Classification and kinetic analysis of viscosity growth processes for NaOH-gelatinized rice starches

Using capillary viscometry, viscosity growth processes were studied for non-glutinous rice starches gelatinized with different NaOH solution concentrations. The viscosity-time data series generally conformed to sigmoid curves with an arbitrary inflection point (IP) for each curve, and were analyzed using a kinetic model that incorporated a first-order reaction rate equation and a mixing rule of a power-law type. The shapes of curves were classified with the exponent ν or the ratio η(*)/ηG, where η(*) and ηG were viscosities at IP and at equilibrium, respectively. It was argued that these parameters were related to the complex formation arising from NaOH-starch interactions. The rate constant K defined uniformly for an entire process increased with NaOH concentration and was power-law dependent. Furthermore, it was suggested that gelatinization evolved non-uniformly over time. A non-uniform analysis was then performed by disassembling the entire process into several elementary stages and revealed the evolutionary process for K.