Wheat starch-tannic acid complexes modulate physicochemical and rheological properties of wheat starch and its digestibility

In this study, the interactions of wheat starch (WS) and tannic acid (TA) were investigated for their gelatinization, pasting, structural, rheological properties and digestibility of wheat starch. TA was either complexed with starch (WS-TA complexes) or mixed with starch (WS-TA mixtures) right before the characterization of its properties. The increase of melting enthalpy and temperature range (T_peak - T_onset) associated with melting of amylose-lipid complex and the increase in the X-ray diffraction peak at 2θ = 20° possibly indicated the formation of inclusion types of complexes. The appearance of an endothermic transition at 130–160 °C indicated the formation of non-inclusion types of complexes. Non-inclusion type of complexes were mostly formed by co-gelatinization of WS-TA mixtures, while inclusion complexes would be mostly formed by complexation of TA with ungelatinized starch. TA in mixtures increased G′ and viscosity and decreased frequency dependency of the moduli, thus producing a stronger gel. TA in complexes decreased G′ and viscosity at low TA%, thus producing a weak gel, but increased gel strength at high TA%. The storage moduli G′ increased depending on the amount of TA involved in non-inclusion complexes. The formation of complexes between WS and TA largely slowed down the digestibility of gelatinized starch. The insights gained in this study provide opportunities to modulate starch techno-functional properties and digestibility in processing of starchy food.