Human Foods and Their Nutritive Value | Page 9

are supposed to be proteins that are in feeble chemical combination with acid and alkaline compounds, and they are sometimes called acid and alkali proteids. Some are precipitated from their solutions by acids and others by alkalies. Peas and beans contain quite large amounts of a casein-like proteid called legumin. Proteoses and peptones are proteins soluble in water, but not coagulated by heat. They are produced from other proteids by ferment action during the digestion of food and the germination of seeds, and are often due to the changes resulting from the action of the natural ferments or enzymes inherent in the food materials. As previously stated, the insoluble proteids are present in far the largest amount of any of the nitrogenous materials of foods. Lean meat and the gluten of wheat and other grains are examples of the insoluble proteids. The various insoluble proteids from different food materials each has its own composition and distinctive chemical and physical properties, and from each a different class and percentage amount of derivative products are obtained.[1] While in general it is held that the various proteins have practically the same nutritive value, it is possible that because differences in structural composition and the products formed during digestion there may exist notable differences in nutritive value. During digestion the insoluble proteids undergo an extended series of chemical changes. They are partially oxidized, and the nitrogenous portion of the molecule is eliminated mainly in the form of amids, as urea. The insoluble proteins constitute the main source of the nitrogenous food supply of both humans and animals.
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