Effect of polarity of complexing agents on thermal and rheological properties of rice starch gels

The ability of rice starch to complex with ligands of various polarities was studied to examine the mechanism of complex formation in an aqueous solution. Differential scanning calorimetry (DSC) showed that TNuS19 rice starch (27.9% amylose) formed inclusion complexes with all 12-C complexing agents. The onset melting temperatures (T-o) of the complexes were approximate to 93-96 degrees C. The saturation concentrations of added ligands with high polarity, lauric acid (LA), and lauryl alcohol (LOH), had a range of 2-4% (w/w) Of the starch, and both of the corresponding melting enthalpies (Delta H) were approximate to 3.0 J/g. In contrast, the saturation concentrations of ligands with low polarity, methyl laurate (ML) and dodecane (DO), were approximate to 1-2% (w/w), and the Delta H were 1.87 and 1.80 J/g, respectively. This implied that solubility of ligands had a significant effect on the extent of complexation. The T-o and Delta H increased with an increase of annealing time at 85 degrees C, and the optima for the partially reversible complex formation were 2 hr of annealing in all cases. When measured by a dynamic rheometer, the TNuS19 rice starch gel with added LA or LOH showed a higher storage modulus (G') than that with no complexing agent added during heating. The G' and tan delta of the complexed gel were further increased during 12 hr of storage. The increase of G' indicated that the elastic structure of the concentrated rice starch gels could be improved by complex formation and annealing, whereas the increase of tan delta suggested incompatibility of starch components during storage.