Apple volatiles monitoring and control in microwave drying

Apple volatiles were online monitored with a zNose™ during microwave drying. The samples were first dried at three fixed temperatures of 60 °C, 70 °C, and 80 °C. Four typical volatile peaks were recorded every 4 min for all the three replicates. Apple samples could be dried in a shorter time at high temperatures but were more susceptible to burning. A fuzzy logic controller was then designed and employed to control the drying process. The controller used online detected apple volatiles and burning signals as variables and tried to retain apple volatiles, avoid burning, shorten drying time, and save energy. Based on the temperature curve achieved under fuzzy logic control, a simple linear control method was further developed where zNose™ online assistance could be omitted. Color and sensory evaluation by an untrained taste panel were performed after rehydration. With the new control strategies, apple color and overall appearance were intact and less time and energy were consumed.