Rheological Properties of a Heat-reversible Gel of Water-soluble Soybean Polysaccharide Extracted under Acidic Conditions

Abstract

  We studied the rheological properties of a heat-reversible gel formed with water-soluble soybean polysaccharides (RG-SSPS) which had been extracted from purified polysaccharides of soybean cotyledon meal at pH 2 and 80°C for 90 min. It was found that the gelling point and melting point of this heat-reversible gel were at approximately 51°C and 46°C, respectively. Gelation was observed in aqueous solutions with a concentration of over 3%, but not at 1%, at 5°C. The influence of pH on the gel strength of RG-SSPS was strong. Especially on the acidic side, the G′ and G″ values, which were highest at pH 7, were remarkably low and no gel was formed at pH 5. The addition of EDTA lowered the G′ and G″ values, while the addition of Ca raised them. These findings and the analytical results for the fractions obtained from RG-SSPS after reheating at pH 5 and 120°C for 90 min, under which conditions the non-gel-forming polysaccharides were extracted, suggest that gelation was due to weak bonding and cross-linking between polyvalent cations and low-methoxyl uronic acid regions in the homogalacturonic or rhamnogalacturonic main chain of the RG-SSPS structure. The results also suggest that the heat reversibility was due to weakness of the bonding which did not conform to the egg-box model, like the mechanism for low-methoxyl pectin or sodium alginate, because the neutral sugar side chains of the RG-SSPS structure are thought to limit their mutual proximity.