Pilot-scale extraction of Norway spruce bark polyphenols with purification for tannin-enriched fractions

Abstract

Softwood bark represents an abundant and promising source of high-value bioactive polyphenols. However, it is currently used mainly for energy production, and its efficient extraction remains a major bottleneck for the industrial valorization of tannins and other polyphenols. The aim of this study was to demonstrate the pilot-scale extraction and purification of antibacterial polyphenols from industrially sourced Norway spruce (Picea abies) bark using pressurized hot water extraction and XAD7HP polymeric resin. Nine batches of winter- and summer-harvested bark were processed in a 300 L reactor at 58–122 °C for 40–80 min, with characterization of moisture, particle size, and bark-to-wood ratio to account for feedstock variability. The maximum overall extraction yield (140.4 mg/g) on a dry bark basis (DB) was obtained at 122 °C. Seasonal differences were evident, as winter bark consistently provided higher yields than summer bark. Resin purification significantly enriched the polyphenol content, increasing condensed tannins from 2.1 to 4.6 mg/g to 23.4–59.2 mg/g (DB) while markedly reducing carbohydrate contamination. The tannin-enriched fractions also exhibited strong antibacterial activity against bioluminescent Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) biosensor strains. These results confirm the technical feasibility of producing polyphenol-rich fractions from spruce bark at pilot scale, supporting their use in biobased industrial applications such as cosmetics, feed, and water treatment.