Nitrogen-removal with protease as a method to improve the selective delignification of hemp stemwood by the white-rot fungus Bjerkandera sp. strain BOS55

Abstract

Certain white-rot fungi cause selective removal of lignin from woody substrates. Selective delignification can potentially be applied to biopulping and upgrading animal feeds. Nitrogen nutrient limitation is known to enhance the selectivity of lignin degradation. The relatively high N-content of annual fiber crops is an important drawback for utilizing white-rot fungi for their selective delignification. In this study, removal of N from hemp stemwood with protease was explored as a means of improving the selectivity of lignin degradation by the white-rot fungus Bjerkandera sp. strain BOS55. Various protease treatments followed by hot-water extraction were found to be suitable in lowering the N-content of hemp stemwood by up to 70%. The removal was significantly higher than with hot-water extraction alone, which caused a 39% N-removal. The selectivity of lignin degradation was compared in protease-treated, hot-water treated, untreated and ammonium-spiked hemp stemwood, providing N levels that were, respectively, 0.32-, 0.61-, 1.0- and 5.0-fold relative to the natural N-content in the substrate. Removal of N by hot-water extraction alone or in combination with protease greatly protected the holocellulose fraction from excessive decay during 10 weeks of solid state fermentation. However, the selectivity of lignin decay was only greatly enhanced (three-fold) by the protease treatment, due mostly to a highly improved lignin degradation at the lowest N-level.