Advantages of MW-assisted water extraction, combined with steam explosion, of black alder bark in terms of isolating valuable compounds and energy efficiency

“Green” extraction of black alder bark using deionized water as an alternative to organic solvents and microwave (MW) - induced heating instead of convective/conductive heating was studied. An extractor of original construction with the MW capacity of 2.4 kW, equipped with a pressurized extraction chamber (V = 1350 cm³) and a condenser section, was used. The MW-assisted heating of the suspension up to the desirable temperature was performed with a uniform high rate of ~ 30 ℃ min⁻¹. The yield of the black alder bark water extracts obtained by MW-assisted extraction varied from 15.4% to 26.4% (on bark dry matter) increasing with increasing extraction temperature (70–150 ℃) and increasing duration of the isothermal step of extraction (0–30 min). The electricity consumption per dry weight of obtained products needed for MW-assisted heating of the substrate containing suspension varied from 6.2 to 32.0 kWh kg⁻¹ being higher at higher extraction temperature and longer duration of isothermal heating step. UHPLC-ESI-MS/MS, GC-FID, Py-GS/MS/FID, UV spectroscopy methods revealed that the main constituents of the obtained black alder bark water extracts were diarylheptanoids, condensed tannins, carbohydrates, organic acids, flavonoids, lignin related phenols. The MW-induced depolymerization of alder bark cell wall constituents (hemicellulose, lignin) evidenced by Py-GC/MS/FID, GC-FID, wet chemistry analyses of extracts and bark residues after extraction, resulted in the disruption of the bark cell wall and isolation of metabolites bound within it by microwaves-assisted extraction (MAE). Therefore, the black alder bark water extracts, obtained by MAE at 70–110 ℃, compared to those isolated by advanced accelerated solvent extraction (ASE) at a given temperature, have higher diversity and higher contents of different groups of phenolic extractives, including a higher content of their major class in alders – diarylheptanoids. The increase of the extraction temperature above 110 ℃ and the duration of extraction up to 30 min led to an increase of the content of carbohydrates and noticeable decrease of phenolics content in extracts, including the dominating diarylheptanoid - oregonin. The content of oregonin, having a high antioxidant activity and valuable biological activities, was in focus of our research and practical interest. MAE with dynamic heating to 90 ℃ without keeping of isothermal heating is the most attractive for obtaining of oregonin-enriched extracts, containing up to 57% of oregonin, in contrast to ASE water extracts with the maximum oregonin content of 39%. In this regime, two-fold less energy consumption (7 vs 15 kWh kg⁻¹) and two and a half times more productivity (1.5 vs 0.6)% min⁻¹ was achieved in comparison to the case of the ASE method used as reference.