Effect of Microwave-Treatment Time on the Drying Rate of Sprouted Lentil in Different Dryers

In the food drying industry, it is especially important to use various pretreatments to reduce drying time and improve nutritional properties. As a fast and effective heating source with together thermal and non-thermal impacts, microwave can directly affect the food material, thereby speed upping physicochemical reactions, and drying rate, and produce high-quality dried products. Using new methods such as infrared reduces drying time and improves the quality of the dried product. Sprouting is one of the most traditional methods used for reducing most of the anti-nutritional constituents in legumes. In addition, sprouting is considered to enhance the nutritional and quality characteristics of legumes. For the preparation of sprouts, lentils were purchased from a market, cleaned, soaked for 24 hours, and then germinated at 25°C for 48 hours. In this study, the effects of microwave time and dryer type (hot-air and infrared) on drying time, effective moisture diffusivity coefficient and rehydration of sprouted lentils were studied to model drying kinetics. To apply microwave pretreatment, sprouted lentils were placed in a microwave oven for 0, 20, 40, and 60 seconds, and after exiting from device, they were placed in a hot-air (with a temperature of 70°C) and an infrared (power of 250 W) dryers in a thin layer. The results of this research showed that microwave pretreatment increases the outflow rate of moisture from the sprouts, increases the effective moisture diffusivity coefficient, and shortens the drying time. By increasing the microwave treatment time from 0 to 60 s, the sprouts drying time in hot-air and infrared dryers decreased by 38.46% and 25.53%, respectively. The average drying time for the samples was 130.8 minutes for the hot air dryer and 26.3 minutes for the infrared dryer. The average effective moisture diffusivity coefficient calculated for the samples placed in the hot air dryer was 2.82×10-10 m2s-1 and for the infrared dryer it was 1.76×10-9 m2s-1. Microwave exposure time and drying method had a significant effect on samples rehydration, with higher values for this parameter in microwave-treated and hot-air dried samples. To study the drying kinetics of sprouted lentils, the Page model was selected as the best model because a mathematical model was fitted to the laboratory data and had the highest accuracy. In general, the optimal conditions for drying sprouted lentils were pretreatment in a microwave oven for 60 seconds followed by the use of an infrared dryer.