20. Crude Protein.—In the analysis of foods, the term "crude protein" is used to designate the total nitrogenous compounds considered collectively; it is composed largely of protein, but also includes the amids, alkaloids, and albuminoids. "Crude protein" and "total nitrogenous compounds" are practically synonymous terms. The various proteins all contain about 16 per cent of nitrogen; that is, one part of nitrogen is equivalent to 6.25 parts of protein. In analyzing a food material, the total organic nitrogen is determined and the amount multiplied by 6.25 to obtain the crude protein. In some food materials, as cereals, the crude protein is largely pure protein, while in others, as potatoes, it is less than half pure protein, the larger portion being amids and other compounds. In comparing the crude protein content of one food with that of another, the nature of both proteids should be considered and also the amounts of non-proteid constituents. The factor 6.25 for calculating the protein equivalent of foods is not strictly applicable to all foods. For example, the proteids of wheat—gliadin and glutenin—contain over 18 per cent of nitrogen, making the nitrogen factor about 5.68 instead of 6.25. If wheat contains 2 per cent of nitrogen, it is equivalent to 12.5 per cent of crude protein, using the factor 6.25; or to 11.4, using the factor 5.7. The nitrogen content of foods is absolute; the protein content is only relative.[[9]]
21. Food Value of Protein.—Because of its complexity in composition, protein is capable of being used by the body in a greater variety of ways than starch, sugar, or fat. In addition to producing heat and energy, protein serves the unique function of furnishing material for the construction of new muscular tissue and the repair of that which is worn out. It is distinctly a tissue-building nutrient. It also enters into the composition of all the vital fluids of the body, as the blood, chyme, chyle, and the various digestive fluids. Hence it is that protein is required as a nutrient by the animal body, and it cannot be produced from non-nitrogenous compounds. In vegetable bodies, the protein can be produced synthetically from amids, which in turn are formed from ammonium compounds. While protein is necessary in the ration, an excessive amount should be avoided. When there is more than is needed for functional purposes, it is used for heat and energy, and as foods rich in protein are usually the most expensive, an excess adds unnecessarily to the cost of the ration. Excess of protein in the ration may also result in a diseased condition, due to imperfect elimination of the protein residual products from the body.[[10]]
22. Albuminoids differ from proteids in general composition and, to some extent, in nutritive value. They are found in animal bodies mainly in the connective tissue and in the skin, hair, and nails. Some of the albuminoids, as nuclein, are equal in food value to protein, while others have a lower food value. In general, albuminoids are capable of conserving the protein of the body, and hence are called "protein sparers," but they cannot in every way enter into the composition of the body, as do the true proteins.
23. Amids and Amines.—These are nitrogenous compounds of simpler structure than the proteins and albuminoids. They are sometimes called compound ammonia in that they are derived from ammonia by the replacement of one of the hydrogen atoms with an organic radical. In plants, amids are intermediate compounds in the production of the proteids, and in some vegetables a large portion of the nitrogen is amids. In animal bodies amids are formed during oxidation, digestion, and disintegration of proteids. It is not definitely known whether or not a protein in the animal body when broken down into amid form can again be reconstructed into protein. The amids have a lower food value than the proteids and albuminoids. It is generally held that, to a certain extent, they are capable, when combined with proteids, of preventing rapid conversion of the body proteid into soluble form. When they are used in large amounts in a ration, they tend to hasten oxidation rather than conservation of the proteids.
24. Alkaloids.—In some plant bodies there are small amounts of nitrogenous compounds called alkaloids. They are not found to any appreciable extent in food plants. The alkaloids, like ammonia, are basic in character and unite with acids to form salts. Many medicinal plants owe their value to the alkaloids which they contain. In animal bodies alkaloids are formed when the tissue undergoes fermentation changes, and also during disease, the products being known as ptomaines. Alkaloids have no food value, but act physiologically as irritants on the nerve centers, making them useful from a medicinal rather than from a nutritive point of view. To medical and pharmaceutical students the alkaloids form a very important group of compounds.
Fig. 5.—Graphic Composition of Flour.
1, flour; 2, starch; 3, gluten; 4, water; 5, fat; 6, ash.
25. General Relationship of the Nitrogenous Compounds.—Among the various subdivisions of the nitrogenous compounds there exists a relationship similar to that among the non-nitrogenous compounds. From proteids, amids and alkaloids may be formed, just as invert sugars and their products are formed from sucrose. Although glucose products are derived from sucrose, it is not possible to reverse the process and obtain sucrose or cane sugar from starch. So it is with proteins, while the amid may be obtained from the proteid in animal nutrition, as far as known the process cannot be reversed and proteids be obtained from amids. In the construction of the protein molecule of plants, nitrogen is absorbed from the soil in soluble forms, as compounds of nitrates and nitrites and ammonium salts. These are converted, first, into amids and then into proteids. In the animal body just the reverse of this process takes place,—the protein of the food undergoes a series of changes, and is finally eliminated from the body as an amid, which in turn undergoes oxidation and nitrification, and is converted into nitrites, nitrates, and ammonium salts. These forms of nitrogen are then ready to begin again in plant and animal bodies the same cycle of changes. Thus it is that nitrogen may enter a number of times into the composition of plant and animal tissues. Nature is very economical in her use of this element.[[5]]