Influence of maize starch granule-associated protein on the rheological properties of starch pastes. Part I. Large deformation measurements of paste properties

Starch granule-associated proteins, mainly consisting of granule bound starch synthase (GBSS), were shown to influence the rheological properties of starch pastes measured under large deformation steady shear conditions. A normal, a waxy null mutant, and two GBSS-containing waxy mutant maize starches (waxy protein was synthesized with no GBSS activity) were isolated (isolation procedure I, IP I) and further purified (isolation procedure II, IP II) using the toluene method. Further purification removed additional residual proteins in starches including the majority of GBSS in the two GBSS-containing waxy starches, and increased the paste viscosity in all four starches, especially at high shear rates. IP I starch paste of the waxy null mutant exhibited higher viscosity at high shear rate than starch pastes of the GBSS-containing waxy mutants that was attributed to the absence of GBSS in the former. Shear sweep tests (up to a shear rate of 120 l/s) showed that the IP I GBSS-containing waxy mutant starches had less shear-induced breakdown than the waxy null mutant starch; removal of additional granule-associated protein in the former, including GBSS, caused an increase in shear-induced breakdown. This study showed that granule-associated protein reduced paste viscosity, and GBSS specifically reduced shear-induced breakdown of starch pastes.