Influence of solvent additives on the aqueous extraction of tannins from pine bark: potential extracts for leather tanning

BACKGROUND: Pine bark, which represents approximately 10 to 20% in weight of the tree trunk, is an abundant and low cost agro-industrial waste. In order to obtain condensed tannin-rich pine bark extracts (for future application in leather tanning/retanning processes), conventional aqueous extractions were performed, varying the type and amount of the solvent additive: sodium hydroxide (0.5, 1.0 and 1.5%, w/v), formic acid (0.5, 1.0 and 1.5%, v/v) and ethanol (5, 10 and 15%, v/v). Extractions were performed with and without the addition of sodium sulfite (1.0%, w/v). RESULTS: The most favorable solvent additive in terms of total phenolics and condensed tannins was ethanol: extracts achieved 34.8% of gallic acid equivalents and 62.8% of catechin monohydrate equivalents. Moreover, the favorable pH values (similar to 3.5) of the resultant aqueous solutions and their relatively low viscosities are suitable for subsequent tanning applications. CONCLUSIONS: The results of this study suggest that the high added-value aqueous extracts of Pinus pinaster bark, which were achieved with small amounts of ethanol, have appropriate characteristics for leather tanning, and therefore they may be used as eco-friendly tanning agents to partially replace chromium salts, thus reducing some of the environmental impacts typically associated with the leather industry. (C) 2017 Society of Chemical Industry