牛奶及其用途

每个孩子都喜欢喝鲜牛奶。那香浓的味道，甜美的滋味是多么诱人！对于这样一种极为有用的饮品，你肯定想要了解更多吧? 我们喝的牛奶几乎全部来自奶牛的乳房。不过，人们还饮用其他动物的奶，例如羊、驴以及骆驼。

Every child likes to drink new milk. How pleasant it smells and how sweet it tastes! Do you not want to know something about this useful liquid? Nearly all the milk we use comes from the milk-bag or udder of the cow. But people also drink the milk of other animals, such as the goat, the ass, and the camel.

All these animals have a plentiful flow of milk, which is intended for their own young ones. Animals which bring up or nourish their young on milk are called mammals.

Milk is an excellent food. It has been called a "model " food, for it contains just the things that we need to nourish our bodies, and these are mixed in just the right way.

Milk is a white fluid a little heavier than water. We call it a fluid because it will flow. All liquids are fluids, and so are all gases; for both liquids and gases will readily flow from one place to another. But solids, such as wood and iron, do not flow.

Of what is milk composed? Let your new milk stand all night in a glass jug, say twelve inches in height; then in the morning you will see—or you ought to see—a rich, yellowish-white layer of cream one inch in thickness on the top of the milk. If your jug is six inches high, you should have a layer of cream half an inch thick.

This cream contains thousands of tiny balls of milk-fat, each ball having its own covering or skin. These balls rise to the top because they are lighter than the rest of the milk, just as a cork rises to the surface of water.

People skim off the cream and put it into a kind of wooden tub or barrel called a churn, in which they beat and dash it about by moving a plunger up and down, or by turning a handle.

Soon all the little balls of cream have their skins or coats broken, and then the tiny pieces of milk-fat inside them stick together and make a lump of yellow butter. You can make a little butter for yourself by putting a few spoonfuls of cream into a bottle, and shaking it as hard as you can for a while; but you will likely feel very tired before the butter appears.

The "skim-milk" which remains after the cream has been taken away is of a bluish-white colour; and while new milk sells for, perhaps, threepence per quart, skim-milk fetches only a penny. Yet there is good nourishment in skim-milk.

Let us add a little acid—vinegar will do—to some skim-milk. This makes it curdle , and we see little white jelly-like clots or "curds" forming. We will strain off the curds by squeezing the curdled milk through a piece of muslin. The white and solid curds that remain only need pressing to turn them into "cheese. "

The watery liquid that runs through the muslin is called "whey. " Dissolved in the whey there is a little milk-sugar, the substance that makes new milk taste sweet, and also some lime.

Now we know why new milk is such an excellent food. The milk-fat and the milk-sugar which it contains warm our bodies and help us to do our work, the curd helps to form our flesh, and the lime helps to form our bones. You can now see how good it is for us to drink milk, and why we add milk to many other kinds of food, such as tea, coffee, and cocoa.

Have you ever seen condensed milk ? This kind of milk is made by boiling fresh milk for some time, so as to evaporate part of the water contained in it, and then adding some sugar. The sweet, thick liquid thus made is then put into tins, which are carefully closed and sealed, so as to keep out the air. If kept in those air-tight tins, condensed milk will remain good for years.