Thermal treatment of dry zein to improve rheological properties in gluten-free dough

Abstract

Zein is a gluten-free storage protein from corn that provides extensibility to starch-based doughs. However, zein does not confer a strong elastic behavior, a fundamental feature to produce high quality bread. Previous research showed that extrusion above 160 °C can improve zein dough viscoelasticity through the generation of a high molecular weight fraction. To determine whether thermal treatments alone could similarly improve zein function in doughs, dry zein was heated in a vacuum oven at temperatures of 160–200 °C. Gel electrophoresis of treated samples showed that thermal treatment promoted development of higher molecular weight zein fractions, which were resistant to dissolution in reducing conditions, presence of surfactant, and urea. Infrared spectroscopy also revealed that thermal treatments increased β-sheet content and decreased subsequent rearrangement following hydration. Zein hydrated with equal parts of water displayed increased elasticity when the zein was thermally treated at 160–190 °C. Doughs prepared with rice starch and zein treated at 160 °C exhibited improved strain-hardening behavior at higher extensional strain when compared to doughs prepared with unheated zein; no improvements were observed for higher temperature treatments. Electron microscopy revealed formation of discrete protein bodies among doughs prepared with zein treated at high temperatures, unlike fibrous strands observed for lower temperature treatment. Controlling heat-induced cross-linking and structural rearrangement of zein enhances its function in gluten-free doughs.