Carbohydrate Fractions of Grapefruit Peel

Abstract

1. Whole peel of grapefruit was extracted with 80% ethyl alcohol and separated into two distinct fractions, the alcohol-soluble and alcohol-insoluble solids. The carbohydrate constituents of these two fractions were investigated, and certain of their physical and chemical properties were determined. All analyses were made on fruit samples obtained from trees that had been fumigated with hydrocyanic acid or sprayed with oil for pest control during the growing season. 2. The alcohol-soluble fraction of the peel contained substances which liberated a small amount of carbon dioxide on hydrolysis with 12% hydrochloric acid. The total solids of this fraction composed 66.31% and 67.35% of the dry matter of the peel of fumigated and oil-sprayed fruit, respectively. 3. The soluble sugars in the peel averaged 40.29% and 41.28% of the dry weight of the fumigated and oil-sprayed fruit, respectively. These amounts of soluble sugars accounted for 60.76% and 61.28% (mean values) of the total alcoholic extractives. Approximately 40% of the dry weight of the alcohol-soluble fraction was composed of substances other than sugars, namely, essential oil, waxes, organic acids, naringin, and various concentrations of undetermined constituents. 4. The alcohol-insoluble fraction of the peel contains the cell-wall constituents-cellulose, hemicellulose, and pectin. This fraction comprised 33.69% and 32.65% of the dry weight of the peel. Lignin and starch were not present in sufficient quantities to be determined by the prevailing chemical methods. 5. Since the alcohol-insoluble fraction contains the pectin, most of the carbon dioxide which can be liberated by hydrolysis with 12% HCl and all the methoxyl groups occur in this fraction. The ratio of methoxyl to carbon dioxide was much lower in grapefruit peel than in lemon peel. 6. The carbon dioxide equivalent to the sum of the esterified and non-esterified carboxyl groups was equal to the total carbon dioxide of the alcohol-insoluble fraction obtained on hydrolysis with 12% hydrochloric acid. 7. The sum of the water-soluble and acid-soluble pectin (as calcium pectate) of the alcohol-insoluble fraction of the peel of the fumigated and oil-sprayed fruit amounted to 43.79% and 43.95%, respectively. These values are much lower than those for the total pectin calculated from the carbon dioxide and the methoxyl values. 8. To establish criteria of purity, the calcium pectate values are accompanied by corresponding values for carbon dioxide, calcium, and furfural. The percentages of calcium in the calcium pectates are somewhat higher than the 7.50% usually reported for pure calcium pectate. The percentages of carbon dioxide are slightly higher than the 17.40% reported for pure calcium pectate. The furfural values are in accord with those reported by other investigators. 9. The residue that remains after extracting the pectin from the alcohol-insoluble solids of the peel is composed of cellulose and hemicellulose and a comparatively small amount of firmly bound pectin that is difficult to extract and determine quantitatively. 10. The sum of the carbon dioxide in the aqueous extract, acid extract, and residue amounted to 94.26% and 95.19% of the total carbon dioxide of the alcoholinsoluble solids in the peel of fumigated and oil-sprayed fruit, respectively. In the aqueous and acid extracts, calcium pectate accounted for more than 90% of the carbon dioxide dissolved by the extractants.