The flow curves and time-dependent rheological behavior of two traditional Iranian hydrocolloids obtained from Salep tubers (wild terrestrial orchids) and Balangu seeds (Lallemantia rolyleana) have been investigated. All samples exhibited thixotropic behavior at the concentrations used and both in forward and backward measurements were characterized by the power law model. At the constant shear rate, the apparent viscosity decreased rapidly with time of shearing within the first 100 s for Salep and 50 s for Balangu samples, and approached to a constant value corresponding to a steady state after approximately 450 and 250 s, respectively. The breakdown rate of all samples accelerated at higher shear rates. Four time-dependent models namely the second order structural kinetic model, Weltman model, first order stress decay models with zero and non-zero equilibrium stress values were used to describe the thixotropy behavior and different parameters of these models were analyzed. The results showed that the rate of the thixotropic breakdown increased with increasing shear rate for Salep. Whereas, the extent of thixotrpoy increased with shear rate for Salep and decreased for Balangu. In this study, the selected hydrocolloids were adequately modeled by the first order stress decay model with non-zero equilibrium stress value.

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