The effect of thermal drying on the contents of condensed tannins and stilbenes in Norway spruce (Picea abies [L.] Karst.) sawmill bark

Abstract Norway spruce (Picea abies (L.) Karst.) bark contains marked amounts of polyphenolic compounds. Condensed tannins (CTs) and stilbenes show commercial potential as antioxidants, antimicrobials, preservatives in food and cosmetic applications, technochemical products, and pharmaceuticals. Storing of bark before the conversion process leads to substantial losses of extractives compounds. In the present study, the potential of thermal drying for maintaining extractives content was assessed based on an experiment in which bark samples were dried in convection kilns at 40, 50, 60, and 70 °C temperatures. The development of CTs and stilbene contents and CT degradation were followed for 28–34 h. CTs were analysed from bark samples with thiolysis. Quantities of stilbene glycosides and stilbene aglycones in water-acetone extracts were analysed applying gas chromatography with flame-ionization detection (GC-FID). Multilevel regression analysis was used to analyse the statistical differences in moisture content and extractives composition between the drying schemes. The initial CT content of 35–36 mg g−1 in dry bark material declined to 25–31 mg g-1 in 28–34 h. The average degree of polymerisation (DP) decreased slightly, and the relative proportion of prodelphinidins in CTs increased significantly in the 60 and 70 °C schemes. The proportion of A-type linkages slightly increased with the increase in drying temperature. The initial mean stilbene contents varied from 19 mg g−1 to 22 mg g−1 in dry bark mass. Isorhapontin was the major stilbene constituent, with a proportion of 45–49 % of the total stilbenes. Stilbene losses of up to 60 % were detected during the drying processes. In 10 h, for example, 36–43 % of total stilbenes were lost. Degradation activities by enzymes released from the bark and oxidative reactions after crushing at the debarking phase were concluded to be the primary mode of degradation. The results indicate that bark CT content can be preserved at a moderate temperature not exceeding 50 °C, but the degradation of CTs may affect their suitability for various applications. Sufficient stilbene content for industrial processes is unlikely to be maintainable through thermal drying. The permanence of the post-drying extractives content should be assessed based on a practical-scale storage experiment using bark dried to varying moisture contents.