Factors affecting texture and structure formation of texturized soy flour and their possible mechanisms

Abstract

Extrusion texturized products were prepared from untreated and modified (physically, chemically and/or enzymatically) defatted soy flours. Some high selective physical property measurement, Instron rheological testing and light microscopic techniques were used to investigate the effects of protein modification on texture and structure of extruded soy flour. Reduction of protein content in soy flower weakened the extrudates' texture and reduced the water holding ad retention capacities but increased the bulk densities of the extrudates. Completely opposite effects were observed when protein contents of flours were increased. Decreasing the nitrogen solubility index of oy flour reduced the surface smoothness, diameter, water holding and retention capacities and fibrousness of protein-carbohydrate matrix of extrudates, but the bulk density was increased. No relationship was found between nitrogen solubility index of soy flour and texture measurement. Texture and bulk density of soy extrudates were decreased when the pH values of soy flour was increased; whereas water holding and retention capacities were increased. Extrudate diameter, surface smoothness and interior fibrousness of the protein-carbohydrate matrix increased when pH of soy flour was increased and vice versa. Enzyme (papain) modified soy flour did not produce a retortable extrudate. Transmitted light micrographs showed that this product had a very discontinuous protein-carbohydrate matrix. Proteins with relatively large molecular weights were required for strong texturized extrudate. Effect of NaCl on extrusion of soy flour was different from CaCl2 at concentractions of 1 and 2%. Addition of NaCl deceased the texture, surface smoothness and fibrousness of the extrudates; whereas addition of CaCl2 has opposite effects. Surfactants such as sodium and calcium stearoyl lactylates (0.2% and 0.4%) decreased the texture, water holding and retention capacities, surface smoothness and continuity of the protein-carbohydrate matrix, but increased the bulk density of the extrudate. Reducing agents such as Na2SO3 and cysteine-HCl at 0.1 and 0.2% levels increased the water holding and retention capacities, diameter, surface smoothness and fibrousness of the protein-carbohydrate matrix, but decreased the bulk density...