Rheology of mixed systems of sweet potato starch and galactomannans

The effect of galactomannans (guar gum and locust bean gum) at different concentrations (0,0.2, 0.4 and 0.6%, w/w) on rheological properties of sweet potato starch (SPS) was studied. The flow behaviors of SPS-galactomannan mixtures were determined from the rheological parameters of power law and Casson models. The SPS-galacto-mannan mixtures had high shear-thinning fluid characteristics (n=0.30-0.36) exhibiting yield stress at 25°C. The presence of galactomannans resulted in the increase in consistency index (K), apparent viscosity (η_a,100) and Casson yield stress (σ_OC) . In the temperature range of 25-70°C, the mixtures followed the Arrhenius temperature relationship. Dynamic rheological tests at 25°C indicated that the SPS-galactomannan mixtures had weak gel-like behavior with storage moduli (G′) higher than loss moduli (G″) over most of the frequency range (0.63-62.8 rad/s) with frequency dependency. The magnitudes of dynamic moduli (G′, G″ and η* of the SPS-galactomannan mixtures were higher than those of the control (0% gum), and increased with an increase in gum concentration. The tan S (ratio of G″G′) values (0.41 -0.46) of SPS-guar gum mixtures were much lower than those (0.50-0.63) of SPS-locust bean gum mixtures, indicating that there was a more pronounced effect of guar gum on the elastic properties of SPS.