Effect of Radio Frequency Vacuum Drying on Drying Characteristics and Physicochemical Quality of Codonopsis pilosula Slices

In this study, the radio frequency vacuum drying (RFVD) technique was used to dry Codonopsis pilosula slices. The effects of the drying temperature, slice thickness, plate spacing, and vacuum degree on the drying characteristics and physicochemical properties of the slices were investigated. The results showed that as the drying temperature and vacuum degree increased and the slice thickness and plate distance decreased, the drying rate and effective moisture diffusion coefficient of the Codonopsis pilosula slices improved, and the required drying time was shortened by 11.11% to 29.41% compared to that after hot air drying (HAD). Through comparison, it was found that the Midilli and Weibull models could better describe the moisture variation trend during the RFVD of Codonopsis pilosula. After RFVD, the retention of lobetyolin and syringin in Codonopsis pilosula significantly increased, with maximum contents of 135.74 mg/100 g and 19.16 mg/100 g respectively, which were 75.2% and 124.28% higher than those obtained by HAD. The contents of polysaccharides, total phenolics, and total flavonoids and antioxidant performance were also enhanced. The color, shrinkage rate, and internal tissue structure were significantly improved. In conclusion, RFVD not only increases the drying speed of Codonopsis pilosula slices but also ensures the good quality of the dried products.