Pretreatment is a crucial step in the conversion of lignocellulosic biomass to fermentable sugars. Microbial pretreatment with white rot fungi can delignify lignocellulosic biomass under atmospheric conditions and, thus, has the potential to be applied to on-farm wet storage for cost-effective cellulosic ethanol production. The present research investigated the feasibility of solid-state microbial pretreatment of corn stover and other types of lignocellulosic biomass feedstocks with the white rot fungus Ceriporiopsis subvermispora.
The glucose and ethanol yields, based on the theoretical yield of untreated corn stover, reached 66.61% and 57.80%, respectively, after a 35-d pretreatment, which were more than three times of that obtained with the untreated corn stover. Washing of fungal-pretreated corn stover did not cause significant improvement in ethanol yield, suggesting no water insoluble inhibitory compounds were formed during fungal pretreatment. A high correlation was obtained between the remaining lignin content in the treated corn stover and its cellulose digestibility, indicating that lignin removal facilitated enzymatic hydrolysis.
The delignification process of corn stover was investigated throughout a 42-d pretreatment by monitoring changes in composition and enzyme production. Lignin degradation increased with pretreatment time, reaching 39.20% at the end of pretreatment while cellulose degradation was less than 4.52% throughout the process, indicating a high selectivity of the fungus for lignin in the corn stover. However, hemicellulose degradation, mainly xylose loss, was substantial with up to 27.52% hemicellulose simultaneously degraded with lignin. Manganese peroxidase (MnP) and laccase were two detected oxidative enzymes while xylanase was the major hydrolytic enzyme. Observation by scanning electron microscopy showed that fungal pretreatment changed the microstructure of corn stover significantly. At the later stage of pretreatment, corn stover became light, soft, and spongy and the color changed to whitish-yellow.
To evaluate the robustness of the fungal pretreatment process with C. subvermispora , other feedstocks, including wheat straw, soybean straw, switchgrass, and hardwood, were also examined. After pretreatment for 18 d, switchgrass and hardwood were effectively delignified with significant production of manganese peroxidase and laccase. Wheat straw was greatly resistant to fungal pretreatment unless glucose and malt extract were added to the substrate. In contrast, no fungal degradation occurred in soybean straw even with addition of external carbon sources and enzyme inducers (Mn2+, H2O2).
Hot water extraction and/or hydrothermal pretreatment were applied to improve fungal degradation of wheat straw and soybean straw. Hot water extraction removed extractives in substrates, which facilitated fungal degradation of wheat straw but not of soybean straw. Fungal degradation of soybean straw was finally improved by the liquid hot water pretreatment, which resulted in 36.70% lignin removal and 64.25% glucose yield.
Empirical models satisfactorily predicted fungal growth, oxygen uptake, holocellulose consumption, and lignin degradation; however, the prediction of enzyme production was poor.
The knowledge obtained from this study is important for the development of concurrent wet storage and microbial pretreatment with white rot fungi for lignocellulosic biomass and of a combined fungal and thermal/physical pretreatment process that can potentially overcome the problems associated with existing pretreatment methods.