Abstract: Combined microwave-hot air drying is a potential method to extend the shelf life of burdock. During combined microwave-hot air drying, there is a temperature gradient between the hotter center and the colder surface inside the sample. In order to investigate the effect of the temperature gradient, drying processes with fixed core and hot air temperatures were carried out, and the drying rate and quality of dried product were analyzed. It was found that the drying rate did not change monotonously as the temperature gradient increased. When the drying time was the shortest, the product quality was poor due to the high drying rate. To further improve the drying process, temperature gradient control strategy was proposed for drying process. With this strategy, drying efficiency and product quality could be balanced.

Key words: microwave-hot air combined drying, temperature gradient, burdock, vapor diffusion