What is food? - This would seem to be a difficult question to answer as we look about a modern grocery or market with its bewildering assortment of foods. It seems hardly possible to describe such a variety of articles in a brief sentence, or to find a definition that will apply to all. Yet we seem to know instinctively what food is, although we may not find it easy to give a definition. Even the lower animals are guided in selecting food by some natural instinct and seldom make a mistake.
A widely used government bulletin gives this definition: "Food is that which taken into the body builds tissue or yields energy or does both." Probably we have learned this in our physiology, and admit it to be true, but for practical purposes, we need a more complete statement than this. Let us carefully determine what our foods really are, and what elements they contain, in order that we may select wisely for purposes of nutrition, and also that we may learn how to prepare food materials in a way that will utilize everything in them and waste nothing.
It is easy to divide food materials in a general way into those derived from the vegetable kingdom and those derived from the animal kingdom. In the vegetable group we have first, the different parts of many plants, and second, substances manufactured from plants. While we do not usually eat the whole of any one plant, yet there is not any part of the plant that we have not adopted as food. We use roots and tubers in beets, carrots, and potatoes, and the onion is a bulb. In celery and asparagus we eat the plant stalk. Plant leaves give us lettuce and other salads, cabbage and the like. Peas and beans and nuts are seeds, and cauliflower is a part of the flower. The fruit as a whole is familiar in many forms. Manufactured vegetable food materials include flour, meals, breakfast cereals, starch, sugar, molasses and sirups. The animal kingdom gives us the flesh of animals, fish and shell fish, and substances derived from animals, like eggs, milk, and the milk products, cream, butter, and cheese.
These materials vary so much in appearance that they would seem to have nothing in common. If, however, we compare the food of different animals and different races of men, we cannot but conclude that this is a mistaken judgment. We find an animal like the lion feeding entirely upon the flesh of other animals, and a strong creature like the ox, eating nothing but grass and grain. We also note that one race of men includes meat in its diet, and another subsists almost entirely upon vegetable food, such as rice and beans. Yet in both cases, these diverse kinds of food accomplish the same end, - body building and the supplying of energy. Let us study two common foods, from the two kingdoms, and see if through this study we can discover in what ways they are alike.
A moment's thought enables us to see that in milk we have a food that must have all the elements needed in nutrition, since it is the only food taken by many young animals. The baby and the young calf find in it everything that is needed to build the growing body, and to give them energy. If you see a young calf frisking about the field, you can appreciate how well his food supplies his needs.
A simple experiment will help us to find some of the sub-stances contained in milk. Let the milk stand until the cream rises on the top. Skim the cream, warm it slightly and beat it with an egg beater. Butter will soon "come," and butter, we know, is a form of fat. Warm a pint of the skimmed milk, add to it a dissolved rennet tablet, and set it in a warm place. In a short time, the milk becomes solidified to a consistency like that of jelly. If allowed to stand longer, a watery liquid will separate itself from the solid portion. These are the "curds and whey" that result, also, from the souring of milk. The whey can be squeezed out of the curd, leaving it quite dry. We have now found at least three constituents of milk, - water, fat, and curd.
You may then surmise from the sweet taste of milk that sugar is present; the chemist knows how to obtain it in pure form as "sugar of milk." The chemist also finds certain mineral substances which remain behind when all the water is evaporated and the curds and sugar burned away. These mineral substances are spoken of by the chemist as "ash," because this is what remains after burning the other portions of a food material, as ashes remain from a wood fire. Figure 1 shows you these substances in the amounts in which each occurs in a pint of milk. The sugar is one of a class of substances to which the chemist gives the name carbohydrate. To the substance in the curd that is different from all the other substances in the milk the name "protein" is given.
Fig. 1. - Composition of milk.
1. Whole milk.
2. Water. 3 Fat.
6. Mineral matter or ash.
We will now turn to the composition of beans, for in beans we find food stored up to nourish the young plant, which we, also, appropriate as food. The composition of both the milk and the beans is given in this table. Compare also Figures 35 and 41.