radicle in the molecule like simple olein, stearin, or palmitin are called "simple glycerides"; and it may also be true that two or more kinds of fatty acid may combine with the same molecule of glycerol forming what is called a "mixed glyceride." "Mixed glyceride" is thus used as a technical term to mean something more than merely a mixture of glycerides. If one molecule of glycerol be combined with one molecule each of oleic, stearic, and palmitic acids, the resulting mixed glyceride, oleo-stearopalmitin, is different from a mere mixture of simple olein, palmitin, and stearin. Ordinary natural and commercial fats are mixtures of both simple and mixed glycerides.

Fats in foods and nutrition. In food analysis, fat is usually determined by extraction with ether. All ether-soluble substances are therefore likely to be counted as fat. In this way some small quantities of materials of less food value are likely to be counted along with the fat of the food. A more serious source of misunderstanding arises from the fact that some food fats are important sources of fat-soluble vitamin while others are not. As will be shown more fully in Chapter X, the familiar edible fats and oils of commerce are rather similar in their chemical composition and digestibility and therefore in their fuel value; but they are very different in their food value because of the great differences in their fat-soluble vitamin

content.

The fat of the food after digestion and absorption is again found in the blood in the form of glycerides or neutral fat which disappears partly by being burned in the muscles and other active tissues where it is used as fuel for the same purposes as carbohydrate; and if in excess of the fuel requirements of the body, the fat obtained from the food may also be stored in the tissues. The body fat obtained thus directly from the food may show somewhat different characters from the fat which has been formed in the body from carbohydrate, but its nutritive relations appear to be exactly the same. In either case, the fat

thus stored in the body may be drawn upon for use as fuel at any future time when the energy requirements of the body demand it.

The energy value to the body of average food fat is 9.0 Calories per gram, or 4082 Calories per pound.

Proteins (Nitrogen Compounds)

Among the nitrogenous constituents of foods, the proteins usually so far predominate that the term "protein" is often used as practically synonymous with the nitrogen compounds of food materials. For this reason, and because the great majority of proteins contain from 15 to 18 per cent, averaging about 16 per cent, of nitrogen, the protein content of food materials is usually estimated by determining nitrogen and multiplying the percentage of nitrogen found by 6.25.

The proteins are very complex substances and in no case is the chemical constitution of a natural protein fully and exactly known. It has, however, been determined that the typical proteins are essentially anhydrides of amino acids. Thus the relation of the protein molecule to the amino acids, from which it is derived and into which it can be resolved, is analogous to the relation of starch to glucose. There is, however, this striking difference: that the molecules of monosaccharide derived from the complete hydrolysis of the starch are all alike (glucose), whereas the complete hydrolysis of a protein always yields several different amino acids, usually from twelve to fifteen.

1

The names of the amino acids commonly met as products of hydrolysis of proteins are: glycine (glycocoll), alanine, serine, valine, leucine, proline, phenylalanine, tyrosine, aspartic acid,

1 The names of the amino acids are sometimes spelled without the final e; for example, glycine as glycin, alanine as alanin. The names of proteins should always be spelled without the final e to distinguish them from amino acids and some other groups of nitrogen compounds; thus always gelatin (never gelatine).

glutamic (glutaminic) acid, lysine, arginine, histidine, tryptophane, cystine. The strict chemical names and structural formulæ of these amino acids are given in Chapter III of Chemistry of Food and Nutrition, Revised Edition.

Classification of the proteins. There has been considerable confusion in the classification and terminology of the proteins, and even in the publications of the present day, the same terms may sometimes be found employed with different meanings by different writers. The classification now generally approved is as follows:

I. Simple proteins. Protein substances which yield only amino acids or their derivatives on hydrolysis.

(a) Albumins. Simple proteins soluble in pure water and coagulable by heat. Examples: egg albumin, lact-albumin (milk), serum albumin (blood), leucosin (wheat), legumelin (peas).

(b) Globulins. Simple proteins insoluble in pure water, but soluble in neutral salt solutions. Examples: muscle globulin, serum globulin (blood), edestin (wheat, hemp seed, and other seeds), phaseolin (beans), legumin (beans and peas), vignin (cow peas), tuberin (potato), amandin (almonds), excelsin (Brazil nuts).

(c) Glutelins. Simple proteins insoluble in all neutral solvents, but readily soluble in very dilute acids and alkalies. The best known and most important member of this group is the glutenin of wheat.

(d) Alcohol-soluble proteins. Simple proteins soluble in relatively strong alcohol (70-80 per cent) but insoluble in water, absolute alcohol, and other neutral solvents. Examples: gliadin (wheat), zein (corn), hordein (barley).

(e) Albuminoids. These are the simple proteins characteristic of the skeletal structures of animals (for which reason they are also called scleroproteins) and also of the external protective tissues such as the skin, hair, etc. None of these proteins is

commonly used for food in the natural state, but collagen when boiled with water yields gelatin so that these two are of considerable importance in food chemistry.

(f) Histons. Soluble in water, and insoluble in very dilute ammonia, and in the absence of ammonium salts insoluble even in an excess of ammonia; yield precipitates with solutions of other proteins and a coagulum on heating which is easily soluble in very dilute acids. On hydrolysis they yield several amino acids, among which the basic ones predominate. The only members of this group which have any considerable importance as food are the thymus histon and the globin derived from the hemoglobin of the blood.

(g) Protamins. These are simpler substances than the preceding groups, are soluble in water, uncoagulable by heat, possess strong basic properties, and on hydrolysis yield a few amino acids, among which the basic amino acids greatly predominate. While basic amino acids are important in nutrition, the protamins themselves are of practically no importance as food.

II. Conjugated proteins. Substances which contain the protein molecule united to some other molecule or molecules otherwise than as a salt.

(a) Nucleoproteins. Compounds of one or more protein molecules with nucleic acid. Examples of the nucleic acids thus found united with proteins are thymo-nucleic acid (thymus gland), tritico-nucleic acid (wheat germ).

(b) Glycoproteins. Compounds of the protein molecule with a substance or substances containing a carbohydrate group other than a nucleic acid. Example: mucins.

(c) Phosphoproteins. Compounds in which the phosphorus is in organic union with the protein molecule otherwise than in a nucleic acid or lecithin. Examples: caseinogen (milk), ovovitellin (egg yolk).

(d) Hemoglobins. Compounds of the protein molecule with. hematin or some similar substance. Example: hemoglobin of

blood.

(The redness of meat is due chiefly to the hemoglobin of the blood which the meat still retains.)

(e) Lecithoproteins. Compounds of the protein molecule with lecithins or related substances.

III. Derived proteins.

1. Primary protein derivatives. Derivatives of the protein molecule apparently formed through hydrolytic changes which involve only slight alterations of the protein molecule.

(a) Proteans. Insoluble products which apparently result from the incipient action of water, very dilute acids, or enzymes. Examples: casein (curdled milk), fibrin (coagulated blood).

(b) Metaproteins. Products of the further action of acids and alkalies whereby the molecule is sufficiently altered to form proteins soluble in very weak acids and alkalies, but insoluble in neutral solvents. This group includes the substances which have been called " acid proteins," "acid albumins," "syntonin," "alkali proteins," " alkali albumins," and "albuminates."

(c) Coagulated proteins. Insoluble products which result from (1) the action of heat on protein solutions, or (2) the action of alcohol on the protein. Example: cooked egg albumin, or egg albumin precipitated by means of alcohol.

2. Secondary protein derivatives. Products of the further hydrolytic cleavage of the protein molecule.

(a) Proteoses. Soluble in water, not coagulable by heat, precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. The products commercially known as "peptones" consist largely of proteoses.

(b) Peptones. Soluble in water, not coagulable by heat, and not precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. These represent a further stage of cleavage than the proteoses.

(c) Peptids. Definitely characterized combinations of two or more amino acids. An anhydride of two amino acid radicles is called a "di-peptid "; one having three amino acid radicles, a