This study examined the steady flow and dynamic rheological behaviors of hydroxypropylated sweet potato starch (HPSPS) pastes mixed with guar gum (GG), locust bean gum (LBG), and xanthan gum (XG) at different concentrations (0, 0.3, and 0.6%). The HPSPS-gum mixtures had higher shear-thinning fluid characteristics than the control (0% gum) at 258C. The addition of the gums resulted in an increase in the consistency index (K) and apparent viscosity (h a,100 ). The dynamic moduli (G 0 , G 00 ) and complex viscosity (h à ) values of the HPSPS-gum mixtures were higher than those of the control, and they increased with an increase in gum concentration. In particular, the presence of XG at 0.6% in the HPSPS-gum mixture systems gave rise to the greatest viscoelastic properties among the gums examined at different concentrations. The tan d (ratio of G 00 /G 0 ) values (0.35-0.57) of the HPSPS-GG and HPSPS-XG mixtures were much lower than those of the control (0.82) and HPSPS-LBG (0.88-1.06), indicating that the elastic properties in the HPSPS-gum mixture systems were strongly affected by the additions of GG and XG. These steady flow and dynamic rheological parameters indicated there were synergistic interactions between the HPSPS and gums. The synergistic effects of the gums and modified starch were hypothesized by considering the molecular incompatibility and molecular interactions between the gums and HPSPS.

The effects of guar gum (GG) and xanthan gum (XG) at different concentrations (0, 0.2, 0.4, and 0.6% w/w) on the rheological properties of Korean waxy rice starch (WRS) pastes were evaluated under both steady and dynamic shear conditions. The flow properties of WRS-gum mixtures were determined from the rheological parameters of the power law model. The addition of GG and XG to WRS resulted in an increase in the apparent viscosity (ηa,100) and consistency index (K) values obtained from power law model. The flow behavior index (n) values of the WRS-XG mixtures decreased with an increase in gum concentration while there was only a marginal difference between n values for the WRS-GG mixtures. Dynamic moduli (G', G", and η *) values in the WRS-gum mixture systems also increased with an increase in gum concentration. WRS-XG mixtures had higher dynamic moduli and lower tan δ (ratio of G"/G') values than WRS-GG mixtures, indicating that the higher dynamic rheological properties of WRS-XG can be attributed to an increase in the viscoelasticity of the continuous phase in the starch-gum mixture systems, which was due to the higher viscoleastic properties of XG compared to GG. The dynamic (η *) and steady shear (ηa) viscosities of the WRS-XG paste at a 0.2% gum concentration followed the Cox-Merz superposition rule.

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