Effect of Microwave Roasting on the Physicochemical, Functional, Rheological, and Antioxidant Properties of Kodo Millet Flour

Abstract

In this study, Kodo millet grains were roasted in the microwave at 360, 720, and 900 W for 3 and 5 min and pan-roasted at 195°C for 5 min. The Kodo millet flours (KMFs) were characterized on the basis of physical, functional, pasting, rheological, antioxidant, morphological, and structural properties. After roasting, the bulk density (BD) of all treated samples decreased, while the tap density, Hausner’s ratio, and compressibility index (CI) increased with roasting. Higher microwave power (720 and 900 W) and duration time (5 min) resulted in lower L∗ values and higher a∗ and b∗ values. The roasting in pan and microwave significantly improved the water absorption capacity (WAC) of flour. However, the destruction of hydrophobic functional groups resulted in a significant reduction in the oil absorption capacity (OAC). The pasting and rheological curves confirmed that low power can improve the pasting properties, producing thick pastes. However, the pan and microwave roasting (MWR) at 900 W for 5 min degraded starch and resulted in a flat pasting curve. Scanning electron microscope (SEM) micrographs also confirmed this, showing pore space formation, initiated at a power of 720 W and becoming more noticeable until 900 W, when the structure is destroyed. The X-ray diffraction (XRD) patterns of all samples suggested a type A crystalline structure, and the degree of crystallinity increased with microwave power, except for the 900 W for 5 min and the pan-roasted KMF. The antioxidant activity (AOA) of all roasted samples was higher than that of the control sample, except for 900 W for 5 min. Total flavonoid content (TFC) decreased in all KMF compared to the control, except 360 W for 3 min. Furthermore, all microwave-roasted KMF showed significantly higher total phenolic content (TPC) than control and pan-roasted KMF. These findings provide valuable information on optimizing the roasting process to improve the nutritional and functional properties of KMF.