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Evaluation of Compatibility of Rice Starch and Pectins by Glass Transition and Sub-T_g Endotherms and the Effect of Compatibility on Gel Viscosity and Water Loss

¹Dept. Nutrition and Food Sciences, Fu-Jen University, Taipei, Taiwan, R.O.C. ²Dept. Chemistry, Fu-Jen University, Taipei, Taiwan, R.O.C. ³Corresponding author. Phone: 886-2-29031111 x3627. Fax: 886-2-22093271. E-mail: nutr1000@mails.fju.edu.tw

Differences in molecular structure and hydrophilicity may affect the compatibility of food components in a highly concentrated solution. Mixtures of TNuS19 rice starch (RS) and pectins with three different degrees of esterification (22, 64, and 92%) were used as a model system to evaluate the components' compatibility in a low-moisture system. When analyzed individually by differential scanning calorimetry (DSC), RS, low methoxyl pectin (LMP), intermediate methoxyl pectin (IMP), and high methoxyl pectin (HMP) showed the presence of a glass transition temperature (T_g) at 75.2, 96.2, 96.4, and 93.5°C, respectively. Among mixtures, the compatible RS-HMP exhibited only a single T_g between the T_g values of the two components, whereas the incompatible RS-LMP showed two T_g values that were close to those of the individual components. The sub-T_g endotherms of all three mixtures (1:1) were lower than the means of the corresponding components. The degree of decrease was more pronounced in the RS-HMP mixture than in the others. The above results imply that the interaction, which led to close contact between side chains of the two components, was more intense in the compatible RS-HMP mixture than in the RS-IMP and RS-LMP mixtures. The decrease of the sub-T_g endotherm can be used as an index to evaluate the degree of compatibility as well as the interaction occurring between the two molecules. The above findings were further verified by dynamic mechanical analyses. Both viscosity and water retention of the compatible RS-HMP mixed gel were significantly higher than those of the RS-IMP and RS-LMP mixed gels. This evidence further suggests that RS and HMP are compatible and exhibit a strong intermolecular interaction that increases gel viscosity and decreases water loss during high-temperature heating.