Properties Of Peel Research Articles

GROWTH AND DEVELOPMENT IN VITRO OF EXCISED OVARIES

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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.

Ether-Soluble Organic Acids and Buffer Properties of Citrus Peels

1. The concentrations of ether-soluble organic acids are much lower in peels of citrus fruits (oranges, grapefruit, and lemons) than in juice samples of corresponding pulps. The high pH values of the peel saps indicate that most of the organic acids are present in the salt form and not in the free state. The amounts of citric and malic acids in the expressed saps agreed satisfactorily with the total organic acids titrated between pH 7.8 and 2.6. The organic acids precipitated from the peel sap with lead acetate agreed with the total organic acids titrated between pH 7.8 and 2.6 on a water solution of the ether extract. The citric and malic acids values of the ether extract agreed substantially with similar values of the lead acetate precipitate. The ether liquid extraction method is satisfactory for the isolation of organic acids from citrus-peel saps. 2. The total nonvolatile organic acids were extracted from dried peel of citrus fruits with absolute ether at pH 1. The organic acid content consisted chiefly of citric, malic, and oxalic acids. The citric acid content is very low in comparison with the concentrations of malic and oxalic acids. The oxalic acid is present in the peel as insoluble calcium oxalate, with the exception of a trace occurring in the peel sap. 3. The titration curves of peel saps of citrus fruits (navel and Valencia oranges, grapefruit, and lemons) show that these systems contain comparatively small amounts of organic acids and their salts. In the absence of these constituents, the pH values of the peels are high, and these systems consequently have only a slight buffer capacity.