Multiple steps and critical behaviors of the binding of tannic acid to wheat starch: Effect of the concentration of wheat starch and the mass ratio of tannic acid to wheat starch

The objective of this study was to investigate the effect of concentration of wheat starch (WS) and mass ratio of tannic acid (TA) to WS on interaction between TA and WS, using isothermal titration calorimetry (ITC), UV-vis spectrophotometer and dynamic light scattering (DLS). The interaction process and structural properties of complexes formed with different mass ratios of TA to WS ranged from 0.8 to 2.4 were also investigated through X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and rheological measurements. The results of turbidity, particle size and ITC revealed that TA added into WS could spontaneously form complexes through three steps with the increasing of TA/WS ratio. They were assigned to (i) interaction of TA with WS forming soluble complexes; (ii) propagation and formation of insoluble complexes via cross-linking these soluble complexes; and (iii) lateral association of soluble/insoluble complexes, generating larger aggregates even precipitation. The interaction process would be delayed with decreasing the WS concentration. The results of SEM further revealed the three-step interaction process and showed that the TA/WS complexes appearance was approximately spherical. The results of dynamic rheological, TA loading amount and XPS showed that the complexes had highly intermolecular cross-linked gel-like network structures and contained lower TA inside and higher TA outside.