Value Adding of Poplar Hemicellulosic Prehydrolyzates: Laccase Production by Botryosphaeria rhodina MAMB-05 and Its Application in the Detoxification of Prehydrolyzates

Abstract

Fast-growing hybrid poplar species are attractive as feedstocks for the production of bio-based chemicals including biofuels. Pretreatment makes cellulose accessible to enzymatic hydrolysis, producing hemicellulose and lignin as major co-products. Poplar woodchips pretreated hydrothermally in an experimental industrial pilot-plant generated two prehydrolyzate streams; purge-condensate collected from cooking the woodchips and press-filtrate collected after the fiber explosion step. Both prehydrolyzates contain hemicellulose sugars (pentoses, hexoses, oligosaccharides), acetic acid, and degradation products from hemicellulose and lignin (furanic and phenolic compounds, respectively). Purge prehydrolyzate was highly diluted, of low sugar content and high biochemical/chemical oxygen demand, and constituted a process waste that was addressed for value-adding through production of enzymes by fermentation. Wood-decay fungi were screened for laccases on purge prehydrolyzate, with Botryosphaeria rhodina producing highest titers. B. rhodina detoxified the prehydrolyzate but at expense of sugar consumption. Adding xylose and glycerol to purge did not inhibit fungal growth or impede laccase production. Phenolics in the purge had inducing effect on enhancing laccase titers. Laccase production was optimized by Box-Behnken-(33)-factorial design varying the concentration of xylose in the purge, together with adding copper as laccase inducer and glycerol to increase the level of fermentable substrate, resulting in optimal enzyme titers (36.37 ± 3.52U/mL; validated). The crude laccase preparation was employed to detoxify the more concentrated press prehydrolyzate, reducing the overall content of phenolics by ∼30 %. Chromatographic (high-performance liquid chromatography-ultraviolet, gas chromatography/mass spectrometry) analysis identified various phenolic compounds present in the press prehydrolyzate following laccase treatment, and in the presence of the mediator, ABTS, the phenolics content decreased further.