Changes in water status and microstructure reveal the mechanisms by which tempering affects drying characteristics and quality attributes of medicinal chrysanthemums

To elucidate the mechanism behind the heterogeneity in the water distribution during the drying process of medicinal chrysanthemum (Imperial chrysanthemum), and thereby optimize the drying process, this study introduced an innovative approach by incorporating tempering into the drying process and evaluated the effects of hot air drying (HAD), infrared-assisted hot air drying (IR-HAD), and tempering-incorporated IR-HAD (TD+IR-HAD) on the water status, microstructure, and phytochemicals of dried chrysanthemums. Results showed a significant difference in the rate of dehydration between the central and marginal parts of chrysanthemums. The highest difference in moisture content value (1.34 g water/g dry matter) was observed between these two parts for samples subjected to 2.5 h of IR-HAD drying. Consequently, the sample subjected to 2.5 h of IR-HAD drying was selected for tempering. After the optimal tempering treatment (12 h), the samples were subjected to an additional 0.5 h of IR-HAD drying. This incorporated drying method (TD+IR-HAD) effectively reduced the drying time and energy consumption. Transverse relaxation curves and microstructural observations further confirmed that these reductions were related to the enhancement of water mobility and the formation of microchannels during tempering. As a result, chrysanthemums dried using TD+IR-HAD showed increased contents of chlorogenic acid, luteolin, total phenolic, total flavonoid, antioxidant capacity, and the number of volatile compounds compared to HAD and IR-HAD. Overall, these results highlight the potential of TD+IR-HAD as a promising drying technique for enhancing the value and applications of medicinal chrysanthemums in industrial settings.