Pectic Polysaccharides from Pumpkin Fruit

Abstract

The aim of this work was to study the influence of the hydrolysis-extraction process parameters on the structural characteristics of pectic polysaccharides (PP) from pumpkin. After hydrolysis by rapid steam assisted flash extraction (SAFE) the solubilized PP was separated from the cell wall residue (CWR) and the supernatant was further separated into three fractions. On standing, pectin gel (PG) precipitated and was separated. On addition of alcohol to the supernatant, a second pectin substance (PS) precipitated. Alcohol soluble oligosaccharides (OS) remained in the supernatant. The structure of pumpkin PP (PS, PG and MG fractions) was investigated using 1H,13C and gHSQC NMR; ATR FT-IR spectra; and HPSEC. From 1H, 13C and 2D gHSQC data we observed the major components present were homogalacturonans (HG) and the minority were rhamnogalacturonans (RG). Analysing NMR data from the literature one might expect the presence of α-D-Galp-(1→3)-β-L-Arap-(1→4)-β-D-Galp attached to the HG or RG chain (minor resonance of terminal →4)-α-L-Rha). Further work is required to determine the primary and secondary structure of pumpkin fruit pectin. The molecular structure of pectin components from this source appear to be affected not only by the extraction mode, but also depend on the degree of esterification (DE) and the ability of the components to undergo intermolecular aggregation. The molar mass distribution (MMD) of PS fractions indicate the presence of two species. One has a high and the other a low molar mass. Furthermore, these species have narrow distribution profiles. Conformation plot analyses of sub-fractions showed that PS is a mixture of extended and spherical shaped molecules. Our results suggest that pumpkin pectin is comprised of three macromolecular patterns differing by DE. Changes in DE and the presence of divalent cations appear to lead to various kinds of aggregation. The yield of PP is about 15-16 %, which includes soluble PS, an alcohol insoluble fraction a part of which is pectin gel and OS. The soluble pectin has use as a food thickener and emulsifier. Pectin gel as a composite with OS fractions could be a functional food additive.