Fractionation of soybean functional glycosides from soy-waste based on the chemical reaction of soyasaponin β g

We studied the chemical characteristics of soyasaponin β g with a view to establishing an effective fractionating method for soybean glycoside. Under an acidic condition of below pH 3, soyasaponin β g precipitated with unknown sugar and glycoside compounds of soybean. In addition, soyasaponin β g was hydrolyzed to soyasaponin Bb and 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one moieties under alkaline conditions. 1.0 mM NaOH hydrolyzed soyasaponin β g at the rate of 0.44 μmol/min. Metal ion-binding activity of soyasaponin β g resulted in 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one moiety and 1 mM soyasaponin β g chelated with 1.57 mM Fe2+ and 0.2 mM Cu2+. Based on these chemical characteristics of soyasaponin β g, we fractionated soybean saponin from soy-waste after oil extraction (industrial waste) by four steps: (1) precipitation of glycosides under acidic conditions (pH 2), (2) separation of hydrophobic functional compound (isoflavonoid), (3) chelate precipitation of 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one conjugated saponin using FeCl2 and (4) removal of the 2,3-dhydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one and Fe2+ complex by an alkaline hydrolysis (pH 12). Finally, 375 mg of group B saponin (84% purity) were obtained from 100 g of soy-waste after oil extraction. This fractionating method is a simple and useful method for producing medicinal foodstuffs from soybean.