Batch-type Cone-plate Viscometry of Corn and Wheat Starches Gelatinized by NaOH Solution at 20°C

Abstract

Batch-type cone-plate rotational viscometry was performed for 4.0% (carbohydrate content) dispersions of nonglutinous corn and wheat starches gelatinized by NaOH solutions at 20°C. The flow characteristics at different NaOH concentrations and the growth process of viscosity at a constant NaOH concentration were studied. Power-law model analysis of the samples stored for a certain time after adding NaOH solutions of different concentrations, showed that the corn and wheat starch samples showed an apparently dilatant flow in the 0.171-0.183 and 0.122-0.137 M regions respectively, in which an increase of NaOH concentration greatly enhanced the consistency coefficient. Then, the storage-time dependence of viscosity was investigated for batch samples gelatinized by 0.170 M (corn) and 0.133 M (wheat) NaOH solutions. For both cases, viscosity was found to increase exponentially with time in the early stage of gelatinization. This exponential growth of viscosity was theoretically described by the kinetic model, composed of the first-order reaction rate equation for the degree of gelatinization and the fluidity-type mixing rule for ungelatinized and gelatinized parts. Although some quantitative differences existed, the present results for corn and wheat starches were qualitatively the same with those previously observed for rice starch, and hence supported that such a novel growth process of viscosity has a physicochemical origin, not being characteristic of a specific plant origin.