened, and, by the addition of certain substances, it very soon takes place; thus acid and spirits of wine curdle it, which is owing to the albumen it contains being acted upon by them in the same way as blood or white of eggs. By far the most powerful coagulator, however, is the substance called rennet. When the milk is previously heated, and rennet added, it instantly coagulates. If the coagulum be cut, a thinnish fluid oozes from it; and if it is put into a bag and squeezed, the whole of this is forced out, and a whitish tough matter remains; the former is whey, and the latter curd.
On this depends the process of making cheese, which varies in richness according to the mode followed in preparing it. When milk is heated gradually, and merely to the temperature at which it curdles, and the curd freed gently from the whey, it retains almost the whole of the cream, which adds to its richness and flavour; but when it is curdled quickly, and the whey is speedily removed by cutting the curd, a great deal, or nearly the whole of the cream is carried off, and the cheese is poor, and has not the rich flavour of cheese made in the other way. The latter is the method usually followed in Scotland, where both cheese and butter are obtained from milk, - the whey procured in the process yielding a considerable quantity of the latter; and hence the comparative poorness of Scotch cheese. In making cheese, having obtained the curd, and freed it from its whey, the remaining part of the process is merely to subject it to pressure, by which the whole of the whey is forced out; the colouring ingredient is generally annotta, to give it the desired tint.
Milk, according to the analysis of Berzelius, consists of -
The same chemist found cream of spec. grav. 1.0244, by analysis to consist of butter 4 5, cheese 3.5, whey 92.