Flow characteristics and gelatinization kinetics of rice starch under strong alkali conditions

Abstract

The normality dependence and the kinetics of flow behavior in the rice starch dispersion under strong alkali conditions were studied. Non-Newtonian flow (power-law) quantities n and μ ( σ = μ ( ε ˙ ) n ; σ, shear stress; ε ˙ , shear rate) of the samples stored (gelatinized) at 20 °C for 10 min after the addition of NaOH solution with the normality 0.090–0.175 N, were measured by a cone-plate type rotational viscometer. For the flow analysis a care was taken for the ambiguity of the analysis range of flow data. The samples added with 0.14–0.16 N NaOH displayed an apparently dilatant flow. As the normality of added NaOH increased, apparent viscosity η a was steeply enhanced in the vicinity of 0.14 N and started to grow almost exponentially. Next, the storage time dependence of flow properties was investigated for samples gelatinized for 6≤ t (min)≤230 after the addition of 0.146 N NaOH solution. A Newtonian analysis of the flow ( σ − ε ˙ ) data indicated that viscosity η increased exponentially in the early stage (6≤ t≤35) and did linearly in the middle. It was argued that the observed time dependence of viscosity could be described by a first-order reaction hypothesis combined with the fluidity ( η −1) mixing rule with respect to ungelatinized and gelatinized fractions of starch. The application of this theoretical treatment to the present data suggested that the gelatinization process was composed of plural phases with different values of a rate constant. For the observed kinetic characteristic of alkali gelatinization, the discussions were presented on the basis of amylose complex formation.