Foods, their selection and preparation

There are some 22 amino acids present from different sources in varying proportions in the proteins. Only ten of these are essential; the others can be built in the body if the essential amino acids are present in adequate amounts. The possible combinations of amino acids in any one food are so great that as yet any classification of proteins based on amino-acid content is impossible. The efficiency of any protein in tissue building depends upon its amino-acid content. The more nearly the amino-acid content of a food approaches that of the body proteins, the more economically it is used in building body tissues and fluids, or the more effi¬ cient it is. The term high quality also is used to describe an efficient protein. Proteins vary in efficiency. Study of individual proteins has been hampered since no natural foods contain only protein or only one protein; most foods contain several pro¬ teins combined with other nutrients. Since the first decade of this century rapid ad¬ vances have been made in the isolation of individual proteins, and in recent years con¬ siderable knowledge has been built up as to the amino-acid content of these individual proteins. But because we eat foods, not separated proteins, emphasis has shifted from the chem¬ ical to the biological study of these important nutrients. In biological studies of proteins, animals are fed diets containing definite weights of protein from a single food source — milk, eggs, meat, beans, cereal, etc. When the proteins from a single food source are added to a basal diet supplying the essentials other than protein, the amount of growth obtained from each gram of protein is a measure of the efficiency of the food as a source of protein. These biological studies have built up our knowledge of the compara¬ tive efficiency of the proteins present in different foods and in some cases of single proteins. Some foods also contain simple nitroge¬ nous compounds called "extractives,” formed from the breaking down of the proteins. They are present in small amounts and are important chiefly as sources of flavor in foods. Minerals. Minerals are usually closely as¬ sociated with the organic portion of the food, though some are present in inorganic forms. The mineral constituents do not yield energy in the body, but are useful as body-building and regulating materials. The minerals known to be essential are calcium, magne¬ sium, sodium, potassium, phosphorus, sulfur, chlorine, iron, copper, iodine, manganese. cobalt, and zinc. Aluminum, nickel, fluorine, silicon, and boron have been shown to be present in traces in body tissues and in foods. None of these latter minerals have been proved essential. The proximate components in any food are determined by chemical analysis. The average results of analyses are brought together in tables of food composition.1 These tables give the percentage of waste in the food as purchased and the composition of the edible portion in terms of protein, carbohydrates, fat, ash, or total minerals. Selected data from these tables will be used in appro¬ priate chapters throughout this text. DIGESTION AND ABSORPTION Before energy and building materials are available for use in the body, food must be digested and absorbed.