Carbohydrate Fractions of Grapefruit Peel

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Abstract
Grapefruit peel was extracted with 80% ethyl alcohol and separated into 2 fractions. The alcohol-soluble fraction of the peel contained substances which liberated a small amount of carbon dioxide on hydrolysis with 12% HC1. 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. 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 amts. of soluble sugars accounted for 60.76% and 61.28% (mean values) of the total alcoholic extractives. Approx. 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 concns. of undetermined constituents. 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 wt. of the peel. Lignin and starch were not present in sufficient quantities to be detd. by the prevailing chemical methods. The carbon dioxide equivalent to the sum of the esteri-fied and nonesterified carboxyl groups was equal to the total CO2 of the alcohol-insoluble fraction obtained on hydrolysis with 12% HC1. The sum of the water-soluble and acid-soluble pectin (as Ca 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 CO2 and the methoxyl values. 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 amt. of firmly bound pectin that is difficult to extract and determine quantitatively. The sum of the CO2 in the water extract, acid extract, and residue amounted to 94.26% and 95.19% of the total CO2 of the alcohol-insoluble solids in the peel of fumigated and oil-sprayed fruit, respectively. In the water and acid extracts, Ca pectate accounted for more than 90% of the CO2 dissolved by the extractants.

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