Ether-Soluble Organic Acids and Buffer Properties of Citrus Peels

Abstract

The concs. 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 amts. 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 soln. 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 liquid extraction method is satisfactory for the isolation of the organic acids from citrus-peel saps. 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 concs. of malic and oxalic adds. The oxalic acid is present in the peel as insoluble Ca oxalate, with the exception of a trace occurring in the peel sap. The titration curves of peel saps of citrus fruits (navel and Valencia oranges, grapefruit, and lemons) show that these systems contain comparatively small amts. 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.