Starch Gelatinization Under Thermal Stress

Abstract

The behavior under thermal stress of starch dispersed in water was studied by differential scanning calorimetry (DSC) to estimate the heat transported through the aqueous medium in gelatinization, and to characterize the range of gelatinization temperatures. In DSC scanning mode, the endotherm of 10% starch in aqueous dispersion showed the tracing of gelatinization at between 67 and 80°C, having an onset at approximately 69°C. In the isothermal mode, characteristically distinct isothermal heat flow profiles were revealed. It was hypothesized that the thermal influx proposed as being analogous to the diffusion process may affect the profiles. The profiles were transformed and nonlinearly fitted according to the square root of time model to characterize a so-called t-parameter, which was related to mean square displacement of molecular distribution. The t-parameter of starch in excess of water decreased compared to that of water only. The plot of difference in these t-parameters, expressed as Δ, against temperature showed a dramatically decreased Δ at the temperature between 66.7 and 75.2°C, which coincided with the findings from scanning mode DSC. It was further hypothesized that the decreased Δ may be due to the gelatinizing process. According to the theory of polymer solution, the critical temperature (Θ) at 75.2°C, where the free energy became theoretically negative, i.e., the starch became spontaneously dissolved, was drawn. This Θ was located within the range of gelatinizing temperatures. It was deduced that starch polymer may have dissolved during gelatinization. The dissolution from acetaminophen tablets prepared by starch paste was lower compared with that of negative controls (without paste). Moreover, the paste prepared at gelatinizing temperature (70°C) seemed to inhibit acetaminophen dissolution from tablet matrices more than that prepared at subgelatinizing temperature (50°C).