Optimization and characterization of membrane-processed soy protein concentrate

Soy proteins were concentrated and purified from soy flour by a process that involved first pectinase treatment followed by ultrafiltration and diafiltration. An initial solids concentration of 10% (w/w) was optimal in terms of processing time with marginal decreases in flux and permeability. The composition of the soy concentrate (MSC) produced was consistent with the co-efficient of variation being less than 1.5%, thereby establishing the reproducibility of the membrane separation process. The functional properties of MSC were very similar to that of soy flour in terms of solubility and water hydration capacity. The high emulsifying activity index (EAI) of soy flour is believed to be reflective of its higher solubility, while surface hydrophobicity is believed to be responsible for an equally high EAI in acid precipitated soy isolate (APSI). Even though, the EAI of MSC was less than that of soy flour and APSI, the emulsion formed was the most stable. The proteins of soy flour and membrane soy concentrate seem to have most of their hydrophobic residues buried in the interior, while in the acid precipitated soy isolate hydrophobic residues are exposed on the surface. Heating resulted in a decrease in solubility but improved the hydration capacity and the emulsifying activity of both soy flour and MSC. The improvement in emulsifying activity associated with heating is thought to be related to increases in surface hydrophobicity. Based on the amino acid content, the membrane soy concentrate was found to be deficient (50%) in methionine, when compared to casein, reflective of the limiting amino acid in soy flour from which the concentrate is processed. However, the essential amino acid profile of the concentrate was comparable to current commercial isolates manufactured by acid precipitation. On comparing the electrophoretic patterns of the flour and MSC, it was observed that majority of the polypeptides present in soy flour were present in the concentrate. MSC was determined to have the least soybean aroma when compared to both soy flour and APSL The membrane soy concentrate was described as possessing a distinctive 'toasted grain' flavor that was not detected in either the soy flour or the acid precipitated soy isolate. Membrane processing contributed to a 44% decrease in the total isoflavone content. MSC was found to contain higher amounts of the aglucons, daidzein and genistein, and significantly lesser amounts of the glucosides, when compared to soy flour.