Testing the effects of drying methods on willow flavonoids, tannins, and salicylates

In this study, we compared the effects of several preservation methods on the secondary phenolics of the mature leaves of purple willow (Salix purpurea L., Salicaceae) with results obtained with fresh leaf analyses. Conventional freeze-drying, in which the leaves were first frozen with liquid nitrogen and then placed in a freeze-dryer, produced substantial qualitative and quantitative changes in purple willow flavonoids and salicylates. Modified freeze-drying, in which leaves were put into a freeze-dryer without being prefrozen, gave concentrations that, for most secondary components, were comparable with those found in fresh leaves. Reducing the freeze-dryer chamber temperature hindered the decomposition of phenolics in prefrozen leaves and in leaves dried without prefreezing. Heat drying induced substantial changes in the composition of all phenolics, except for apigenin-7-glucoside. Vacuum drying at room temperature gave the highest concentrations for nearly all phenolics, while room-drying with desiccation gave results that were comparable with those obtained by fresh leaf analyses.