Rheological properties of water-soluble soybean polysaccharides extracted under weak acidic condition

We studied the rheological properties of water-soluble soybean polysaccharides (SSPS) directly extracted from the residual by-product of isolated soybean protein for 1.5 h at pH 5.0 and 120 degrees C. The purity was relatively high, and they were composed mostly of saccharides. The molecular weight of the SSPS, as determined by a viscometric method, was approximately 114 000. The SSPS solution was lower in viscosity than that reported by Thompson et al., (Thompson, D.B., Huang, C., & Sieglaff, C. (1987). Rheological behavior of soluble polysaccharides fractions from soybean. Food Hydrocolloids, 1, 333-337) who investigated the polysaccharides extracted by the method of Morita (Morita, M. (1965). Polysaccharides of soybean seeds: polysaccharide constituents of "hot-water-extract'' fraction of soybean seed and an arabinogalactan as its major component. Agric. Biol. Chem., 29, 564-573), and the viscosity was higher than polysaccharides extracted by the method of Aspinall et al. (Aspinall, G.O., Begbie, R., Hamilton, A., & Whyte, J.N.C. (1967). Polysaccharides of soybeans: extraction and fractionation of polysaccharides from cotyledon meal. J. Chem. Sec. (C), 1065-1070) The SSPS could be dissolved in water at 30% concentration at 20 degrees C. A 10% solution of SSPS showed Newtonian behavior. This solution did not undergo gelation in the range of 580 degrees C. Although this SSPS contained approximately 20% uronic acid, it did not show reactivity with polyvalent cations. The viscosity of the SSPS solution increased slightly with increasing pH, and this increase was reversible. We conclude that SSPS have strong glucosidic bonds that enable them to endure the sterilization at 120 degrees C for 15 min at pH 3-6. Further, the behavior of the viscosities of dilute solutions suggests that galacturonic acids, such as uronic acid, exist closely together in the SSPS structure. (C) 1999 Elsevier Science Ltd. All rights reserved.