Carbohydrate Fractions of Lemon Peel

The peel of citrus fruits is physiologically important in fruit growth and maturity in that the vascular system is confined chiefly to the mesocarp (albedo), which thus serves the important function of transporting water and solutes from the trees to the juice vesicles of the pulp. To understand the principles of the biochemical mechanisms occurring in the fruit during growth and development, one has to know the chemical composition of the peel and the relation between the constituents. Such data are also essential for the understanding of certain physiological relation? between peel and pulp, and for the understanding of the metabolic reactions of the peel under storage conditions. The phase of the work which has to do with the organic acids and buffer properties of citrus peels has already been published (21). The present studies have been concerned with the carbohydrate fractions of lemon fruit peel. Samples of peel from lemons have been thoroughly extracted with 80 per cent, ethyl alcohol and separated into alcohol-soluble and alcohol-insoluble fractions. The alcohol-soluble fraction contains chiefly the monoand di-saccharides ; it also contains other substances such as essential oils and waxes, and various concentrations of undetermined substances. The alcohol-insoluble fraction contains the cell-wall constituents, such as cellulose, lignin, pectin, and hemicellulose (sometimes designated as polysaccharides or acid-hydrolyzable material). These substances, particularly pectin and hemicellulose, appear to be closely related. The pectins are not usually considered as part of the carbohydrate reserve of the plant. The readily available carbohydrate fractions serving as sources of energy consist of the total-soluble-sugars and the starch-dextrin fractions. The carbohydrate material of the cell walls is not necessarily composed of condensation products of the simple sugars, but may consist of uronic acids united with the hexose and pentose sugars. Pectins belong to this general class of compounds, and any hemicellulose fraction that may accompany the pectins on weak-acid extraction of the tissues probably contains a uronic acid group. Cellulose, hemicellulose, and lignin compose the structural, and not the energy-yielding, units of the tissues; consequently, they are relatively inert to the metabolic activity of the cells.