Physicochemical, rheological, morphological and in vitro digestibility properties of pearl millet starch modified at varying levels of acetylation.

Pearl millet (PM) starch was reacted with acetic anhydride at different concentrations (1.25, 2.5, 3.75 & 5.0%) and its physicochemical, rheological, morphological and in vitro digestibility properties were compared with native starch. The acetyl (%) and degree of substitution (DS) of acetylated starches ranged between 1.07 and 2.15% and 0.040-0.081, respectively. Swelling power and solubility of acetylated PM starch increased progressively upto 3.75% level of acetylation, however, further increase in acetylation levels resulted in a decrease. Peak, setback and final viscosities of acetylated PM starches were higher than their native counterpart starch. Both native and acetylated PM starches showed similar A-type X-ray diffraction patterns. During heating, storage modulus (G') and loss modulus (G″) of acetylated starches ranged between 753 and 1177Pa and 96-129Pa, respectively. G' was much higher than G'' at all the values of angular frequency studied. Both native and acetylated PM starch pastes showed flow behaviour index value of <1. For acetylated starches, both, yield stress and consistency index values were higher than native starch. Readily digestible and slowly digestible starch contents of acetylated PM starches varied between 44.3 and 45.5% and 27.3-32.5%, respectively, the highest values observed for starch acetylated at 3.75% and 1.25% acetic anhydride concentrations.