Evaluation of pretreatment with Pleurotus ostreatus for enzymatic hydrolysis of rice straw

Synergistic action between extracellular products from white-rot fungus and cellulase significantly improves enzymatic hydrolysis

With a set of perfect extracellular lignocellulolytic enzymes, white-rot fungus has been recognized as playing an important role in the degradation of lignocellulose materials, which leads to the possibility of creating a composite enzymatic system with high hydrolysis efficiency in vitro. Echinodontium taxodii is a promising white-rot fungus for biologic pretreatment. In this study, we extracted the extracellular products of E. taxodii under solid-state fermentation conditions, mixed the extracellular products with cellulase to build a composite enzymatic system, and systematically evaluated the effect of this system on the hydrolysis of acid-pretreated and raw maize stovers. We found that the extracellular products from E. taxodii could significantly improve the hydrolysis efficiency of cellulase, with a synergistic action between the extracellular products and cellulase. Corn stovers treated with extracellular products were suitable for the enzymatic hydrolysis of cellulase. Furthermore, we found that pure proteins from the extracellular products were not sufficient to generate synergistic action. This finding suggests that non-protein substances may also be involved in the synergistic action between the extracellular products and cellulase.

Effects of cre1 modification in the white-rot fungus Pleurotus ostreatus PC9: altering substrate preference during biological pretreatment

BACKGROUND

During the process of bioethanol production, cellulose is hydrolyzed into its monomeric soluble units. For efficient hydrolysis, a chemical and/or mechanical pretreatment step is required. Such pretreatment is designed to increase enzymatic digestibility of the cellulose chains inter alia by de-crystallization of the cellulose chains and by removing barriers, such as lignin from the plant cell wall. Biological pretreatment, in which lignin is decomposed or modified by white-rot fungi, has also been considered. One disadvantage in biological pretreatment, however, is the consumption of the cellulose by the fungus. Thus, fungal species that attack lignin with only minimal cellulose loss are advantageous. The secretomes of white-rot fungi contain carbohydrate-active enzymes (CAZymes) including lignin-modifying enzymes. Thus, modification of secretome composition can alter the ratio of lignin/cellulose degradation.

RESULTS

Pleurotus ostreatus PC9 was genetically modified to either overexpress or eliminate (by gene replacement) the transcriptional regulator CRE1, known to act as a repressor in the process of carbon catabolite repression. The cre1-overexpressing transformant demonstrated lower secreted cellulolytic activity and slightly increased selectivity (based on the chemical composition of pretreated wheat straw), whereas the knockout transformant demonstrated increased cellulolytic activity and significantly reduced residual cellulose, thereby displaying lower selectivity. Pretreatment of wheat straw using the wild-type PC9 resulted in 2.8-fold higher yields of soluble sugar compared to untreated wheat straw. The overexpression transformant showed similar yields (2.6-fold), but the knockout transformant exhibited lower yields (1.2-fold) of soluble sugar. Based on proteomic secretome analysis, production of numerous CAZymes was affected by modification of the expression level of cre1.

CONCLUSIONS

The gene cre1 functions as a regulator for expression of fungal CAZymes active against plant cell wall lignocelluloses, hence altering the substrate preference of the fungi tested. While the cre1 knockout resulted in a less efficient biological pretreatment, i.e., less saccharification of the treated biomass, the converse manipulation of cre1 (overexpression) failed to improve efficiency. Despite the inverse nature of the two genetic alterations, the expected "mirror image" (i.e., opposite regulatory response) was not observed, indicating that the secretion level of CAZymes, was not exclusively dependent on CRE1 activity.

Reducing biomass recalcitrance by heterologous expression of a bacterial peroxidase in tobacco (Nicotiana benthamiana)

Commercial scale production of biofuels from lignocellulosic feed stocks has been hampered by the resistance of plant cell walls to enzymatic conversion, primarily owing to lignin. This study investigated whether DypB, the lignin-degrading peroxidase from Rodococcus jostii, depolymerizes lignin and reduces recalcitrance in transgenic tobacco (Nicotiana benthamiana). The protein was targeted to the cytosol or the ER using ER-targeting and retention signal peptides. For each construct, five independent transgenic lines were characterized phenotypically and genotypically. Our findings reveal that expression of DypB in the cytosol and ER does not affect plant development. ER-targeting increased protein accumulation, and extracts from transgenic leaves showed higher activity on classic peroxidase substrates than the control. Intriguingly, in situ DypB activation and subsequent saccharification released nearly 200% more fermentable sugars from transgenic lines than controls, which were not explained by variation in initial structural and non-structural carbohydrates and lignin content. Pyrolysis-GC-MS analysis showed more reduction in the level of lignin associated pyrolysates in the transgenic lines than the control primarily when the enzyme is activated prior to pyrolysis, consistent with increased lignin degradation and improved saccharification. The findings reveal for the first time that accumulation and in situ activation of a peroxidase improves biomass digestibility.

Development of natural cellulase inhibitor mediated intensified biological pretreatment technology using Pleurotus florida for maximum recovery of cellulose from paddy straw under solid state condition

Inhibitor mediated intensified bio-pretreatment (IMBP) technology using natural cellulase inhibitor (NCI) for maximum cellulose recovery from paddy straw was studied. Pretreatment was carried out under solid state condition. Supplementation of 8% NCI in pretreatment medium improves cellulose recovery and delignification by 1.2 and 1.5-fold respectively, compared to conventional bio-pretreatment due to inhibition of 61% of cellulase activity in IMBP. Further increase in NCI concentration showed negative effect on Pleurotus florida growth and suppress the laccase productivity by 1.1-fold. Laccase activity in IMBP was found to be 2.0U/mL on 19^thday, which is higher than (1.5U/mL) conventional bio-pretreatment. Physico-chemical modifications in paddy straw before and after pretreatment were analysed by SEM, ATR-FTIR, XRD and TGA. According to these findings, the IMBP technology can be a viable eco-friendly technology for sustainable production of bioethanol with maximum cellulose recovery.

White-rot fungi pretreatment combined with alkaline/oxidative pretreatment to improve enzymatic saccharification of industrial hemp

White-rot fungi combined with alkaline/oxidative (A/O) pretreatments of industrial hemp woody core were proposed to improve enzymatic saccharification. In this study, hemp woody core were treated with only white rot fungi, only A/O and combined with the two methods. The results showed that Pleurotus eryngii (P. eryngii) was the most effective fungus for pretreatment. Reducing sugars yield was 329mg/g with 30 Filter Paper Unit (FPU)/g cellulase loading when treated 21day. In the A/O groups, the results showed that when treated with 3% NaOH and 3% H₂O₂, the yield of reducing sugars was 288mg/g with 30FPU/g cellulase loading. After combination pretreatment with P. eryngii and A/O pretreatment, the reducing sugar yield from enzymatic hydrolysis of combined sample increased 1.10-1.29-fold than that of bio-treated or A/O pretreatment sample at the same conditions, suggesting that P. eryngii combined with A/O pretreatment was an effective method to improve enzyme hydrolysis.

Chemical Pretreatment-Independent Saccharifications of Xylan and Cellulose of Rice Straw by Bacterial Weak Lignin-Binding Xylanolytic and Cellulolytic Enzymes

Complete utilization of carbohydrate fractions is one of the prerequisites for obtaining economically favorable lignocellulosic biomass conversion. This study shows that xylan in untreated rice straw was saccharified to xylose in one step without chemical pretreatment, yielding 58.2% of the theoretically maximum value by Paenibacillus curdlanolyticus B-6 PcAxy43A, a weak lignin-binding trifunctional xylanolytic enzyme, endoxylanase/β-xylosidase/arabinoxylan arabinofuranohydrolase. Moreover, xylose yield from untreated rice straw was enhanced to 78.9% by adding endoxylanases PcXyn10C and PcXyn11A from the same bacterium, resulting in improvement of cellulose accessibility to cellulolytic enzyme. After autoclaving the xylanolytic enzyme-treated rice straw, it was subjected to subsequent saccharification by a combination of the Clostridium thermocellum endoglucanase CtCel9R and Thermoanaerobacter brockii β-glucosidase TbCglT, yielding 88.5% of the maximum glucose yield, which was higher than the glucose yield obtained from ammonia-treated rice straw saccharification (59.6%). Moreover, this work presents a new environment-friendly xylanolytic enzyme pretreatment for beneficial hydrolysis of xylan in various agricultural residues, such as rice straw and corn hull. It not only could improve cellulose saccharification but also produced xylose, leading to an improvement of the overall fermentable sugar yields without chemical pretreatment.IMPORTANCE Ongoing research is focused on improving "green" pretreatment technologies in order to reduce energy demands and environmental impact and to develop an economically feasible biorefinery. The present study showed that PcAxy43A, a weak lignin-binding trifunctional xylanolytic enzyme, endoxylanase/β-xylosidase/arabinoxylan arabinofuranohydrolase from P. curdlanolyticus B-6, was capable of conversion of xylan in lignocellulosic biomass such as untreated rice straw to xylose in one step without chemical pretreatment. It demonstrates efficient synergism with endoxylanases PcXyn10C and PcXyn11A to depolymerize xylan in untreated rice straw and enhanced the xylose production and improved cellulose hydrolysis. Therefore, it can be considered an enzymatic pretreatment. Furthermore, the studies here show that glucose yield released from steam- and xylanolytic enzyme-treated rice straw by the combination of CtCel9R and TbCglT was higher than the glucose yield obtained from ammonia-treated rice straw saccharification. This work presents a novel environment-friendly xylanolytic enzyme pretreatment not only as a green pretreatment but also as an economically feasible biorefinery method.

A novel and efficient fungal delignification strategy based on versatile peroxidase for lignocellulose bioconversion

BACKGROUND

The selective lignin-degrading white-rot fungi are regarded to be the best lignin degraders and have been widely used for reducing the saccharification recalcitrance of lignocellulose. However, the biological delignification and conversion of lignocellulose in biorefinery is still limited. It is necessary to develop novel and more efficient bio-delignification systems.

RESULTS

Physisporinus vitreus relies on a new versatile peroxidase (VP)-based delignification strategy to remove enzymatic recalcitrance of corn stover efficiently, so that saccharification of corn stover was significantly enhanced to 349.1 mg/g biomass (yield of glucose) and 91.5% (hydrolysis yield of cellulose) at 28 days, as high as levels reached by thermochemical treatment. Analysis of the lignin structure using pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) showed that the total abundance of lignin-derived compounds decreased by 54.0% and revealed a notable demethylation during lignin degradation by P. vitreus. Monomeric and dimeric lignin model compounds were used to confirm the ligninolytic capabilities of extracellular ligninases secreted by P. vitreus. The laccase (Lac) from P. vitreus could not oxidize nonphenolic lignin compounds and polymerized β-O-4 and 5-5' dimers to precipitate which had a negative effect on the enzymatic hydrolysis of corn stover in vitro. However, the VP from P. vitreus could oxidize both phenolic and nonphenolic lignin model compounds as well as break the β-O-4 and 5-5' dimers into monomeric compounds, which were measured by high-performance liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). Moreover, we showed that addition of purified VP in vitro improved the enzymatic hydrolysis of corn stover by 14.1%.

CONCLUSIONS

From the highly efficient system of enzymatic recalcitrance removal by new white-rot fungus, we identified a new delignification strategy based on VP which could oxidize both phenolic and nonphenolic lignin units and break different linkages in lignin. In addition, this is the first evidence that VP could break 5-5' linkage efficiently in vitro. Moreover, VP improved the enzymatic hydrolysis of corn stover in vitro. The remarkable lignin-degradative potential makes VP attractive for biotechnological applications.

Bioethanol production from spent mushroom compost derived from chaff of millet and sorghum

BACKGROUND

In Uganda, the chaff remaining from threshed panicles of millet and sorghum is a low value, lignocellulose-rich agricultural by-product. Currently, it is used as a substrate for the cultivation of edible Oyster mushrooms (Pleurotus ostreatus). The aim of this study was to assess the potential to exploit the residual post-harvest compost for saccharification and fermentation to produce ethanol.

RESULTS

Sorghum and millet chaff-derived spent oyster mushroom composts minus large mycelium particles were assessed at small-scale and low substrate concentrations (5% w/v) for optimal severity hydrothermal pre-treatment, enzyme loading and fermentation with robust yeasts to produce ethanol. These conditions were then used as a basis for larger scale assessments with high substrate concentrations (30% w/v). Millet-based compost had a low cellulose content and, at a high substrate concentration, did not liquefy effectively. The ethanol yield was 63.9 g/kg dry matter (DM) of original material with a low concentration (19.6 g/L). Compost derived from sorghum chaff had a higher cellulose content and could be liquefied at high substrate concentration (30% w/v). This enabled selected furfural-resistant yeasts to produce ethanol at up to 186.9 g/kg DM of original material and a concentration of 45.8 g/L.

CONCLUSIONS

Spent mushroom compost derived from sorghum chaff has the potential to be an industrially useful substrate for producing second-generation bioethanol. This might be improved further through fractionation and exploitation of hemicellulosic moieties, and possibly the exploitation of the mycelium-containing final residue for animal feed. However, spent compost derived from millet does not provide a suitably high concentration of ethanol to make it industrially attractive. Further research on the difficulty in quantitatively saccharifying cellulose from composted millet chaff and other similar substrates such as rice husk is required.

Accumulation of sugar from pulp and xylitol from xylose by pyruvate decarboxylase-negative white-rot fungus Phlebia sp. MG-60

Phlebia sp. MG-60 is a white-rot fungus that produces ethanol with high efficiency from lignocellulosic biomass without additional enzymes. Through engineering of this powerful metabolic pathway for fermentation in Phlebia sp. MG-60, chemical compounds other than ethanol could be produced. Here, we demonstrate sugar accumulation from unbleached hardwood kraft pulp and conversion of xylose to xylitol by pyruvate decarboxylase (pdc)-negative Phlebia sp. MG-60. We isolated Phlebia sp. strain MG-60-P2 from protoplasts to unify the protoplast phenotypes of the regenerated strains. Homologous recombination achieved a stable pdc-knockout line, designated KO77. The KO77 line produced traces of ethanol, but accumulated xylitol from xylose or glucose from unbleached hardwood kraft pulp. These metabolic changes in the pdc-knockout strain reflect the potential of metabolic engineering in Phlebia sp. MG-60 for direct production of chemical compounds from lignocellulosic biomass.

Enhancement in multiple lignolytic enzymes production for optimized lignin degradation and selectivity in fungal pretreatment of sweet sorghum bagasse

The objective of this work was to study the increase in multiple lignolytic enzyme productions through the use of supplements in combination in pretreatment of sweet sorghum bagasse (SSB) by Coriolus versicolor such that enzymes act synergistically to maximize the lignin degradation and selectivity. Enzyme activities were enhanced by metallic salts and phenolic compound supplements in SSF. Supplement of syringic acid increased the activities of LiP, AAO and laccase; gallic acid increased MnP; CuSO₄ increased laccase and PPO to improve the lignin degradations and selectivity individually, higher than control. Combination of supplements optimized by RSM increased the production of laccase, LiP, MnP, PPO and AAO by 17.2, 45.5, 3.5, 2.4 and 3.6 folds respectively for synergistic action leading to highest lignin degradation (2.3 folds) and selectivity (7.1 folds). Enzymatic hydrolysis of pretreated SSB yielded ∼2.43 times fermentable sugar. This technique could be widely applied for pretreatment and enzyme productions.

Fungal pretreatment of sweet sorghum bagasse with supplements: improvement in lignin degradation, selectivity and enzymatic saccharification

Sweet sorghum bagasse (SSB) from food processing and agricultural industry has attracted the attention for uses in production of biofuel, enzymes and other products. The alteration in lignocellulolytic enzymes by use of supplements in fungal pretreatment of SSB to achieve higher lignin degradation, selectivity value and enzymatic hydrolysis to fermentable sugar was studied. Fungal strain Coriolus versicolor was selected for pretreatment due to high ligninolytic and low cellulolytic enzyme production resulting in high lignin degradation and selectivity value. SSB was pretreated with supplements of veratryl alcohol, syringic acid, catechol, gallic acid, vanillin, guaiacol, CuSO₄ and MnSO₄. The best results were obtained with CuSO₄, gallic acid and syringic acid supplements. CuSO₄ increased the activities of laccase (4.9-fold) and polyphenol oxidase (1.9-fold); gallic acid increased laccase (3.5-fold) and manganese peroxidase (2.5-fold); and syringic acid increased laccase (5.6-fold), lignin peroxidase (13-fold) and arylalcohol oxidase (2.8-fold) resulting in enhanced lignin degradations and selectivity values than the control. Reduced cellulolytic enzyme activities resulted in high cellulose recovery. Enzymatic hydrolysis of pretreated SSB yielded higher sugar due to degradation of lignin and reduced the crystallinity of cellulose. The study showed that supplements could be used to improve the pretreatment process. The results were confirmed by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric/differential thermogravimetric analysis of SSB.

Improvement of selective lignin degradation in fungal pretreatment of sweet sorghum bagasse using synergistic CuSO4-syringic acid supplements

Sweet sorghum bagasse (SSB) generated in large quantities could be hydrolyzed to sugar and then fermented to green fuels. The hydrolysis of SSB polysaccharides interlocked in recalcitrant lignin network is the major problem. Pretreatment of SSB in SSF by using Coriolus versicolor with CuSO₄-syringic acid supplements for effects on production of ligninocellulolytic enzymes, lignin degradation and selectivity values (SV) were studied. C. versicolor was selected based on high ligninolytic and low cellulolytic abilily. Individually, CuSO₄ increased the activities of laccase (4.9 folds) and PPO (1.9 folds); syringic acid increased LiP (13 folds), AAO (2.8 folds) and laccase (5.6 folds) resulting in increased lignin degradation and SVs. Combined syringic acid (4.4 μmol g^-1 SSB) and CuSO₄ (4.4 μmol g^-1 SSB) increased the activities of laccase, LiP, MnP, PPO and AAO by 11.2, 17.6, 2.8, 2.4 and 2.3 folds respectively due to synergistic effect, resulting in maximum lignin degradation 35.9 ± 1.3% (w w^-1) (1.86 fold) and highest SV 3.07 (4.7 fold). Enzymatic hydrolysis of pretreated SSB yielded higher (∼2.2 times) fermentable sugar. Pretreated SSB was characterized by XRD, SEM, FTIR and TGA/DTG analysis to confirm results. It is possible to improve fungal pretreatment of agricultural waste by combination of supplements.

Biotransformation of lignocellulosic materials into value-added products-A review

In the past decade, with the key biotechnological advancements, lignocellulosic materials have gained a particular importance. In serious consideration of global economic, environmental and energy issues, research scientists have been re-directing their interests in (re)-valorizing naturally occurring lignocellulosic-based materials. In this context, lignin-modifying enzymes (LMEs) have gained considerable attention in numerous industrial and biotechnological processes. However, their lower catalytic efficiencies and operational stabilities limit their practical and multipurpose applications in various sectors. Therefore, to expand the range of natural industrial biocatalysts e.g. LMEs, significant progress related to the enzyme biotechnology has appeared. Owing to the abundant lignocellulose availability along with LMEs in combination with the scientific advances in the biotechnological era, solid-phase biocatalysts can be economically tailored on a large scale. This review article outlines first briefly on the lignocellulose materials as a potential source for biotransformation into value-added products including composites, fine chemicals, nutraceutical, delignification, and enzymes. Comprehensive information is also given on the purification and characterization of LMEs to present their potential for the industrial and biotechnological sector.

Metal accumulation by sunflower (Helianthus annuus L.) and the efficacy of its biomass in enzymatic saccharification

Accumulation of metal contaminants in soil as a result of various industrial and anthropogenic activities has reduced soil fertility significantly. Phytoextraction of metal contaminants can improve soil fertility and provide inexpensive feedstock for biorefineries. We investigated the hyperaccumulation capacity of sunflower (Helianthus annuus) biomass by cultivating these plants in various concentrations of metal contaminants. Sunflowers were grown in soils contaminated with various levels of heavy metals (10–2,000 mg/kg dry soil). The degree of metal uptake by different parts of the biomass and the residual concentration in the soil were estimated through inductively coupled plasma mass spectrometry. An almost 2.5-fold hyperaccumulation of Zn²⁺ was observed in the leaf and flower biomass compared with the concentration in the soil. For the subsequent saccharification of biomass with hyperaccumulated contaminants, a fungal lignocellulosic consortium was used. The fungal consortium cocktail retained more than 95% filter paper activity with 100 mM Ni²⁺ ions even after 36 h. The highest saccharification yield (SY, 87.4%) was observed with Ni²⁺ as the contaminant (10 mg/kg dry wt), whereas Pb²⁺ (251.9 mg/kg dry wt) was the strongest inhibitor of biomass hydrolysis, resulting in only a 30% SY. Importantly, the enzyme cocktail produced by the fungal consortium resulted in almost the same SY (%) as that obtained from a combination of commercial cellulase and β-glucosidase. Significant sugar conversion (61.7%) from H. annuus biomass hydrolysate occurred, resulting in the production of 11.4 g/L of bioethanol. This is the first study to assess the suitability of phytoremediated sunflower biomass for bioethanol production.

Differential Proteomic Profiles of Pleurotus ostreatus in Response to Lignocellulosic Components Provide Insights into Divergent Adaptive Mechanisms

Pleurotus ostreatus is a white rot fungus that grows on lignocellulosic biomass by metabolizing the main constituents. Extracellular enzymes play a key role in this process. During the hydrolysis of lignocellulose, potentially toxic molecules are released from lignin, and the molecules are derived from hemicellulose or cellulose that trigger various responses in fungus, thereby influencing mycelial growth. In order to characterize the mechanism underlying the response of P. ostreatus to lignin, we conducted a comparative proteomic analysis of P. ostreatus grown on different lignocellulose substrates. In this work, the mycelium proteome of P. ostreatus grown in liquid minimal medium with lignin, xylan, and carboxymethyl cellulose (CMC) was analyzed using the complementary two-dimensional gel electrophoresis (2-DE) approach; 115 proteins were identified, most of which were classified into five types according to their function. Proteins with an antioxidant function that play a role in the stress response were upregulated in response to lignin. Most proteins involving in carbohydrate and energy metabolism were less abundant in lignin. Xylan and CMC may enhanced the process of carbohydrate metabolism by regulating the level of expression of various carbohydrate metabolism-related proteins. The change of protein expression level was related to the adaptability of P. ostreatus to lignocellulose. These findings provide novel insights into the mechanisms underlying the response of white-rot fungus to lignocellulose.

Combinations of fungal and milling pretreatments for enhancing rice straw biogas production during solid-state anaerobic digestion

Rice straw was pretreated by different combinations of physical (milling) and biological (incubation with Pleurotus ostreatus fungus) treatment to improve its biodegradability and biogas production during solid-state anaerobic digestion (SS-AD). Effects of milling (⩽2mm) and incubation time (10, 20 and 30d), on lignin, cellulose, and hemicellulose degradation during fungal pretreatment and methane yield during digestion were assessed by comparison with untreated rice straw. Both incubation time and milling had significant impacts on both lignin removal during fungal pre-treatment and methane yield during digestion. A combination of fungal pretreatment at 30days followed by milling prior to anaerobic digestion resulted in 30.4% lignin removal, the highest selectivity value (the ratio between relative lignin removal and relative cellulose removal) of 4.22, and the highest methane yield of 258L/kgVS. This was equivalent to a 165% increase in methane yield from SS-AD compared to untreated rice straw.

Unravelling the capability of Pyrenophora phaeocomes S-1 for the production of ligno-hemicellulolytic enzyme cocktail and simultaneous bio-delignification of rice straw for enhanced enzymatic saccharification

A natural variant of Pyrenophora phaeocomes isolated from natural biodiversity was able to grow on various agricultural residues by co-producing laccase, xylanase and mannanase. Solid state fermentation of rice straw induced the highest productivities corresponding to 10,859.51±46.74, 22.01±1.00 and 10.45±0.128IUgds^-1 for laccase, xylanase and mannanase respectively after 4days. Besides producing the ligno-hemicellulolytic enzyme cocktail, 40days cultivation of P. phaeocomes S-1 on rice straw brought about the 63 and 51% degradation of lignin and hemicellulose. These components were further removed with mild alkali extraction revealing the overall losses amounting to 78 and 60% respectively for lignin, and hemicellulose. The biologically pretreated straw upon enzymatic hydrolysis revealed 50% saccharification efficiency releasing 470mgg^-1 sugars. Application of this knowledge will lead to efficient management of waste rice straw with low cost production of industrially important enzymes cocktail and its biological delignification for effective enzymatic hydrolysis to free sugars.

Unleashing the potential of ligninolytic bacterial contributions towards pulp and paper industry: key challenges and new insights

Lignocellulose biomass predominantly constitutes the main feedstock for pulp and paper industry. Though some products of pulp and paper industry require the presence of lignin content, for most of the useful products formation lies in the efficient and selective removal of lignin component to make use of the intact cellulose fraction during the pretreatment of pulp. Lignin is a recalcitrant heteropolymer comprised of several complex stable bonds and linkages. The chemicals or intense energy processes used for delignification process release the hazardous chemicals compounds in the wastewater which cause toxicity and environmental pollution. The implementation of bacterial species has elucidated an effective approach in the generation of value-added products while degrading lignin from pulp biomass as well as detoxification of effluent. The direct use of bacterial cells in lignocellulose biomass and wastewater streams is promising as it outperforms the practical and technical constraints largely confronted by fungal and enzymatic means. The present review paper thus unleashed the potential of ligninolytic bacteria towards delignification of pulp biomass and treatment of effluent together with bioconversion of biomass and lignin into value-added products. Graphical abstract Schematic illustration of potential possible contribution of ligninolytic bacteria towards pulp and paper industry.

Cotton Stalk Pretreatment Using Daedalea flavida, Phlebia radiata, and Flavodon flavus: Lignin Degradation, Cellulose Recovery, and Enzymatic Saccharification

Lignocellulolytic enzyme activities of selective fungi Daedalea flavida MTCC 145 (DF-2), Phlebia radiata MTCC 2791 (PR), and non-selective fungus Flavodon flavus MTCC 168 (FF) were studied for pretreatment of cotton stalks. Simultaneous productions of high LiP and laccase activities by DF-2 during early phase of growth were effective for lignin degradation 27.83 ± 1.25 % (w/w of lignin) in 20-day pretreatment. Production of high MnP activity without laccase in the early growth phase of PR was ineffective and delayed lignin degradation 24.93 ± 1.53 % in 25 days due to laccase production at later phase. With no LiP activity, low activities of MnP and laccase by FF yielded poor lignin degradation 15.09 ± 0.6 % in 20 days. Xylanase was predominant cellulolytic enzyme produced by DF-2, resulting hemicellulose as main carbon and energy source with 83 % of cellulose recovery after 40 days of pretreatment. The glucose yield improved more than two fold from 20-day DF-2 pretreated cotton stalks after enzymatic saccharification.

Green methods of lignocellulose pretreatment for biorefinery development

Lignocellulosic biomass is the most abundant, low-cost, bio-renewable resource that holds enormous importance as alternative source for production of biofuels and other biochemicals that can be utilized as building blocks for production of new materials. Enzymatic hydrolysis is an essential step involved in the bioconversion of lignocellulose to produce fermentable monosaccharides. However, to allow the enzymatic hydrolysis, a pretreatment step is needed in order to remove the lignin barrier and break down the crystalline structure of cellulose. The present manuscript is dedicated to reviewing the most commonly applied "green" pretreatment processes used in bioconversion of lignocellulosic biomasses within the "biorefinery" concept. In this frame, the effects of different pretreatment methods on lignocellulosic biomass are described along with an in-depth discussion on the benefits and drawbacks of each method, including generation of potentially inhibitory compounds for enzymatic hydrolysis, effect on cellulose digestibility, and generation of compounds toxic for the environment, and energy and economic demand.

Biologically pretreated sugarcane top as a potential raw material for the enhancement of gaseous energy recovery by two stage biohythane process

The aim of the present study was to develop a suitable pretreatment method to enhance the microbial degradation of lignocellulosic biomass and to maximize the overall energy recovery by using biohythane process. An efficient and eco-friendly biological pretreatment was used. Maximum lignin removal using biological pretreatment of sugarcane top was 60.4% w/w after 21d incubation at 28°C in static condition. Confocal microscopy observation and FTIR analysis confirmed the removal of lignin from sugarcane top. The maximum hydrogen production rate (Rm), hydrogen production potential (P) and lag time (λ) using pretreated sugarcane top were 16.76mL/g-VS/h, 87.40mL/g-VS and 3.38h respectively. The maximum methane production potential using spent medium of dark fermentation was 180.86mL/g-VS with the lag time of 2.9d. The overall gaseous energy recovery was 37.7% which is 54% higher than that of the untreated one.

Corn forage biological pretreatment by Trametes versicolor in a tray bioreactor

Trametes versicolor is a white-rot fungus known to be efficient in lignin removal due to its complex extracellular lignocellulolytic enzymatic system. Therefore, it can be used in the treatment of lignocellulose waste from agro, food, and wood industries. In a first experiment, corn forage treatment with T. versicolor was investigated in laboratory jars. In a second experiment, the process was scaled up to a tray bioreactor. In the tray bioreactor, the process of lignin degradation was improved, resulting in an increase in lignin conversion of up to 71% during seven days' treatment.

Purification and characterisation of processive-type endoglucanase and β-glucosidase from Aspergillus ochraceus MTCC 1810 through saccharification of delignified coir pith to glucose

The study describes purification and characterisation of processive-type endoglucanase and β-glucosidase from Aspergillus ochraceus MTCC 1810 through bioconversion of delignified coir pith to fermentable glucose. The purified processive endoglucanase (AS-HT-Celuz A) and β-glucosidase (AS-HT-Celuz B) were found to have molecular mass of ≈78-kDa and 43-kDa respectively with optimum endoglucanase (35.63U/ml), total cellulase (28.15FPU/ml) and β-glucosidase (15.19U/ml) activities at 40°C/pH 6. The unique feature of AS-HT-Celuz A is the multiple substrate specificity and processivity towards both amorphous and crystalline cellulose. Zymogram indicated both endo and exoglucanase activities residing in different binding sites of a single protein exhibiting sequential synergy with its own β-glucosidase. Accordingly, the identified enzymes could be implemented as synergistic cellulases for complete cellulose saccharification which still considered an unresolved issue in bio-refineries.

Optimization of Bioethanol Production Using Whole Plant of Water Hyacinth as Substrate in Simultaneous Saccharification and Fermentation Process

Water hyacinth was used as substrate for bioethanol production in the present study. Combination of acid pretreatment and enzymatic hydrolysis was the most effective process for sugar production that resulted in the production of 402.93 mg reducing sugar at optimal condition. A regression model was built to optimize the fermentation factors according to response surface method in saccharification and fermentation (SSF) process. The optimized condition for ethanol production by SSF process was fermented at 38.87°C in 81.87 h when inoculated with 6.11 ml yeast, where 1.291 g/L bioethanol was produced. Meanwhile, 1.289 g/L ethanol was produced during experimentation, which showed reliability of presented regression model in this research. The optimization method discussed in the present study leading to relatively high bioethanol production could provide a promising way for Alien Invasive Species with high cellulose content.

Emerging Technologies for the Production of Renewable Liquid Transport Fuels from Biomass Sources Enriched in Plant Cell Walls

Plant cell walls are composed predominantly of cellulose, a range of non-cellulosic polysaccharides and lignin. The walls account for a large proportion not only of crop residues such as wheat straw and sugarcane bagasse, but also of residues of the timber industry and specialist grasses and other plants being grown specifically for biofuel production. The polysaccharide components of plant cell walls have long been recognized as an extraordinarily large source of fermentable sugars that might be used for the production of bioethanol and other renewable liquid transport fuels. Estimates place annual plant cellulose production from captured light energy in the order of hundreds of billions of tons. Lignin is synthesized in the same order of magnitude and, as a very large polymer of phenylpropanoid residues, lignin is also an abundant, high energy macromolecule. However, one of the major functions of these cell wall constituents in plants is to provide the extreme tensile and compressive strengths that enable plants to resist the forces of gravity and a broad range of other mechanical forces. Over millions of years these wall constituents have evolved under natural selection to generate extremely tough and resilient biomaterials. The rapid degradation of these tough cell wall composites to fermentable sugars is therefore a difficult task and has significantly slowed the development of a viable lignocellulose-based biofuels industry. However, good progress has been made in overcoming this so-called recalcitrance of lignocellulosic feedstocks for the biofuels industry, through modifications to the lignocellulose itself, innovative pre-treatments of the biomass, improved enzymes and the development of superior yeasts and other microorganisms for the fermentation process. Nevertheless, it has been argued that bioethanol might not be the best or only biofuel that can be generated from lignocellulosic biomass sources and that hydrocarbons with intrinsically higher energy densities might be produced using emerging and continuous flow systems that are capable of converting a broad range of plant and other biomasses to bio-oils through so-called 'agnostic' technologies such as hydrothermal liquefaction. Continued attention to regulatory frameworks and ongoing government support will be required for the next phase of development of internationally viable biofuels industries.

The role of pretreatment in improving the enzymatic hydrolysis of lignocellulosic materials

Lignocellulosic materials are among the most promising alternative energy resources that can be utilized to produce cellulosic ethanol. However, the physical and chemical structure of lignocellulosic materials forms strong native recalcitrance and results in relatively low yield of ethanol from raw lignocellulosic materials. An appropriate pretreatment method is required to overcome this recalcitrance. For decades various pretreatment processes have been developed to improve the digestibility of lignocellulosic biomass. Each pretreatment process has a different specificity on altering the physical and chemical structure of lignocellulosic materials. In this paper, the chemical structure of lignocellulosic biomass and factors likely affect the digestibility of lignocellulosic materials are discussed, and then an overview about the most important pretreatment processes available are provided. In particular, the combined pretreatment strategies are reviewed for improving the enzymatic hydrolysis of lignocellulose and realizing the comprehensive utilization of lignocellulosic materials.

Simultaneous pretreatment and saccharification: green technology for enhanced sugar yields from biomass using a fungal consortium

Two different biomasses were subjected to simultaneous pretreatment and saccharification (SPS) using a cocktail of hydrolytic and oxidizing enzymes. Application of a novel laccase as a detoxifying agent caused the removal of 49.8% and 32.6% of phenolic contents from the soaked rice straw and willow, respectively. Hydrolysis of soaked substrates using a newly developed fungal consortium resulted in saccharification yield of up to 74.2% and 63.6% for rice straw and willow, respectively. A high saccharification yield was obtained with soaked rice straw and willow without using any hazardous chemicals. The efficiency of each step related to SPS was confirmed by atomic force microscopy. The suitability of the developed SPS process was further confirmed by converting the hydrolysate from the process into bioethanol with 72.4% sugar conversion efficiency. To the best of our knowledge, this is the first report on the development of a less tedious, single-pot, and eco-friendly SPS methodology.

Production of a lignocellulolytic enzyme system for simultaneous bio-delignification and saccharification of corn stover employing co-culture of fungi

Aiming at improving the efficiency of transferring corn stover into sugars, an efficient lignocellulolytic enzyme system was developed and investigated by co-cultivation of the Coprinus comatus with Trichoderma reesei in a single bioreactor. The results showed that the lignocellulolytic enzyme activities of the co-culture exceeded that of the monoculture, suggesting synergistic interaction between two fungi. The highest laccase activity from the co-culture was 2.6-fold increase over that of the C. comatus monoculture and reached a peak 3days earlier. The maximum delignification obtained was 66.5% and about 82% of the original polysaccharides were converted into fermentable sugars by simultaneous bio-delignification and saccharification process. Correlation analysis showed that sugar yields were directly proportional to the lignin degradation. Our results suggested that co-fungi cultivation was a valuable technique for corn stover bioconversion, which could produce high efficiency of lignocellulolytic enzyme system as a cheaper alternative to commercial enzymes for industrial utilization.

The use of steam explosion to increase the nutrition available from rice straw

In the present study, rice straw was pretreated using steam-explosion (ST) technique to improve the enzymatic hydrolysis of potential reducing sugars for feed utilization. The response surface methodology based on central composite design was used to optimize the effects of steam pressure, pressure retention time, and straw moisture content on the yield of reducing sugar. All the investigated variables had significant effects (P < 0.001) on the reducing sugar yield. The optimum yield of 30.86% was obtained under the following pretreatment conditions: steam pressure, 1.54 MPa; pressure retention time, 140.5 Sec; and straw moisture content, 41.6%. The yield after thermal treatment under the same conditions was approximately 16%. Infrared (IR) radiation analysis showed a decrease in the cellulose IR crystallization index. ST noticeably increases reducing sugars in rice straw, and this technique may also be applicable to other cellulose/lignin sources of biomass.

Interactions between fungal growth, substrate utilization, and enzyme production during solid substrate cultivation of Phanerochaete chrysosporium on cotton stalks

Fungal pretreatment, using lignin-degrading microorganisms to improve lignocellulosic feedstocks with minimal energy input, is a potential alternative to physiochemical pretreatment methods. Identifying the kinetics for fungal pretreatment during solid substrate cultivation is needed to help establish the processing conditions for effective scale up of this technology. In this study, a set of mathematical models were proposed for describing the interactions between holocellulose consumption, lignin degradation, cellulase, ligninolytic enzyme, and the growth of Phanerochaete chrysosporium during a 14 day fungal pretreatment process. Model parameters were estimated and validated by the System Biology Toolbox in MatLab. Developed models provided sufficiently accurate predictions for fungal growth (R (2) = 0.97), holocellulose consumption (R (2) = 0.97), lignin degradation (R (2) = 0.93) and ligninolytic enzyme production (R (2) = 0.92), and fair prediction for cellulase production (R (2) = 0.61). The models provide valuable information for understanding the interactive mechanisms in biological systems as well as for fungal pretreatment process scale up and improvement.

Evaluation of selected white-rot fungal isolates for improving the sugar yield from wheat straw

Biological pretreatment of lignocellulosic biomass by fungi can represent a low-cost and eco-friendly alternative to physicochemical methods to facilitate enzymatic hydrolysis. However, fungal metabolism can cause cellulose loss and it is therefore necessary to use the appropriate fungal strain-biomass type combination. In this work, the effects of biological pretreatments carried out by five different fungi on enzymatic hydrolysis of wheat straw were investigated. The best results were obtained with a Ceriporiopsis subvermispora strain, which minimized weight and cellulose losses and gave the highest net sugar yield (calculated with respect to the holocellulose content of the untreated straw), up to 44 % after a 10-week pretreatment, more than doubling the yields obtained with the other isolates. Moreover, prolonging the pretreatment from 4 up to 10 weeks produced a 2-fold increase, up to 60 %, in digestibility (sugar yield, calculated considering the holocellulose content of the pretreated material). The hemicellulose content of the pretreated material resulted inversely correlated with digestibility, and it could thus be utilized as an index of the pretreatment efficacy. Finally, a correlation was also found between digestibility and the difference between the absorbance values at 290 and 320 nm of pretreated wheat straw extracts.

Characterization of a β-1,4-mannanase from a newly isolated strain of Pholiota adiposa and its application for biomass pretreatment

A highly efficient β-1,4-mannanase-secreting strain, Pholiota adiposa SKU0714, was isolated and identified on the basis of its morphological features and sequence analysis of internal transcribed spacer rDNA. P. adiposa β-1,4-mannanase was purified to homogeneity from P. adiposa culture supernatants by one-step chromatography on a Sephacryl gel filtration column. P. adiposa β-1,4-mannanase showed the highest activity toward locust bean gum (V max = 1,990 U/mg protein, K m = 0.12 mg/mL) ever reported. Its internal amino acid sequence showed homology with hydrolases from the glycoside hydrolase family 5 (GH5), indicating that the enzyme is a member of the GH5 family. The saccharification of commercial mannanase and P. adiposa β-1,4-mannanase-pretreated rice straw by Celluclast 1.5L (Novozymes) was compared. In comparison with the commercial Novo Mannaway(®) (113 mg/g-substrate), P. adiposa β-1,4-mannanase-pretreated rice straw released more reducing sugars (141 mg/g-substrate). These properties make P. adiposa β-1,4-mannanase a good candidate as a new commercial β-1,4-mannanase to improve biomass pretreatment.

Bioconversion of xylose, hexoses and biomass to ethanol by a new isolate of the white rot basidiomycete Trametes versicolor

Second-generation bioethanol production requires the development of economically feasible and sustainable processes that use renewable lignocellulosic biomass as a starting material. However, the microbial fermentation of xylose, which is the principal pentose sugar in hemicellulose, is a limiting factor in developing such processes. Here, a strain of the white rot basidiomycete Trametes versicolor that was capable of efficiently fermenting xylose was newly isolated and characterized. This strain, designated KT9427, was capable of assimilating and converting xylose to ethanol under anaerobic conditions with a yield of 0.44 g ethanol per 1 g of sugar consumed. In culture medium containing low yeast extract concentrations, xylose consumption and ethanol productivity were enhanced. Adjusting the initial pH between 3.0 and 5.0 did not markedly influence xylose fermentation. T. versicolor KT9427 also produced ethanol from glucose, mannose, fructose, cellobiose and maltose at yields ranging from 0.45 to 0.49 g ethanol per 1 g of sugar consumed. In addition, strain KT9427 exhibited favourable conversion of non-pretreated starch, cellulose, xylan, wheat bran and rice straw into ethanol compared to common recombinant yeast strains. Taken together, the present findings suggest that T. versicolor KT9427 is a promising candidate for environmentally friendly ethanol production directly from lignocellulosic biomass.

Degradation and selective ligninolysis of wheat straw and banana stem for an efficient bioethanol production using fungal and chemical pretreatment

Lignocelluloses from agricultural, industrial, and forest residues constitute a majority of the total biomass present in the world. Environmental concerns of disposal, costly pretreatment options prior to disposal, and increased need to save valuable resources have led to the development of value-added alternate technologies such as bioethanol production from lignocellulosic wastes. In the present study, biologically pretreated (with the fungus, Pleurotus ostreatus HP-1) and chemically pretreated (with mild acid or dilute alkali) wheat straw (WS) and banana stem (BS) were subsequently subjected to enzymatic saccharification (with mixture of 6.0 U/g of filter paper cellulase and 17 U/g of β-glucosidase) and were evaluated for bioethanol production using Saccharomyces cerevisiae NCIM 3570. Biological and chemical pretreatments removed up to 4.0–49.2 % lignin from the WS and BS which was comparatively higher than that for cellulose (0.3–12.4 %) and for hemicellulose (0.7–21.8 %) removal with an average 5.6–49.5 % dry matter loss. Enzymatic hydrolysis yielded 64–306.6 mg/g (1.5–15 g/L) reducing sugars from which 0.15–0.54 g/g ethanol was produced from Saccharomyces cerevisiae NCIM 3570.