Design of microwave hot-airflow vibrating drying equipment for initial drying and enzymatic inactivation of fresh fruits and vegetables

Abstract

We developed and optimized microwave hot-airflow vibrating drying (MHAVD) equipment to improve drying efficiency and control the quality of dried agricultural products such as fruits and vegetables. Through three-dimensional modeling and finite element analysis, we simulated and demonstrated microwave source distribution modes and field intensity uniformity. We proposed two flow diversion modes, parallel deflection mode and V-shaped deflection mode, for the hot air distribution system, and we used finite element modeling to analyze them and determine the most optimal modes for use with the dryer. The microwave field strength that achieved the greatest uniformity on the surface of the material tray was used with feeding ports at X = 60 mm, Y = 80 mm, and H = 70 mm. The V-shaped deflector reduced the difference in the wind speeds on the material from 1.1 m/s to 0.36 m/s, thereby significantly increasing the uniformity, although no significant improvement in the local uniformity was noted when the parallel deflector was used. We used fresh balsam pear slices as a representative raw material to measure temperature distribution variations, moisture states, and distribution variations, and we also measured the polyphenol oxidase (PPO) enzymatic activity of the material during the primary drying with MHAVD. The results showed that balsam pear slices could be uniformly and efficiently dried in an environment below 70 °C, and the dry basis moisture content can be reduced from 15.82 to 12.35 in 12 minutes. The NMR/MRI results indicate that mainly free water in the balsam pear slices was decreased in the MHAVD process, with good moisture reduction uniformity. The PPO enzymatic activity decreased from 15.92 U/g to 0 U/g after drying. MHAVD provides a feasible method of enzyme killing, which also has the function of pre-drying for fruits and vegetables. Highlights A microwave hot-airflow vibrating drying experimental apparatus was designed Hot-air and vibrated can improve the uniformity of microwave drying There is a perfect correlation between moisture content and signal amplitude Free water can transform into immobilized water and combined water Microwave pretreatment can inactivate PPO enzyme