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

We studied the rheological properties of water-soluble soybean polysacchari des (SSPS) directly extracted from the residual by-product of isolated soyb ean protein for 1.5 h at pH 5.0 and 120 degrees C. The purity was relativel y high, and they were composed mostly of saccharides. The molecular weight of the SSPS, as determined by a viscometric method, was approximately 114 0 00. The SSPS solution was lower in viscosity than that reported by Thompson et al., (Thompson, D.B., Huang, C., & Sieglaff, C. (1987). Rheological beh avior 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: polysacchar ide constituents of "hot-water-extract'' fraction of soybean seed and an ar abinogalactan as its major component. Agric. Biol. Chem., 29, 564-573), and the viscosity was higher than polysaccharides extracted by the method of A spinall et al. (Aspinall, G.O., Begbie, R., Hamilton, A., & Whyte, J.N.C. ( 1967). Polysaccharides of soybeans: extraction and fractionation of polysac charides 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 i n the range of 580 degrees C. Although this SSPS contained approximately 20 % uronic acid, it did not show reactivity with polyvalent cations. The visc osity 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 a t pH 3-6. Further, the behavior of the viscosities of dilute solutions sugg ests that galacturonic acids, such as uronic acid, exist closely together i n the SSPS structure. (C) 1999 Elsevier Science Ltd. All rights reserved.