Multifunctional antioxidant activity of polysaccharide fractions from the soybean byproduct okara

An alcohol insoluble residue (AIR) from okara, a byproduct from soymilk, was mainly composed by indigestible carbohydrate (55.7%). After sequential extraction, three alkali-soluble fractions (12.7% yield) and an insoluble residue (RES, 58.7% yield) were obtained. Soluble polysaccharide fractions showed in vitro reduction power (11–26 μmol Trolox Equivalent (TE)/g dry weight (dw)) and free radical scavenging activity (63–78 μmol TE/g dw). The highest antioxidant activity was exhibited by 0.05 M NaOH-soluble fraction, rich in pectins. The 1 M KOH-soluble fraction had a mixture of hemicellulosic and pectic polysaccharides, and 4 M KOH-soluble fraction contained the bulk of xyloglucans, although some pectins could also be present. RES contained cellulose along with residual pectins. FT-IR spectra of okara and AIR exhibited an absorption band at 1740 cm−1 of carboxylic ester from pectins, which lacked in the fractions. Moreover, they showed absorption bands at 1650 and 1550 cm−1 of proteins, and at 900–890 cm−1 of β-glycosidic linkages. Potential antioxidant activity of okara cell-wall polysaccharides could be attributed to pectins, although the contribution of residual proteins cannot be ruled out.