THE PECTOSE GROUP

The pectins resemble somewhat the gums in composition and properties, and are found in fruits, especially those that are partially ripe. On account of the presence of these substances in fruits the making of jelly becomes possible. (See p. 218.)

FATS AND OILS

These very important food substances are widely distributed in both plants and animals. The fat may be partially extracted from any of the finely ground vegetable or animal foods, by pressure and more completely by digestion with ether, chloroform or similar solvents. The term "ether extract," which is often used in tables giving the composition of foods, refers to the material, mostly fat, which is extracted in this way from the food. (See p. 307.)

THE ORGANIC ACIDS

The acids which exist in various parts of plants are sometimes present as salts of the metals, sometimes as acid salts, and oc- . casionally are found free. The most important acids are malic, tartaric, racemic, and citric. They play an important part in the diet. They are found in most fruits and vegetables and their presence has much to do with the agreeable taste and odor. (See p. 209.)

NITROGEN COMPOUNDS1

Proteins

Protein is of very complex composition, and contains the elements carbon, hydrogen, oxygen, nitrogen, and sulfur with sometimes phosphorus and iron. Different proteins from different sources are of such varied composition that there may seem little excuse for classifying them together, except for the fact that they all contain nitrogen. These bodies, which exist both 1 Am. Jour. Phys., Vol. 21.

in animals and plants, are built up through the vital energies of plants. They can be used by the animal body only after this preliminary construction in the plant cell. The average composition of protein, although it varies within rather wide limits, may be stated as follows.1

Familiar examples of the occurrence of protein are in lean beef, in the white of eggs, in blood and in the gluten of wheat. The proteins may be classified as follows:

(a) Simple proteins include albumins, globulins, alcoholsoluble proteins, glutelins, albuminoids, histones and protamines.

One of the most important properties of the albumins is that they are soluble in pure cold water, and when this solution is heated to the boiling point, the liquid coagulates, and the albumins separate out as an insoluble substance. The ablumins can, therefore, be extracted from lean beef, from blood, from milk and from eggs by cold water. Plants contain a small amount of vegetable albumin which can also be extracted by soaking with water.

When plant or animal tissues are treated with water containing 10 per cent. of common salt, another considerable portion of the protein may be dissolved. This is called globulin, and is especially abundant in plants. Kidney beans contain 20 per cent., peas 10 per cent., lentils 13 per cent. and wheat 0.6 per cent. of globulin. Special names have been given to the globulins from different seeds, as phaseolin to the one found in some species of beans, vignin to the one in the cow pea.

There are also animal globulins existing in the muscle and in the blood. The name myosin has been given to the globulin which is obtained from lean meat, fibrinogen to that found in blood. 1 Neumeister, Principles of Human Nutrition, Jordan, p. 46.

Fibrin as such does not occur in living blood, but is one of the products into which the fibrinogen breaks up when blood is exposed to the air. Vitellin is the principal protein found in egg yolk.

The glutenins1 constitute the largest part of the nitrogen compounds of cereals. The tenacious substance which is left after washing the starch out of wheat flour (see p. 34) is a protein of this class. There are also alcohol-soluble proteins in cereals, such as gliadin from wheat and zein from corn. The gliadin and glutenins together constitute about 80 per cent. of the total nitrogenous material of wheat.

According to the recent classification mentioned, the term albuminoids should be restricted to the proteins found in cartilage, bones, feathers, hair, hoofs, horns and nails. The one which occurs in cartilage and bone is called collagen; that in horns, hoofs and similar tissues, a substance that contains considerable sulfur, is called keratin. Commercial gelatin is derived from the collagen which is extracted especially from the tendons. (See p. 358.)

(b) Conjugated proteins, include nucleoproteins, glyco-proteins, proteins, phosphoproteins, hæmoglobins, and lecithoproteins.

The nucleoproteins are relatively abundant in such glandular tissues as the spleen, pancreas, and liver. One of the most important phosphoproteins is the casein of milk, which is insoluble in water but exists in suspension in milk. This coagulates, not by heat, but by coming in contact with a ferment, as in the human stomach, or by the action of rennet in cheese-making.

A very peculiar compound, existing in the blood, is known as hæmoglobin. This, when decomposed, separates into a protein (globin) and a coloring matter (hæmatin), which when charged with oxygen is called oxyhæmoglobin. This coloring matter or blood pigment has the property of very readily taking up and releasing oxygen, and plays an important part in the transfer of oxygen in the lungs from the air to the blood.

(c) Derived proteins, include proteans, metaproteins, coagulated proteins, proteoses, peptones, and peptides.

Lecithoproteins are derived from the yolk of eggs, the mucous membranes, the kidneys and other sources, and contain lecithin, a peculiar phosphorized fat.

From this brief description it will be seen that, a knowledge of the different proteins, particularly as to solubility and effect of heat, acids and ferments, is necessary to a proper understanding of foods and their uses.

Non-proteins

(d) Extractives, amides and amino acids.

There are also existing in foods a few nitrogen compounds that are not proteins. Among these may be mentioned the amides, bodies found especially in plants, such as the asparagine of asparagus. These substances seem to be of importance in the transfer of the nitrogen compounds from one part of the plant to another, as from the stem to the seed,1 and they are supposed to be of less value as muscle-formers than the proteins. The extractives that are obtained from beef, after the precipitation of the albumins by boiling, contain the compounds creatin (C4H7N3O2) and creatinin (C1H2N2O) which seem to have little or no food value (p. 356).

MINERAL SALTS

The mineral salts, especially the compounds of the elements iron, calcium, magnesium, potassium, sodium, chlorine, sulfur and phosphorus, enter into the animal body and take part in the process of metabolism as essential constituents of the organic materials of the food. They are of importance in three ways: (1) "as the constituents which give rigidity and comparative permanence to the skeleton, (2) as essential elements of the proto

1 Loc. cit.

plasm of the active tissues, (3) as salts, held in solution in the fluids of the body, giving these fluids their characteristic influence upon the elasticity and irritability of muscle and nerve, supplying the material for the acidity or alkalinity of the digestive juices and other secretions, and yet maintaining the neutrality or slight alkalescence of the internal fluids as well as their osmotic pressure and solvent power." In the study of nutrition very careful attention has been given to the rôle which the various salts, as the chlorides, sulfur and phosphorus compounds, calcium and iron compounds play in the process of animal nutrition. These inorganic materials are abundant in both vegetable and animal food-stuffs, and their occurrence, and relative quantity as a part of the diet should be always considered.

HEAT UNITS OF FOOD COMPOUNDS

о

Since reference is frequently made to the amount of energy available from different foods it is important briefly to define the use of this term as used in dietetics. The unit of energy adopted for a comparative study of foods is the "calorie." This is the energy in terms of heat which is sufficient to raise the temperature of 1 pound of water 4° F, or to raise the temperature of one kilogram of water from o° to 1° C. This energy or the number of heat units is determined by the use of an instrument called a calorimeter, in which a known weight of the food is actually burned, and the heat evolved is used to raise the temperature of a known weight of water. By very elaborate experiments it has been shown that there is a close relation between the results obtained by the calorimeter, and the actual energy produced in the body by the proper digestion and assimilation of the food.

As an illustration of the use of this energy unit, the energy value of a pound of edible material from a few food-stuffs is quoted from Jordan.2

1 Chemistry of Food and Nutrition, Sherman.

2 Principles of Human Nutrition, Jordan, p. 163.