Modification of soybean proteins by immobilized proteases

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Virginia Polytechnic Institute and State University

Trypsin and alpha-chymotrypsin were immobilized on nylon pellets or porous glass by covalent methods to change molecular properties and functional characteristics of soybean proteins. The amount of trypsin immobilized on nylon pellets using the glutaraldehyde method was high when the pellets were treated with methanolic solution and 6 - 8% glutaraldehyde as well as high concentrations of soluble trypsin. Immobilized trypsin and chymotrypsin had uniform pKm and were stable at high temperatures. The optimum pH for activity of immobilized enzymes could be changed by using different supports and different methods of immobilization. A multi-enzyme system with immobilized trypsin and chymotrypsin was designed to produce an efficient hydrolysis and various desirable products of hydrolysis.

Controlled hydrolysis of soybean proteins by immobilized enzyme(s) increased water holding capacity, oil holding capacity, and relative viscosity, and improved emulsifying and foaming characteristics. Hydrolysis by immobilized protease(.s) increased solubility, relative viscosity and foaming ability of partially purified fractions. Succinylated soybean proteins had high oil holding capacity, viscosity, emulsifying ability, emulsion stability, and foaming ability. The order in which succinylation and hydrolysis by immobilized enzymes were done, conferred on soybean proteins various functional properties.

Evaluation of the molecular size of modified soybean proteins with sodium dodecyl sulfate (SDS) indicated that immobilized trypsin and chymotrypsin preferentially hydrolyzed specific protein components, and that succinylation enhanced hydrolysis, expanded protein molecules, and dissociated subunits. Measurement of molecular size and charge of the modified soybean proteins without SDS showed that immobilized trypsin hydrolyzed the intermediate subunits relatively fast, and succinylation separated the intermediate subunits. Succinylation increased the average molecular charge of soybean proteins, while hydrolysis decreased their average molecular size and their average molecular charge. The ratio of the average molecular weight to the average molecular charge could explain various functional properties. When the ratio was less than 5.0 x 10⁵, the modified soybean proteins had high soluble amino groups, high foaming ability, low water holding capacity and low foam stability. When the ratio was 9.0 x 10⁵, oil holding capacity, emulsifying ability, and emulsion stability were maximum. Relative viscosity was high at a constant value of 2.5 x 10⁵.