Optimizing the saccharification of sugar cane bagasse using dilute phosphoric acid followed by fungal cellulases

Dysregulation of Cell Envelope Homeostasis in Staphylococcus aureus Exposed to Solvated Lignin

Lignin is an aromatic plant cell wall polymer that facilitates water transport through the vasculature of plants and is generated in large quantities as an inexpensive by-product of pulp and paper manufacturing and biorefineries. Although lignin's ability to reduce bacterial growth has been reported previously, its hydrophobicity complicates the ability to examine its biological effects on living cells in aqueous growth media. We recently described the ability to solvate lignin in Good's buffers with neutral pH, a breakthrough that allowed examination of lignin's antimicrobial effects against the human pathogen Staphylococcus aureus. These analyses showed that lignin damages the S. aureus cell membrane, causes increased cell clustering, and inhibits growth synergistically with tunicamycin, a teichoic acid synthesis inhibitor. In the present study, we examined the physiological and transcriptomic responses of S. aureus to lignin. Intriguingly, lignin restored the susceptibility of genetically resistant S. aureus isolates to penicillin and oxacillin, decreased intracellular pH, impaired normal cell division, and rendered cells more resistant to detergent-induced lysis. Additionally, transcriptome sequencing (RNA-Seq) differential expression (DE) analysis of lignin-treated cultures revealed significant gene expression changes (P < 0.05 with 5% false discovery rate [FDR]) related to the cell envelope, cell wall physiology, fatty acid metabolism, and stress resistance. Moreover, a pattern of concurrent up- and downregulation of genes within biochemical pathways involved in transmembrane transport and cell wall physiology was observed, which likely reflects an attempt to tolerate or compensate for lignin-induced damage. Together, these results represent the first comprehensive analysis of lignin's antibacterial activity against S. aureus. IMPORTANCE S. aureus is a leading cause of skin and soft tissue infections. The ability of S. aureus to acquire genetic resistance to antibiotics further compounds its ability to cause life-threatening infections. While the historical response to antibiotic resistance has been to develop new antibiotics, bacterial pathogens are notorious for rapidly acquiring genetic resistance mechanisms. As such, the development of adjuvants represents a viable way of extending the life span of current antibiotics to which pathogens may already be resistant. Here, we describe the phenotypic and transcriptomic response of S. aureus to treatment with lignin. Our results demonstrate that lignin extracted from sugarcane and sorghum bagasse restores S. aureus susceptibility to β-lactams, providing a premise for repurposing these antibiotics in treatment of resistant S. aureus strains, possibly in the form of topical lignin/β-lactam formulations.

Extrapolation of design strategies for lignocellulosic biomass conversion to the challenge of plastic waste

The goal of cost-effective production of fuels and chemicals from biomass has been a substantial driver of the development of the field of Metabolic Engineering. The resulting design principles and procedures provide a guide for the development of cost-effective methods for degradation, and possibly even valorization, of plastic wastes. Here we highlight these parallels, using the creative work of Lonnie O'Neal (Neal) Ingram in enabling production of fuels and chemicals from lignocellulosic biomass, with a focus on ethanol production as an exemplar process.

Dual phase statistical optimization of biological pre-treatment of sugarcane bagasse with Pycnoporus coccineus MScMS1 for polyhydroxyalkanoates production

Biological pre-treatment is the removal of recalcitrant lignin from lignocellulose through the action of lignin degrading organisms and/or their ligninolytic enzymes system. Despite numerous environmental benefits, biological pre-treatment has been side-lined due to its prolonged periods of fermentation, ascribed to the slow growth rate of lignin degrading organisms. Thus, the present work adopted a dual phase statistical optimization approach for the biological pre-treatment of sugarcane bagasse, with Pycnoporus coccineus MScMS1, using Taguchi Orthogonal Array, in conjunction with Response Surface Methodology, to address this issue. Amplification of the organism's functioning resulted in an enhancement of sugar productivity and yield accompanied by a significant reduction in fermentation time. Optimized sugar concentration was approx. 18 g/L within 4 days of pre-treatment, with productivity of 4.5 g/(L.day). Substrate compositional analysis revealed significant (p < 0.05) reduction of lignin by 70% in the biologically pre-treated substrate, along with significantly (p < 0.05) higher quantities of water soluble components (35 ± 0.95 g) and cellulose content (33 ± 0.18 g), as compared to the untreated substrate. Appreciable levels of xylose, arabinose, glucose and galactose were detected in hydrolysates from biologically pre-treated bagasse. Furthermore, Bacillus megaterium Ti3, a potent polyhydroxyalkanoates (PHA) producer, was grown on these sugar-rich hydrolysates and generated 0.58 g/L PHA in 24 h of fermentation accompanied by 0.88 g/L dry cell weight and 65% PHA accumulation. These results were comparable with those from a glucose medium. Thus, the present study was successful in optimizing the biological pre-treatment of sugarcane bagasse and utilizing the resultant sugar-rich hydrolysates, as inexpensive and renewable raw materials, for PHA production.

Recent Developments and Current Status of Commercial Production of Fuel Ethanol

Ethanol produced from various biobased sources (bioethanol) has been gaining high attention lately due to its potential to cut down net emissions of carbon dioxide while reducing burgeoning world dependence on fossil fuels. Global ethanol production has increased more than six-fold from 18 billion liters at the turn of the century to 110 billion liters in 2019, only to fall to 98.6 billion liters in 2020 due to the pandemic. Sugar cane and corn have been used as the major feedstocks for ethanol production. Lignocellulosic biomass has recently been considered as another potential feedstock due to its non-food competing status and its availability in very large quantities. This paper reviews recent developments and current status of commercial production of ethanol across the world with a focus on the technological aspects. The review includes the ethanol production processes used for each type of feedstock, both currently practiced at commercial scale and still under developments, and current production trends in various regions and countries in the world.

Principles and practice of designing microbial biocatalysts for fuel and chemical production

The finite nature of fossil fuels and the environmental impact of its use have raised interest in alternate renewable energy sources. Specifically, non-food carbohydrates, such as lignocellulosic biomass, can be used to produce next generation biofuels, including cellulosic ethanol and other non-ethanol fuels like butanol. However, currently there is no native microorganism that can ferment all lignocellulosic sugars to fuel molecules. Thus, research is focused on engineering improved microbial biocatalysts for production of liquid fuels at high productivity, titer and yield. A clear understanding and application of the basic principles of microbial physiology and biochemistry are crucial to achieve this goal. In this review, we present and discuss the construction of microbial biocatalysts that integrate these principles with ethanol-producing Escherichia coli as an example of metabolic engineering. These principles also apply to fermentation of lignocellulosic sugars to other chemicals that are currently produced from petroleum.

Pretreatment and fermentation of lignocellulosic biomass: reaction mechanisms and process engineering

At a time of rapid depletion of oil resources, global food shortages and solid waste problems, it is imperative to encourage research into the use of appropriate pre-treatment techniques using regenerative raw materials such as lignocellulosic biomass.

Combined in situ Physical and ex-situ Biochemical Approaches to Investigate in vitro Deconstruction of Destarched Wheat Bran by Enzymes Cocktail Used in Animal Nutrition

Wheat bran is a foodstuff containing more than 40% of non-starch polysaccharides (NSPs) that are hardly digestible by monogastric animals. Therefore, cocktails enriched of hydrolytic enzymes (termed NSPases) are commonly provided as feed additives in animal nutrition. However, how these enzymes cocktails contribute to NSPs deconstruction remains largely unknown. This question was addressed by employing an original methodology that makes use of a multi-instrumented bioreactor that allows to dynamically monitor enzymes in action and to extract in-situ physical and ex-situ biochemical data from this monitoring. We report here that the deconstruction of destarched wheat bran by an industrial enzymes cocktail termed Rovabio® was entailed by two concurrent events: a particles fragmentation that caused in <2 h a 70% drop of the suspension viscosity and a solubilization that released <30 % of the wheat bran NSPs. Upon longer exposure, the fragmentation of particles continued at a very slow rate without any further solubilization. Contrary to this cocktail, xylanase C alone caused a moderate 25% drop of viscosity and a very weak fragmentation. However, the amount of xylose and arabinose from solubilized sugars after 6 h treatment with this enzyme was similar to that obtained after 2 h with Rovabio®. Altogether, this multi-scale analysis supported the synergistic action of enzymes mixture to readily solubilize complex polysaccharides, and revealed that in spite of the richness and diversity of hydrolytic enzymes in the cocktail, the deconstruction of NSPs in wheat bran was largely incomplete.

Liquefaction of Biomass and Upgrading of Bio-Oil: A Review

The liquefaction of biomass is an important technology to converse the biomass into valuable biofuel. The common technologies for liquefaction of biomass are indirect liquefaction and direct liquefaction. The indirect liquefaction refers to the Fischer-Tropsch (F-T) process using the syngas of biomass as the raw material to produce the liquid fuel, including methyl alcohol, ethyl alcohol, and dimethyl ether. The direct liquefaction of biomass refers to the conversion biomass into bio-oil, and the main technologies are hydrolysis fermentation and thermodynamic liquefaction. For thermodynamic liquefaction, it could be divided into fast pyrolysis and hydrothermal liquefaction. In addition, this review provides an overview of the physicochemical properties and common upgrading methods of bio-oil.

Simultaneous fermentation of biomass-derived sugars to ethanol by a co-culture of an engineered Escherichia coli and Saccharomyces cerevisiae

Microorganisms ferment xylose at high rate only when glucose concentration in the medium falls below a critical level. Since the specific productivity of product is highest during exponential to early stationary phase of growth, a glucose utilization negative ethanologenic E. coli (strain LW419a) was constructed for high rate of xylose fermentation in combination with Turbo yeast. This co-culture fermented all the released sugars in an acid/enzyme-treated sugar cane bagasse slurry (10% solids) to an ethanol titer of 24.9 ± 0.8 g.L^-1 (70% of the theoretical yield) in <30 h. Ethanol titer increased to 48.6 ± 1.04 g.L^-1 (yield, 0.45 g.g^-1 sugars) at a solids content of 20% and the highest rate of xylose consumption was 1.58 ± 0.21 g.L^-1.h^-1. This study demonstrates the potential of a co-culture of strain LW419a and yeast to rapidly ferment all the sugars in pretreated biomass slurries to ethanol at their respective highest rates.

Evaluation of inhibitory effect and feasible utilization of dilute acid-pretreated rice straws on succinate production by metabolically engineered Escherichia coli AS1600a

This work demonstrated a pioneer work in the pre-treatment of rice straw by phosphoric acid (H₃PO₄) for succinate production. The optimized pre-treatment condition of rice straw was at 121 °C for 30 min with 2 N H₃PO₄. With this condition, total sugar concentration of 31.2 g/L with the highest hemicellulose saccharification yield of 94% was obtained. The physicochemical analysis of the pre-treated rice straw showed significant changes in its structure thus enhancing enzymatic saccharification. Succinate concentrations of 78.5 and 63.8 g/L were produced from hydrolysate liquor (L) and solid fraction (S) of the pre-treated rice straw respectively, with a comparable yield of 86% by E. coli AS1600a. Use of a combined L + S fraction in simultaneous saccharification and fermentation (LS + SSF) further improved succinate production at a concentration and yield of 85.6 g/L and 90% respectively. The results suggested that H₃PO₄ pre-treated rice straw may be utilized for economical succinate production by E. coli AS1600a.

Optimization of a pretreatment and hydrolysis process for the efficient recovery of recycled sugars and unknown compounds from agricultural sweet sorghum bagasse stem pith solid waste

Background

Sweet sorghum bagasse (SSB), comprising both a dermal layer and pith, is a solid waste generated by agricultural activities. Open burning was previously used to treat agricultural solid waste but is harmful to the environment and human health. Recent reports showed that certain techniques can convert this agricultural waste into valuable products. While SSB has been considered an attractive raw material for sugar extraction and the production of value-added products, the pith root in the SSB can be difficult to process. Therefore, it is necessary to pretreat bagasse before conventional hydrolysis.

Methods

A thorough analysis and comparison of various pretreatment methods were conducted based on physicochemical and microscopic approaches. The responses of agricultural SSB stem pith with different particle sizes to pretreatment temperature, acid and alkali concentration and enzyme dosage were investigated to determine the optimal pretreatment. The integrated methods are beneficial to the utilization of carbohydrate-based and unknown compounds in agricultural solid waste.

Results

Acid (1.5−4.5%, v/v) and alkali (5−8%, w/v) reagents were used to collect cellulose from different meshes of pith at 25–100 °C. The results showed that the use of 100 mesh pith soaked in 8% (w/v) NaOH solution at 100 °C resulted in 32.47% ± 0.01% solid recovery. Follow-up fermentation with 3% (v/v) acid and 6.5% (w/v) alkali at 50 °C for enzymolysis was performed with the optimal enzyme ratio. An analysis of the surface topography and porosity before and after pretreatment showed that both the pore size of the pith and the amount of exposed cellulose increased as the mesh size increased. Interestingly, various compounds, including 42 compounds previously known to be present and 13 compounds not previously known to be present, were detected in the pretreatment liquid, while 10 types of monosaccharides, including D-glucose, D-xylose and D-arabinose, were found in the enzymatic solution. The total monosaccharide content of the pith was 149.48 ± 0.3 mg/g dry matter.

Discussion

An integrated technique for obtaining value-added products from sweet sorghum pith is presented in this work. Based on this technique, lignin and hemicellulose were effectively broken down, amorphous cellulose was obtained and all sugars in the sweet sorghum pith were hydrolysed into monosaccharides. A total of 42 compounds previously found in these materials, including alcohol, ester, acid, alkene, aldehyde ketone, alkene, phenolic and benzene ring compounds, were detected in the pretreatment pith. In addition, several compounds that had not been previously observed in these materials were found in the pretreatment solution. These findings will improve the transformation of lignocellulosic biomass into sugar to create a high-value-added coproduct during the integrated process and to maximize the potential utilization of agricultural waste in current biorefinery processing.

Fermentation of undetoxified sugarcane bagasse hydrolyzates using a two stage hydrothermal and mechanical refining pretreatment

In this study, liquid hot water pretreatment was combined with disk milling for pretreatment of sugarcane bagasse. Sugarcane bagasse was pretreated using liquid hot water (LHW) at 140-180 °C for 10 min (20% w/w solids content) and then disk milled. Disk milling improved glucose release 41-177% and ethanol production from glucose/xylose cofermentation by 80% compared to only using LHW pretreatment. The highest ethanol conversion efficiency achieved was 94%, which was observed when bagasse was treated at 180 °C with LHW and disk milled. However, a small amount of residual xylose (3 g/L) was indicative that further improvement could be achieved to increase ethanol production.

Phosphoric acid based pretreatment of switchgrass and fermentation of entire slurry to ethanol using a simplified process

Switchgrass (Alamo) was pretreated with phosphoric acid (0.75 and 1%, w/w) at three temperatures (160, 175 and 190 °C) and time (5, 7.5 and 10 min) using a steam gun. The slurry after pretreatment was liquefied by enzymes and the released sugars were fermented in a simultaneous saccharification and co-fermentation process to ethanol using ethanologenic Escherichia coli strain SL100. Among the three variables in pretreatment, temperature and time were critical in supporting ethanol titer and yield. Enzyme hydrolysis significantly increased the concentration of furans in slurries, apparently due to release of furans bound to the solids. The highest ethanol titer of 21.2 ± 0.3 g/L ethanol obtained at the pretreatment condition of 190-1-7.5 (temperature-acid concentration-time) and 10% solids loading accounted for 190 ± 2.9 g ethanol/kg of raw switch grass. This converts to 61.7 gallons of ethanol per ton of dry switchgrass, a value that is comparable to other published pretreatment conditions.

Direct hydrogen production from dilute-acid pretreated sugarcane bagasse hydrolysate using the newly isolated Thermoanaerobacterium thermosaccharolyticum MJ1

Background

Energy shortage and environmental pollution are two severe global problems, and biological hydrogen production from lignocellulose shows great potential as a promising alternative biofuel to replace the fossil fuels. Currently, most studies on hydrogen production from lignocellulose concentrate on cellulolytic microbe, pretreatment method, process optimization and development of new raw materials. Due to no effective approaches to relieve the inhibiting effect of inhibitors, the acid pretreated lignocellulose hydrolysate was directly discarded and caused environmental problems, suggesting that isolation of inhibitor-tolerant strains may facilitate the utilization of acid pretreated lignocellulose hydrolysate.

Results

Thermophilic bacteria for producing hydrogen from various kinds of sugars were screened, and the new strain named MJ1 was isolated from paper sludge, with 99% identity to Thermoanaerobacterium thermosaccharolyticum by 16S rRNA gene analysis. The hydrogen yields of 11.18, 4.25 and 2.15 mol-H₂/mol sugar can be reached at an initial concentration of 5 g/L cellobiose, glucose and xylose, respectively. The main metabolites were acetate and butyrate. More important, MJ1 had an excellent tolerance to inhibitors of dilute-acid (1%, g/v) pretreated sugarcane bagasse hydrolysate (DAPSBH) and could efficiently utilize DAPSBH for hydrogen production without detoxication, with a production higher than that of pure sugars. The hydrogen could be quickly produced with the maximum hydrogen production reached at 24 h. The hydrogen production reached 39.64, 105.42, 111.75 and 110.44 mM at 20, 40, 60 and 80% of DAPSBH, respectively. Supplementation of CaCO₃ enhanced the hydrogen production by 21.32% versus the control.

Conclusions

These results demonstrate that MJ1 could directly utilize DAPSBH for biohydrogen production without detoxication and can serve as an excellent candidate for industrialization of hydrogen production from DAPSBH. The results also suggest that isolating unique strains from a particular environment offers an ideal way to conquer the related problems.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0692-y) contains supplementary material, which is available to authorized users.

Two-stage dilute-acid and organic-solvent lignocellulosic pretreatment for enhanced bioprocessing

A two stage pretreatment approach for biomass is developed in the current work in which dilute acid (DA) pretreatment is followed by a solvent based pretreatment (N-methyl morpholine N oxide - NMMO). When the combined pretreatment (DAWNT) is applied to sugarcane bagasse and corn stover, the rates of hydrolysis and overall yields (>90%) are seen to dramatically improve and under certain conditions 48h can be taken off the time of hydrolysis with the additional NMMO step to reach similar conversions. DAWNT shows a 2-fold increase in characteristic rates and also fractionates different components of biomass - DA treatment removes the hemicellulose while the remaining cellulose is broken down by enzymatic hydrolysis after NMMO treatment to simple sugars. The remaining residual solid is high purity lignin. Future work will focus on developing a full scale economic analysis of DAWNT for use in biomass fractionation.

Non-severe thermochemical hydrolysis of stover from white corn and sequential enzymatic saccharification and fermentation to ethanol

A parametric study, with an initial load of 15%w/w of dry stover from white corn, was conducted to evaluate the sequential thermochemical hydrolysis (TH), enzymatic saccharification (ES) and fermentation of the whole slurry with ethanologenic Escherichia coli. The TH was designed to release the maximum amount of xylose with a concomitant formation of minimal amounts of furans. It was found that 29.0% or 93.2% of the xylan was recovered as free xylose at 130°C after 8 min in the presence of 1% or 2%w/w H2SO4 and produced only 0.06 or 0.44 g/L of total furans, respectively. After 24h of ES, 76.14-77.18 g/L of monosaccharides (pentoses and hexoses) were obtained. These slurries, which contained 0.03-0.26 g/L of total furans and 5.14-5.91 g/L of acetate, were fermented with 3.7 g/L of ethanologenic E. coli to produce 24.5-23.5 g/L of ethanol.

Microbial conversion of pyrolytic products to biofuels: a novel and sustainable approach toward second-generation biofuels

This review highlights the potential of the pyrolysis-based biofuels production, bio-ethanol in particular, and lipid in general as an alternative and sustainable solution for the rising environmental concerns and rapidly depleting natural fuel resources. Levoglucosan (1,6-anhydrous-β-D-glucopyranose) is the major anhydrosugar compound resulting from the degradation of cellulose during the fast pyrolysis process of biomass and thus the most attractive fermentation substrate in the bio-oil. The challenges for pyrolysis-based biorefineries are the inefficient detoxification strategies, and the lack of naturally available efficient and suitable fermentation organisms that could ferment the levoglucosan directly into bio-ethanol. In case of indirect fermentation, acid hydrolysis is used to convert levoglucosan into glucose and subsequently to ethanol and lipids via fermentation biocatalysts, however the presence of fermentation inhibitors poses a big hurdle to successful fermentation relative to pure glucose. Among the detoxification strategies studied so far, over-liming, extraction with solvents like (n-butanol, ethyl acetate), and activated carbon seem very promising, but still further research is required for the optimization of existing detoxification strategies as well as developing new ones. In order to make the pyrolysis-based biofuel production a more efficient as well as cost-effective process, direct fermentation of pyrolysis oil-associated fermentable sugars, especially levoglucosan is highlly desirable. This can be achieved either by expanding the search to identify naturally available direct levoglusoan utilizers or modify the existing fermentation biocatalysts (yeasts and bacteria) with direct levoglucosan pathway coupled with tolerance engineering could significantly improve the overall performance of these microorganisms.

Improving furfural tolerance of Zymomonas mobilis by rewiring a sigma factor RpoD protein

Furfural from lignocellulosic hydrolysates is the key inhibitor for bio-ethanol fermentation. In this study, we report a strategy of improving the furfural tolerance in Zymomonas mobilis on the transcriptional level by engineering its global transcription sigma factor (σ(70), RpoD) protein. Three furfural tolerance RpoD mutants (ZM4-MF1, ZM4-MF2, and ZM4-MF3) were identified from error-prone PCR libraries. The best furfural-tolerance strain ZM4-MF2 reached to the maximal cell density (OD600) about 2.0 after approximately 30 h, while control strain ZM4-rpoD reached its highest cell density of about 1.3 under the same conditions. ZM4-MF2 also consumed glucose faster and yield higher ethanol; expression levels and key Entner-Doudoroff (ED) pathway enzymatic activities were also compared to control strain under furfural stress condition. Our results suggest that global transcription machinery engineering could potentially be used to improve stress tolerance and ethanol production in Z. mobilis.

Engineering Sugar Utilization and Microbial Tolerance toward Lignocellulose Conversion

Production of fuels and chemicals through a fermentation-based manufacturing process that uses renewable feedstock such as lignocellulosic biomass is a desirable alternative to petrochemicals. Although it is still in its infancy, synthetic biology offers great potential to overcome the challenges associated with lignocellulose conversion. In this review, we will summarize the identification and optimization of synthetic biological parts used to enhance the utilization of lignocellulose-derived sugars and to increase the biocatalyst tolerance for lignocellulose-derived fermentation inhibitors. We will also discuss the ongoing efforts and future applications of synthetic integrated biological systems used to improve lignocellulose conversion.

Effects of production and market factors on ethanol profitability for an integrated first and second generation ethanol plant using the whole sugarcane as feedstock

BACKGROUND

Sugarcane is an attractive feedstock for ethanol production, especially if the lignocellulosic fraction can also be treated in second generation (2G) ethanol plants. However, the profitability of 2G ethanol is affected by the processing conditions, operating costs and market prices. This study focuses on the minimum ethanol selling price (MESP) and maximum profitability of ethanol production in an integrated first and second generation (1G + 2G) sugarcane-to-ethanol plant. The feedstock used was sugarcane juice, bagasse and leaves. The lignocellulosic fraction was hydrolysed with enzymes. Yields were assumed to be 95% of the theoretical for each of the critical steps in the process (steam pretreatment, enzymatic hydrolysis (EH), fermentation, solid/liquid separation, anaerobic digestion) in order to obtain the best conditions possible for ethanol production, to assess the lowest production costs. Techno-economic analysis was performed for various combinations of process options (for example use of pentoses, addition of leaves), EH conditions (water-insoluble solids (WIS) and residence time), operating cost (enzymes) and market factors (wholesale prices of electricity and ethanol, cost of the feedstock).

RESULTS

The greatest reduction in 2G MESP was achieved when using the pentoses for the production of ethanol rather than biogas. This was followed, in decreasing order, by higher enzymatic hydrolysis efficiency (EHE), by increasing the WIS to 30% and by a short residence time (48 hours) in the EH. The addition of leaves was found to have a slightly negative impact on 1G + 2G MESP, but the effect on 2G MESP was negligible. Sugarcane price significantly affected 1G + 2G MESP, while the price of leaves had a much lower impact. Net present value (NPV) analysis of the most interesting case showed that integrated 1G + 2G ethanol production including leaves could be more profitable than 1G ethanol, despite the fact that the MESP was higher than in 1G ethanol production.

CONCLUSIONS

A combined 1G + 2G ethanol plant could potentially outperform a 1G plant in terms of NPV, depending on market wholesale prices of ethanol and electricity. Therefore, although it is more expensive than 1G ethanol production, 2G ethanol production can make the integrated 1G + 2G process more profitable.

Isolation and structural characterization of sugarcane bagasse lignin after dilute phosphoric acid plus steam explosion pretreatment and its effect on cellulose hydrolysis

The structure of lignin after dilute phosphoric acid plus steam explosion pretreatment process of sugarcane bagasse in a pilot scale and the effect of the lignin extracted by ethanol on subsequent cellulose hydrolysis were investigated. The lignin structural changes caused by pretreatment were identified using advanced nondestructive techniques such as gel permeation chromatography (GPC), quantitative (13)C, and 2-D nuclear magnetic resonance (NMR). The structural analysis revealed that ethanol extractable lignin preserved basic lignin structure, but had relatively lower amount of β-O-4 linkages, syringyl/guaiacyl units ratio (S/G), p-coumarate/ferulate ratio, and other ending structures. The results also indicated that approximately 8% of mass weight was extracted by pure ethanol. The bagasse after ethanol extraction had an approximate 22% higher glucose yield after enzyme hydrolysis compared to pretreated bagasse without extraction.

Ethanol production from glucose and xylose obtained from steam exploded water-extracted olive tree pruning using phosphoric acid as catalyst

In this work, the effect of phosphoric acid (1% w/w) in steam explosion pretreatment of water extracted olive tree pruning at 175°C and 195°C was evaluated. The objective is to produce ethanol from all sugars (mainly glucose and xylose) contained in the pretreated material. The water insoluble fraction obtained after pretreatment was used as substrate in a simultaneous saccharification and fermentation (SSF) process by a commercial strain of Saccharomyces cerevisiae. The liquid fraction, containing mainly xylose, was detoxified by alkali and ion-exchange resin and then fermented by the xylose fermenting yeast Scheffersomyces stipitis. Ethanol yields reached in a SSF process were close to 80% when using 15% (w/w) substrate consistency and about 70% of theoretical when using prehydrolysates detoxified by ion-exchange resins. Considering sugars recovery and ethanol yields about 160g of ethanol from kg of water extracted olive tree pruning could be obtained.

Utilization of household food waste for the production of ethanol at high dry material content

BACKGROUND

Environmental issues and shortage of fossil fuels have turned the public interest to the utilization of renewable, environmentally friendly fuels, such as ethanol. In order to minimize the competition between fuels and food production, researchers are focusing their efforts to the utilization of wastes and by-products as raw materials for the production of ethanol. household food wastes are being produced in great quantities in European Union and their handling can be a challenge. Moreover, their disposal can cause severe environmental issues (for example emission of greenhouse gasses). On the other hand, they contain significant amounts of sugars (both soluble and insoluble) and they can be used as raw material for the production of ethanol.

RESULTS

Household food wastes were utilized as raw material for the production of ethanol at high dry material consistencies. A distinct liquefaction/saccharification step has been included to the process, which rapidly reduced the viscosity of the high solid content substrate, resulting in better mixing of the fermenting microorganism. This step had a positive effect in both ethanol production and productivity, leading to a significant increase in both values, which was up to 40.81% and 4.46 fold, respectively. Remaining solids (residue) after fermentation at 45% w/v dry material (which contained also the unhydrolyzed fraction of cellulose), were subjected to a hydrothermal pretreatment in order to be utilized as raw material for a subsequent ethanol fermentation. This led to an increase of 13.16% in the ethanol production levels achieving a final ethanol yield of 107.58 g/kg dry material.

CONCLUSIONS

In conclusion, the ability of utilizing household food waste for the production of ethanol at elevated dry material content has been demonstrated. A separate liquefaction/saccharification process can increase both ethanol production and productivity. Finally, subsequent fermentation of the remaining solids could lead to an increase of the overall ethanol production yield.

Genome-wide RNAi screen reveals the E3 SUMO-protein ligase gene SIZ1 as a novel determinant of furfural tolerance in Saccharomyces cerevisiae

BACKGROUND

Furfural is a major growth inhibitor in lignocellulosic hydrolysates and improving furfural tolerance of microorganisms is critical for rapid and efficient fermentation of lignocellulosic biomass. In this study, we used the RNAi-Assisted Genome Evolution (RAGE) method to select for furfural resistant mutants of Saccharomyces cerevisiae, and identified a new determinant of furfural tolerance.

RESULTS

By using a genome-wide RNAi (RNA-interference) screen in S. cerevisiae for genes involved in furfural tolerance, we identified SIZ1, a gene encoding an E3 SUMO-protein ligase. Disruption of SIZ1 gene function by knockdown or deletion conferred significantly higher furfural tolerance compared to other previously reported metabolic engineering strategies in S. cerevisiae. This improved furfural tolerance of siz1Δ cells is accompanied by rapid furfural reduction to furfuryl alcohol and leads to higher ethanol productivity in the presence of furfural. In addition, the siz1Δ mutant also exhibited tolerance towards oxidative stress, suggesting that oxidative stress tolerance related proteins may be under the SUMO regulation of SIZ1p and responsible for furfural tolerance.

CONCLUSIONS

Using a genome-wide approach, we identified a novel determinant for furfural tolerance, providing valuable insights into the design of recombinant microbes for efficient lignocellulose fermentation.

Diluted phosphoric acid pretreatment for production of fermentable sugars in a sugarcane-based biorefinery

The influence of time (8-24 min), temperature (144-186 °C) and phosphoric acid concentration (0.05-0.20%, w/v) on the pretreatment of sugarcane bagasse in a 20 L batch rotary reactor was investigated. The efficiency of the pretreatment was verified by chemical characterization of the solid fraction of the pretreated bagasse and the conversion of cellulose to glucose by enzymatic hydrolysis. Models representing the percentage of cellulose, hemicelluloses, lignin, solubilized hemicellulose and the enzymatic conversion of cellulose to glucose were predictive and significant. Phosphoric acid concentration of 0.20% at temperature of 186 °C, during 8 and 24 min, was shown to be very effective in solubilizing hemicellulose from sugarcane bagasse, reaching solubilization of 96% and 98%, respectively. Relatively low amounts of inhibitors were produced, and the phosphoric acid remaining in the hemicellulosic hydrolysate is at adequate levels for supplying phosphorous requirement during subsequent fermentation.

Mapping the lignin distribution in pretreated sugarcane bagasse by confocal and fluorescence lifetime imaging microscopy

BACKGROUND

Delignification pretreatments of biomass and methods to assess their efficacy are crucial for biomass-to-biofuels research and technology. Here, we applied confocal and fluorescence lifetime imaging microscopy (FLIM) using one- and two-photon excitation to map the lignin distribution within bagasse fibers pretreated with acid and alkali. The evaluated spectra and decay times are correlated with previously calculated lignin fractions. We have also investigated the influence of the pretreatment on the lignin distribution in the cell wall by analyzing the changes in the fluorescence characteristics using two-photon excitation. Eucalyptus fibers were also analyzed for comparison.

RESULTS

Fluorescence spectra and variations of the decay time correlate well with the delignification yield and the lignin distribution. The decay dependences are considered two-exponential, one with a rapid (τ1) and the other with a slow (τ2) decay time. The fastest decay is associated to concentrated lignin in the bagasse and has a low sensitivity to the treatment. The fluorescence decay time became longer with the increase of the alkali concentration used in the treatment, which corresponds to lignin emission in a less concentrated environment. In addition, the two-photon fluorescence spectrum is very sensitive to lignin content and accumulation in the cell wall, broadening with the acid pretreatment and narrowing with the alkali one. Heterogeneity of the pretreated cell wall was observed.

CONCLUSIONS

Our results reveal lignin domains with different concentration levels. The acid pretreatment caused a disorder in the arrangement of lignin and its accumulation in the external border of the cell wall. The alkali pretreatment efficiently removed lignin from the middle of the bagasse fibers, but was less effective in its removal from their surfaces. Our results evidenced a strong correlation between the decay times of the lignin fluorescence and its distribution within the cell wall. A new variety of lignin fluorescence states were accessed by two-photon excitation, which allowed an even broader, but complementary, optical characterization of lignocellulosic materials. These results suggest that the lignin arrangement in untreated bagasse fiber is based on a well-organized nanoenvironment that favors a very low level of interaction between the molecules.

Response surface optimization of corn stover pretreatment using dilute phosphoric acid for enzymatic hydrolysis and ethanol production

Dilute H(3)PO(4) (0.0-2.0%, v/v) was used to pretreat corn stover (10%, w/w) for conversion to ethanol. Pretreatment conditions were optimized for temperature, acid loading, and time using central composite design. Optimal pretreatment conditions were chosen to promote sugar yields following enzymatic digestion while minimizing formation of furans, which are potent inhibitors of fermentation. The maximum glucose yield (85%) was obtained after enzymatic hydrolysis of corn stover pretreated with 0.5% (v/v) acid at 180°C for 15min while highest yield for xylose (91.4%) was observed from corn stover pretreated with 1% (v/v) acid at 160°C for 10min. About 26.4±0.1g ethanol was produced per L by recombinant Escherichia coli strain FBR5 from 55.1±1.0g sugars generated from enzymatically hydrolyzed corn stover (10%, w/w) pretreated under a balanced optimized condition (161.81°C, 0.78% acid, 9.78min) where only 0.4±0.0g furfural and 0.1±0.0 hydroxylmethyl furfural were produced.

Mass balance and transformation of corn stover by pretreatment with different dilute organic acids

Previous studies indicated high xylose yield could be achieved after pretreatment using organic acids, but it is necessary to systematically investigate the effects of different parameters during organic acid pretreatments. Corn stover was pretreated with sulfuric, oxalic, citric, tartaric and acetic acid at 50 and 90 mM from 130 to 190°C. The xylan balance for each different acid was distinct, but all balances were very close to 100% by determining xylan recovery, xylooligomer yield, xylose yield and furfural yield. The effects of combined severity on the recovery or yields of these components were also studied. The acid pK(a) value affected the proportion of xylan degradation products. The maximum value of xylose and xylooligomer yield for specific acid pretreatment was also determined by pK(a) value. The maximum xylose yield was obtained after pretreatment with sulfuric and oxalic acid, but more xylooligomers were obtained after pretreatment with weaker acids.

Increase in furfural tolerance in ethanologenic Escherichia coli LY180 by plasmid-based expression of thyA

Furfural is an inhibitory side product formed during the depolymerization of hemicellulose by mineral acids. Genomic libraries from three different bacteria (Bacillus subtilis YB886, Escherichia coli NC3, and Zymomonas mobilis CP4) were screened for genes that conferred furfural resistance on plates. Beneficial plasmids containing the thyA gene (coding for thymidylate synthase) were recovered from all three organisms. Expression of this key gene in the de novo pathway for dTMP biosynthesis improved furfural resistance on plates and during fermentation. A similar benefit was observed by supplementation with thymine, thymidine, or the combination of tetrahydrofolate and serine (precursors for 5,10-methylenetetrahydrofolate, the methyl donor for ThyA). Supplementation with deoxyuridine provided a small benefit, and deoxyribose was of no benefit for furfural tolerance. A combination of thymidine and plasmid expression of thyA was no more effective than either alone. Together, these results demonstrate that furfural tolerance is increased by approaches that increase the supply of pyrimidine deoxyribonucleotides. However, ThyA activity was not directly affected by the addition of furfural. Furfural has been previously shown to damage DNA in E. coli and to activate a cellular response to oxidative damage in yeast. The added burden of repairing furfural-damaged DNA in E. coli would be expected to increase the cellular requirement for dTMP. Increased expression of thyA (E. coli, B. subtilis, or Z. mobilis), supplementation of cultures with thymidine, and supplementation with precursors for 5,10-methylenetetrahydrofolate (methyl donor) are each proposed to increase furfural tolerance by increasing the availability of dTMP for DNA repair.

Chemical and morphological characterization of sugarcane bagasse submitted to a delignification process for enhanced enzymatic digestibility

BACKGROUND

In recent years, biorefining of lignocellulosic biomass to produce multi-products such as ethanol and other biomaterials has become a dynamic research area. Pretreatment technologies that fractionate sugarcane bagasse are essential for the successful use of this feedstock in ethanol production. In this paper, we investigate modifications in the morphology and chemical composition of sugarcane bagasse submitted to a two-step treatment, using diluted acid followed by a delignification process with increasing sodium hydroxide concentrations. Detailed chemical and morphological characterization of the samples after each pretreatment condition, studied by high performance liquid chromatography, solid-state nuclear magnetic resonance, diffuse reflectance Fourier transformed infrared spectroscopy and scanning electron microscopy, is reported, together with sample crystallinity and enzymatic digestibility.

RESULTS

Chemical composition analysis performed on samples obtained after different pretreatment conditions showed that up to 96% and 85% of hemicellulose and lignin fractions, respectively, were removed by this two-step method when sodium hydroxide concentrations of 1% (m/v) or higher were used. The efficient lignin removal resulted in an enhanced hydrolysis yield reaching values around 100%. Considering the cellulose loss due to the pretreatment (maximum of 30%, depending on the process), the total cellulose conversion increases significantly from 22.0% (value for the untreated bagasse) to 72.4%. The delignification process, with consequent increase in the cellulose to lignin ratio, is also clearly observed by nuclear magnetic resonance and diffuse reflectance Fourier transformed infrared spectroscopy experiments. We also demonstrated that the morphological changes contributing to this remarkable improvement occur as a consequence of lignin removal from the sample. Bagasse unstructuring is favored by the loss of cohesion between neighboring cell walls, as well as by changes in the inner cell wall structure, such as damaging, hole formation and loss of mechanical resistance, facilitating liquid and enzyme access to crystalline cellulose.

CONCLUSIONS

The results presented herewith show the efficiency of the proposed method for improving the enzymatic digestibility of sugarcane bagasse and provide understanding of the pretreatment action mechanism. Combining the different techniques applied in this work warranted thorough information about the undergoing morphological and chemical changes and was an efficient approach to understand the morphological effects resulting from sample delignification and its influence on the enhanced hydrolysis results.

Injection of air into the headspace improves fermentation of phosphoric acid pretreated sugarcane bagasse by Escherichia coli MM170

Microaeration (injecting air into the headspace) improved the fermentation of hemicellulose hydrolysates obtained from the phosphoric acid pretreatment of sugarcane bagasse at 170°C for 10 min. In addition, with 10% slurries of phosphoric acid pretreated bagasse (180°C, 10 min), air injection into the headspace promoted xylose utilization and increased ethanol yields from 0.16 to 0.20 g ethanol/g bagasse dry weight using a liquefaction plus simultaneous saccharification and co-fermentation process (L+SScF). This process was scaled up to 80 L using slurries of acid pretreated bagasse (96 h incubation; 0.6L of air/min into the headspace) with ethanol yields of 312-347 L (82-92 gal) per tone (dry matter), corresponding to 0.25 and 0.27 g/g bagasse (dry weight). Injection of small amounts of air into the headspace may provide a convenient alternative to subsurface sparging that avoids problems of foaming, sparger hygiene, flotation of particulates, and phase separation.

High-temperature enzymatic breakdown of cellulose

Cellulose is an abundant and renewable biopolymer that can be used for biofuel generation; however, structural entrapment with other cell wall components hinders enzyme-substrate interactions, a key bottleneck for ethanol production. Biomass is routinely subjected to treatments that facilitate cellulase-cellulose contacts. Cellulases and glucosidases act by hydrolyzing glycosidic bonds of linear glucose β-1,4-linked polymers, producing glucose. Here we describe eight high-temperature-operating cellulases (TCel enzymes) identified from a survey of thermobacterial and archaeal genomes. Three TCel enzymes preferentially hydrolyzed soluble cellulose, while two preferred insoluble cellulose such as cotton linters and filter paper. TCel enzymes had temperature optima ranging from 85°C to 102°C. TCel enzymes were stable, retaining 80% of initial activity after 120 h at 85°C. Two modes of cellulose breakdown, i.e., with endo- and exo-acting glucanases, were detected, and with two-enzyme combinations at 85°C, synergistic cellulase activity was observed for some enzyme combinations.

Increased furfural tolerance due to overexpression of NADH-dependent oxidoreductase FucO in Escherichia coli strains engineered for the production of ethanol and lactate

Furfural is an important fermentation inhibitor in hemicellulose sugar syrups derived from woody biomass. The metabolism of furfural by NADPH-dependent oxidoreductases, such as YqhD (low K(m) for NADPH), is proposed to inhibit the growth and fermentation of xylose in Escherichia coli by competing with biosynthesis for NADPH. The discovery that the NADH-dependent propanediol oxidoreductase (FucO) can reduce furfural provided a new approach to improve furfural tolerance. Strains that produced ethanol or lactate efficiently as primary products from xylose were developed. These strains included chromosomal mutations in yqhD expression that permitted the fermentation of xylose broths containing up to 10 mM furfural. Expression of fucO from plasmids was shown to increase furfural tolerance by 50% and to permit the fermentation of 15 mM furfural. Product yields with 15 mM furfural were equivalent to those of control strains without added furfural (85% to 90% of the theoretical maximum). These two defined genetic traits can be readily transferred to enteric biocatalysts designed to produce other products. A similar strategy that minimizes the depletion of NADPH pools by native detoxification enzymes may be generally useful for other inhibitory compounds in lignocellulosic sugar streams and with other organisms.

Effect of reduced sulfur compounds on the fermentation of phosphoric acid pretreated sugarcane bagasse by ethanologenic Escherichia coli

The addition of reduced sulfur compounds (thiosulfate, cysteine, sodium hydrosulfite, and sodium metabisulfite) increased growth and fermentation of dilute acid hydrolysate of sugarcane bagasse by ethanologenic Escherichia coli (strains LY180, EMFR9, and MM160). With sodium metabisulfite (0.5mM), toxicity was sufficiently reduced that slurries of pretreated biomass (10% dry weight including fiber and solubles) could be fermented by E. coli strain MM160 without solid-liquid separation or cleanup of sugars. A 6-h liquefaction step was added to improve mixing. Sodium metabisulfite also caused spectral changes at wavelengths corresponding to furfural and soluble products from lignin. Glucose and cellobiose were rapidly metabolized. Xylose utilization was improved by sodium metabisulfite but remained incomplete after 144 h. The overall ethanol yield for this liquefaction plus simultaneous saccharification and co-fermentation process was 0.20 g ethanol/g bagasse dry weight, 250 L/tonne (61 gal/US ton).

Enzyme hydrolysis and ethanol fermentation of dilute ammonia pretreated energy cane

This study is the first one ever to report on the use of high fiber sugarcane (a.k.a. energy cane) bagasse as feedstock for the production of cellulosic ethanol. Energy cane bagasse was pretreated with ammonium hydroxide (28% v/v solution), and water at a ratio of 1:0.5:8 at 160°C for 1h under 0.9-1.1 MPa. Approximately, 55% lignin, 30% hemicellulose, 9% cellulose, and 6% other (e.g., ash, proteins) were removed during the process. The maximum glucan conversion of dilute ammonia treated energy cane bagasse by cellulases was 87% with an ethanol yield (glucose only) of 23 g ethanol/100g dry biomass. The enzymatic digestibility was related to the removal of lignin and hemicellulose, perhaps due to increased surface area and porosity resulting in the deformation and swelling of exposed fibers as shown in the SEM pictures.

Simplified process for ethanol production from sugarcane bagasse using hydrolysate-resistant Escherichia coli strain MM160

Hexose and pentose sugars from phosphoric acid pretreated sugarcane bagasse were co-fermented to ethanol in a single vessel (SScF), eliminating process steps for solid-liquid separation and sugar cleanup. An initial liquefaction step (L) with cellulase was included to improve mixing and saccharification (L+SScF), analogous to a corn ethanol process. Fermentation was enabled by the development of a hydrolysate-resistant mutant of Escherichia coli LY180, designated MM160. Strain MM160 was more resistant than the parent to inhibitors (furfural, 5-hydroxymethylfurfural, and acetate) formed during pretreatment. Bagasse slurries containing 10% and 14% dry weight (fiber plus solubles) were tested using pretreatment temperatures of 160-190°C (1% phosphoric acid, 10 min). Enzymatic saccharification and inhibitor production both increased with pretreatment temperature. The highest titer (30 g/L ethanol) and yield (0.21 g ethanol/g bagasse dry weight) were obtained after incubation for 122 h using 14% dry weight slurries of pretreated bagasse (180°C).

Optimizing cellulase usage for improved mixing and rheological properties of acid-pretreated sugarcane bagasse

Consolidation of bioprocessing steps with lignocellulose is limited by hydrolysate toxicity, the fibrous nature of suspensions, and low activity of cellulase enzymes. Combinations of enzyme dose and treatment conditions improved the flow properties and pumping of acid-pretreated sugarcane bagasse slurries (10% dry weight). Low levels of cellulase enzyme (0.1 and 0.5 FPU/g dry weight acid-pretreated bagasse) were found to reduce viscosities by 77-95% after 6 h, solubilizing 3.5% of the bagasse dry weight. Flow of slurries through small funnels was a useful predictor of success with centrifugal and diaphragm pumps. Equations were derived that describe viscosity and solubilized carbohydrates as a function of time and cellulase dosage. Blending of acid-pretreated bagasse (10% dry weight) with suspensions of acid-pretreated bagasse (10% dry weight) that had been previously digested with cellulase enzymes (low viscosity) did not increase viscosity in a linear fashion. Viscosity of these mixtures remained relatively constant until a threshold level of new fiber was reached, followed by a rapid increase with further additions. Up to 35% fresh acid-pretreated bagasse could be blended with enzyme-digested fiber (5.0 FPU/g dry weight acid-pretreated fiber; 6 h) with only a modest increase in viscosity. The smooth surfaces of enzyme-treated fiber are proposed to hinder the frequency and extent of interactions between fibrils of fresh fiber particles (acid-pretreated) until a threshold concentration is achieved, after which fiber interactions and viscosity increase dramatically. These results were used to model the viscosity in an ideal continuous stirred tank reactor (liquefaction) as a function of residence time and enzyme dosage.

Comparative study of mechanical, hydrothermal, chemical and enzymatic treatments of digested biofibers to improve biogas production

Organic waste such as manure is an important resource for biogas production. The biodegradability of manures is however limited because of the recalcitrant nature of the biofibers it contains. To increase the biogas potential of the biofibers in digested manure, we investigated physical treatment (milling), chemical treatment (CaO), biological treatment (enzymatic and partial aerobic microbial conversion), steam treatment with catalyst (H(3)PO(4) or NaOH) and combination of biological and steam treatments (biofibers steam-treated with catalyst were treated with laccase enzyme). We obtained the highest methane yield increase through the chemical treatment that resulted in 66% higher methane production compared to untreated biofibers. The combination of steam treatment with NaOH and subsequent enzymatic treatment increased the methane yield by 34%. To choose the optimal treatment, the energy requirements relative to the energy gain as extra biogas production have to be taken into account, as well as the costs of chemicals or enzymes.