Effects of cross-linking on the rheological and thermal properties of sweet potato starch

Sweet potato starches were modified with three different concentrations of phosphorus oxychloride (POCl3) (0.01, 0.02, and 0.03%, based on dry weight of starch) as a cross-linking agent. The effects of crosslinking on rheological and thermal properties of sweet potato starch (SPS) pastes were evaluated. Cross-linking considerably reduced the swelling power, consistency index (K), apparent viscosity (eta(a)), and yield stress (sigma(oc)) values of SPS, which significantly decreased with increase in POCl3 concentration. The gelatinization temperature (T-p) and enthalpy (Delta H) values of the cross-linked SPS, which were determined using differential scanning calorimetry, were higher than those of native SPS. Storage modulus (G'), loss modulus (G ''), and complex viscosity (eta*) of the cross-linked SPS pastes determined using small deformation oscillatory rheometry, were higher than native starch, and they also decreased with increase in POCl3 concentration from 0.01 to 0.03%. The tan delta (ratio of G ''/G') values (0.15-0.19) of the cross-linked SPS samples were much lower than that (0.37) of the native SPS, indicating that the elastic properties of the SPS pastes were strongly influenced by modifications from cross-linking. Finally, Cox-Merz plots showed that eta* was much higher than eta(a) for the cross-linked SPS pastes.