Optimization and modeling of the stale bread drying with three different dryers

The present work is aimed to study the drying process of stale bread to a moisture level of 8% wet basis using three different dryers: single convective, single microwave and combined convective-microwave. The experiments were designed and analyzed using response surface methodology with central composite design. Different operating parameters were investigated including air temperature (40–60 °C) and velocity (0.5–1.5 m/s) for single convective and combined convective-microwave dryers and microwave power (90-900 W) for single microwave and combined convective-microwave dryers. The drying time, energy consumption, and water activity of the stale bread were selected as the responses of the drying process. Comparing the results of these three drying methods, it was indicated that the shortest drying time was occurred in the combined microwave-convective dryer. The lowest energy consumption was related to the single microwave dryer (0.019-0.116 kWh), followed by the combined convective-microwave dryer (0.046-0.584 kWh) and single convective dryer (0.588-1.255 kWh). Range of the water activity was obtained desirable for all of the three dryers. The process was then optimized to achieve the minimum drying time, energy consumption, and water activity. The optimal conditions were found to be 616 W for single microwave dryer, 57 ℃ and 1.1 m/s for single convective dryer and 580 W, 50 ℃, and 1.2 m/s for combined convective-microwave dryer. The combined convective-microwave dryer also showed the best performance in the optimal conditions consisted of 1.47 min drying time, 0.083 kWh energy consumption and 0.201 water activity.