Processing of soybean hulls to enhance the distribution and extraction of value-added proteins

Soybean hulls contain peroxidase (SBP) and Bowman-Birk type protease inhibitor (BBI). SBP is used in the European bread-baking industry, and BBI isolated from dehulled, defatted soy flour possesses cancer-preventive and anticarcinogenic properties. Because hulls possess a low percentage of nitrogen, with fibre and other carbohydrates being the major components, our objective was to determine whether air classification of milled hulls could be used to fractionate and enrich these value-added proteins. Hulls were pin milled three times at 18 000 rpm, followed by air classification to yield fractions with particle size distributions ranging from < 15 to > 30 mum. The finely ground fractions with particle sizes ranging from < 15 to 18 mum were enriched with nitrogen and lipid when compared with the more coarsely ground fractions. Yields of extractable nitrogenous components from aqueous extracts were quantified at a variety of pHs, homogenisation speeds and times. BBI and SBP were confirmed by polyacrylamide gel electrophoresis with known standards. Based on assays for SBP and BBI activities, BBI, though concentrated in the finer fractions, also appeared in the highest-mass, coarsest fraction, whereas SBP resided mainly in the coarsest fraction. Dry milling and air classification of soybean hulls proved beneficial to distribute SBP into a product stream that facilitates its isolation and purification. Published in 2003 for SCI by John Wiley Sons, Ltd.