Effect of temperature difference on the aroma and quality of carrots processed through microwave drying combined with hot air drying

A real-time aroma -detecting system for carrot processing through microwave combined with hot-air drying was constructed to study the aroma diffusion and quality of carrots by controlling the material temperature and temperature differences between material core and hot air. Various pairs of material temperatures (60 degrees C, 70 degrees C, 80 degrees C) and temperature differences between material core and hot air (10 degrees C, 20 degrees C, 30 degrees C) were applied. The best quality evaluated by total vitamin C content, colour change, drying time, sensory evaluation and rehydration ratio was obtained with material temperature at 70 degrees C and temperature difference at 20 degrees C. Seven typical volatile peaks were detected and recorded during drying for all the three replicates. The spread of aroma is divided into two phases: the first phase is the change phase of the first hour; the second phase is the constant phase after the first phase. The emission of carrot and charring peaks increases with the increasing material temperature. When the material temperature is at 60 degrees C which is the best material temperature for aroma, with the least aroma emission, the best quality was obtained with temperature difference at 20 degrees C. Based on the fixed aroma release, a mixed drying condition was employed to control the drying process. During first hour it was set using a material temperature of 60 degrees C and temperature difference of 20 degrees C. After one hour, the material temperature was set at 70 degrees C and temperature difference was set at 20 degrees C. It was found that the quality were better than those of the fixed material temperature and temperature difference. Therefore, on the basis of ensuring the quality of the product, the temperature and temperature difference of the material can be adjusted by the odor law to obtain a product having a better odor and quality. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.