Continuous microwave drying of germinated brown rice: Effects of drying conditions on fissure and color, and modeling of moisture content and stress inside kernel

Suitable fissures in kernels and the golden yellow color are the desired visual quality to evaluate germinated brown rice (GBR) subjected to drying processing. To improve the visual quality of GBR dried in a continuous microwave dryer, the effects of microwave intensity (1–5 W/g), air velocity (0–2 m/s), drying duration for per drying pass (2–10 min), and material layer thickness (4–12 mm) on the formation of visual quality of GBR in terms of fissure and color were investigated by using bench experiments and numerical modeling. The results showed that temperature and moisture content of the final dried GBR had high uniformity in the continuous microwave dryer, and most of the dried GBR kernels were in a “mild” (1–2 fissures in kernels) and “moderate” (3–4 fissures in kernels) fissure degree at the microwave intensity of 2–3 W/g. The increasing air velocity may weaken the effects of interior stress caused by moisture content gradients on fissures in GBR kernel. The chroma b*, representing the golden yellow color index of GBR, significantly (P < 0.05) increased with the microwave intensity and the drying duration for per drying pass of 4–6 min. The material layer thickness lower than 10 mm was suitable for the formation of golden yellow color appearance of GBR. The simulation results confirmed that microwave intensity had a positive effect on the formation of fissures for GBR in a continuous microwave dryer due to the rising stress along the short-axis direction of GBR kernel. The distribution of moisture content inside GBR kernel was relatively uniform under the microwave intensity lower than 3 W/g, and the higher moisture content gradients aggravated the evolution of fissures in kernels. Considering the desired visual quality of the final GBR, the appropriate microwave drying conditions of GBR were developed as microwave intensity of 2 W/g, air velocity of 1.0–1.5 m/s, drying duration of 6 min for per drying pass and 8 mm material layer thickness, which induced the generation of suitable fissures and the formation of golden yellow color for GBR under the continuous microwave dryer. This study may provide a preference for the industrial application of microwave drying of cereal materials such as GBR.