Role of extractable lignin in enzymatic hydrolysis of hydrothermally pretreated hardwood

The role of solvent extractable lignin in enzymatic hydrolysis of hydrothermally pretreated sweetgum was investigated. Lignin extraction with acetone, methanol or acetone/methanol removed 27–33% of lignin in hydrothermally pretreated sweetgum pulp, resulting in total sugar yield reduction. Comprehensive characterization indicated that the extractable lignin is highly degraded with low molecular weight, high phenolic hydroxyls, and low native lignin interunit linkages and exhibited neither stimulation nor inhibition effects on enzymatic hydrolysis of filter paper, Avicel, and bleached eucalyptus kraft pulp. The extractable lignin was mainly deposited on the surface of the fibers and pores, which removal caused significant morphological change and collapse of the mesopores and macropores. Thus, the reduction in enzymatic digestion efficiency was more likely caused by the pore collapse in fiber and decreased accessible area, rather than the stimulative action of the extractable lignin. [Display omitted] •Lignin extraction with organic solvents from hydrothermally pretreated sweetgum pulp resulting in total sugar yield reduction.•Extractable lignin exhibited neither stimulation nor inhibition effects on enzymatic hydrolysis of cellulose.•Lignin extraction caused significant morphological change and collapse of the mesopores and macropores.•Reduction in enzymatic digestion was caused by the pore collapse rather than the stimulative action of the extractable lignin.