Combined production of edible mushrooms and biofuels from lignocellulosic residues

• Forest residues, wood-processing by-products and other lignocellulosic materials are promising renewable resources for bioconversion to advanced biofuels such as cellulosic ethanol. Increased production of biofuels from lignocellulosic materials would allow reducing environmental problems caused by the use of fossil resources while supporting development of bioeconomy. Enzymatic saccharification of cellulose is crucial in the production of cellulosic ethanol, since it produces the carbohydrates needed for microbial fermentation. However, due to feedstock recalcitrance, enzymatic saccharification of raw lignocellulose results in low rates and yield. In this thesis, the possibility of pretreating lignocellulosic biomass with edible white-rot fungi for integrated production of edible mushroom and cellulosic ethanol, as primary products, were investigated. This thesis was able to conclude that the effectiveness of fungal pretreatment, using shiitake as a model species, varied with substrate composition. Shiitake (Lentinula edodes) cultivation on woodbased substrates resulted in major degradation of lignin and hemicelluloses, while cellulose was degraded to a minor extent. Nitrogen content of the substrate was an important factor since low nitrogen content resulted in increased delignification and minimal cellulose consumption but compromised the production of mushroom fruiting bodies. Compared to the impact of nitrogen content, the choice of hardwood species had a less significant role, as indicated by the comparable performance in mushroom yield or delignification, across substrates composed of birch, alder and aspen and their combinations. Differences in chemical features between stemwood and bark were significant, which had a major effect on the degree of lignocellulose degradation by shiitake. Delignification by shiitake was correlated with degradation of syringyl lignin unit, as indicated by a significant reduction of the syringyl-to-guaiacyl ratio during cultivation. Shiitake cultivation did not cause statistically significant changes in the substrate crystallinity. Cellulose contained in the spent mushroom substrate (SMS) resulting from shiitake cultivation displayed higher susceptibility to enzymatic saccharification than cellulose contained in the raw woody material of initial substrate. The enhancement of the enzymatic saccharification of cellulose was related to the low mass fractions of lignin and hemicelluloses in the SMS, and it was neither affected by the relative content of lignin phenylpropane units nor by substrate crystallinity. Considerable formation of phenolic compounds was resulted from lignin degradation during fungal cultivation. Formed phenols ended up in the SMS hydrolysate after enzymatic saccharification. Acetic acid was hardly detectable in substrates, but it was found in the SMS hydrolysates as result of the hydrolysis of remaining hemicelluloses during enzymatic saccharification. The concentrations of phenolic compounds and acetic acid in the enzymatic hydrolysates were low and, consequently, their inhibitory effect on fermentation was limited.  Fungal pretreatment caused accumulation of nitrogen, including soluble forms, in the substrates. The nitrogen source was further enriched in the SMS hydrolysates, which ensured high ethanol yield in fermentation by Saccharomyces cerevisiae even without supplementing additional nutrients. Furthermore, studies on the potential of wood ear (Auricularia auricular-judae) and summer oyster (Pleurotus pulmonarius) for food and biofuel production suggested that the both species adapted well to the hot-air pasteurization based novel mushroom cultivation system. Comparable production of fruiting bodies and lignocellulose degradation were achieved for each studied mushroom, regardless pasteurization regimes. However, before applying these two species for the food and cellulosic ethanol concept, further studies are needed to investigate the factors that may improve their efficiency of fungal pretreatment while obtaining acceptable quantity mushroom fruiting bodies. The solid leftover generated after SMS enzymatic saccharification, being one major side-stream, showed high thermal energy value and promising combustion characteristics, demonstrating a plausibility to be recycled as solid fuel for self-supporting energy system and space heating.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Industriell bioteknik -- Bioenergi (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Industrial Biotechnology -- Bioenergy (hsv//eng)

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