Milk. Milk consists of water holding in solution casein or cheese, sugar of milk, various salts, and, in suspension, fatty matter in the form of myriads of semi-opaque globules, to which the colour and opacity of milk is due.
Skim-milk, butter-milk, cream, butter, curd3 and whey, cream-cheese, and ordinary cheese, are mere modifications of milk, differing only from each other either in the abstraction of one or more of its constituents, or else in the variation of their proportions.
Butter differs little from cream, but is more completely separated from the sugar, cheese, and salts ; and the fat globules in place of being free and distinct have all run together, so as to form a semi-solid substance.
Cheese is made from skim-milk, entire milk, or cream; it consists of the casein and butter. The cheese prepared from skim-milk containing the smallest quantity of butter; that from entire milk, as Cheshire cheese, a larger quantity ; and that from cream, as Stilton cheese, the most of all.
Now, although the casein and sugar of milk, as well as the butter, vary in quantity in different cases, yet ordinarily the quality of milk is estimated by the amount of cream which it yields.
For the determination of the quality of milk it is, however, requisite not only to ascertain the amount of cream which it yields, but also to take the specific gravity or density of the milk.
In estimating the specific gravity of any liquid, distilled water is taken as the standard, being reckoned at 1,000. Now milk, holding as it does in solution a large quantity of sugar, casein, and salts, is of course much heavier than water; and it is stated that milk of good quality should have a specific gravity of about 1,031. But milk, as we have seen, contains also a large proportion of fatty matter, and which, being much lighter than distilled water, serves when equally suspended through the fluid, to decrease its density. The larger therefore the quantity of cream, the lower will be the specific gravity - some milks, owing to the large quantity of cream contained in them, possessing a density of only 1,020, or even less.
We have said that the butter is suspended in milk in the form of innumerable droplets of various sizes; in rich milk, these are particularly abundant, so that when a drop of such milk is viewed under an object-glass of high magnifying power, the field is crowded with myriads of these globules, as shown in fig. 1.
Fig. 1-Good Milk.
In an impoverished milk, the globules will be smaller in size and fewer, and the field of vision will present the appearance of fig. 2
Fig. 2.-Poor Milk.
Fig. 3. - Curd of Milk.
Of all articles of food, none is so much adulterated as milk. We find different writers naming a variety of ingredients as commonly employed in the adulteration of it - amongst which may be mentioned flour, milk of almonds, gum arable, gum tragacanth, chalk, turmeric carbonate of soda, sugar, emulsion of hemp-seed, and sheep and horses' brains, rubbed up with water into au emulsion.