Changes induced by shear and gel formation in the viscoelastic behaviour of potato, wheat and maize starch dispersions

Abstract

The effect of shear and gel formation on the behaviour of heat-treated wheat, maize and potato starch dispersions was studied using dynamic viscoelastic measurements. The starch types were compared both at a concentration of 10% and at concentrations where the same level of shear forces was applied during the paste preparation procedure (11% wheat, 10% maize, and 4% potato). The cereal and the potato starch pastes initially showed similar viscoelastic properties after gelatinization, but the cereal starch pastes underwent a transition in their viscoelastic behaviour when cooled. This transition was believed to be due to the gelation of amylose. The main changes observed were a sharp increase in the shear modulus and a drop in the phase angle to below 2 degrees. No such transition occurred on cooling the potato starch pastes, at 25°C for 1 h, in the concentration range studied (4–10%). When the warm potato starch pastes were subjected to mechanical treatment, their viscoelastic behaviour changed significantly. Under the shear conditions used, the complex modulus of the 4% potato starch was decreased to as much as 7% of its initial value, and the phase angle was increased. The warm cereal starch pastes were considerably less affected than the potato starch pastes by the shear treatment. The complex shear modulus of the hot cereal starch pastes decreased approximately 50% and the phase angles increased somewhat. The shear-induced changes in the maize starch at 90°C were partly reversible. The gels formed from sheared cereal pastes had a lower complex modulus than those formed from low sheared pastes.