Microwave irradiation alters the rheological properties and molecular structure of hull-less barley starch

Abstract

Hull-less barley kernels and their isolated starches were irradiated under different microwave conditions (power 640, 720, and 800 W, time 60, 120 and 180 s) and changes in rheological properties and molecular structures were studied. Steady shear rheological results showed that, as the microwave energy input increased, K values decreased and n values increased, displaying a less thickening, more thinning effect. Microwave irradiation of in-kernel starches may result in more amylose breakdown than that of isolated starches, resulting in smaller K values. Dynamic rheological results showed starch pastes tended to have lower TGʹmax after treatment. The Gʹmax and Gʹ90°C of most in-kernel MWI HBS and isolated MWI HBS increased significantly. In-kernel MWI HBS had higher strength, harder gel, and higher Gʹ25°C, Gʹ0.1Hz and Gʹ20Hz than untreated starches. However, chain-length distributions of A and B chains showed no significant changes. Furthermore, molecular size distributions of the treated starches showed that peaks (Rh = ~100 nm, due to amylopectin) shifted slightly to the left, indicating that amylopectin molecules were degraded. For all treated starches, no new peaks appeared. When the treatment time was 180 s, the peak (Rh = ~10 nm, due to amylose) of in-kernel MWI HBS increased significantly with power, while for isolated MWI HBS, this peak increased dramatically at relatively low power. The primary effect of microwave irradiation may result from cutting C chains.