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 (∼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. © 2017 Society of Chemical Industry