Effect of Isothermal Heat Treatment on Oregonin Content in Black Alder Bark Extract

Black alder (Alnus glutinosa (L.) Gaertn.) wood is characterized by rapid and frequently non-uniform color changes after felling and during hydrothermal processing. These changes are largely associated with extractive compounds, among which oregonin, a phenolic diarylheptanoid glycoside, has been identified as a precursor of the characteristic orange-red coloration. However, information on the thermal stability of oregonin remains limited. This study investigated the effect of temperature and exposure time on oregonin content in black alder bark extract. Aliquots of the extract were exposed to isothermal conditions of 30, 40, 50, and 60 °C for up to 24 h. In a separate trial, a treatment at 98 °C was conducted to simulate the temperature conditions associated with wood saturated water steaming at atmospheric pressure. Oregonin concentration was determined using reversed-phase high-performance liquid chromatography with diode-array detection. Oregonin content remained comparatively stable during 24 h exposure at 30, 40, 50 and 60 °C, with reductions of 6.00%, 3.00%, 2.78%, and 1.64%, respectively, without clear increase in oregonin loss with increasing temperature. Exposure to 98 °C resulted in the most pronounced decrease in oregonin content, with a reduction of 43.93%. The results indicate comparatively high stability of oregonin during 24 h exposure at 30–60 °C, whereas a pronounced decrease was observed in the separate aqueous treatment at 98 °C. These findings are relevant for understanding chemical changes in alder extractives during wood drying and steaming and demonstrate the importance of minimizing unnecessary heat exposure during sample preparation, extraction, and storage.