Both of these omelets may be varied by the addition of chopped parsley or chopped ham, or grated cheese. _Laboratory management._ When the price of eggs is high, some of the experiments can be demonstrated by the teacher. Eggs should be used when the price is at its lowest, even if this interferes with the logical sequence of lessons. MILK AND MILK PRODUCTS Milk is the natural food of the young mammal, and contains all the foodstuffs in a form easily assimilable. Starch is not present, the carbohydrate being found in the form of lactose, or milk sugar, a sugar differing somewhat from the sugars found in vegetables and fruit (see Chapter X). Whole milk and the milk products, cream, butter, and cheese, are all important food materials among the nations of the western world; and the manufacture of milk products, such as condensed milk, butter, and cheese, has developed large industries. While the Chinese and Japanese are two great peoples who have not utilized milk or any of its products as food for grown people to any extent, yet we are fully justified in counting these foods among the necessities. Nothing can fully take the place of milk in the family dietary. Figure 41 shows how all the foodstuffs are represented in milk. When milk first comes from the cow the fat is suspended in tiny, invisible particles throughout the water, giving the milk its yellow tint, and the fat rises to the top in the form of cream after a few hours. The protein, sugar, and ash are dissolved in the water. When milk reaches the stomach, the protein separates from the water in the form of curd. This change is brought about by an enzyme (soluble ferment) called rennin, which is present, along with pepsin, in the gastric juice. Curd is also formed by the souring of milk through the action of bacteria, or by adding acid directly to the milk. Milk should never be gulped down, but taken in sips, so that only small portions of curd are formed in the stomach, because these are much easier to digest than large ones. Sometimes milk is soured purposely, as in buttermilk or zoolak or matzoon, that curds may form and be beaten fine before it is drunk. This is very easy to digest, because then no large curds can form. For the same reason, it is often better to take milk with bread or some other food, or to cook it in some dish. Skim milk is a valuable food, for it has everything found in whole milk but the fat. We miss the flavor of the fat in drinking it, hence it is better to use it in pudding or soup or in cooking cereals where we do not care so much about the milk flavor. Study Fig. 41, comparing the percentages of the foodstuffs in whole, skim, and buttermilk, and cream. Notice that the skim milk is higher than the whole milk in protein and sugar, that it has as much ash, and a trace of fat even. It does not tell us, however, that the forms of ash in milk are most valuable, and that it is richer in calcium than any other food material.