Free sugars in fruits and vegetables.

The carbohydrates are especially prominent constituents of plants and usually form over one-half of the total plant substance. They serve not only as a source of available energy but also as reserve food and as structural materials. They are one of the main groups of food substances (carbohydrates, proteins, and fats) to be synthesized in the plant from simple organic substances. The empirical composition of carbohydrates may be expressed by the formula CnH2nOn. With regard to their specific chemical properties, carbohydrates may contain a potential aldehyde, -CHO, or ketone, C=O, group. In general, the substances belonging to this class of compounds may be divided into three broad groups: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides have five carbon atoms (pentoses) or six carbon atoms (hexoses) and have a sweet taste. The second group of carbohydrates, oligosaccharides, is made up of two or more monosaccharide units linked to one another through a glycosidic bond. These are the disaccharides, trisaccharides, tetrasaccharides, etc., and may or may not have reducing properties. No sharp line of distinction can be drawn between the oligosaccharides and the third group of carbohydrates, the polysaccharides, which represent large aggregates of monosaccharide units (starch, cellulose, pectin, etc.). The main function of carbohydrates upon ingestion by an animal organism is that of a fuel. They are metabolized to other products with the release of carbon dioxide, water, and energy. In addition, certain products of carbohydrate metabolism aid in the breakdown of many food stuffs, acting as catalysts in biological oxidations. Carbohydrates can also be used as a starting material for the biological synthesis of other types of compounds in the body, such as fatty acids and certain amino acids. Regardless of the form in which a carbohydrate happens to be ingested, it must be transformed into a monosaccharide for absorption and metabolism, thus emphasizing the significance of monosaccharides in food stuffs. In spite of the increasing awareness of the specific carbohydrate role as human food, however, many problems in this field are far from being solved owing to chemical and structural complexity of the sugars. The types of sugar and their concentration in foods are only known approximately and in general terms. The free hexose and pentose content of many foods is generally reported only as "total reducing sugars,' and the oligosaccharide content of foods is usually reported as "nonreducing sugar" calculated to be sucrose. It is now apparent that the "nonreducing sugar" fraction of foods may have, as the major sugar present, higher molecular weight and more complex homologues of sucrose such as raffinose, stachyose, and verbascose. Recently, Shallenberger and Moores (1) and Shallen-berger and Moyer (2) were able to apply paper chromatographic techniques to identify those common monosaccharides and oligosaccharides in foods by quantitative chromatographic methods in the range of standard deviation of less than of sugar. With the rapidly developing role of individual specific sugars in metabolic processes, it is becoming increasingly imperative that the specific sugars in foods be identified and their concentration tabulated. The specific purpose of this study was to identify and determine the concentration of the major free sugars which occur in common foods of plant origin.