Texture-Structure Relationships in Textured Soy Protein. V. Influence of pH and Protein Acylation on Extrusion Texturization

Abstract

Data were gathered to further support the hypothesis that formation of texture, measured in terms of shear values and sensory responses, during the thermal extrusion of soy protein involves protein-protein interactions based on NH2 groups. It was shown that ninhydrin, a reagent capable of irreversibly binding with free amino groups, significantly inhibited texture formation. When texturization was studied as a function of pH adjustment (~4–10), maximum Warner-Bratzler shear values and sensory response were found to occur close to pH 8, where amino groups are unprotonated and more reactive. At the extremes of the pH range extrudates lost structural integrity, were more soluble, and an examination of their microstructure showed that the interconnected longitudinal vacuoles separated by thick cell walls normally seen were replaced by a denser globoid system. Soy protein isolate was acylated using either acetic or succinic anhydride. Both treatments resulted in a significant decrease in extrudate texture but increases in extrudate solubility; the latter finding tends to discount the importance of solubility in texture formation. Scanning electron microscopy revealed that the acylated extrudates exhibited less cellularity and were denser than the control materials. The soluble fraction of acylated proteins demonstrated increased electrophoretic mobility as a result of the loss of the cationic amino groups.