Polyuronide Fraction and Soluble and Insoluble Carbohydrates of Orange Peel

Abstract

1. The whole peel of Valencia oranges and the albedo of navel oranges were separated into alcohol-soluble and alcohol-insoluble fractions by extraction of the fresh material with hot 80% ethyl alcohol. The principal constituents of the two fractions were determined, and some of their chemical properties studied. 2. The alcohol-soluble fraction of the whole peel of Valencia orange and of the albedo of navel orange averaged 57.31% and 55.19%, respectively, of the total dry weights. Soluble sugars accounted for 55.4% of the total extractable material in Valencia peel and for 72.69% in navel albedo. This fraction also contained substances which produced a small amount of carbon dioxide on hydrolysis with 19% hydrochloric acid. This carbon dioxide was probably produced by low polymer galacturonides soluble in hot 80% alcohol. 3. The alcohol-insoluble fraction of Valencia orange whole peel and of navel albedo averaged 42.69% and 44.81%, respectively, of the total dry weights. The sum of the water-soluble and acid-soluble pectic substances (as calcium pectate) of the alcohol-insoluble fraction averaged 39.76% in Valencia peel and 34.87% in navel albedo; the insoluble residues averaged 30.53% and 30.41%, respectively. The undetermined portion consisted of hemicelluloses which contained small amounts of polyuronides and which were rendered soluble by the extraction treatment. 4. The purity of the extracted pectic material (as calcium pectate) was ascertained on each sample by determining contents of carbon dioxide, calcium, and furfural. 5. As the polyuronides yield carbon dioxide quantitatively on hydrolysis with hydrochloric acid, the carbon dioxide values determined are a fair measure of the total pectic substances in the alcohol-insoluble solids. When the carbon dioxide values were accompanied by corresponding values for methoxyl, the degree of methylation of the pectic substances was subsequently calculated by dividing the determined methoxyl by that equivalent to the total carbon dioxide. Methylation in Valencia peel averaged 83.38%, and in navel albedo, 82.39%. 6. The sum of the carbon dioxide in the aqueous extract, acid extract, and residue amounted to 95.20% and 97.47% (mean values) of the total carbon dioxide of the alcohol-insoluble solids in the Valencia orange peel and navel orange albedo, respectively. The total calcium pectate precipitated from the aqueous and acid extracts of Valencia peel yielded 94.12% of the carbon dioxide dissolved by the extractants. The corresponding value for navel albedo was 88.00%. 7. Since the alcohol-insoluble fraction contained the pectic substances, most of the carbon dioxide which could be liberated by hydrolysis with 19% hydrochloric acid, and all the methoxyl groups, occurred in this fraction. The ratio of methoxyl to carbon dioxide was approximately the same in Valencia orange peel and in navel orange albedo. 8. The carbon dioxide equivalent to the sum of the esterified and nonesterified carboxyl groups was equal to the total carbon dioxide of the alcohol-in-soluble fraction obtained on hydrolysis with 19% hydrochloric acid.