Cheese. Cheese is the curd of milk squeezed dry of its liquid (_whey_), salted, pressed into a mould, and allowed to ferment slowly, or "ripen," in which process a considerable part of its casein is turned into fat. It is a cheap, concentrated, and very nutritious food, and in small amounts is quite appetizing. But unfortunately, the acids and extracts which have formed in the process of fermentation and ripening are so irritating to the stomach, that it can usually be eaten only in small amounts, without upsetting the digestion. Its chief value is as a relish with bread, crackers, potatoes, or macaroni. In moderate amounts, it is not only appetizing and digestible, but will assist in the digestion of other foods; hence the custom of eating a small piece of "ripe" cheese at the end of a heavy meal. CHAPTER V THE COAL FOODS (_Continued_) STARCHES Sources of Starch. The starches are valuable and wholesome foods. They form the largest part, both in bulk and in fuel value, of our diet, and have done so ever since man learned how to cultivate the soil and grow crops of grain. The reason is clear: One acre of good land will grow from ten to fifteen times the amount of food in the form of starch in grains or roots, as of meat in the shape of cattle or sheep. Consequently, starch is far cheaper, and this is its great advantage. Our chief supply of starch is obtained from the seed of certain most useful grasses, which we call wheat, oats, barley, rye, rice, and corn, and from the so-called "roots" of the potato. Potatoes are really underground buds packed with starch, and their proper name is tubers. Starch, when pure or extracted, is a soft, white powder, which you have often seen as cornstarch, or laundry starch. As found in grains, it is mixed with a certain amount of vegetable fibre, covered with husks, or skin, and has the little germ or budlet of the coming plant inside it. It has been manufactured and laid down by little cells inside their own bodies, which make up the grains; so that each particular grain of starch is surrounded by a delicate husk--the wall of the cell that made it. This means that grains and other starch foods have to be prepared for eating by grinding and cooking. The grinding crushes the grains into a powder so that the starch can be sifted out from the husks and coating of the grain, and the fibres which hold it together; and the cooking causes the tiny starch grain to swell and burst the cell wall, or bag, which surrounds it. Starches as Fuel. The starches contain no nitrogen except a mere trace in the framework of the grains or roots they grow in. They burn very clean; that is, almost the whole of them is turned into carbon dioxid gas and water.[7]