Effects of alkali treatment and subsequent acidic extraction on the properties of soybean soluble polysaccharides

Soybean soluble polysaccharides (SSPS) were extracted under acidic condition from the okara of producing soybean protein isolate. The methyl esterificated carboxylic group of galacturonic acid (GalA) in SSPS could be demethoxylated by alkali treatment when heated, which had effects on SSPS properties. In this study, okara was treated in six selected alkali conditions, and then SSPS were extracted under the acidic condition (115 degrees C, 100 min, pH 4.5). The obtained SSPS were determined to have degree of esterification (DE) of 49, 55, 60, 66, 74 and 83%, respectively. Violent alkali treatment produced SSPS with not only lower DE but also lower molecular weight (MW) compared to mild alkali treatment. MW was increased from 396 to 489 kDa with increasing DE. It was found that SSPS with lower DE possessed lower aqueous viscosity, more negative charges, better emulsifying activity in oil-in-water emulsion and stabilizing activity in acidic milk drink. When DE changed from 49 to 83%, aqueous viscosity (10% (w/w) SSPS), average particle size of fresh oil-in-water emulsion, and centrifugation precipitating rate (CPR) of acidic milk drink (pH 4.5; two-stage 30 MPa homogeneous pressure) were increased from 25.8 to 108.2 mPa s, 1.5 to 4.0 pm, and 0.20 to 0.47%, respectively; and zeta potential (pH 7) was decreased from 17 to 10 mV. These results suggested that SSPS obtained from different alkali treatment had different functional properties. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.