Temperature gradient control during microwave combined with hot air drying

Fresh carrot cubes were blanched and dried by microwave processing combined with hot air drying. Material temperature, temperature difference and size of cube were optimised in terms of total vitamin C content, colour change, drying time, sensory evaluation and rehydration ratio of dried samples. During a stage under the optimal drying condition, drying rate was affected by temperature gradient, as evidenced by changed shrinkage. The effect of different temperature gradients on the drying rate and quality was compared with that under the optimal condition by using fitted shrinkage models of the optimised material temperature, temperature difference and size of cube. The quality of dried carrot cubes under the average temperature gradient (ATG) of 6 °C mm−1 was better than that under other ATGs. A simple linear control method was developed based on the control for industrial production. Results showed that the product quality and drying time of samples in linear control were similar to those dried at 6 °C mm−1.