Pulsed Vacuum Drying (PVD) of Hawthorn (Crataegus spp.) Slices: Effects on Drying Characteristics and Physicochemical Quality

This research investigated the effects of drying temperature and vacuum pressure duration (t-VPD) on drying characteristics, rehydration ratio (RR), microstructure, color, vitamin C (VC), and citric acid (CA) during the pulsed vacuum-drying (PVD) process of hawthorn slices. An extreme learning machine (ELM) model was developed to predict the change of moisture content of hawthorn slices during PVD process. Experimental results indicated that both drying temperature and t-VPD significantly affected the drying characteristics and quality attributes of dried hawthorn slices. As the drying temperature increased from 60 to 70 degrees C, the drying time decreased from 274 to 181 min. Compared to hot air drying (HAD), the dried hawthorn obtained by PVD had higher retention rates of VC and CA and lower color differences. The highest retention rates of VC and CA of dried hawthorn slices occurred at a drying temperature of 60 degrees C and a t-VPD of 8 min, i.e., 80.65% and 94.82%, respectively. PVD promoted the formation of pore structures and improved the rehydration capacity of dried products. The correlation analysis showed that the values of a* and b* were in association with the VC content of hawthorn. Besides, an optimal ELM model with a 3-12-1 topology structure was obtained to predict the change of moisture ratio of hawthorn slices during PVD process. The obtained R-2 and RMSE values between the actual and predicted values were 0.997 and 2.07e(-2), respectively. Overall, these results indicated that PVD could be a desirable technique for the drying of hawthorn slices processing.