Alcohol-Insoluble Solids of Juice Vesicles and Pulp of Citrus Fruit

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Abstract
The alcohol-insoluble fraction of juice vesicles of Valencia and navel oranges, and of pulp of navel oranges and lemons, was isolated from fresh material by extraction with 80% ethyl alcohol. This fraction, which contained cellulose, most of the hemicellulose, and pectin, was lowest in Valencia orange vesicles (mean, 9.87%) and highest in navel orange pulp (mean 15.56%), on a dry-weight basis. Since the alcohol-insoluble fraction contains the pectin, most of the CO2 liberated by hydrolysis with 12% HC1, and all methoxyl groups, occur in this fraction. The ratio of methoxyl to CO2 was much lower in Valencia orange vesicles than in navel orange vesicles, navel orange pulp, or lemon pulp. The CO2 equivalent to the sum of the esterified and non-esterified carboxyl groups was equal to the total CO2 of the alcohol-insoluble fraction obtained on hydrolysis with 12% NCI. The sum of the water-soluble and acid-soluble pectin (as Ca pectate) of the alcohol-insoluble fraction of the vesicles and pulp samples ranged between means of 26.51% in Valencia orange vesicles and 36.47% in lemon pulp. These values are much lower than those for the total pectin calculated from the CO2 and methoxyl values. The residue that remains after extracting the pectin from the alcohol-insoluble solids of the vesicles and pulp is composed of cellulose and hemicellulose and a comparatively small amount of firmly-bound pectin that is difficult to extract and determine quantitatively. The sum of the CO2 in the aqueous extract, acid extract, and residue amounted to 95.4% and 100.9% (mean values) of the total CO2 of the alcohol-insoluble solids in the vesicles and pulp, respectively. In the aqueous and acid extracts, Ca pectate accounted for more than 85% of the CO2 dissolved by the extractants. As a measure of the degree of esterification of the alcohol-insoluble fractions, the ratio of methoxyl togalacturonic acid was lowest in the Valencia orange vesicles (mean, 0.107) and highest (mean, 0.134) in navel orange pulp. In a polygalacturonic acid the number of carboxyl groups that are free, esterified, or combined with cations is not a constant amt. but varies considerably with the method of preparation.

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