On hexenuronic acid (HexA) removal and mediator coupling to pulp fiber in the laccase/mediator treatment

Flax soda/AQ pulps were treated with different fungal laccase–mediator combinations followed by physical and chemical characterization of the pulps to obtain a thorough understanding of the laccase/mediator effects on hexenuronic acid (HexA) removal and the coupling of mediator onto pulps for fiber functionalization. Large differences were found and the presence of lauryl gallate (LG) during Trametes villosa laccase (TvL) treatment (TvL + LG) resulted in a much larger reduction of pulp-linked HexA than the combination of p-coumaric acid (PCA) and Pycnoporus cinnabarinus laccase (PcL). A major portion of LG became attached to the pulp as revealed by an increase in the kappa number and further confirmed by thioacidolysis and 1H NMR analysis of solubilized pulp fractions. Additional experiments with other chemical pulps and isolated pulp xylan and lignin revealed that HexA seems to be the sole pulp component attacked by TvL + LG. As a substrate for TvL, the reaction preference order is PCA > HexA > LG.