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-galactomannan mixtures had high shear-thinning fluid characteristics (n = 0.30-0.36) exhibiting yield stress at 25 degrees C. The presence of galactomannans resulted in the increase in consistency index (K), apparent viscosity (eta a,100) and Casson yield stress (sigma(OC). In the temperature range of 25-70'C, the mixtures followed the Arrhenius temperature relationship. Dynamic rheological tests at 25 degrees 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 eta*) of the SPS-galactomannan mixtures were higher than those of the control (0% gum), and increased with an increase in gum concentration. The tan 6 (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.