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.

This research was supported by the Spanish Ministry of Science and Innovation, through CICYT Projects AGL2005-02447-ALI and AGL2008-0998-ALI.

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