Effects of lignocellulose degradation products on ethanol fermentations of glucose and xylose by Saccharomyces cerevisiae, Zymomonas mobilis, Pichia stipitis, and Candida shehatae

Identification of crucial yeast inhibitors in bio-ethanol and improvement of fermentation at high pH and high total solids

Compounds inhibitory to enzymatic hydrolysis and fermentation are generated from neutral steam exploded corn stover in the process of producing bio-ethanol. In this study, weak acids were identified as main yeast inhibitors, while phenols and aldehyde contribute to the inhibition to a lower degree. Main weak acids in hydrolysates are acetic acid and formic acid, for which critical levels for yeast inhibition are 6 and 4g/L, respectively. The inhibitory effect of these compounds can be greatly overcome by increasing pH of hydrolysates to 6.0-9.0, but there is a risk of bacterial contamination when fermenting at high pH. The relationship of pH, total solids of hydrolysates, fermentation and contamination was studied in detail. Results indicate that the contamination by bacteria when fermenting at high pH can be prevented effectively using hydrolysates with total solids of more than 20%. Meanwhile, ethanol yield is improved significantly.

Oil production by oleaginous yeasts using the hydrolysate from pretreatment of wheat straw with dilute sulfuric acid

This paper explores the use of the hydrolysate from the dilute sulfuric acid pretreatment of wheat straw for microbial oil production. The resulting hydrolysate was composed of pentoses (24.3g/L) and hexoses (4.9 g/L), along with some other degradation products, such as acetic acid, furfural, and hydroxymethylfurfural (HMF). Five oleaginous yeast strains, Cryptococcus curvatus, Rhodotorula glutinis, Rhodosporidium toruloides, Lipomyces starkeyi, and Yarrowia lipolytica, were evaluated by using this hydrolysate as substrates. The results showed that all of these strains could use the detoxified hydrolysate to produce lipids while except R. toruloides non-detoxified hydrolysate could also be used for the growth of all of the selective yeast strains. C. curvatus showed the highest lipid concentrations in medium on both the detoxified (4.2g/L) and non-detoxified (5.8 g/L) hydrolysates. And the inhibitory effect studies on C. curvatus indicated HMF had insignificant impacts at a concentration of up to 3g/L while furfural inhibited cell growth and lipid content by 72.0% and 62.0% at 1g/L, respectively. Our work demonstrates that lipid production is a promising alternative to utilize hemicellulosic sugars obtained during pretreatment of lignocellulosic materials.

Adsorptive removal of fermentation inhibitors from concentrated acid hydrolyzates of lignocellulosic biomass

Adsorptive purification of concentrated acid hydrolyzate of lignocellulose was investigated. Cation exchange resin (CS16GC), neutral polymer adsorbent (XAD-16), and granulated activated carbon (GAC) were studied to remove furfural, HMF, and acetic acid from a synthetic hydrolyzate containing 20 wt.% H(2)SO(4). Adsorption isotherms were determined experimentally. Loading and regeneration were investigated in a laboratory scale column. GAC has the highest adsorption capacity, but regeneration with water was not feasible. XAD-16 and CS16GC had lower adsorption capacities but also shorter cycle times due to easier regeneration. Productivity increased when regenerating with 50 wt.% EtOH(aq) solution. To compare adsorbents, process performance was quantified by productivity and fraction of inhibitors removed. GAC yields highest performance when high purity is required and ethanol can be used in regeneration. For lower purities, XAD-16 and GAC yield approximately equal performance. When using ethanol must be avoided, CS16GC offers highest productivity.

Influence of cultivation procedure for Saccharomyces cerevisiae used as pitching agent in industrial spent sulphite liquor fermentations

The cell viability and fermentation performance often deteriorate in fermentations of spent sulphite liquor (SSL). This investigation therefore addresses the question of how different cultivation conditions for yeast cells influence their ability to survive and boost the ethanol production capacity in an SSL-based fermentation process. The strains used as pitching agents were an industrially harvested Saccharomyces cerevisiae and commercial dry baker's yeast. This study therefore suggests that exposure to SSL in combination with nutrients, prior to the fermentation step, is crucial for the performance of the yeast. Supplying 0.5 g/l fresh yeast cultivated under appropriate cultivation conditions may increase ethanol concentration more than 200%.

An evaluation of the potential of Acacia dealbata as raw material for bioethanol production

In this work, the potential of Acacia dealbata as raw material for ethanol production was evaluated, as well as its composition with regard to cellulose, hemicelluloses, lignin, extractives and ash. The tree samples were subjected to several dilute acid pretreatments using a combined severity parameter ranging from 0.7 to 3.7. The highest ethanol concentration obtained was 10.31 g ethanol/L within 24 h by using a separate hydrolysis and fermentation of the water insoluble fraction after pretreatment at 180 °C with 0.8% of sulfuric acid for 15 min. With simultaneous saccharification and fermentation, results obtained for the washed solids of water insoluble fraction were better than those obtained with the whole slurry.

Conversion of sugars present in rice hull hydrolysates into ethanol by Spathaspora arborariae, Saccharomyces cerevisiae, and their co-fermentations

The production of ethanol by the new yeast Spathaspora arborariae using rice hull hydrolysate (RHH) as substrate, either alone or in co-cultures with Saccharomyces cerevisiae is presented. Cultivations were also carried out in synthetic medium to gather physiological information on these systems, especially concerning their ability to grow and produce ethanol in the presence of acetic acid, furfural, and hydroxymethylfurfural, which are toxic compounds usually present in lignocellulosic hydrolysates. S. arborariae was able to metabolize xilose and glucose present in the hydrolysate, with ethanol yields (Y(P/S)(et)) of 0.45. In co-cultures, ethanol yields peaked to 0.77 and 0.62 in the synthetic medium and in RHH, respectively. When the toxic compounds were added to the synthetic medium, their presence produced negative effects on biomass formation and ethanol productivity. This work shows good prospects for the use of the new yeast S. arborariae alone and in co-cultures with S. cerevisiae for ethanol production.

Stress-related challenges in pentose fermentation to ethanol by the yeast Saccharomyces cerevisiae

Conversion of agricultural residues, energy crops and forest residues into bioethanol requires hydrolysis of the biomass and fermentation of the released sugars. During the hydrolysis of the hemicellulose fraction, substantial amounts of pentose sugars, in particular xylose, are released. Fermentation of these pentose sugars to ethanol by engineered Saccharomyces cerevisiae under industrial process conditions is the subject of this review. First, fermentation challenges originating from the main steps of ethanol production from lignocellulosic feedstocks are discussed, followed by genetic modifications that have been implemented in S. cerevisiae to obtain xylose and arabinose fermenting capacity per se. Finally, the fermentation of a real lignocellulosic medium is discussed in terms of inhibitory effects of furaldehydes, phenolics and weak acids and the presence of contaminating microbiota.

High temperature dilute acid pretreatment of coastal Bermuda grass for enzymatic hydrolysis

Dilute sulfuric acid was used to pretreat coastal Bermuda grass at high temperature prior to enzymatic hydrolysis. After both pretreatment and enzymatic hydrolysis processes, the highest yield of total sugars (combined xylose and glucose) was 97% of the theoretical value. The prehydrolyzate liquor was analyzed for inhibitory compounds (furfural, hydroxymethylfurfural (HMF)) in order to assess potential risk for inhibition during the following fermentation. Accounting for the formation of the inhibitory compounds, a pretreatment with 1.2% acid at 140 °C for 30 min with a total sugar yield of 94% of the theoretical value may be more favorable for fermentation. From this study, it can be concluded that dilute sulfuric acid pretreatment can be successfully applied to coastal Bermuda grass to achieve high yields of monomeric glucose and xylose with acceptable levels of inhibitory compound formation.

Optimization of processing conditions for the fractionation of triticale straw using pressurized low polarity water

Pressurized low polarity water (PLPW) fractionation of triticale straw was optimized to maximize hemicellulose and lignin yield, and to produce a cellulose rich fraction for biofuels production. The optimum PLPW conditions for hemicellulose yield was determined to be 165 °C, with a flow rate of 115 mL/min, and a solvent-to-solid ratio of 60 mL/g. Hemicellulose and lignin yield generally increased with increasing temperature and solvent-to-solid ratio. There was a small decrease in hemicellulose yield with an increase in flow rate. Minimum lignin content of the triticale straw residue after extraction was predicted to occur at a processing condition of 206 °C, 160 mL/min, and 67 mL/g. PLPW was successful in removing 73-78% of the hemicellulose, leaving a cellulose rich fraction (65% glucose concentration). Lignin was equally distributed between the solid residues and the extracts and most of the hemicellulose was extracted in oligomer form. Remaining solid residues after fractionation were highly digestible by cellulase enzymes.

Response surface optimization of oxalic acid pretreatment of yellow poplar (Liriodendron tulipifera) for production of glucose and xylose monosaccarides

The primary goal of this study was to determine the optimal condition to obtain fermentable monosaccharides (xylose and glucose) from hydrolysates of yellow poplar (Liriodendron tulipifera) by oxalic acid pretreatment as a potential bio-ethanol source. Based on 2(3) factorial design, fifteen operations were performed by varying on acid loading, reaction time and temperature, and the components of the solid and liquid fractions were analyzed. The sugar concentration (g/L) in hydrolysates and xylose solubilization (%) were applied to response surface methodology. The optimal condition for producing sugars was 151 °C, 0.042 g/g (weight of oxalic acid/dry matter), 13 min with predicted yield of 37.4 g/L, and for the xylose solubilization was 158 °C, 0.037 g/g, 13 min yielding 72.0% of the predicted value. Severe conditions generated inhibitors. By measuring the concentrations, the possibility utilizing hydrolysates for fermentation were estimated.

Enhanced enzymatic saccharification of barley straw pretreated by ethanosolv technology

The fermentable sugars in lignocellulosic biomass are derived from cellulose and hemicellulose, which are not readily accessible to enzymatic saccharification because of their recalcitrance. An ethanosolv pretreatment method was applied for the enzymatic saccharification of barley straw with an inorganic acid. The effects of four process variables (temperature, time, catalyst dose, and ethanol concentration) on the barley straw pretreatment were analyzed over a broad range using a small composite design and a response surface methodology. The yield of the residual solid and composition of the solid fraction differed as ethanosolv conditions varied within the experimental range. A glucan recovery, xylan recovery, and delignification were 85%, 14%, and 69% at center point conditions (170°C, 60 min, 1.0% (w/w) H(2)SO(4), and 50% (w/w) ethanol), respectively. Ethanosolv pretreatment removed lignin effectively. Additionally, the highest enzymatic digestibility of 85.3% was obtained after 72 h at center point conditions.

Effects of enzyme feeding strategy on ethanol yield in fed-batch simultaneous saccharification and fermentation of spruce at high dry matter

BACKGROUND

To make lignocellulosic fuel ethanol economically competitive with fossil fuels, it is necessary to reduce the production cost. One way to achieve this is by increasing the substrate concentration in the production process, and thus reduce the energy demand in the final distillation of the fermentation broth. However, increased substrate concentration in simultaneous saccharification and fermentation (SSF) processes has been shown to result in reduced ethanol yields and severe stirring problems. Because the SSF medium is being continuously hydrolyzed, running the process in fed-batch mode could potentially reduce the stirring problems and lead to increased ethanol yields in high-solids SSF. Different enzyme feeding strategies, with the enzymes either present in the reactor from start-up or fed into the reactor together with the substrate, have been studied, along with the influence of the enzyme feeding strategy on the final ethanol yield and productivity.

RESULTS

In the present study, SSF was run successfully with 10% and 14% water-insoluble solids (WIS) in batch and fed-batch mode. The mixing of the material in the reactor was significantly better in fed-batch than batch mode, and similarly high or higher ethanol yields were achieved in fed-batch mode compared with batch SSF in some cases. No general trend in the dependence of ethanol yield on enzyme feeding strategy was found.

CONCLUSIONS

The optimum enzyme feeding strategy appears to depend on the conditions during SSF, such as the WIS concentration and the concentration of inhibitory compounds in the SSF medium.

Bioethanol fermentation by recombinant E. coli FBR5 and its robust mutant FBHW using hot-water wood extract hydrolyzate as substrate

Hemicellulose is a potential by-product currently under-utilized in the papermaking industry. It is a hetero-carbohydrate polymer. For hardwood hemicelluloses, D-xylose is the major component upon depolymerization. At SUNY-ESF, wood extracts were obtained by extracting sugar maple wood chips with hot water at an elevated temperature. The wood extracts were then concentrated and acid hydrolyzed. Ethanologenic bacteria, E. coli FBR5, had a good performance in pure xylose medium for ethanol production. However, FBR5 was strongly inhibited in dilute sulfuric acid hydrolyzate of hot-water wood extract. FBR5 was challenged by hot-water wood extract hydrolyzate in this study. After repeated strain adaptation, an improved strain: E. coli FBHW was obtained. Fermentation experiments indicated that FBHW was resistant to the toxicity of hydrolyzate in the fermentation media of concentrated hydrolyzate, and xylose was completely utilized by the strain to produce ethanol. FBHW was grown in the concentrated hydrolyzate without any detoxification treatment and has yielded 36.8g/L ethanol.

Ethanol production from orange peels: two-stage hydrolysis and fermentation studies using optimized parameters through experimental design

Orange peels were evaluated as a fermentation feedstock, and process conditions for enhanced ethanol production were determined. Primary hydrolysis of orange peel powder (OPP) was carried out at acid concentrations from 0 to 1.0% (w/v) at 121 degrees C and 15 psi for 15 min. High-performance liquid chromatography analysis of sugars and inhibitory compounds showed a higher production of hydroxymethyfurfural and acetic acid and a decrease in sugar concentration when the acid level was beyond 0.5% (w/v). Secondary hydrolysis of pretreated biomass obtained from primary hydrolysis was carried out at 0.5% (w/v) acid. Response surface methodology using three factors and a two-level central composite design was employed to optimize the effect of pH, temperature, and fermentation time on ethanol production from OPP hydrolysate at the shake flask level. On the basis of results obtained from the optimization experiment and numerical optimization software, a validation study was carried out in a 2 L batch fermenter at pH 5.4 and a temperature of 34 degrees C for 15 h. The hydrolysate obtained from primary and secondary hydrolysis processes was fermented separately employing parameters optimized through RSM. Ethanol yields of 0.25 g/g on a biomass basis (YP/X) and 0.46 g/g on a substrate-consumed basis (YP/S) and a promising volumetric ethanol productivity of 3.37 g/L/h were attained using this process at the fermenter level, which shows promise for further scale-up studies.

Individual and interaction effects of vanillin and syringaldehyde on the xylitol formation by Candida guilliermondii

The effect of lignin degradation products liberated during chemical hydrolysis of lignocellulosic materials on xylose-to-xylitol bioconversion by Candida guilliermondii FTI 20037 was studied. Two aromatic aldehydes (vanillin and syringaldehyde) were selected as model compounds. A two-level factorial design was employed to evaluate the effects of pH (5.5-7.0), cell concentration (1.0-3.0 g l(-1)), vanillin concentration (0-2.0 g l(-1)) and syringaldehyde concentration (0-2.0 g l(-1)) on this bioprocess. The results showed that in the presence of vanillin or syringaldehyde (up to 2.0 g l(-1)) the cell growth was inhibited to different degrees with a complete inhibition of the yeast growth when the mixture of both (at 2.0 g l(-1) each) was added to the fermentation medium. The xylitol yield was not significantly influenced by vanillin, but was strongly reduced by syringaldehyde, which showed a more pronounced inhibitor effect at pH 7.0. The yeast was also able to convert vanillin and syringaldehyde to the corresponding aromatic acids or alcohols and their formation was dependent of the experimental conditions employed.

Enzymatic hydrolysis of sodium dodecyl sulphate (SDS)-pretreated newspaper for cellulosic ethanol production by Saccharomyces cerevisiae and Pichia stipitis

Fermentation of enzymatic hydrolysate of waste newspaper was investigated for cellulosic ethanol production in this study. Various nonionic and ionic surfactants were applied for waste newspaper pretreatment to increase the enzymatic digestibility. The surfactant-pretreated newspaper was enzymatically digested in 0.05 M sodium citrate buffer (pH 4.8) with varying solid content, filter paper unit loading (FPU/g newspaper), and ratio of filter paper unit/beta-glucosidase unit (FPU/CBU). Newspaper pretreated with the anionic surfactant sodium dodecyl sulphate (SDS) demonstrated the highest sugar yield. The addition of Tween-80 in the enzymatic hydrolysis process enhanced the enzymatic digestibility of newspaper pretreated with all of the surfactants. Enzymatic hydrolysis of SDS-pretreated newspaper with 15% solid content, 15 FPU/g newspaper, and FPU/CBU of 1:4 resulted in a newspaper hydrolysate conditioning 29.07 g/L glucose and 4.08 g/L xylose after 72 h of incubation at 50 degrees C. The fermentation of the enzymatic hydrolysate with Saccharomyces cerevisiae, Pichia stipitis, and their co-culture produced 14.29, 13.45, and 14.03 g/L of ethanol, respectively. Their corresponding ethanol yields were 0.43, 0.41, and 0.42 g/g.

Resistance of Saccharomyces cerevisiae to high concentrations of furfural is based on NADPH-dependent reduction by at least two oxireductases

Biofuels derived from lignocellulosic biomass hold promises for a sustainable fuel economy, but several problems hamper their economical feasibility. One important problem is the presence of toxic compounds in processed lignocellulosic hydrolysates, with furfural as a key toxin. While Saccharomyces cerevisiae has some intrinsic ability to reduce furfural to the less-toxic furfuryl alcohol, higher resistance is necessary for process conditions. By comparing an evolved, furfural-resistant strain and its parent in microaerobic, glucose-limited chemostats at increasing furfural challenge, we elucidate key mechanism and the molecular basis of both natural and high-level furfural resistance. At lower concentrations of furfural, NADH-dependent oxireductases are the main defense mechanism. At furfural concentrations above 15 mM, however, (13)C-flux and global array-based transcript analysis demonstrated that the NADPH-generating flux through the pentose phosphate pathway increases and that NADPH-dependent oxireductases become the major resistance mechanism. The transcript analysis further revealed that iron transmembrane transport is upregulated in response to furfural. While these responses occur in both strains, high-level resistance in the evolved strain was based on strong induction of ADH7, the uncharacterized open reading frame (ORF) YKL071W, and four further, likely NADPH-dependent, oxireductases. By overexpressing the ADH7 gene and the ORF YKL071W, we inversely engineered significantly increased furfural resistance in the parent strain, thereby demonstrating that these two enzymes are key elements of the resistance phenotype.

Effects of biomass hydrolysis by-products on oleaginous yeast Rhodosporidium toruloides

Lignocellulosic biomass hydrolysis inevitably coproduces byproducts that may have various affects on downstream biotransformation. It is imperative to document the inhibitor tolerance ability of microbial strain in order to utilize biomass hydrolysate more effectively. To achieve better lipid production by Rhodosporidium toruloides Y4, we performed fermentation experiments in the presence of some representative inhibitors. We found that acetate, 5-hydroxymethylfurfural and syringaldehyde had slightly inhibitory effects; p-hydroxybenzaldehyde and vanillin were toxic at a concentration over 10 mM; and furfural and its derivatives furfuryl alcohol and furoic acid inhibited cell growth by 45% at around 1 mM. We further demonstrated that inhibition is generally additive, although strong synergistic inhibitions were also observed. Finally, lipid production afforded good results in the presence of six inhibitors at their respective concentrations usually found in biomass hydrolysates. Fatty acid compositional profile of lipid samples indicated that those inhibitors had little effects on lipid biosynthesis. Our work will be useful for optimization of biomass hydrolysis processes and lipid production using lignocellulosic materials.

Extraction and separation of carbohydrates and phenolic compounds in flax shives with pH-controlled pressurized low polarity water

A bench-scale pressurized low polarity water (PLPW) extractor was used for the extraction and separation of hemicellulose, cellulose, lignin, and other phenolic compounds in flax shives. In the first part of this research, the key PLPW extraction process variables of temperature, pH, and flow rate, were optimized using central composite design (CCD). Temperature and pH of water had a significant affect on the fractionation of carbohydrates (cellulose and hemicellulose), lignin, and other phenolics. The optimal extraction conditions for the separation of hemicellulose and lignin, determined by the optimization using CCD, were 170 degrees C, pH 3.0, and a flow rate of 2.5 mL/min. Under these extraction conditions, 39.3% of the initial biomass or feed, 70.1% of the hemicellulose, 35.3% of the lignin, and 5.3% of the cellulose were extracted from the flax shives. In order to improve the purity and yield of the cellulose, a two-stage PLPW extraction was examined. The first stage was designed to remove hemicellulose by water at 170 degrees C and the second stage was intended for delignification by a pH 12 buffer at 220 degrees C. The two-stage PLPW extraction effectively removed 63.2% of the feed, 97.3% of hemicellulose, and 86.3% of lignin, while solubilizing 23.9% of cellulose; resulting in a solid residue containing 0.7 g of hemicellulose, 3.5 g of lignin, and 27.3 g of cellulose/100 g of DFS. The PLPW extraction is able to extract and separate components in flax shives by changing pH and temperature. The best case occurs between pH 9.5 and 12, resulting in maximum solubilization of hemicellulose and lignin.

Identification and characterization of fermentation inhibitors formed during hydrothermal treatment and following SSF of wheat straw

A pilot plant for hydrothermal treatment of wheat straw was compared in reactor systems of two steps (first, 80 degrees C; second, 190-205 degrees C) and of three steps (first, 80 degrees C; second, 170-180 degrees C; third, 195 degrees C). Fermentation (SSF) with Sacharomyces cerevisiae of the pretreated fibers and hydrolysate from the two-step system gave higher ethanol yield (64-75%) than that obtained from the three-step system (61-65%), due to higher enzymatic cellulose convertibility. At the optimal conditions (two steps, 195 degrees C for 6 min), 69% of available C6-sugar could be fermented into ethanol with a high hemicellulose recovery (65%). The concentration of furfural obtained during the pretreatment process increased versus temperature from 50 mg/l at 190 degrees C to 1,200 mg/l at 205 degrees C as a result of xylose degradation. S. cerevisiae detoxified the hydrolysates by degradation of several toxic compounds such as 90-99% furfural and 80-100% phenolic aldehydes, which extended the lag phase to 5 h. Acetic acid concentration increased by 0.2-1 g/l during enzymatic hydrolysis and 0-3.4 g/l during fermentation due to hydrolysis of acetyl groups and minor xylose degradation. Formic acid concentration increased by 0.5-1.5 g/l probably due to degradation of furfural. Phenolic aldehydes were oxidized to the corresponding acids during fermentation reducing the inhibition level.

Separate hydrolysis and fermentation (SHF) of Prosopis juliflora, a woody substrate, for the production of cellulosic ethanol by Saccharomyces cerevisiae and Pichia stipitis-NCIM 3498

Prosopis juliflora (Mesquite) is a raw material for long-term sustainable production of cellulosics ethanol. In this study, we used acid pretreatment, delignification and enzymatic hydrolysis to evaluate the pretreatment to produce more sugar, to be fermented to ethanol. Dilute H(2)SO(4) (3.0%,v/v) treatment resulted in hydrolysis of hemicelluloses from lignocellulosic complex to pentose sugars along with other byproducts such as furfural, hydroxymethyl furfural (HMF), phenolics and acetic acid. The acid pretreated substrate was delignified to the extent of 93.2% by the combined action of sodium sulphite (5.0%,w/v) and sodium chlorite (3.0%,w/v). The remaining cellulosic residue was enzymatically hydrolyzed in 0.05 M citrate phosphate buffer (pH 5.0) using 3.0 U of filter paper cellulase (FPase) and 9.0 U of beta-glucosidase per mL of citrate phosphate buffer. The maximum enzymatic saccharification of cellulosic material (82.8%) was achieved after 28 h incubation at 50 degrees C. The fermentation of both acid and enzymatic hydrolysates, containing 18.24 g/L and 37.47 g/L sugars, with Pichia stipitis and Saccharomyces cerevisiae produced 7.13 g/L and 18.52 g/L of ethanol with corresponding yield of 0.39 g/g and 0.49 g/g, respectively.

Metabolic effects of furaldehydes and impacts on biotechnological processes

There is a growing awareness that lignocellulose will be a major raw material for production of both fuel and chemicals in the coming decades--most likely through various fermentation routes. Considerable attention has been given to the problem of finding efficient means of separating the major constituents in lignocellulose (i.e., lignin, hemicellulose, and cellulose) and to efficiently hydrolyze the carbohydrate parts into sugars. In these processes, by-products will inevitably form to some extent, and these will have to be dealt with in the ensuing microbial processes. One group of compounds in this category is the furaldehydes. 2-Furaldehyde (furfural) and substituted 2-furaldehydes--most importantly 5-hydroxymethyl-2-furaldehyde--are the dominant inhibitory compounds found in lignocellulosic hydrolyzates. The furaldehydes are known to have biological effects and act as inhibitors in fermentation processes. The effects of these compounds will therefore have to be considered in the design of biotechnological processes using lignocellulose. In this short review, we take a look at known metabolic effects, as well as strategies to overcome problems in biotechnological applications caused by furaldehydes.

Soluble and insoluble solids contributions to high-solids enzymatic hydrolysis of lignocellulose

The rates and extents of enzymatic cellulose hydrolysis of dilute acid pretreated corn stover (PCS) decline with increasing slurry concentration. However, mass transfer limitations are not apparent until insoluble solids concentrations approach 20% w/w, indicating that inhibition of enzyme hydrolysis at lower solids concentrations is primarily due to soluble components. Consequently, the inhibitory effects of pH-adjusted pretreatment liquor on the enzymatic hydrolysis of PCS were investigated. A response surface methodology (RSM) was applied to empirically model how hydrolysis performance varied as a function of enzyme loading (12-40 mg protein/g cellulose) and insoluble solids concentration (5-13%) in full-slurry hydrolyzates. Factorial design and analysis of variance (ANOVA) were also used to assess the contribution of the major classes of soluble components (acetic acid, phenolics, furans, sugars) to total inhibition. High sugar concentrations (130 g/L total initial background sugars) were shown to be the primary cause of performance inhibition, with acetic acid (15 g/L) only slightly inhibiting enzymatic hydrolysis and phenolic compounds (9 g/L total including vanillin, syringaldehyde, and 4-hydroxycinnamic acid) and furans (8 g/L total of furfural and hydroxymethylfurfural, HMF) with only a minor effect on reaction kinetics. It was also demonstrated that this enzyme inhibition in high-solids PCS slurries can be approximated using a synthetic hydrolyzate composed of pure sugars supplemented with a mixture of acetic acid, furans, and phenolic compounds, which indicates that generally all of the reaction rate-determining soluble compounds for this system can be approximated synthetically.

Microbial utilization and biopolyester synthesis of bagasse hydrolysates

Cellulosic biomass is a potentially inexpensive renewable feedstock for the biorefineries of fuels, chemicals and materials. Sugarcane bagasse was pretreated in dilute acid solution under moderately severe conditions, releasing sugars and other hydrolysates including volatile organic acids, furfurals and acid soluble lignin. Utilization of the hydrolysates by an aerobic bacterium, Ralstonia eutropha, was investigated to determine if the organic inhibitors can be removed for potential recycling and reuse of the process water. Simultaneous biosynthesis of polyhydroxyalkanoates (PHAs) for the production of value-added bioplastics was also investigated. An inhibitory effect of hydrolysates on microbial activity was observed, but it could be effectively relieved by using (a) a large inoculum, (b) a diluted hydrolysate solution, and (c) a tolerant strain, or a combination of the three. The major organic inhibitors including formic acid, acetic acid, furfural and acid soluble lignin were effectively utilized and removed to low concentration levels (less than 100ppm) while at the same time, PHA biopolyesters were synthesized and accumulated to 57wt% of cell mass under appropriate C/N ratios. Poly(3-hydroxybutyrate) was the predominant biopolyester formed on the hydrolysates, but the cells could also synthesize co-polyesters that exhibit high ductility.

Modeling sucrose hydrolysis in dilute sulfuric acid solutions at pretreatment conditions for lignocellulosic biomass

Agricultural and herbaceous feedstocks may contain appreciable levels of sucrose. The goal of this study was to evaluate the survivability of sucrose and its hydrolysis products, fructose and glucose, during dilute sulfuric acid processing at conditions typically used to pretreat lignocellulose biomass. Solutions containing 25g/l sucrose with 0.1-2.0% (w/w) sulfuric acid concentrations were treated at temperatures of 160-200 degrees C for 3-12min. Sucrose was observed to completely hydrolyze at all treatment conditions. However, appreciable concentrations of fructose and glucose were detected and glucose was found to be significantly more stable than fructose. Different mathematical approaches were used to fit the kinetic parameters for acid-catalyzed thermal degradation of these sugars. Since both sugars may survive dilute acid pretreatment, they could provide an additional carbon source for production of ethanol and other bio-based products.

Effect of Inhibitors Released during Steam-Explosion Treatment of Poplar Wood on Subsequent Enzymatic Hydrolysis and SSF

Steam-exploded (SE) poplar wood biomass was hydrolyzed by means of a blend of Celluclast and Novozym cellulase complexes in the presence of the inhibiting compounds produced during the preceding steam-explosion pretreatment process. The SE temperature and time conditions were 214 degrees C and 6 min, resulting in a log R(0) of 4.13. In enzymatic hydrolysis tests at 45 degrees C, the biomass loading in the bioreactor was 100 g(DW)/L (dry weight) and the enzyme-to-biomass ratio 0.06 g/g(DW). The enzyme activities for endo-glucanase, exo-glucanase, and beta-glucosidase were 5.76, 0.55, and 5.98 U/mg, respectively. The inhibiting effects of components released during SE (formic, acetic, and levulinic acids, furfural, 5-hydroxymethyl furfural (5-HMF), syringaldehyde, 4-hydroxy benzaldehyde, and vanillin) were studied at different concentrations in hydrolysis runs performed with rinsed SE biomass as model substrate. Acetic acid (2 g/L), furfural, 5-HMF, syringaldehyde, 4-hydroxybenzaldehyde, and vanillin (0.5 g/L) did not significantly effect the enzyme activity, whereas formic acid (11.5 g/L) inactivated the enzymes and levulinic acid (29.0 g/L) partially affected the cellulase. Synergism and cumulative concentration effects of these compounds were not detected. SSF experiments show that untreated SE biomass during the enzymatic attack gives rise to a nonfermentable hydrolysate, which becomes fermentable when rinsed SE biomass is used. The presence of acetic acid, vanillin, and 5-HMF (0.5 g/L) in SSF of 100 g(DW) /L biomass gave rise to ethanol yields of 84.0%, 73.5%, and 91.0% respectively, with respective lag phases of 42, 39, and 58 h.

Identification of furfural as a key toxin in lignocellulosic hydrolysates and evolution of a tolerant yeast strain

The production of fuel ethanol from low‐cost lignocellulosic biomass currently suffers from several limitations. One of them is the presence of inhibitors in lignocellulosic hydrolysates that are released during pre‐treatment. These compounds inhibit growth and hamper the production of ethanol, thereby affecting process economics. To delineate the effects of such complex mixtures, we conducted a chemical analysis of four different real‐world lignocellulosic hydrolysates and determined their toxicological effect on yeast. By correlating the potential inhibitor abundance to the growth‐inhibiting properties of the corresponding hydrolysates, we identified furfural as an important contributor to hydrolysate toxicity for yeast. Subsequently, we conducted a targeted evolution experiment to improve growth behaviour of the half industrial Saccharomyces cerevisiae strain TMB3400 in the hydrolysates. After about 300 generations, representative clones from these evolved populations exhibited significantly reduced lag phases in medium containing the single inhibitor furfural, but also in hydrolysate‐supplemented medium. Furthermore, these strains were able to grow at concentrations of hydrolysates that effectively killed the parental strain and exhibited significantly improved bioconversion characteristics under industrially relevant conditions. The improved resistance of our evolved strains was based on their capacity to remain viable in a toxic environment during the prolonged, furfural induced lag phase.

Microbial conversion of sugars from plant biomass to lactic acid or ethanol

Concerns for our environment and unease with our dependence on foreign oil have renewed interest in converting plant biomass into fuels and 'green' chemicals. The volume of plant matter available makes lignocellulose conversion desirable, although no single isolated organism has been shown to depolymerize lignocellulose and efficiently metabolize the resulting sugars into a specific product. This work reviews selected chemicals and fuels that can be produced from microbial fermentation of plant-derived cell-wall sugars and directed engineering for improvement of microbial biocatalysts. Lactic acid and ethanol production are highlighted, with a focus on engineered Escherichia coli.

Effect of furfural, vanillin and syringaldehyde on Candida guilliermondii growth and xylitol biosynthesis

Xylitol is a five-carbon sugar alcohol with established commercial use as an alternative sweetener and can be produced from hemicellulose hydrolysate. However, there are difficulties with microbiological growth and xylitol biosynthesis on hydrolysate because of the inhibitors formed from hydrolysis of hemicellulose. This research focused on the effect of furfural, vanillin, and syringaldehyde on growth of Candida guilliermondii and xylitol accumulation from xylose in a semi-synthetic medium in microwell plate and bioreactor cultivations. All three compounds reduced specific growth rate, increased lag time, and reduced xylitol production rate. In general, increasing concentration of inhibitor increased the severity of inhibition, except in the case of 0.5 g vanillin per liter, which resulted in a faster late batch phase growth rate and increased biomass yield. At concentrations of 1 g/l or higher, furfural was the least inhibitory to growth, followed by syringaldehyde. Vanillin most severely reduced specific growth rate. All three inhibitors reduced xylitol production rate approximately to the same degree.

Detoxification of sugarcane bagasse hydrolysate improves ethanol production by Candida shehatae NCIM 3501

Sugarcane bagasse hydrolysis with 2.5% (v/v) HCl yielded 30.29g/L total reducing sugars along with various fermentation inhibitors such as furans, phenolics and acetic acid. The acid hydrolysate when treated with anion exchange resin brought about maximum reduction in furans (63.4%) and total phenolics (75.8%). Treatment of hydrolysate with activated charcoal caused 38.7% and 57.5% reduction in furans and total phenolics, respectively. Laccase reduced total phenolics (77.5%) without affecting furans and acetic acid content in the hydrolysate. Fermentation of these hydrolysates with Candida shehatae NCIM 3501 showed maximum ethanol yield (0.48g/g) from ion exchange treated hydrolysate, followed by activated charcoal (0.42g/g), laccase (0.37g/g), overliming (0.30g/g) and neutralized hydrolysate (0.22g/g).

Effect of lignocellulose-derived inhibitors on growth of and ethanol production by growth-arrested Corynebacterium glutamicum R

In cellulosic ethanol production, pretreatment of a biomass to facilitate enzymatic hydrolysis inevitably yields fermentation inhibitors such as organic acids, furans, and phenols. With representative inhibitors included in the medium at various concentrations, individually or in various combinations, ethanol production by Corynebacterium glutamicum R under growth-arrested conditions was investigated. In the presence of various inhibitors, the 62 to 100% ethanol productivity retained by the C. glutamicum R-dependent method far exceeded that retained by previously reported methods.

Influence of inhibitory compounds and minor sugars on xylitol production by Debaryomyces hansenii

To obtain in-depth information on the overall metabolic behavior of the new good xylitol producer Debaryomyces hansenii UFV-170, batch bioconversions were carried out using semisynthetic media with compositions simulating those of typical acidic hemicellulose hydrolysates of sugarcane bagasse. For this purpose, we used media containing glucose (4.3-6.5 g/L), xylose (60.1-92.1 g/L), or arabinose (5.9-9.2 g/L), or binary or ternary mixtures of them in either the presence or absence of typical inhibitors of acidic hydrolysates, such as furfural (1.0-5.0 g/L), hydroxymethylfurfural (0.01- 0.30 g/L), acetic acid (0.5-3.0 g/L), and vanillin (0.5-3.0 g/L). D. hansenii exhibited a good tolerance to high sugar concentrations as well as to the presence of inhibiting compounds in the fermentation media. It was able to produce xylitol only from xylose, arabitol from arabinose, and no glucitol from glucose. Arabinose metabolization was incomplete, while ethanol was mainly produced from glucose and, to a lesser less extent, from xylose and arabinose. The results suggest potential application of this strain in xyloseto- xylitol bioconversion from complex xylose media from lignocellulosic materials.