A simple experiment will help us to find some of the substances 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.

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.

Composition of Milk and Beans

================================================================= | WATER |PROTEIN | FAT |CARBOHYDRATE| ASH FOOD MATERIALS |PER CENT|PER CENT|PER CENT| PER CENT | PER CENT ---------------|--------|--------|--------|------------|--------- Milk | 87.0 | 3.3 | 4.0 | 5.0 | 0.7 Beans, dried | 12.6 | 22.5 | 1.8 | 59.6 | 3.5 -----------------------------------------------------------------

Notice that the substances in the beans are the same in general nature as those in the milk, although the amounts are different. The water that the young plant needs is, of course, supplied from the earth. There is another difference to note although this is not shown in the table; in the beans the carbohydrate is of two kinds, sugar and starch.

FOODSTUFFS[1]

All the varieties of food with which we are supplied will be found to contain some of these substances: protein, fat, carbohydrate, mineral matter, water; and to these we give the name foodstuffs. Some food materials (like the milk and beans just studied) contain all the foodstuffs, some only one, as in the case of sugar. We can now define food as something that contains one or more of the substances known as foodstuffs. But what are the foodstuffs themselves?

=Elements in the foodstuffs.=——Although we are not chemists, and may not even have taken a course in chemistry, yet through our nature study or physiology lessons, we are familiar with the fact that all the materials about us, including our own bodies and our food, are made up of simple substances that we call “elements.” We know, for instance, that coal is chiefly carbon, and we are familiar with such substances as sulphur, calcium, phosphorus, and iron. We know that the air contains oxygen, which we inhale, and that we breathe out a combination of carbon and oxygen called “carbon dioxide.” Since our bodies are composed of these and other elements, these elements must be supplied by our foods, and therefore, the foodstuffs in turn are composed of these same elements.

Proteins, fats, and carbohydrates all contain large amounts of carbon, and on this account are called fuel foods. But proteins are distinguished because they contain nitrogen in addition, which is found in no other foodstuff. Sulphur, too, we get only from protein, but we need less of it than of nitrogen, so we think about the nitrogen and let the sulphur take care of itself. The nitrogen that we draw in from the air with every breath, we breathe out again without being able to use it. This element is necessary to every living cell, but we can make it ours only through our protein food. Nitrogen is cheapest when obtained from the grains, from dried beans and peas. We pay a higher price for it in milk, eggs, fish, meat, and nuts. Carbon, which is found in all foodstuffs except water and some kinds of mineral matter, costs much less, especially when we take it in the form of carbohydrates such as starches and sugars. Oxygen is also abundant in our foods, but we get it even more cheaply in water and by breathing it in from the air. Phosphorus, iron, and calcium are very important elements, but we do not need them in very large quantities. We can get them cheaply in whole grains, peas and beans, some fruits and green vegetables, but they are worth paying for in milk and eggs. The elements last mentioned are present in the food partly as constituents of certain proteins and fats, partly as mineral salts. Other elements found as mineral matter are sodium and chlorine (which we take as common salt), potassium, magnesium, and traces of iodine and fluorine. These are all necessary to keep our bodies in good working order. We shall see later how to select our food materials so as to have all the different elements in the foodstuffs present in sufficient amounts.

FUNCTIONS OF THE FOODSTUFFS

=Food for energy.=——The first requirement of the body is for fuel, because it has a great deal of work to do. Even when one lies perfectly quiet and appears to be resting, the heart is working to keep up the circulation of the blood, the chest and diaphragm muscles are working to maintain the oxygen supply to the lungs, the alimentary tract is moving food material along, working to digest it and get rid of waste, and the skeletal muscles are being held up to “tone” so as to be ready for further action. All this work that we scarcely realize, may be called involuntary. To it we may add all sorts of voluntary movements, from simply speaking a word to turning somersaults or lifting heavy weights. All work involves energy, which we can obtain only from the fuel foods, proteins, fats, and carbohydrates.

Energy takes different forms. Our supply comes from the sun in the forms of heat and light, and plants store it up in the form of chemical energy when they build carbohydrates, fats, and proteins. This may be changed into the forms of work or of heat when we eat the food. Whenever an attempt is made to change chemical energy to work, some of it will change to heat. So in our bodies, the fuel foods, which enable us to do both involuntary and voluntary work, furnish heat at the same time, to keep our bodies warm. When we are too cold, we can shiver or run or jump, and thus, by doing more work, get more heat too.

=The unit of fuel value.=——In our studies of food materials, we must find out just how much energy, or working power, can be obtained from each kind. We must have a measure of energy or fuel value; and just as the inch is a measure of length, and the pound of weight, so the Calorie serves as a measure of fuel value. This unit[2] measures energy as heat, being the amount of heat required to raise 1 kilogram of water 1° C. (or 1 pound of water about 4° F.), but we can express it also as work, being sufficient energy to raise a 1-pound weight 3087 feet into the air (or 1 ton about 1-1/2 feet) if it were possible to convert it into mechanical work without loss. By burning foods in pure oxygen in a vessel placed in water so that all the heat is given off to the water, and then noting the change in temperature of the water, it is possible to find out just how much energy each will yield. Such a device is called a calorimeter. In the body there is usually a small portion of each kind of foodstuff which escapes digestion, and protein is not quite so completely burned as in the calorimeter. When allowance for the probable loss is made, the energy values of the fuel foodstuffs are as follows:

Protein 4 Calories per gram or 1814 per pound. Fat 9 Calories per gram or 4082 per pound. Carbohydrate 4 Calories per gram or 1814 per pound.

=The standard portion.=——Knowing the composition of any food material, it is possible from these figures to calculate the total fuel value, or we can refer to tables in which this has been calculated, and save ourselves labor. For comparison of different foods the Standard, or 100-Calorie, Portion is used, as this corresponds very closely with the amount of food for a single serving in many cases. In the sections treating of different foods the Standard Portion will be stated.

=Food for body building.=——Every living cell has a little life history of its own, and constantly demands a certain amount of new material to replace old which it has worn out. Besides this, old cells die, and new ones have to be made to replace them. Hence even a full-grown person needs building material, and much more is required in proportion when the person is growing and perhaps adding several ounces a week to his weight. The foodstuffs which have especial value as building material are protein and mineral matter.