The effects of drying temperature and air velocity on the drying characteristics, color, bioactive compounds, rehydration ratio, and microstructure of Ophiopogonis Radix during hot air impingement drying (HAID) were explored in the current study. The experimental results showed that the drying temperature and air velocity had a significant impact on the drying characteristics and quality attributes of dried products except for the rehydration ratio. The drying time decreased from 720 to 240 min with the increase of drying temperature from 50 to 70 °C. Increasing the air velocity from 6 to 12 m/s enhanced the drying process of Ophiopogonis Radix, while the extension of air velocity to 15 m/s lowered the drying rate. The samples that were dried at a lower drying temperature obtained lower color difference. Properly increasing the drying temperature or air velocity could increase the total polysaccharide and flavonoid contents of dried products. Additionally, a back-propagation neural network (BPNN) model was developed to predict the moisture ratio of Ophiopogonis Radix during the drying process. The optimal BPNN with 3-11-1 topology were obtained to predict the moisture ratio of Ophiopogonis Radix during HAID and performed with an acceptable performance.

In order to explore an alternative drying method to enhance the drying process and quality of persimmon slices, pulsed vacuum drying (PVD) was employed and the effects of different drying temperatures (60, 65, 70, and 75 °C) on drying kinetics, color, rehydration ratio (RR), microstructure, bioactive compounds, and the antioxidant capacity of sliced persimmons were investigated in the current work. Results showed that the rehydration ratio (RR) of the samples under PVD was significantly higher than that of the traditional hot air-dried ones. Compared to the fresh samples, the dried persimmon slices indicated a decrease in the bioactive compounds and antioxidant capacity. The total phenolic content (TPC) of PVD samples at 70 °C was 87.96% higher than that of the hot air-dried persimmon slices at 65 °C. Interestingly, at 70 °C, the soluble tannin content and TPC of the PVD samples reached the maximum values of 6.09 and 6.97 mg GAE/g, respectively. The findings in the current work indicate that PVD is a promising drying method for persimmon slices as it not only enhances the drying process but also the quality attributes.

Effects of ultrasound-assisted pretreatment combined with air impingement drying on drying kinetics, microstructure, crystallinity, color, total phenolic content (TPC), and antioxidant capacity of Chaenomeles sinensis slices were investigated in the current study. Experimental results showed that ultrasonic pretreatment significantly enhanced heat and mass transfer, reducing total drying time by 12.50% to 43.75% compared to untreated samples. Optical microscopy and scanning electron microscopy revealed that ultrasound disrupted cell walls, loosened tissue structure, and increased porosity, creating microchannels favorable for moisture diffusion. X-ray diffraction and color analysis indicated that moderate ultrasonic treatments improved crystallinity and reduced color degradation. Furthermore, increasing the ultrasonic time and power could greatly increase the TPC and FRAP values in fresh samples. The highest TPC, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, and ferric-ion reducing antioxidant power (FRAP) values in dried products were achieved with ultrasonic pretreatment at 160 W for 40 min, i.e., 8.71 mg/g, 6.14, and 23.18 mmol Trolox/g, respectively. These findings suggest that appropriate ultrasonic pretreatment facilitates the release and retention of bioactive compounds by altering the microstructure and crystallinity, as well as shortening drying duration. In summary, ultrasound-assisted air impingement drying is an effective approach to processing C. sinensis slices with the improvement of drying efficiency and physicochemical properties of products.

An emerging drying method, far-infrared radiation heating-assisted pulsed vacuum drying (FIR-PVD), was employed to dry raspberries. In this study, the impacts of FIR-PVD, freeze drying (FD), and hot air drying (HAD) on drying characteristics and quality attributes, including color, rehydration ratio, content of total phenolics (TP), content of total monomeric anthocyanins (TMA), antioxidant activity, and microstructural attributes of dried raspberries, were examined. Results indicated that FIR-PVD notably shortened the drying time by 47.78% compared to HAD and by 73.89% compared to FD. The FIR-PVD samples exhibited the highest TP content, DPPH radical scavenging activity, and FRAP value, which were 17.73%, 6.09%, and 38.16% higher than those of the FD samples, respectively, and 2.78%, 2.77%, and 18.74% higher than those of the HAD samples. Significant correlations (p < 0.05) were observed between antioxidant capacity, as measured by DPPH and FRAP assays, and TP content. However, FD at a low temperature led to a higher TMA content than FIR-PVD and HAD. FIR-PVD resulted in the highest ΔE values of dried products due to the lightness enhancement. In addition, the dried products obtained by FIR-PVD had better rehydration capacity. These findings indicate that FIR-PVD presents a promising alternative method for drying raspberries, as it enhances drying efficiency and improves the quality attributes of the dried products.