Effect of glucose and storage time on the viscosity of wheat starch dispersions

The effect of glucose addition, time of constant temperature heating (t_CTH) and storage time on the rheological properties of starch dispersion systems was examined. A fixed starch mixture of 5% (w/v), and glucose concentrations of 0-6% (w/v) were used. The starch dispersion was heated up to 80°C, then kept for 0, 10, 20, or 30 min, then cooled and kept at 8°C for 1, 2, 3, or 4 days. The apparent viscosity was then measured at 25°C as a function of shear rate. The apparent viscosity was also measured, at constant shear rate, as a function of shearing time. The Herschel-Bulkley model was used to fit the flow behavior of the above starch dispersions. It was found that the flow behavior index approached unity for 0 and 1% dextrose concentrations at low t_CTH values. It was also found that as t_CTH was increased, for a given glucose concentration, both yield stress, τ_o, and consistency coefficient, m, increased. The presence of glucose also resulted in an increase of both τ_o and m. Regarding the time-dependent rheological behavior, it was found that the examined starch dispersions were adequately described by the Weltman model and found to be thixotropic. In addition, as storage time increased, in the absence of dextrose, starch dispersions changed their flow behavior from Bingham to shear-thinning fluids. On the other hand, the presence of glucose almost kept the flow behavior constant.