Enzyme-assisted nanofiltration to enrich tannins from softwood bark extract

Abstract

Softwood bark is a rich source of renewable chemicals, including phenolic compounds known as tannins. Tannins can be extracted from bark with hot water, but the yield can be relatively low, leading to dilute extracts. Next to tannin, a considerable amount of extracted dry matter comprised of carbohydrates, and bark carbohydrates limit the applicability of the extract in applications that demand high purity. This study used membrane filtration to refine and concentrate tannins produced from hot water extracts of softwood bark. Enzymatic hydrolysis of bark carbohydrates prior to membrane filtration was assessed as means to improve the separation of bark sugars from bark phenols. Enzymatic hydrolysis using an optimised enzyme mixture liberated 55% of bark carbohydrates mainly by degradation of pectin and other polysaccharides in spruce unrefined tannins. Separation of bark phenols from the enzyme-treated extract by precipitation at acidic pH and subsequent microfiltration produced low tannin yield. Instead, keeping bark phenols soluble at alkaline pH allowed us to concentrate them 15-fold with good and stable flux using nanofiltration. Additionally, enzymatically liberated sugars permeated well through the nanofiltration membrane, decreasing the free sugar content of the concentrated tannin from 31% to 7.4%. However, diafiltration did not seem to decrease the proportion of free sugars in the concentrate further. This enzyme-assisted nanofiltration concept was successfully upscaled to a pilot scale which was a promising result for improved industrial applicability of softwood bark tannins in the future.