CHARACTERIZATION OF BARK RICH-IN-TANNINS EXTRACTS FROM DECIDUOUS TREES WITH EMPHASIS ON THEIR ANTIOXIDANT ACTIVITY

Abstract

The barks of deciduous trees spread over a wide area of Northern Europe, were sequentially extracted using solvents of increasing polarity. The highest yields of hydrophilic extractives were found for ash tree and goat willow (23.6% and 23.8%, respectively). The highest total contents of polyphenols were found for black alder and goat willow bark (0.18 GAE·g-1 and 0.12 GAE·g-1, respectively). Hydrophilic extract from the both grey and black alder barks contained high amount (up to 7% on bark dry mass) of condensed tannins (CT) or oligomeric proanthocyanidins (OPC). In tests with free radicals (ABTS•+, DPPH•), the high radical scavenging capacities of the hydrophilic extracts enriched with OPC were demonstrated. The antioxidant efficiency of the hydrophilic extracts and purified OPC from alder barks was tested by their influence on thermo-oxidative destruction of model polyurethane (PU) films. The hydrophilic extracts enriched with tannins have good potential as a technical antioxidant for polyurethanes, with the most prominent activity for the hydrophilic extract from black alder and goat willow bark.