Human Foods and Their Nutritive Value | Page 5

is in excess of the demands of the body, and it is only in rare instances, as in cases of restricted diet, or convalescence from some disease, that special attention need be given to increasing the mineral content of the ration. An excess of mineral matter in foods is equally as objectionable as a scant amount, elimination of the excess entailing additional work on the body.
The composition of the ash of different food materials varies widely, both in amount, and form of the individual elements. When for any reason it is necessary to increase the phosphates in a ration, milk and eggs do this to a greater extent than almost any other foods. Common salt, or sodium chloride, is one of the most essential of the mineral constituents of the body. It is necessary for giving the blood its normal composition, furnishing acid and basic constituents for the production of the digestive fluids, and for the nutrition of the cells. While salt is a necessary food, in large amounts, as when the attempt is made to use sea water as a beverage, it acts as a poison, suggesting that a material may be both a food and a poison. When sodium chloride is entirely withheld from an animal, death from salt starvation ensues. Many foods contain naturally small amounts of sodium chloride.
4. Organic Matter.--That portion of a food material which is converted into gaseous or volatile products during combustion is called the organic matter. It is a mechanical mixture of compounds made up of carbon, hydrogen, oxygen, nitrogen, and sulphur, and is composed of various individual organic compounds, as cellulose, starch, sugar, albumin, and fat. The amount in a food is determined by subtracting the ash and water from 100. The organic matter varies widely in composition; in some foods it is largely starch, as in potatoes and rice, while in others, as forage crops consumed by animals, cellulose predominates. The nature of the prevailing organic compound, as sugar or starch, determines the nutritive value of a food. Each has a definite chemical composition capable of being expressed by a formula. Considered collectively, the organic compounds are termed organic matter. When burned, the organic compounds are converted into gases, the carbon uniting with the oxygen of the air to form carbon dioxide, hydrogen to form water, sulphur to form sulphur dioxide, and the nitrogen to form oxides of nitrogen and ammonia.
5. Classification of Organic Compounds.--All food materials are composed of a large number of organic compounds. For purposes of study these are divided into classes. The element nitrogen is taken as the basis of the division. Compounds which contain this element are called nitrogenous, while those from which it is absent are called non-nitrogenous.[2] The nitrogenous organic compounds are composed of the elements nitrogen, hydrogen, carbon, oxygen, and sulphur, while the non-nitrogenous compounds are composed of carbon, hydrogen, and oxygen. In vegetable foods the non-nitrogenous compounds predominate, there being usually from six to twelve parts of non-nitrogenous to every one part of nitrogenous, while in animal foods the nitrogenous compounds are present in larger amount.
NON-NITROGENOUS COMPOUNDS
6. Occurrence.--The non-nitrogenous compounds of foods consist mainly of cellulose, starch, sugar, and fat. For purposes of study, they are divided into subdivisions, as carbohydrates, pectose substances or jellies, fats, organic acids, essential oils, and mixed compounds. In plants the carbohydrates predominate, while in animal tissue the fats are the chief non-nitrogenous constituents.
7. Carbohydrates.--This term is applied to a class of compounds similar in general composition, but differing widely in structural composition and physical properties. Carbohydrates make up the bulk of vegetable foods and, except in milk, are found only in traces in animal foods. They are all represented by the general formula CH2n2n, there being twice as many hydrogen as oxygen atoms, the hydrogen and oxygen being present in the same proportion as in water. As a class, the carbohydrates are neutral bodies, and, when burned, form carbon dioxide and water.
[Illustration: FIG. 2.--CELLULAR STRUCTURE OF PLANT CELL.]
8. Cellulose is the basis of the cell structure of plants, and is found in various physical forms in food materials.[3] Sometimes it is hard and dense, resisting digestive action and mechanically inclosing other nutrients and thus preventing their being available as food. In the earlier stages of plant growth a part 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. Ordinarily, however, cellulose adds but little in the way of nutritive value, although it is often beneficial mechanically and imparts bulk to some foods otherwise too concentrated. The mechanical action of cellulose on the digestion of food is discussed in Chapter XV. Cellulose usually makes up a very small part of human food, less than 1 per cent. In refined