Mechanism of gliadin–glutenin cross-linking during hydrothermal treatment

Abstract

The gluten proteins, gliadin and glutenin, are important for wheat flour functionality and they undergo changes during heat treatment involving sulfhydryl (SH) groups. To change the level of SH-groups during hydrothermal treatment, the oxidant, potassium iodate (2.1 μmol/g protein) and the reducing agent dithiothreitol (DTT, 6.1 μmol/g protein) were added to 20% (w/w) gluten-in-water suspensions at room temperature, at 90 °C and after 15 min at 95 °C, and the viscosity was measured by the Rapid Visco Analyser (RVA). Protein extractabilities after hydrothermal treatment were determined by size-exclusion and reversed-phase HPLC. DTT decreased maximal RVA viscosity and the levels of extractable α- and γ-gliadin and this decrease was independent of the time of addition during hydrothermal treatment. In contrast, potassium iodate increased the levels of extractable α- and γ-gliadin. Its impact was less when added at later times during RVA analysis. A SH-blocking agent ( N-ethylmaleimide, 8.0 μmol/g protein), added at room temperature to the gluten suspension, decreased RVA viscosity at 95 °C and increased the extractabilities of glutenin and α- and γ-gliadin after hydrothermal treatment. Subsequent addition, at 90 °C, of a reducing agent (glutathione, 3.1 and 6.2 μmol/g protein) recovered the control RVA profile and restored the control protein extractabilities after RVA analysis. This shows the importance of heat-induced gliadin–glutenin reactions for gluten viscosity and of the presence of free SH-groups for the polymerization of gluten proteins. A model explaining gliadin–glutenin polymerization through a sulfhydryl-disulfide exchange mechanism and demonstrating the effects of redox agents is put forward.