英文科學(xué)讀本 第五冊·Lesson 29 Animal Food—Milk, Butter, Cheese, Eggs

Lesson 29 Animal Food—Milk, Butter, Cheese, Eggs

We have already been taught to regard milk and eggs as perfect foods, said Mr. Wilson. "They are the distinctive foods which Nature supplies for the sustenance of the young, helpless, growing progeny of her creatures. One of the earliest conquests of civilized man was accomplished when he domesticated the useful cow, and compelled her to contribute to his daily needs by supplying him with the milk, intended originally for the support of her own offspring.

In all civilized countries today milk, in one form or other, is a very important article of the daily diet. We drink it as a beverage; we add it to our tea, coffee, cocoa, and other drinks; we use it with various other things to make nutritious puddings, cakes, and custards. In whatever way we take it, it becomes a highly useful article of diet. In the dairy the milk, or rather one of its constituent parts, the cream, is converted into butter with the help of the churn. The milk is left to stand quietly for a time in shallow pans, until the cream, which consists of tiny globules or bladders of fatty or oily matter, rises to the surface. These tiny globules rise in this way, because they are lighter than the rest of the liquid.

The object of the churning process is to burst the delicate skin which encloses the little globules, thus setting free the oily, fatty matter within, which forms into lumps of butter. It is clear then that butter, which consists entirely of fat, is a fuel food. It is no part of our business now to follow up the process of butter-making. We are only concerned in learning the use of the butter itself as an article of our daily food.

When the cream has been removed from the surface, the skim milk left behind in the pan may be still further separated into two distinct parts. In the dairy this is done by pouring in rennet, but vinegar or any acid will separate them. In fact, they separate themselves when the milk turns sour. One of these parts, as we saw in our lesson on the different kinds of food, is a white, opaque, solid substance— the curd. Its scientific name is casein. This is the tissue-forming principle of milk. In the dairy it is collected and made into cheese, by squeezing out the water, and drying and pressing it into moulds.

Cheese may be made either of new or skim milk. When made from skim milk it contains as much of the tissue-forming casein as that made from new milk, but it is less palatable, because it contains no cream. It is sometimes called skim-milk cheese. Suffolk and Dutch cheeses are made from skim milk. If made of new milk the cheese contains a large amount of the fatty element, as well as the essential principle, casein. This adds to its flavor, and thus to its market price, but it does not make it more valuable as a tissue-forming food. Gloucester, Cheshire, and most of the fine cheeses of commerce are all made of new milk.

Cream cheeses are made from new milk too, but, besides the cream which that milk contains, an additional quantity of cream is added. Cheese taken in large quantities is neither a good nor an economical article of food. Few persons are able to digest a large piece of cheese at one time, although cheese is said to be a good aid to the digestion of other food if taken in small quantity, certainly not exceeding an ounce at a meal.

We may, in passing, notice the thin watery fluid which is left behind when these more essential parts of the milk have been removed. It is called whey. Dissolved in the liquid are certain mineral salts, as well as from 3 to 5 per cent of milk-sugar. Our lessons have, of course, made clear to you the purpose of these constituents. Let us now glance at the last of these animal foods— eggs. We have already examined the egg as to its structure, and the purpose for which it was designed.

What becomes of the clear, sticky, colorless portion when it is boiled?

It changes into a white, opaque, solid substance, sir, which we call albumen, from the Latin word albus, which means white.

Quite right. Just one word more about this albumen, and then we have done. Albumen is an important constituent of the yolk, as well as of the rest of the egg; 20 per cent of the yolk itself consists of albumen. What else did we find in the egg? We have already noticed a yellow fatty oil. This oil forms 30 per cent of the yolk of the egg. The albumen is the tissue-forming constituent, the fatty oil the heat-giving part of the egg.

As an article of food, eggs should be but lightly cooked. Albumen in its raw state is easily digestible, but when boiled it becomes hard and more or less indigestible."