3. The glucose or monosaccharid group (C&H12O6).

a. Glucose (grape sugar or dextrose).

b. Fructose (fruit sugar or levulose).
c. Galactose.

d. Mannose.

e. Sorbinose.

4. The pectose group. a. Pectin.

b. Pectosic acid.

c. Pectic acid.

II. Fats

1. Animal.

a. Hard (tallow, suet).

b. Soft or liquid (milk fat, fish oils).

2. Vegetable.

a. Oils.

b. Butters (cacao, cocoanut).

c. Artificially hardened (Crisco). (Snow Drift.)

a. Malic.

b. Citric.

c. Tartaric.

d. Racemic.
e. Lactic.

III. Organic Acids

IV. Nitrogenous Compounds (proteins)

1. Simple proteins.

2. Conjugated proteins.

3. Derived proteins.

Mineral Salts are also important inorganic constituents of

foods.

CARBOHYDRATES

The carbohydrates are found in various vegetable substances, and are of exceedingly varied structure and composition. They are all composed however, essentially, of oxygen, hydrogen and carbon, but so combined as to make a very complicated molecule. In some cases we know how the different parts of the molecule are related, and in others, although we know the percentage composition, it is not possible with our present knowledge to determine the molecular weight, or, as we sometimes say, to know how many atoms go to make up the molecule.

The carbohydrates are among the best known and the most. important foods, both for man and the lower animals. They are the most important nutrients in cereals, roots, stalks, fruits and seeds. Although the full mechanism of the building up of carbohydrates in plants is not understood, we know that through the energy of the sun's rays acting on the chlorophyl, carbon dioxide and water, which contain the bulk of the elements needed, unite to form these organic compounds. Some nitrogenous substances and the mineral constituents are drawn from the soil. The volume of oxygen liberated is equal to the volume of carbon dioxide which disappears.1 The "net" results of the process may be represented thus:

6CO2+6H2O=C6H12O6+602

It is probable that formaldehyde is an intermediate product, and hydrogen peroxide is produced, as shown thus:

CO2+3H2O=CH2O+2H2O2.

Under the influence of sunlight in the chlorophyl cell the hydrogen peroxide is rapidly decomposed into water and oxygen, and the formaldehyde built up into a carbohydrate. Glucose appears as a direct polymer of formaldehyde thus:

(Formaldehyde) CH2O × 6 equals C6H12O6 (glucose).

1 Chemistry of Food and Nutrition, Sherman, p. 4.

POLYSACCHARIDS

Cellulose (C6H1005), a carbohydrate of very complex character, is the predominating constituent of vegetable tissues. This next to water, is the most abundant substance in the vegetable kingdom, forming as it does the basis of wood, cotton, linen and similar fibrous bodies. Absorbent cotton and "washed" filter paper are examples of almost pure cellulose. - A peculiarity which distinguishes cellulose from most other vegetable substances is its insolubility in ordinary liquids, such as hot and cold water, alcohol, ether or dilute acids and alkalies. To be appreciably affected cellulose must be heated with acids or alkalies, an ammoniacal solution of cupric oxide, or other strong chemicals.

As cellulose is the substance from which the framework of the plant is built,1 it gives rigidity to certain parts, and if these plants are used as food, the greater the proportion of cellulose contained the more difficult will be their digestion. There is more cellulose in the older parts of the plant, as the stem, than in the leaves and fruit; more cellulose in the leaves of the potato vine than in the tuber. In grains and seeds it is most abundant in the outer coatings, so that in the process of milling, the cellulose is concentrated in the bran and the starch in the flour.

The use of foods which contain considerable cellulose, but not so much as to render them indigestible, is illustrated in tender asparagus, or celery, young beets and radishes, which however become tough and inedible, as the proportion of cellulose increases with age. In reports of the analyses of foods, cellulose forms the major part of the portion called "crude fiber," but this term includes also small quantities of other substances of the same general character.

When we study the action of the digestive fluids on vegetable foods it is important to remember that in the vegetable cells cellulose encloses the starch grains, and although it is to a certain extent permeable to the digestive fluids, it is very insoluble. 1 Principles of Human Nutrition, Jordan, p. 76.

While the plant is young, some of the cellulose is in chemical combination with water, forming hydrated cellulose, a portion of which undergoes digestion, and produces heat and energy in the body.1

If food rich in cellulose is very finely ground it is more readily acted upon by the digestive fluids. "Wood flour" was mixed with certain foods in Germany during the War, for the purpose of making the meager rations go farther.

Some of the lower animals seem to have the ability to digest cellulose quite readily, but it apparently adds very little to the nutritive value of the food of man. Whatever digestion of cellulose does take place, in the intestines says Lohrisch,2 seems to be due to the action of certain microorganisms by which fatty acids are produced, which upon absorption yield nutrients.

As will be seen in the later discussion of the various foods, whatever may be said of the lack of nutritive value of cellulose, it is of value mechanically, as it dilutes and gives more bulk to those foods which are rich in starch, sugar and protein. The eating of bulky vegetable foods, low in nutritive value, may however be carried too far, as their constituents are mainly digested in the intestines, where they are liable to ferment and produce gases which will cause the distention of the intestinal organs. In this way the use of an exclusively vegetarian diet may lead to unpleasant results.

Starch (C6H10O5)n is the form in which the digestible carbohydrates are finally stored in the plants after they have circulated through the ducts and cells as the more soluble sugars. The food plants contain it, frequently to the extent of 60 or 70 per cent. It is stored especially in the seeds, but also in the roots, tubers, fruits, stems and leaves of the plants, where in many cases it plays an important part in feeding the young, germinating plant. The starch grains which are found in the interior of the cells are of a characteristic size, shape and appearance, for each 1 Human Foods, Snyder, p. 8.

2 Zeitsch. physiolog. Chemie, 47 (1907), through Sherman, Chem. of Food and Nutrition, p. 311.

particular variety of plants, a fact that aids greatly in the detection of adulteration, especially in cereals.

Starch with sugar may be called the "basic foods" of man, as they are the central and most important constituents of the vegetable foods which he utilizes. These foods also contain fats and nitrogenous constituents, which of course are essential nutrients.

STARCH CONTENT OF FOODS

Starch can be kept if perfectly dry for a long time when stored in a dry place. The starch grains when suspended in cold water are not dissolved, and can readily be filtered from the liquid.