Comparative effects of conventional and modern drying methods on drying time, energy consumption, and physicochemical properties of lavender (Lavandula stricta Del.)

Lavender is known as a natural antidepressant and sleep-promoting herbal medicine. In this study, we compared six distinct drying techniques: Refractance window drying (RW), freeze drying (FD), microwave drying (MW), infrared drying (IR), hot air drying (HA), and shade drying (SD), to evaluate their effects on drying time, energy and physicochemical and phytochemical properties of dried lavender. Moreover, we compared drying methods and their influence on water activity, rehydration ratio, total phenolic content, flavonoid content, antioxidant activity, color, and essential oil. RW and MW drying illustrated short-term drying processes with energy saving. All dried lavender samples demonstrated water activity levels within acceptable limits (less than 0.6). The results showed that FD and RW could preserve the total phenolic and total flavonoid contents compared to other techniques. In addition, there was no significant difference (p < 0.05) between RW and frozen samples regarding essential oil yield and rehydration ratio. The total antioxidant capacity of the fresh sample was significantly higher than that of dried samples, but FD significantly preserved the antioxidant content better than other treatments. Moreover, SD-dried samples had the lowest physicochemical and bioactive properties, which were attributed to oxidation reactions and long drying times. The RW sample showed the least color difference. Among the studied methods, FD and RW are recommended due to their superior ability to preserve bioactive compounds and increase the rehydration ratio. In addition, the RW drying method can provide a suitable alternative to FD by providing less time and energy consumption, practical preservation of physicochemical properties, and fewer color changes.