Fungal pretreatment of lignocellulosic biomass

Effect of phase-separation and thin-slurry recirculation on flexible biogas production from maize silage and bedding straw

This study investigates a two-stage anaerobic digestion (AD) process to enhance methane production from lignocellulosic biomass, in this case bedding straw, co-fed with maize silage. The system combines phase separation and double thin-slurry recirculation under mesophilic conditions. The first hydrolysis-acidogenesis stage produced over 10 g L-1 short-chain carboxylic acids (SCCA) at an organic loading rate of 1.3-3.0 g (L d)-1 when fed with a content of 50 wt% straw. The subsequent methanogenesis stage achieved a maximum methane yield of 260 mL CH4 gCOD-1, with 50 % variation in methane production within 24 h. Thin-slurry recirculation was shown to improve the methane content by 20 %. Cell activity and quick recovery of gas production was proven after fasting periods of several weeks. The approach demonstrates the potential for flexible, efficient processing of bedding straw in AD without further treatment beyond cutting.

Linking the Metabolic Activity of Plastic-Degrading Fungi to Their Taxonomy and Evolution

Plastic, a ubiquitous part of our daily lives, has become a global necessity, with annual production exceeding 300 million tons. However, the accumulation of synthetic polymers in our environment poses a pressing global challenge. To address this urgent issue, fungi have emerged as potential agents for plastic degradation. In our previous manuscript, 'A Review of the Fungi That Degrade Plastic', we explored the taxonomic placement of plastic-degrading fungi across three main phyla: Ascomycota, Basidiomycota, and Mucoromycota. In this review, we built upon that foundation and aimed to further explore the taxonomic relationships of these fungi in a comprehensive and detailed manner, leaving no stone unturned. Moreover, we linked metabolic activity and enzyme production of plastic-degrading fungi to their taxonomy and summarized a phylogenetic tree and a detailed table on enzyme production of plastic-degrading fungi presented here. Microbial enzymes are key players in polymer degradation, operating intra-cellularly and extra-cellularly. Fungi, one of the well-studied groups of microbes with respect to plastic degradation, are at the forefront of addressing the global issue of plastic accumulation. Their unique ability to hydrolyze synthetic plastic polymers and produce a wide range of specific enzymes is a testament to their potential. In this review, we gather and synthesize information concerning the metabolic pathways of fungi involved in the degradation of plastics. The manuscript explores the diverse range of specific enzymes that fungi can produce for plastic degradation and the major pathways of plastic metabolism. We provide a listing of 14 fungal enzymes (Esterase, Cutinase, Laccase, Peroxidases, Manganese peroxidase, Lignin peroxidase, Oxidoreductases, Urease, Protease, Lipase, Polyesterase, Dehydrogenase, Serine hydrolase, and PETase) involved in pathways for plastic degradation alongside the relevant fungi known to produce these enzymes. Furthermore, we integrate the fungi's enzyme-producing capabilities with their taxonomy and phylogeny. Taxonomic and phylogenetic investigations have pinpointed three primary fungal classes (Eurotiomycetes, Sordariomycetes (Ascomycota), and Agaricomycetes (Basidiomycota)) as significant plastic degraders that produce the vital enzymes mentioned earlier. This paper provides a foundational resource for recognizing fungal involvement in the biodegradation of synthetic polymers. It will ultimately advance fungal biotechnology efforts to address the global issue of plastic accumulation in natural environments.

Pure mycelium materials production from agri-processing water: Effects of feedstock composition on material properties for packaging applications

In this work, pure mycelium materials (PMMs) were produced by cultivating fungi Trametes versicolor and Irpex lacteus on lignocellulose-rich agricultural processing water. This water was a side stream from the alkali treatment of purposely grown biomass (miscanthus) for cellulose fiber extraction and contained lignocellulosic residues. Agri-processing water yielded ~75-mg/L PMMs with superior mechano-physical properties than synthetic medium-based ones. These properties were further enhanced by PMM post-processing with glycerol. The thermal stability of PMMs was demonstrated by their higher melting temperature than low density polyethylene (LDPE) while their degradation between 200-380°C, and density of <1.0 g/cm3, like LDPE. Their mechanical performance was studied on filmlike specimens via dynamic mechanical analyzer. PMMs showed a viscoelastic behavior with a high storage modulus of 34 MPa at 65°C suggesting their suitability for packaging applications. This work provides guidelines on optimizing PMM production using agri-processing water to obtain tunable mechano-physical properties. PRACTITIONER POINTS: Valorization of agri-processing water to produce high-value PMM packaging products. No pure or expensive nutrient supplementation was needed for agri-based feedstock. Relationships between feedstock composition and PMM properties were established. PMMs showed a similar thermal profile and density as typical petro-based packaging materials. The addition of glycerol postproduction induced flexibility in PMMs.

Conversion of Lignocellulosic Biomass Into Valuable Feed for Ruminants Using White Rot Fungi

White rot fungi can degrade lignin and improve the nutritional value of highly lignified biomass for ruminants. We screened for excellent fungi-biomass combinations by investigating the improvement of digestibility of wheat straw, barley straw, oat straw, rapeseed straw, miscanthus, new reed, spent reed from thatched roofs, and cocoa shells after colonisation by Ceriporiopsis subvermispora (CS), Lentinula edodes (LE), and Pleurotus eryngii (PE) (indicated by increased in vitro gas production [IVGP]). First, growth was evaluated for three fungi on all types of biomass, over a period of 17 days in race tubes. CS grew faster than LE and PE on all types of biomass. LE did not grow on cocoa shells, while growth rate of CS and PE on cocoa shells was lower compared to other types of biomass. After this first screening, all types of biomass, excluding the cocoa shells, were colonised by the three fungal strains for 8 weeks. Treatment with CS and LE improved IVGP more than treatment with PE. Methane production was reduced in six combinations of biomass with CS, four with LE, and three with PE. Six types of biomass were selected for treatment with CS and four were selected for treatment with CS and LE, to determine the net improvement of nutritional value (increased IVGP corrected for dry matter loss) after 2, 4, 6, 7 and 8 weeks of treatment. The highest net improvement was found for CS and LE treated rapeseed straw (86% and 20%, respectively) and spent reed (80% and 43%, respectively). All treatments decreased dry matter, lignin and hemicellulose, the latter two both in absolute amount and content. In conclusion, net improvement of highly lignified biomasses by CS was greater than LE, with the nutritional value of rapeseed straw and spent reed being significantly improved by both fungi.

Advances in the biological production of sugar alcohols from biomass-derived xylose

Sugar alcohols are a common class of low-calorie sweeteners. The advancement of technologies utilizing renewable resources has heightened interest in synthesizing sugar alcohols from biomass-derived xylose for cost down of process and sustainability. This review focuses on the potential of biomass-derived xylose and its effective conversion into sugar alcohols, underscoring the significance of this process in sustainable industrial applications. The two main approaches for producing sugar alcohols which include enzyme catalysis and microbial fermentation are thoroughly discussed. The microbial fermentation pathway relies on genetically engineered strains, which are modified to efficiently convert xylose into target sugar alcohols. Enzyme catalysis, on the other hand, directly converts xylose to sugar alcohols through specific reactions. In addition, strategies to improve product selectivity and reduce by-products are discussed in the paper, which are crucial for improving the economic viability and environmental sustainability of sugar alcohol production. Overall, utilizing xylose from biomass to produce sugar alcohols manifests environmental and economic benefits, indicating its substantial potential in the shift towards a low-carbon economy. Future studies may further explore cutting edge technologies to maximize the utilization of biomass-derived xylose and the sustainable production of sugar alcohols.

Intraspecies Variation Offers Potential to Improve White Rot Fungi for Increasing Degradability of Lignocellulose for Ruminants

The aim of fungal treatment of organic matter for ruminants is the improvement of its degradability. So far, such treatment appears to be time-consuming and improvement has been modest. In previous work, we observed within three white rot species that there is modest (Ceriporiopsis subvermispora) or low (Lentinula edodes and Pleurotus eryngii) variation in fiber degradation in wheat straw during seven weeks of incubation. By extending and re-examining the data from all three species, we see that strains of C. subvermispora show the largest variation and improvement in the degradability of treated wheat straw. In addition, C. subvermispora also generated the highest absolute amount of degradable organic matter, a parameter not calculated before, but is very relevant for the economic feasibility of fungal treatment. In estimating fungal growth, we found no good correlation between an increase in ergosterol and a decrease in plant biomass, indicating a variation within fungal species of the ergosterol/fungal biomass ratio and/or a variation in carbon use efficiency, which has also not been analyzed before. This work contributes to the knowledge of how fungi degrade lignocellulose and further specifies what can be targeted for breeding to make fungal pretreatment economically feasible for upgrading organic waste streams into ruminal feed.

Recent advances in methane and hydrogen production from lignocellulosic degradation with anaerobic fungi

Anaerobic fungi (AF) efficiently degrade lignocellulosic biomass with unique pseudoroot system and enzymatic properties that can remove polysaccharides and some lignified components from plant cell walls, further releasing acetate, lactate, ethanol, hydrogen (H2), etc. As research on AF for bioengineering has become a hot topic, a review of lignocellulosic conversion with AF for methane (CH4) and H2 production is needed. Efficient degradation of lignocellulose with AF mainly relies on multiple free carbohydrate-active enzymes and cellulosomes in the free and bound state. Meanwhile, co-cultivation of AF and methanogens significantly improves the lignocellulose degradation and CH4 production, and the maximum CH4 yield reached 315 mL/g. Bioaugmentation of AF in anaerobic digestion increases the maximum CH4 yield by 330 %. Also, AF show H2 production potential, however, H2 yield from anaerobic fungal fermentation of lignocellulose remains low. Therefore, anaerobic fungi have great potential in the conversion of lignocellulosic biomass to CH4 and H2.

Biocalorimetry-aided monitoring of fungal pretreatment of lignocellulosic agricultural residues

The present study aimed to investigate whether and how non-invasive biocalorimetric measurements could serve for process monitoring of fungal pretreatment during solid-state fermentation (SSF) of lignocellulosic agricultural residues such as wheat straw. Seven filamentous fungi representing different lignocellulose decay types were employed. Water-soluble sugars being immediately available after fungal pretreatment and those becoming water-extractable after enzymatic digestion of pretreated wheat straw with hydrolysing (hemi)cellulases were considered to constitute the total bioaccessible sugar fraction. The latter was used to indicate the success of pretreatments and linked to corresponding species-specific metabolic heat yield coefficients (YQ/X) derived from metabolic heat flux measurements during fungal wheat straw colonisation. An YQ/X range of about 120 to 140 kJ/g was seemingly optimal for pretreatment upon consideration of all investigated fungi and application of a non-linear Gaussian fitting model. Upon exclusion from analysis of the brown-rot basidiomycete Gloeophyllum trabeum, which differs from all other here investigated fungi in employing extracellular Fenton chemistry for lignocellulose decomposition, a linear relationship where amounts of total bioaccessible sugars were suggested to increase with increasing YQ/X values was obtained. It remains to be elucidated whether an YQ/X range being optimal for fungal pretreatment could firmly be established, or if the sugar accessibility for post-treatment generally increases with increasing YQ/X values as long as "conventional" enzymatic, i.e. (hemi)cellulase-based, lignocellulose decomposition mechanisms are operative. In any case, metabolic heat measurement-derived parameters such as YQ/X values may become very valuable tools supporting the assessment of the suitability of different fungal species for pretreatment of lignocellulosic substrates. KEY POINTS: • Biocalorimetry was used to monitor wheat straw pretreatment with seven filamentous fungi. • Metabolic heat yield coefficients (YQ/X) seem to indicate pretreatment success. • YQ/X values may support the selection of suitable fungal strains for pretreatment.

Fungal behavior and recent developments in biopulping technology

Biological pretreatment of wood chips by fungi is a well-known approach prior to mechanical- or chemical pulp production. For this biological approach, a limited number of white-rot fungi with an ability to colonize and selectively degrade lignin are used to pretreat wood chips allowing the remaining cellulose to be processed for further applications. Biopulping is an environmentally friendly technology that can reduce the energy consumption of traditional pulping processes. Fungal pretreatment also reduces the pitch content in the wood chips and improves the pulp quality in terms of brightness, strength, and bleachability. The bleached biopulps are easier to refine compared to pulps produced by conventional methodology. In the last decades, biopulping has been scaled up with pilot trials towards industrial level, with optimization of several intermediate steps and improvement of economic feasibility. Nevertheless, fundamental knowledge on the biochemical mechanisms involved in biopulping is still lacking. Overall, biopulping technology has advanced rapidly during recent decades and pilot mill trials have been implemented. The use of fungi as pretreatment for pulp production is in line with modern circular economy strategies and can be implemented in existing production plants. In this review, we discuss some recent advances in biopulping technology, which can improve mechanical-, chemical-, and organosolv pulping processes along with their mechanisms.

Valorization of crop residues and animal wastes: Anaerobic co-digestion technology

To switch the over-reliance on fossil-based resources, curb environmental quality deterioration, and promote the use of renewable fuels, much attention has recently been directed toward the implementation of sustainable and environmentally benign 'waste-to-energy' technology exploiting a clean, inexhaustible, carbon-neutral, and renewable energy source, namely agricultural biomass. From this perspective, anaerobic co-digestion (AcoD) technology emerges as a potent and plausible approach to attain sustainable energy development, foster environmental sustainability, and, most importantly, circumvent the key challenges associated with mono-digestion. This review article provides a comprehensive overview of AcoD as a biochemical valorization pathway of crop residues and livestock manure for biogas production. Furthermore, this manuscript aims to assess the different biotic and abiotic parameters affecting co-digestion efficiency and present recent advancements in pretreatment technologies designed to enhance feedstock biodegradability and conversion rate. It can be concluded that the substantial quantities of crop residues and animal waste generated annually from agricultural practices represent valuable bioenergy resources that can contribute to meeting global targets for affordable renewable energy. Nevertheless, extensive and multidisciplinary research is needed to evolve the industrial-scale implementation of AcoD technology of livestock waste and crop residues, particularly when a pretreatment phase is included, and bridge the gap between small-scale studies and real-world applications.

Transcriptomic and metabolomic analysis reveals the influence of carbohydrates on lignin degradation mediated by Bacillus amyloliquefaciens

Introduction

Ligninolytic bacteria can secrete extracellular enzymes to depolymerize lignin into small-molecular aromatics that are subsequently metabolized and funneled into the TCA cycle. Carbohydrates, which are the preferred carbon sources of bacteria, influence the metabolism of lignin-derived aromatics through bacteria.

Methods

In this study, untargeted metabolomics and transcriptomics analyses were performed to investigate the effect of carbohydrates on lignin degradation mediated by Bacillus amyloliquefaciens MN-13, a strain with lignin-degrading activity that was isolated in our previous work.

Results

The results demonstrated that the cell growth of the MN-13 strain and lignin removal were promoted when carbohydrates such as glucose and sodium carboxymethyl cellulose were added to an alkaline lignin-minimal salt medium (AL-MSM) culture. Metabolomics analysis showed that lignin depolymerization took place outside the cells, and the addition of glucose regulated the uptake and metabolism of lignin-derived monomers and activated the downstream metabolism process in cells. In the transcriptomics analysis, 299 DEGs were screened after 24 h of inoculation in AL-MSM with free glucose and 2 g/L glucose, respectively, accounting for 8.3% of the total amount of annotated genes. These DEGs were primarily assigned to 30 subcategories, including flagellar assembly, the PTS system, RNA degradation, glycolysis/gluconeogenesis, the TCA cycle, pyruvate metabolism, and tryptophan metabolism. These subcategories were closely associated with the cell structure, generation of cellular energy, and precursors for biosynthetic pathways, based on a - log 10 (P adjust) value in the KEGG pathway analysis.

Conclusion

In summary, the addition of glucose increased lignin degradation mediated by the MN-13 strain through regulating glycolysis, TCA cycle, and central carbon metabolism.

Whole genome sequencing and annotation of Daedaleopsis sinensis, a wood-decaying fungus significantly degrading lignocellulose

Daedaleopsis sinensis is a fungus that grows on wood and secretes a series of enzymes to degrade cellulose, hemicellulose, and lignin and cause wood rot decay. Wood-decaying fungi have ecological, economic, edible, and medicinal functions. Furthermore, the use of microorganisms to biodegrade lignocellulose has high application value. Genome sequencing has allowed microorganisms to be analyzed from the aspects of genome characteristics, genome function annotation, metabolic pathways, and comparative genomics. Subsequently, the relevant information regarding lignocellulosic degradation has been mined by bioinformatics. Here, we sequenced and analyzed the genome of D. sinensis for the first time. A 51.67-Mb genome sequence was assembled to 24 contigs, which led to the prediction of 12,153 protein-coding genes. Kyoto Encyclopedia of Genes and Genomes database analysis of the D. sinensis data revealed that 3,831 genes are involved in almost 120 metabolic pathways. According to the Carbohydrate-Active Enzyme database, 481 enzymes are found in D. sinensis, of which glycoside hydrolases are the most abundant. The genome sequence of D. sinensis provides insights into its lignocellulosic degradation and subsequent applications.

Green chemical and hybrid enzymatic pretreatments for lignocellulosic biorefineries: Mechanism and challenges

The greener chemical and enzymatic pretreatments for lignocellulosic biomasses are portraying a crucial role owing to their recalcitrant nature. Traditional pretreatments lead to partial degradation of lignin and hemicellulose moieties from the pretreated biomass. But it still restricts the enzyme accessibility for the digestibility towards the celluloses and the interaction of lignin-enzymes, nonproductively. Moreover, incursion of certain special chemical treatments and other lignin sulfonation techniques to the enzymatic pretreatment (hybrid enzymatic pretreatment) enhances the lignin structural modification, solubilization of the hemicelluloses and both saccharification and fermentation processes (SAF). This article concentrates on recent developments in various chemical and hybrid enzymatic pretreatments on biomass materials with their mode of activities. Furthermore, the issues on strategies of the existing pretreatments towards their industrial applications are highlighted, which could lead to innovative ideas to overcome the challenges and give guideline for the researchers towards the lignocellulosic biorefineries.

Structural alteration of cocoa bean shell fibers through biological treatment using Penicillium roqueforti

Lignocellulosic residues, such as cocoa bean shell (FI), are generated in large quantities during agro-industrial activities. Proper management of residual biomass through solid state fermentation (SSF) can be effective in obtaining value-added products. The hypothesis of the present work is that the bioprocess promoted by P. roqueforti can lead to structural changes in the fibers of the fermented cocoa bean shell (FF) that confer characteristics of industrial interest. To unveil such changes, the techniques of FTIR, SEM, XRD, TGA/TG were used. After SSF, an increase of 36.6% in the crystallinity index was observed, reflecting the reduction of amorphous components such as lignin in the FI residue. Furthermore, an increase in porosity was observed through the reduction of the 2θ angle, which gives the FF a potential candidate for applications of porous products. The FTIR results confirm the reduction in hemicellulose content after SSF. The thermal and thermogravimetric tests showed an increase in the hydrophilicity and thermal stability of FF (15% decomposition) in relation to the by-product FI (40% decomposition). These data provided important information regarding changes in the crystallinity of the residue, existing functional groups and changes in degradation temperatures.

Effect of Microbial Consortium Constructed with Lignolytic Ascomycetes Fungi on Degradation of Rice Stubble

Microbial degradation is an effective, eco-friendly and sustainable approach for management of the rice residue. After harvesting a rice crop, removal of stubble from the ground is a challenging task, that forces the farmers to burn the residue in-situ. Therefore, accelerated degradation using an eco-friendly alternative is a necessity. White rot fungi are the most explored group of microbes for accelerated degradation of lignin but they are very slow in growth. The present investigation focuses on degradation of rice stubble using a fungal consortium constructed with highly sporulating ascomycetes fungi, namely, Aspergillus terreus, Aspergillus fumigatus and Alternaria spp. All three species were successful at colonizing the rice stubble. Periodical HPLC analysis of rice stubble alkali extracts revealed that incubation with ligninolytic consortium released various lignin degradation products such as vanillin, vanillic acid, coniferyl alcohol, syringic acid and ferulic acid. The efficiency of the consortium was further studied at different dosages on paddy straw. Maximum lignin degradation was observed when the consortium was applied at 15% volume by weight of rice stubble. Maximum activity of different lignolytic enzymes such as lignin peroxidase, laccase and total phenols was also found with the same treatment. FTIR analysis also supported the observed results. Hence, the presently developed consortium for degrading rice stubble was found to be effective in both laboratory and field conditions. The developed consortium or its oxidative enzymes can be used alone or combined with other commercial cellulolytic consortia to manage the accumulating rice stubble effectively.

In vitro ruminal degradability of wheat straw cultivated with white-rot fungi adapted to mushroom farming conditions

Biological treatment of cereal straw for ruminant nutrition purposes might present an environmentally friendly option of valorizing a widely available by-product of grain production for farming systems with low external input. Several strains of white-rot fungi have been selected in the past under mostly controlled laboratory conditions for their capacity of lignin degradation. The study adapted to conditions on farm for upscaling purposes. The development of the in vitro straw digestibility with two different moistening pre-treatments and inoculated with three different fungi species, namely Pleurotus ostreatus, Ceriporiopsis subvermispora and Volvariella volvacea, was determined up to 42 days of fermentation with five sampling times. The effect of physical straw pre-treatments on nutritional parameters was evaluated. The neutral detergent fiber digestibility (NDFD_30h), enzymatically soluble organic substance (ELOS) and the gas production (Hohenheim Feed value Test, HFT) as indicators for in vitro ruminal degradability decreased over time independent of the fungus: HFT, ELOS and NDFD_30h by up to 50, 35 and 30% of the original straw. Remoistening and autoclaving the straw increased the gas production significantly by 2.6 mL/200 g dry matter (DM), and ELOS and NDFD_30h by 45 and 51 g/kg DM compared to the original straw (34.9 mL/200 mg DM, 342 g/kg DM, 313 g/kg NDF).

Modeling the microbial pretreatment of camelina straw and switchgrass by Trametes versicolor and Phanerochaete chrysosporium via solid-state fermentation process: A growth kinetic sub-model in the context of biomass-based biorefineries

Advancing microbial pretreatment of lignocellulose has the potential not only to reduce the carbon footprint and environmental impacts of the pretreatment processes from cradle-to-grave, but also increase biomass valorization, support agricultural growers, and boost the bioeconomy. Mathematical modeling of microbial pretreatment of lignocellulose provides insights into the metabolic activities of the microorganisms as responses to substrate and environment and provides baseline targets for the design, development, and optimization of solid-state-fermentation (SSF) bioreactors, including substrate concentrations, heat and mass transfer. In this study, the growth of Trametes versicolor 52J (TV52J), Trametes versicolor m4D (TVm4D), and Phanerochaete chrysosporium (PC) on camelina straw (CS) and switchgrass (SG) during an SSF process was examined. While TV52J illustrated the highest specific growth rate and maximum cell concentration, a mutant strain deficient in cellulose catabolism, TVm4D, performed best in terms of holocellulose preservation and delignification. The hybrid logistic-Monod equation along with holocellulose consumption and delignification models described well the growth kinetics. The oxygen uptake rate and carbon dioxide production rate were directly correlated to the fungal biomass concentration; however, a more sophisticated non-linear relationship might explain those correlations better than a linear model. This study provides an informative baseline for developing SSF systems to integrate fungal pretreatment into a large-scale, on-farm, wet-storage process for the utilization of agricultural residues as feedstocks for biofuel production.

Screening the Carbon Source Type in Solid-State Fermentation with Phanerochaete chrysosporium to Improve the Forage Value of Corn Straw and Rice Straw

Poor quality straw can be made more digestible and palatable through delignification using white rot fungi as a biological treatment in SSF. The decomposition of organic matter by white rot fungi is improved when a carbon source is added. Reducing the fermentation cycle can also help retain more nutrients in straw feed. To increase rumen digestibility and nutrient utilization, corn straw and rice straw were subjected to SSF with white rot fungi (Phanerochaete chrysosporium) for 21 days. The type of carbon source (glucose, sucrose, molasses, or soluble starch) was optimized, and the nutrient composition and in vitro fermentation parameters of the fermented straw were assessed. In the fermented corn straw and rice straw supplemented with different carbon sources, the results showed a decrease in lignin content, dry matter, cellulose, and hemicellulose loss, and an increase in crude protein content after 21 days. Total volatile fatty acid and ammonium nitrogen concentrations increased significantly (p < 0.01) during in vitro fermentation. Overall, the most enhanced nutritional values for corn straw and rice straw were observed after 14 days of SSF in the groups using molasses or glucose as a carbon source.

Laccase mediated delignification of wasted and non-food agricultural biomass: Recent developments and challenges

Utilization of microbial laccases is considered as the cleaner and target specific biocatalytic mechanism for the recovery of cellulose and hemicelluloses from nonfood and wasted agricultural, lignocellulosic biomass (LCB). The extent of lignin removal by laccase depends on the biochemical composition of biomass and the redox potential (E⁰) of the biocatalyst. Intensive research efforts are going on all over the world for the recognition of appropriate and easily available agricultural lignocellulosic feedstocks to exploit maximally for the production of value-added bioproducts and biofuels. In such circumstances, laccase can play a major role as a leading biocatalyst and potent substitute for chemical based deconstruction of the lignocellulosic materials. The limited commercialization of laccase at an industrial scale has been feasible due to its full working efficiency mostly expressed in the presence of cost intensive redox mediators only. Although, recently there are some reports that came on the mediator free biocatalysis of enzyme but still not considerably explored and neither understood in depth. The present review will address the various research gaps and shortcomings that acted as the big hurdles before the complete exploitation of laccases at an industrial scale. Further, this article also reveals insights on different microbial laccases and their diverse functional environmental conditions that affect the deconstruction process of LCB.

Fungal Assisted Valorisation of Polymeric Lignin: Mechanism, Enzymes and Perspectives

Lignocellulose is considered one of the significant recalcitrant materials and also is difficult to break down because of its complex structure. Different microbes such as bacteria and fungi are responsible for breaking down these complex lignin structures. This article discussed briefly the lignin-degrading bacteria and their critical steps involved in lignin depolymerization. In addition, fungi are regarded as the ideal microorganism for the degradation of lignin because of their highly effective hydrolytic and oxidative enzyme systems for the breakdown of lignocellulosic materials. The white rot fungi, mainly belonging to basidiomycetes, is the main degrader of lignin among various microorganisms. This could be achieved because of the presence of lignolytic enzymes such as laccases, lignin peroxidases, and manganese peroxidases. The significance of the fungi and lignolytic enzyme’s role in lignin depolymerization, along with its mechanism and chemical pathways, are emphasized in this article.

A critical review on pretreatment and detoxification techniques required for biofuel production from the organic fraction of municipal solid waste

The organic fraction of municipal solid waste (OFMSW) is a widely-available promising feedstock for biofuel production. However, the presence of different inhibitors originating from fruit and food/beverage wastes as well as recalcitrant lignocellulosic fractions hampers its bioconversion. This necessitates a pretreatment to augment the biodigestibility and fermentability of OFMSW. Hence, this review aims to provide the in-vogue inhibitory compound removal and pretreatment techniques that have been employed for efficient OFMSW conversion into biofuels, i.e., hydrogen, biogas, ethanol, and butanol. The techniques are compared concerning their mode of action, chemical and energy consumption, inhibitor formation and removal, economic feasibility, and environmental sustainability. This critique also reviews the existing knowledge gap and future perspectives for efficient OFMSW valorization. The insights provided pave the way toward developing energy-resilient cities while addressing environmental crises related to generating OFMSW.

Lignocellulose dissociation with biological pretreatment towards the biochemical platform: A review

Lignocellulose utilization has been gaining great attention worldwide due to its abundance, accessibility, renewability and recyclability. Destruction and dissociation of the cross-linked, hierarchical structure within cellulose hemicellulose and lignin is the key procedure during chemical utilization of lignocellulose. Of the pretreatments, biological treatment, which can effectively target the complex structures, is attractive due to its mild reaction conditions and environmentally friendly characteristics. Herein, we report a comprehensive review of the current biological pretreatments for lignocellulose dissociation and their corresponding degradation mechanisms. Firstly, we analyze the layered, hierarchical structure of cell wall, and the cross-linked network between cellulose, hemicellulose and lignin, then highlight that the cracking of β-aryl ether is considered the key to lignin degradation because of its dominant position. Secondly, we explore the effect of biological pretreatments, such as fungi, bacteria, microbial consortium, and enzymes, on substrate structure and degradation efficiency. Additionally, combining biological pretreatment with other methods (chemical methods and catalytic materials) may reduce the time necessary for the whole process, which also help to strengthen the lignocellulose dissociation efficiency. Thirdly, we summarize the related applications of lignocellulose, such as fuel production, chemicals platform, and bio-pulping, which could effectively alleviate the energy pressure through bioconversion into high value-added products. Based on reviewing of current progress of lignocellulose pretreatment, the challenges and future prospects are emphasized. Genetic engineering and other technologies to modify strains or enzymes for improved biotransformation efficiency will be the focus of future research.

Enhancement of separation selectivity of hemicellulose from bamboo using freeze-thaw-assisted p-toluenesulfonic acid treatment at low acid concentration and high temperature

The cellulose-rich residual solids are obtained with p-toluenesulfonic acid (p-TsOH) treatment. However, better fractionation of hemicellulose and separation is difficult to obtain during treatment. This study aims at investigating the separation selectivity of bamboo hemicellulose using freeze-thaw-assisted p-TsOH (F/p-TsOH) treatment. The desired separation effect was achieved at freezing temperature -40 °C, freezing time 20 h, p-TsOH concentration 3.0 %, treatment temperature 130 °C and time 80 min. 93.26 % hemicellulose separation was found, which was 32.88 % higher than that of conventional p-TsOH treatment. Furthermore, the separation yield of lignin decreased significantly from 69.29 % to 13.98 %. The distinct lignin characteristic absorption peaks were found, while that of hemicellulose was difficult to observe. The fiber crystallinity index increased from 50.42 to 56.55 %. Furthermore, greater selectivity for hemicellulose separation was achieved. The results provide a new research thinking for efficient fractionation of lignocellulosic biomass by organic acid treatment.

Potential of lignocellulose degrading microorganisms for agricultural residue decomposition in soil: A review

Lignocellulosic crop residues (LCCRs) hold a significant share of the terrestrial biomass, estimated at 5 billion Mg per annum globally. A massive amount of these LCCRs are burnt in many countries resulting in immense environmental pollution; hence, its proper disposal in a cost-effective and eco-friendly manner is a significant challenge. Among the different options for management of LCCRs, the use of lignocellulose degrading microorganisms (LCDMOs), like fungi and bacteria, has emerged as an eco-friendly and effective way for its on-site disposal. LCDMOs achieve degradation through various mechanisms, including multiple supportive enzymes, causing oxidative attacks by which recalcitrance of lignocellulose material is reduced, paving the way to further activity by depolymerizing enzymes. This improves the physical properties of soil, recycles plant nutrients, promotes plant growth and thus helps improve productivity. Rapid and proper microbial degradation may be achieved through the correct combination of the LCDMOs, supplementing nutrients and controlling different factors affecting microbial activity in the field. The review is a critical discussion of previous studies revealing the potential of individuals or a set of LCDMOs, factors controlling the rate of degradation and the key researchable areas for better understanding of the role of these decomposers for future use.

An insight into the principles of lignocellulosic biomass-based zero-waste biorefineries: a green leap towards imperishable energy-based future

Lignocellulosic biomass (LCB) is an energy source that has a huge impact in today's world. The depletion of fossil fuels, increased pollution, climatic changes, etc. have led the public and private sectors to move towards sustainability i.e. using LCB for the production of biofuels and value-added compounds. A major bottleneck of the process is the recalcitrant nature of LCB. This can be overcome by using various pretreatment strategies like physical, chemical, biological, physicochemical, etc. Further, the pretreated biomass is made to undergo various steps like hydrolysis, saccharification, etc. for the conversion of value-added products and the remaining waste residues can be further utilized for the synthesis of secondary products thus favouring the zero-waste biorefinery concept. Currently, microorganisms are being explored for their use in biorefinery but the unavailability of commercial strains is a major limitation. Thus, the use of metagenomics can be used to overcome the limitation which is both cost-effective and environmentally friendly. The review deliberates the composition of LCBs, and their recalcitrance nature, followed by the structural changes caused by various pretreatment methods. The further steps in biorefineries, strategies for the development of zero-waste refineries, bottlenecks, and suggestions are also discussed. Special emphasis is given to the use of metagenomics for the discovery of microorganisms efficient for zero-waste biorefineries.

Combined Biological and Chemical/Physicochemical Pretreatment Methods of Lignocellulosic Biomass for Bioethanol and Biomethane Energy Production—A Review

Lignocellulosic biomass is a low-cost and environmentally-friendly resource that can be used to produce biofuels such as bioethanol and biogas, which are the leading candidates for the partial substitution of fossil fuels. However, the main challenge of using lignocellulosic materials for biofuel production is the low accessibility to cellulose for hydrolysis of enzymes and microorganisms, which can be overcome by pretreatment. Biological and chemical pretreatments have their own disadvantages, which could be reduced by combining the two methods. In this article, we review biological–chemical combined pretreatment strategies for biogas and bioethanol production. The synergy of fungal/enzyme–NaOH pretreatment is the only biological–chemical combination studied for biogas production and has proven to be effective. The use of enzyme, which is relatively expensive, has the advantage of hydrolysis efficiency compared to fungi. Nonetheless, there is vast scope for research and development of other chemical–biological combinations for biogas production. With respect to ethanol production, fungal–organosolv combination is widely studied and can achieve a maximum of 82% theoretical yield. Order of pretreatment is also important, as fungi may reduce the accessibility of cellulose made available by prior chemical strategies and suppress lignin degradation. The biofuel yield of similarly pretreated biomass can vary depending on the downstream process. Therefore, new strategies, such as bioaugmentation and genetically engineered strains, could help to further intensify biofuel yields.

Enzymatic Potential of Filamentous Fungi as a Biological Pretreatment for Acidogenic Fermentation of Coffee Waste

Spent coffee grounds (SCGs) are a promising substrate that can be valorized by biotechnological processes, such as for short-chain organic acid (SCOA) production, but their complex structure implies the application of a pretreatment step to increase their biodegradability. Physicochemical pretreatments are widely studied but have multiple drawbacks. An alternative is the application of biological pretreatments that include using fungi Trametes versicolor and Paecilomyces variotii that naturally can degrade complex substrates such as SCGs. This study intended to compare acidic and basic hydrolysis and supercritical CO₂ extraction with the application of these fungi. The highest concentration of SCOAs, 2.52 gCOD/L, was achieved after the acidification of SCGs pretreated with acid hydrolysis, but a very similar result, 2.44 gCOD/L, was obtained after submerged fermentation of SCGs by T. versicolor. This pretreatment also resulted in the best acidification degree, 48%, a very promising result compared to the 13% obtained with the control, untreated SCGs, highlighting the potential of biological pretreatments.

Efficient biodegradation of straw and persistent organic pollutants by a novel strategy using recombinant Trichoderma reesei

Efficient biodegradation of lignocellulosic biomass needs a battery of enzymes targeting cellulose, hemicellulose, and lignin. In this study, recombinant Trichoderma reesei ZJ-09 with Pycnoporus sanguineus laccase gene was used to degrade rice straw by in situ production of laccase, xylanase, and cellulases under solid-state fermentation (SSF). Effects of parameters on key enzymes (cellulase, xylanase, and laccase) in biodegradation during SSF were investigated. Under the optimized SSF conditions, the FPA, xylanase activity, and laccase activity reached 110.47 FPU/g, 5787.59 IU/g, and 24.45 IU/g, respectively, on day 12. The obtained recombinant T. reesei SSF system achieved efficient degradation of rice straw with the final mass loss up to 51.16% which was 1.4-fold higher than the host strain. Further, bioconversion of rice straw into a novel laccase-enriched koji for persistent organic pollutants bioremediation (LKPB) was conducted by the optimized SSF system. LKPB was found to degrade persistent organic pollutants (POPs) effectively without the addition of mediators. 4-h removal rates of three POPs mediated by LKPB (87.21% for 2,4,5-trichlorophenol, 92.45% for nonylphenol, and 90.73% for oxytetracycline) were comparable to those achieved by laccase-co-mediator system. The newly established recombinant T. reesei SSF system could be potential to effectively degrade lignocellulosic wastes as well as organic pollutants.

Community succession and straw degradation characteristics using a microbial decomposer at low temperature

This study explored changes in the microbial community structure during straw degradation by a microbial decomposer, M44. The microbial community succession at different degradation periods was analyzed using MiSeq high-throughput sequencing. The results showed that 14 days after inoculation, the filter paper enzyme and endoglucanase activities increased to 2.55 U·mL^-1 and 2.34 U·mL^-1. The xylanase, laccase, and lignin peroxidase activities rose to 9.86 U·mL^-1, 132.16 U·L^-1, and 85.43 U·L^-1 after 28 d, which was consistent with changes in the straw degradation rate. The degradation rates of straw, lignin, cellulose, and hemicellulose were 31.43%, 13.67%, 25.04%, and 21.69%, respectively, after 28 d of fermentation at 15°C. Proteobacteria, Firmicutes, and Bacteroidetes were the main bacterial species in samples at different degradation stages. The dominant genera included Pseudomonas, Delftia, and Paenibacillus during the initial stage (1 d, 7 d) and the mid-term stage (14 d). The key functional microbes during the late stage (21 d, 28 d) were Rhizobium, Chryseobacterium, Sphingobacterium, Brevundimonas, and Devosia. Changes in the bacterial consortium structure and straw degradation characteristics during different degradation periods were clarified to provide a theoretical basis for the rational utilization of microbial decomposer M44.

Biovalorization of Grape Stalks as Animal Feed by Solid State Fermentation Using White-Rot Fungi

This work aimed to evaluate the potential of three fungi strains, Lentinula edodes, Pleurotus eryngii, and Pleurotus citrinopileatus, to degrade lignin and enhance the nutritive value of grape stalks (GS). The GS was inoculated with the fungi and incubated under solid-state fermentation at 28 °C and 85% relative humidity for 7, 14, 21, 28, 35, and 42 days, in an incubation chamber. The influence of the treatments was evaluated by analyzing the potential modifications in the chemical composition, in vitro organic matter digestibility (IVOMD) and enzymatic kinetics. An increase (p < 0.001) in the crude protein content was observed in the GS treated with L. edodes and P. citrinopileatus at 42 days of incubation (50 and 75%, respectively). The treatment performed with L. edodes decreased (p < 0.001) lignin content by 52%, and led to higher (p < 0.001) IVOMD values at 42 days of incubation. By contrast, P. eryngii did not affect lignin content and IVOMD. A higher activity of all enzymes was also detected for the treatment with L. edodes. Results indicated that L. edodes has a great potential to enhance the nutritive value of GS as an animal feed, due to its lignin degradation selectivity.

Use of Anthracophyllum Discolor and Stereum Hirsutum as a Suitable Strategy for Delignification and Phenolic Removal of Olive Mill Solid Waste

This study evaluated the use of the white-rot fungi (WRF) Anthracophyllum discolor and Stereum hirsutum as a biological pretreatment for olive mill solid mill waste (OMSW). The WRF strains proposed were added directly to OMSW. The assays consisted of determining the need to add supplementary nutrients, an exogenous carbon source or use agitation systems, and evaluating WRF growth, enzyme activity, phenolic compound removal and lignin degradation. The highest ligninolytic enzyme activity was found at day 10, reaching 176.7 U/L of manganese-independent peroxidase (MniP) produced by A. discolor, and the highest phenolic removal (more than 80% with both strains) was reached after 24 days of incubation. The confocal laser scanning microscopy analysis (CLSM) confirmed lignin degradation through the drop in lignin relative fluorescence units (RFU) from 3967 for untreated OMSW to 235 and 221 RFU, showing a lignin relative degradation of 94.1% and 94.4% after 24 days of treatment by A. discolor and S. hirsutum, respectively. The results demonstrate for the first time that A. discolor and S. hirsutum were able to degrade lignin and remove phenolic compounds from OMSW using this as the sole substrate without adding other nutrients or using agitation systems. This work indicates that it could be possible to design an in situ pretreatment of the valorization of OMSW, avoiding complex systems or transportation. In this sense, future research under non-sterile conditions is needed to evaluate the competition of WRF with other microorganisms present in the OMSW. The main drawbacks of this work are associated with both the low reaction time and the water addition. However, OMSW is seasonal waste produced in one season per year, being stored for a long time. In terms of water addition, the necessary optimization will be addressed in future research.

Pre-treatment of lignocellulosic materials by enzymatic mixture to enhance biogas production

The aim of this study was to evaluate the effect of an enzymatic mixture on the increase of biogas production from lignocellulosic materials as rapeseed straw, maize waste, and wheat straw. For efficient application of the enzymatic mixture, conditions of its use were optimized regarding 50 °C, pH 7 and an enzyme dose of 0.25 % w/v. Biogas potential test confirmed positive effect of the enzymatic mixture on anaerobic digestion of already thermally and alkali pre-treated lignocellulosic materials, as significantly higher biogas production was observed after the enzymatic mixture addition for all monitored substrates. Addition of the enzymatic mixture to the most used substrate at biogas plants — maize silage, had also positive effect on biogas production during the biogas potential test. This fact was not proven during long-term operations of the reactors as the values of total cumulative biogas productions for the whole monitored period from reactors for anaerobic digestion of maize silage with and without addition of enzymatic mixture did not differ significantly.

Experimental and computational studies of cellulases as bioethanol enzymes

Bioethanol industries and bioprocesses have many challenges that constantly impede commercialization of the end product. One of the bottlenecks in the bioethanol industry is the challenge of discovering highly efficient catalysts that can improve biomass conversion. The current promising bioethanol conversion catalysts are microorganism-based cellulolytic enzymes, but lack optimization for high bioethanol conversion, due to biological and other factors. A better understanding of molecular underpinnings of cellulolytic enzyme mechanisms and significant ways to improve them can accelerate the bioethanol commercial production process. In order to do this, experimental methods are the primary choice to evaluate and characterize cellulase’s properties, but they are time-consuming and expensive. A time-saving, complementary approach involves computational methods that evaluate the same properties and improves our atomistic-level understanding of enzymatic mechanism of action. Theoretical methods in many cases have proposed research routes for subsequent experimental testing and validation, reducing the overall research cost. Having a plethora of tools to evaluate cellulases and the yield of the enzymatic process will aid in planning more optimized experimental setups. Thus, there is a need to connect the computational evaluation methods with the experimental methods to overcome the bottlenecks in the bioethanol industry. This review discusses various experimental and computational methods and their use in evaluating the multiple properties of cellulases.

Development of different pretreatments and related technologies for efficient biomass conversion of lignocellulose

Lignocellulose, a kind of biological resource widely existing in nature, which can be transformed into value-added biochemical products through saccharification, fermentation or chemical catalysis. Pretreatments are the necessary step to increase the accessibility and digestibility of lignocellulose. This paper comprehensively reviewed different pretreatment progress of lignocellulose in recent year, including mechanical/thermal, biological, inorganic solvent, organic solvent and unconventional physical-chemical pretreatments, focusing on quantifying the influence of pretreatments on subsequent biomass conversion. In addition, related pretreatment techniques such as genetic engineering, reactor configurations, downstream process and visualization technology of pretreatment were discussed. Finally, this review presented the challenge of lignocellulose pretreatment in the future.

Microbial Degradation and Value Addition to Food and Agriculture Waste

Food and agriculture waste (FAW) is a serious problem that is increasing globally. Wastage of raw materials or processed food due to various man-made activities is huge. This solid waste which is either being discarded by humans in their daily activities or an obligatory residue of agricultural processes is severely harming our environment. This becomes a major concern in densely populated agri-based countries, like India, China, and the USA. It is strongly debated that such issues need to be addressed very emphatically for sustainable development of ourselves and our surroundings. Lots of economic benefits can be obtained by reducing the food loss or converting the agricultural waste into useful products and these advantages can be in the form of better food security, reduced production cost, biodegradable products, and environment sustainability with cleaner options to reduce the ever-increasing global problem of garbage and waste management. Proper management of these substances can considerably lessen the risks to individual health. Reprocessing of waste is of great advantage as FAW has many components which may form an available resource to be converted to another useful product. Several approaches have been made for converting food waste into fruitful products. Bioconversion being the most prominent approach is helping us in a major way to overcome the problem of FAW. Microorganisms are at the forefront of this and have been extensively explored for their bioconversion potential. The present work focuses on the current state of food and agriculture waste and their valorization approaches. Through extensive literature review, we have highlighted and discussed the potential of microorganisms in bioconversion of waste, major types of functional ingredients derived during the process, and potential constraints in implementation of such state-of-the-art technology at industrial scale. The review also gives a brief technical overview of the conversion of waste products into energy generation and biofuels.

Spent mushroom substrates for ethanol production - Effect of chemical and structural factors on enzymatic saccharification and ethanolic fermentation of Lentinula edodes-pretreated hardwood

Spent mushroom substrates (SMS) from cultivation of shiitake (Lentinula edodes) on three hardwood species were investigated regarding their potential for cellulose saccharification and for ethanolic fermentation of the produced hydrolysates. High glucan digestibility was achieved during enzymatic saccharification of the SMSs, which was related to the low mass fractions of lignin and xylan, and it was neither affected by the relative content of lignin guaiacyl units nor the substrate crystallinity. The high nitrogen content in SMS hydrolysates, which was a consequence of the fungal pretreatment, was positive for the fermentation, and it ensured ethanol yields corresponding to 84-87% of the theoretical value in fermentations without nutrient supplementation. Phenolic compounds and acetic acid were detected in the SMS hydrolysates, but due to their low concentrations, the inhibitory effect was limited. The solid leftovers resulting from SMS hydrolysis and the fermentation residues were quantified and characterized for further valorisation.

Solid-State Fermentation of Chestnut Shells and Effect of Explanatory Variables in Predictive Saccharification Models

In this study, chestnut shells (CNS), a recalcitrant and low-value agro-industrial waste obtained during the peeling of Castanea sativa fruits, were subjected to solid-state fermentation by six white-rot fungal strains (Irpex lacteus, Ganoderma resinaceum, Phlebia rufa, Bjerkandera adusta and two Trametes isolates). After being fermented, CNS was subjected to hydrolysis by a commercial enzymatic mix to evaluate the effect of fermentation in saccharification yield. After 48 h hydrolysis with 10 CMCase U mL−1 enzymatic mix, CNS fermented with both Trametes strains was recorded with higher saccharification yield (around 253 mg g−1 fermented CNS), representing 25% w/w increase in reducing sugars as compared to non-fermented controls. To clarify the relationships and general mechanisms of fungal fermentation and its impacts on substrate saccharification, the effects of some independent or explanatory variables in the production of reducing sugars were estimated by general predictive saccharification models. The variables considered were lignocellulolytic activities in fungal fermentation, CNS hydrolysis time, and concentration of enzymatic hydrolysis mix. Multiple linear regression analysis revealed a very high significant effect (p < 0.0001) of fungal laccase and xylanase activities in the saccharification models, thus proving the key potential of these enzymes in CNS solid-state fermentation.

Biodelignification of lignocellulose using ligninolytic enzymes from white-rot fungi

Lignocellulose is the most abundant biomass available on earth, including wood and agricultural wastes such as rice straw, corn cobs, and oil palm empty bunches. The biopolymer content in lignocellulose has a great potential as feedstock for producing industrial raw materials such as glucose, sorbitol, xylose, xylitol, and other pharmaceutical excipients. Currently, scientists and governments agree that the enzymatic delignification method is an environmentally friendly green method to be applied. This review attempts to explain the proper preparation of the enzymes laccase, lignin peroxidase, and manganese peroxidase, as well as the important factors influencing their activity. The recent applications of the enzymes for detoxification of hazardous substances, proper enzyme immobilization technique, and future prospect combination with DESs extraction of lignin are also discussed.

Fungal pretreatment parameters for improving methane generation from anaerobic digestion of corn silage

Corn silage was treated by white rot fungi (WRF) to investigate the effect of pretreatment on material's ability to produce methane in anaerobic digestion (AD). The selective fungi Pleurotus ostreatus and Dichomitus squalens promoted biogas generation, whereas the non-selective Trametes versicolor and Irpex lacteus had negative effect. Cumulative methane production after 10-day pretreatment with P. ostreatus at 28 °C rose 1.55-fold. The longer pretreatments of 30 and 60-days had smaller effect. When the pretreatment with P. ostreatus was carried out at 40 °C a high H₂S release affected the AD process. Effect of WRF action dependent on the type of corn silage. With typical corn silage, the lignin depolymerisation raised the methane generation from 0.301 to 0.465 m³kg_VS^-1. In contrast, extensive decomposition of hemicellulose in hybrid corn silage deteriorated the effect of pretreatment on methane production.

A Review of Pretreatment Methods to Improve Agri-Food Waste Bioconversion by Black Soldier Fly Larvae

As the world population increases, food demand and agricultural activity will also increase. However, ~30–40% of the food produced today is lost or wasted along the production chain. Increasing food demands would only intensify the existing challenges associated with agri-food waste management. An innovative approach to recover the resources lost along the production chain and convert them into value-added product(s) would be beneficial. An alternative solution is the use of the larvae of the black soldier fly (BSFL), Hermetia illucens L., which can grow and convert a wide range of organic waste materials into insect biomass with use as animal feed, fertilizer and/or bioenergy. However, the main concern when creating an economically viable business is the variability in BSFL bioconversion and processing due to the variability of the substrate. Many factors, such as the nutritional composition of the substrate heavily impact BSFL development. Another concern is that substrates with high lignin and cellulose contents have demonstrated poor digestibility by BSFL. Studies suggest that pretreatment methods may improve the digestibility and biodegradability of the substrate by BSFL. However, a systematic review of existing pretreatment methods that could be used for enhancing the bioconversion of these wastes by BSFL is lacking. This paper provides a state-of-the-art review on the potential pretreatment methods that may improve the digestibility of substrates by BSFL and consequently the production of BSFL. These processes include but are not limited to, physical (e.g., mechanical and thermal), chemical (alkaline treatments), and biological (bacterial and fungal) treatments.

Effect of ligninolytic axenic and coculture white-rot fungi on rice straw chemical composition and in vitro fermentation characteristics

The study sought to investigate the potentials of axenic cultures of Pleurotus ostreatus, Phanerochaete chrysosporium and their coculture (P. chrysosporium and P. ostreatus) to break down lignin and to enhance the rumen fermentability of rice straw. Rice straw was fermented by two lignin-degrading fungi, namely, P. ostreatus, P. chrysosporium and its coculture (P. ostreatus and P. chrysosporium) with uninoculated straw as control under solid-state fermentation employing a completely randomized research design. The coculture exhibited a mutual intermingling plus inhibition interaction. The fungi treatment increased the crude protein from (5.1%) in the control to (6.5%, 6.6%, and 6.7%) in the P. ostreatus, P. chrysosporium and coculture respectively. The coculture treated straw had a lower lignin content (5.3%) compared to the P. chrysosporium (6.2%) with the P. ostreatus recording the least (3.3%) lignin fraction. Treatment of rice straw with coculture improved the in vitro dry matter digestibility (68.1%), total volatile fatty acids (35.3 mM), and total gas (57.4 ml/200 mg) compared to P. chrysosporium (45.1%, 32.2 mM, 44.4 ml/200 mg) but was second to P. ostreatus (75.3%, 38.3 mM, 65.6 ml/200 mg). Instead of an anticipated synergistic effect from the coculture, a competitive antagonistic effect was rather observed at the end of the study, a condition that can be attributed to the coculture behavior.

Shiitake cultivation as biological preprocessing of lignocellulosic feedstocks - Substrate changes in crystallinity, syringyl/guaiacyl lignin and degradation-derived by-products

Formulation of substrates based on three hardwood species combined with modulation of nitrogen content by whey addition (0-2%) was investigated in an experiment designed in D-optimal model for their effects on biological preproceesing of lignocellulosic feedstock by shiitake mushroom (Lentinula edodes) cultivation. Nitrogen loading was shown a more significant role than wood species for both mushroom production and lignocellulose degradation. The fastest mycelial colonisation occurred with no nitrogen supplementation, but the highest mushroom yields were achieved when 1% whey was added. Low nitrogen content resulted in increased delignification and minimal glucan consumption. Delignification was correlated with degradation of syringyl lignin unit, as indicated by a significant reduction (41.5%) of the syringyl-to-guaiacyl ratio after cultivation. No significant changes in substrate crystallinity were observed. The formation of furan aldehydes and aliphatic acids was negligible during the pasteurisation and fungal cultivation, while the content of soluble phenolics increased up to seven-fold.

Fungal lignocellulolytic enzymes and lignocellulose: A critical review on their contribution to multiproduct biorefinery and global biofuel research

The continuous increase in the global energy demand has diminished fossil fuel reserves and elevated the risk of environmental deterioration and human health. Biorefinery processes involved in producing bio-based energy-enriched chemicals have paved way to meet the energy demands. Compared to the thermochemical processes, fungal system biorefinery processes seems to be a promising approach for lignocellulose conversion. It also offers an eco-friendly and energy-efficient route for biofuel generation. Essentially, ligninolytic white-rot fungi and their enzyme arsenals degrade the plant biomass into structural constituents with minimal by-products generation. Hemi- or cellulolytic enzymes from certain soft and brown-rot fungi are always favoured to hydrolyze complex polysaccharides into fermentable sugars and other value-added products. However, the cost of saccharifying enzymes remains the major limitation, which hinders their application in lignocellulosic biorefinery. In the past, research has been focused on the role of lignocellulolytic fungi in biofuel production; however, a cumulative study comprising the contribution of the lignocellulolytic enzymes in biorefinery technologies is still lagging. Therefore, the overarching goal of this review article is to discuss the major contribution of lignocellulolytic fungi and their enzyme arsenal in global biofuel research and multiproduct biorefinery.

Combined effect of olive pruning residues and spent coffee grounds on Pleurotus ostreatus production, composition, and nutritional value

No previous study assessed the combined effect of olive pruning residues (OLPR) and spent coffee grounds (SCG) on P. ostreatus production and nutritional value. The aim of this study was to determine the capacity of P. ostreatus to degrade lignocellulosic nature of combined OLPR and SCG as well as their resultant nutrient composition. A complete randomized design was adopted with five treatments: S1:100%wheat straw (WS) (control), S2:33%WS+33%SCG+33%OLPR,S3:66%WS+17%SCG+17%OLPR,S4:17%WS+66%SCG+17%OLPR, and S5:17%WS+17%SCG+66%OLPR, and ten replicates per treatment. Substrate's and mushroom's composition were analyzed on chemical scale, including fatty acids and heavy metals profiles, following international standards. Only S1, S2, and S3 were productive, with comparable biological yield, economical yield, and biological efficiency. Organic matter loss decreased with increasing proportions of OLPR and SCG. Percentage lignin loss was higher in S1 than in S2 and S3 (53.51, 26.25, and 46.15% respectively). Mushrooms of S3 had some enhanced nutritional attributes compared to control: decrease in fat, increase in protein, increase in monounsaturated fatty acids, and lower zinc accumulation. Lead was less accumulated in S2 than S1 mushrooms. Sodium content of mushroom decreased in S2 and S3. The latter substrates yielded mushrooms with lower polyunsaturated fatty acids (PUFA) and higher saturated fatty acids (SFA) contents. All mushrooms had a valuable PUFA/SFA. This study suggests using OLPR and SCG in low proportions as nutritional supplements to the commercial wheat straw.

Phase Separation in Anaerobic Digestion: A Potential for Easier Process Combination?

The flexibilization of bioenergy production has the potential to counteract partly other fluctuating renewable energy sources (such as wind and solar power). As a weather-independent energy source, anaerobic digestion (AD) can offer on-demand energy supply through biogas production. Separation of the stages in anaerobic digestion represents a promising strategy for the flexibilization of the fermentative part of biogas production. Segregation in two reactor systems facilitates monitoring and control of the provision of educts to the second methanogenic stage, thus controlling biogas production. Two-stage operation has proven to reach similar or even higher methane yields and biogas purities than single-stage operation in many different fields of application. It furthermore allows methanation of green hydrogen and an easier combination of material and energy use of many biogenic raw and residual biomass sources. A lot of research has been conducted in recent years regarding the process phase separation in multi-stage AD operation, which includes more than two stages. Reliable monitoring tools, coupled with effluent recirculation, bioaugmentation and simulation have the potential to overcome the current drawbacks of a sophisticated and unstable operation. This review aims to summarize recent developments, new perspectives for coupling processes for energy and material use and a system integration of AD for power-to-gas applications. Thereby, cell physiological and engineering aspects as well as the basic economic feasibility are discussed. As conclusion, monitoring and control concepts as well as suitable separation technologies and finally the data basis for techno-economic and ecologic assessments have to be improved.

Digestibility of Bacillus firmus K-1 pretreated rice straw by different commercial cellulase cocktails

Removal of xylan in plant biomass is believed to increase cellulose hydrolysis by uncovering cellulose surfaces for cellulase adsorption and, in turn, catalysis reaction. Herein, we describe an eco-friendly method by culturing a xylanolytic Bacillus firmus K-1 on rice straw to remove xylan. The bacterium was grown on 2.5% (w/v) rice straw with different biomass particle sizes for two days at 37 °C. We found that the particle sizes ranged from <1 to 5 mm gave a similar xylan removal degree (about 21%). Besides, the porosity and disintegration of the rice straw fibers were observed at the molecular level. The digestibility of pretreated rice straw was tested with different commercial cellulase cocktails. We found that the pretreated rice straw was more susceptible to enzymatic hydrolysis, giving 30-70% glucan conversion than the untreated one. The degree of cellulose hydrolysis depended strongly on the kinds of enzyme and their formulations. HighlightCulturing B. firmus K-1 on rice straw yielded about 21% removal of xylan.Particle sizes (of 1-5 mm) had negligible effects on xylan removal efficiency.The degree of glucan conversion in pretreated biomass relied on enzyme formulation.

Effect of Pretreated Colza Straw on the Growth and Extracellular Ligninolytic Enzymes Production by Lentinula edodes and Ganoderma lucidum

Lentinula edodes 3565 and Ganoderma lucidum 9621 were compared for their ability to produce lignocellulolytic enzymes in submerged (SM) and surface liquid (SL) fermentation of hydrolysed colza straw lignin waste that remained after the production of furfural and bioethanol (CS lignin). Application of cultivated mushrooms to dispose of pretreated colza straw agricultural waste is an approach to decrease the quantity of residual lignin while simultaneously obtaining active substances, e.g., the ligninolytic enzyme complex from mycelium. The effect of adding CS lignin to culture media on the yield of L. edodes and G. lucidum mycelium and extracellular laccase activity was studied. It was revealed that the mycelial growth of G. lucidum on solid media was significantly improved by adding CS lignin. Laccase activity during SL cultivation of L. edodes on medium with CS lignin gradually increased over the experiment starting on day 21 and peaked at 520 U/mL on day 28. G. lucidum expressed the maximum laccase activity, 540 U/mL, during the first 14 days of mycelium SM cultivation. Extracellular laccase activity was enhanced about 35- to 40-fold at cultivation of L. edodes and about 10- to 15-fold in the case of G. lucidum by supplementing liquid culture media with CS lignin.