Fungal growth necessary but not sufficient for effective biopulping of wood for lignocellulosic ethanol applications

A lignocellulosic biomass treatment method utilizing wood rot fungi to improve cellulosic sugar solubilization for ethanol production was modified to provide larger samples for chemical analyses. Liriodendron tulipifera wood chips were treated in a novel application of aerobic polypropylene “spawn bags” as experimental bioreactors for biopulping. Treatments were inoculated with culture suspensions of white rot (Ceriporiopsis subvermispora) and brown rot (Postia placenta) fungi in 40 day incubations. Fungal growth occurred in all treatments, with extensive hyphal coverage of the biomass (ca. >80% of wood chip surfaces). After treatment, physical biodegradation, holocellulose, α-cellulose, and lignin contents of the fungal-biopulped wood chips were comparable to previous results. However, unlike in previous studies, the biopulped wood did not exhibit significant increases in soluble sugars after enzymatic hydrolysis. These results demonstrate that extensive fungal colonization of biomass is not necessarily sufficient for effective biopulping to increase production of free soluble sugars from wood. Environmental controls on hydrolytic activity by wood rot fungi are poorly known, but we hypothesize that treatment inoculation using nitrogen rich medium may have inhibited lignin-specific fungal hydrolysis of the wood. Future development of effective, standardized biopulping techniques for improving soluble sugar production from lignocellulosic biomass will require optimization of methods including characterization of inoculation nutrients and container effects on fungal metabolic activity.