Sustainable vegetable tanning: Ultrasound-assisted tannin extraction of oak bark and analytical approaches to collagen–tannin interactions

Sustainable leather production has long faced the challenge of finding eco-friendly methods maintaining quality standards. This study investigates vegetable tanning with oak bark extracts obtained using eco-friendly Ultrasound-Assisted Extraction (UAE), providing a straightforward analytical approach for a better understanding of the molecular mechanisms by which these extracts stabilise the collagen-tannin interactions. Aqueous extraction of oak bark polyphenols, achieved employing a continuous UAE process, scalable for industrial use, was also boosted using very low concentrations of NaOH (0.1 to 0.5 %) or 0.25 % sulfite/bisulfite. Collagen-tannin matrices were characterized using micro-differential scanning calorimetry (micro-DSC), ATR-FTIR spectroscopy, NMR MOUSE, and thermal microscopy. Key results show that aqueous and low-alkali bark extracts stabilise collagen mainly through hydrogen bonding. Alkali and sulfonated extracts create a mixed network of hydrogen and covalent bonds, likely linked to lignin derivatives. Micro-DSC analysis revealed the highest increase in denaturation temperature, up to 11–14 ◦C, and a narrowing of transition peaks in collagen matrices treated with 0.2 % and 0.5 % NaOH extracts. ATR-FTIR confirmed the formation of ester bonds. NMR MOUSE results showed increased chain packing and water exclusion, linking to enhanced physicomechanical properties. Leather produced using the oak bark extracts showed hydro-thermal and mechanical resistance and structural homogeneity that match those of conventional vegetable-tanned leathers. These findings highlight the technological, commercial and environmental value of UAE-derived oak bark tannins for sustainable leather production. The approach adopted in manuscript fosters sustainability in the tannery sector by reducing the environmental impact and converting tree bark, a low-value by-product, into valuable tannins.