Evaluation of xylose-fermenting yeasts for ethanol production from spent sulfite liquor

Advances on Cellulose Manufacture in Biphasic Reaction Media

Cellulose is produced industrially by the kraft and sulfite processes. The evolution of these technologies in biorefineries is driven by the need to obtain greater added value through the efficient use of raw materials and energy. In this field, organosolv technologies (and within them, those using liquid phases made up of water and one partly miscible organic solvent, known as "biphasic fractionation" in reference to the number of liquid phases) represent an alternative that is receiving increasing interest. This study considers basic aspects of the composition of lignocellulosic materials, describes the fundamentals of industrial cellulose pulp production processes, introduces the organosolv methods, and comprehensively reviews published results on organosolv fractionation based on the use of media containing water and an immiscible solvent (1-butanol, 1-pentanol or 2-methyltetrahydrofuran). Special attention is devoted to aspects related to cellulose recovery and fractionation selectivity, measured through the amount and composition of the treated solids.

Growth of wood-inhabiting yeasts of the Faroe Islands in the presence of spent sulphite liquor

In the microbial community of decaying wood, yeasts are important for the recycling of nutrients. Nevertheless, information on their biodiversity in this niche in the Northern hemisphere is limited. Wood-colonising yeasts encounter identical and similar growth-inhibitory compounds as those in spent sulphite liquor (SSL), an energy-rich, acid hydrolysate and waste product from the paper industry, which may render them well-suited for cultivation in SSL. In the present study, yeasts were isolated from decaying wood on the Faroe Islands and identified based on sequence homology of the ITS and D1/D2 regions. Among the yeasts isolated, Candida argentea, Cystofilobasidium infirmominiatum, Naganishia albidosimilis, Naganishia onofrii, Holtermanniella takashimae and Goffeauzyma gastrica were new to decaying wood in cold and temperate climates. C. argentea and Rhodotorula are rarely-isolated species, with no previous documentation from cold and maritime climates. The isolates were further tested for growth in a medium with increasing concentrations of softwood SSL. Most grew in the presence of 10% SSL. Isolates of Debaryomyces sp., C. argentea and Rhodotorula sp. were the most tolerant. Representatives of Debaryomyces and Rhodotorula have previously been found in decaying wood. In contrast, the least tolerant isolates belonged to species that are rarely reported from decaying wood. The relative importance of individual inhibitors to yeast growth is discussed. To our knowledge, none of the present yeast species have previously been cultivated in SSL medium. Decaying wood can be a useful future source of yeasts for valorisation of various hydrolysates to industrial chemicals and biofuels.

The Effects of Microwave-Assisted Pretreatment and Cofermentation on Bioethanol Production from Elephant Grass

The process of acid hydrolysis using conventional methods at high concentrations results in products having lower yields, and it needs a longer time of process; therefore, it becomes less effective. In this study, we analyzed the effects of microwave-assisted pretreatment and cofermentation on bioethanol production from elephant grass (Pennisetum purpureum). We used a combination of delignification techniques and acid hydrolysis by employing a microwave-assisted pretreatment method on elephant grass (Pennisetum purpureum) as a lignocellulosic material. This was followed by cofermentation with Saccharomyces cerevisiae ITB-R89 and Pichia stipitis ITB-R58 to produce bioethanol. The optimal sugar mixtures (fructose and xylose) of the hydrolysis product were subsequently converted into bioethanol by cofermentation with S. cerevisiae ITB-R89 and P. stipitis ITB-R58, carried out with varying concentrations of inoculum for 5 days (48 h) at 30°C and pH 4.5. The high-power liquid chromatographic analysis revealed that the optimal inoculum concentration capable of converting 76.15% of the sugar mixture substrate (glucose and xylose) to 10.79 g/L (34.74% yield) of bioethanol was 10% (v/v). The optimal rate of ethanol production was 0.45 g/L/d, corresponding to a fermentation efficiency of 69.48%.

Study on inhibitors from acid pretreatment of corn stalk on ethanol fermentation by alcohol yeast

The inhibitory effects of the main inhibitors formed during acid pretreatment of corn stalk were studied through ethanol fermentations of model substrates and hydrolysates from corn stalk by alcohol yeast. Experimental results showed that the tested inhibitors had no significant effect on ethanol fermentations when they were added separately at a concentration according to analysis results from hydrolysate of corn stalk. However, when they were added as a mixture, the inhibitory effects became obvious. With the increase of concentration, there was a delay in ethanol productivity. But complete inhibition was observed at 5.0 g L⁻¹ furfural, 10.0 g L⁻¹ acetic acid, 7.0 g L⁻¹ ferulic acid, and 7.0 g L⁻¹p-coumaric acid, respectively. The inhibitory effect decreased in the order: furfural > acetic acid > ferulic acid > p-coumaric acid > HMF. These results suggest that a high concentration of inhibitor has a strong negative influence on ethanol fermentation, but the inhibiting abilities of various inhibitors are different.

The inhibitory effects of the main inhibitors formed during acid pretreatment of corn stalk were studied through ethanol fermentations of model substrates and hydrolysates from corn stalk by alcohol yeast.

Second Generation Bioethanol Production: On the Use of Pulp and Paper Industry Wastes as Feedstock

Due to the health and environment impacts of fossil fuels utilization, biofuels have been investigated as a potential alternative renewable source of energy. Bioethanol is currently the most produced biofuel, mainly of first generation, resulting in food-fuel competition. Second generation bioethanol is produced from lignocellulosic biomass, but a costly and difficult pretreatment is required. The pulp and paper industry has the biggest income of biomass for non-food-chain production, and, simultaneously generates a high amount of residues. According to the circular economy model, these residues, rich in monosaccharides, or even in polysaccharides besides lignin, can be utilized as a proper feedstock for second generation bioethanol production. Biorefineries can be integrated in the existing pulp and paper industrial plants by exploiting the high level of technology and also the infrastructures and logistics that are required to fractionate and handle woody biomass. This would contribute to the diversification of products and the increase of profitability of pulp and paper industry with additional environmental benefits. This work reviews the literature supporting the feasibility of producing ethanol from Kraft pulp, spent sulfite liquor, and pulp and paper sludge, presenting and discussing the practical attempt of biorefineries implementation in pulp and paper mills for bioethanol production.

Identification of novel metabolic interactions controlling carbon flux from xylose to ethanol in natural and recombinant yeasts

BACKGROUND

Unlike xylose-converting natural yeasts, recombinant strains of Saccharomyces cerevisiae expressing the same xylose assimilation pathway produce under anaerobic conditions xylitol rather than ethanol from xylose at low specific xylose conversion rates. Despite intense research efforts over the last two decades, differences in these phenotypes cannot be explained by current metabolic and kinetic models. To improve our understanding how metabolic flux of xylose carbon to ethanol is controlled, we developed a novel kinetic model based on enzyme mechanisms and applied quantitative metabolite profiling together with enzyme activity analysis to study xylose-to-ethanol metabolisms of Candida tenuis CBS4435 (q xylose = 0.10 g/gdc/h, 25 °C; Y ethanol = 0.44 g/g; Y xylitol = 0.09 g/g) and the recombinant S. cerevisiae strain BP000 (q xylose = 0.07 g/gdc/h, 30 °C; Y ethanol = 0.24 g/g; Y xylitol = 0.43 g/g), both expressing the same xylose reductase (XR), comprehensively.

RESULTS

Results from strain-to-strain metabolic control analysis indicated that activity levels of XR and the maximal flux capacity of the upper glycolysis (UG; both ≥ tenfold higher in CBS4435) contributed predominantly to phenotype differentiation while reactions from the oxidative pentose phosphate pathway played minor roles. Intracellular metabolite profiles supported results obtained from kinetic modeling and indicated a positive correlation between pool sizes of UG metabolites and carbon flux through the UG. For CBS4435, fast carbon flux through the UG could be associated with an allosteric control of 6-phosphofructokinase (PFK) activity by fructose 6-phosphate. The ability of CBS4435 to keep UG metabolites at high levels could be explained by low glycerol 3-phosphate phosphatase (GPP, 17-fold lower in CBS4435) and high XR activities.

CONCLUSIONS

By applying a systems biology approach in which we combined results obtained from metabolic control analysis based on kinetic modeling with data obtained from quantitative metabolite profiling and enzyme activity analyses, we could provide new insights into metabolic and kinetic interactions contributing to the control of carbon flux from xylose to ethanol. Supported by evidences presented two new targets, PFK and GPP, could be identified that aside from XR play pivotal roles in phenotype differentiation. Design of efficient and fast microbial ethanol producers in the future can certainly benefit from results presented in this study.

Pichia anomala 29X: a resistant strain for lignocellulosic biomass hydrolysate fermentation

To efficiently use lignocellulosic biomass hydrolysates as fermentation media for bioethanol production, besides being capable of producing significant amount of ethanol, the fermenting host should also meet the following two requirements: (1) resistant to the inhibitory compounds formed during biomass pretreatment process, (2) capable of utilizing C5 sugars, such as xylose, as carbon source. In our laboratory, a screening was conducted on microorganisms collected from environmental sources for their tolerance to hydrolysate inhibitors. A unique resistant strain was selected and identified as Pichia anomala (Wickerhamomyces anomalus), deposited as CBS 132101. The strain is able to produce ethanol in various biomass hydrolysates, both with and without oxygen. Besides, the strain could assimilate xylose and use nitrate as N source. These physiological characteristics make P. anomala an interesting strain for bioethanol production from lignocellulosic biomass hydrolysates.

Co-fermentation of hexose and pentose sugars in a spent sulfite liquor matrix with genetically modified Saccharomyces cerevisiae

Spent sulfite liquor (SSL) is a by-product of pulp and paper manufacturing and is a promising substrate for second-generation bioethanol production. The Saccharomyces cerevisiae strain IBB10B05 presented herein for SSL fermentation was enabled to xylose utilization by metabolic pathway engineering and laboratory evolution. Two SSLs from different process stages and with variable dry matter content were analyzed; SSL-Thin (14%) and SSL-S2 (30%). Hexose and pentose fermentation by strain IBB10B05 was efficient in 70% SSL matrix without any pretreatment. Ethanol yields varied between 0.31 and 0.44g/g total sugar, depending on substrate and process conditions used. Control of pH at 7.0 effectively reduced the inhibition by the acetic acid contained in the SSLs (up to 9g/L), thus enhancing specific xylose uptake rates (q(Xylose)) as well as ethanol yields. The total molar yield of fermentation by-products (glycerol, xylitol) was constant (0.36±0.03mol/mol xylose) at different q(Xylose). Compound distribution changed with glycerol and xylitol being chiefly formed at low and high q(Xylose), respectively.

Effect of inhibitors formed during wheat straw pretreatment on ethanol fermentation by Pichia stipitis

The inhibitory effect of the main inhibitors (acetic acid, furfural and 5-hydroxymethylfurfural) formed during steam explosion of wheat straw was studied through ethanol fermentations of model substrates and hydrolysates from wheat straw by Pichia stipitis. Experimental results showed that an increase in acetic acid concentration led to a reduction in ethanol productivity and complete inhibition was observed at 3.5 g/L. Furfural produced a delay on sugar consumption rates with increasing concentration and HMF did not exert a significant effect. Fermentations of the whole slurry from steam exploded wheat straw were completely inhibited by a synergistic effect due to the presence of 1.5 g/L acetic acid, 0.15 g/L furfural and 0.05 g/L HMF together with solid fraction. When using only the solid fraction from steam explosion, hydrolysates presented 0.5 g/L of acetic acid, whose fermentations have submitted promising results, providing an ethanol yield of 0.45 g ethanol/g sugars and the final ethanol concentration reached was 12.2 g/L (10.9 g ethanol/100 g DM).

Ethanol production from eucalyptus wood hemicellulose hydrolysate by Pichia stipitis

Ethanol production was evaluated from eucalyptus wood hemicellulose acid hydrolysate using Pichia stipitis NRRL Y-7124. An initial lag phase characterized by flocculation and viability loss of the yeast inoculated was observed. Subsequently, cell regrowth occurred with sequential consumption of sugars and production of ethanol. Polyol formation was detected. Acetic acid present in the hydrolysate was an important inhibitor of the fermentation, reducing the rate and the yield. Its toxic effect was due essentially to its undissociated form. The fermentation was more effective at an oxygen transfer rate between 1.2 and 2.4 mmol/L h and an initial pH of 6.5. The hydrolysate used in the experiences had the following composition (expressed in grams per liter): xylose 30, arabinose 2.8, glucose 1.5, galactose 3.7, mannose 1.0, cellobiose 0.5, acetic acid 10, glucuronic acid 1.5, and galacturonic acid 1.0. The best values obtained were maximum ethanol concentration 12.6 g/L, fermentation time 75 h, fermentable sugar consumption 99% ethanol yield 0.35 g/g sugars consumed, and volumetric ethanol productivity 4 g/L day. (

Pretreatment of biomass by aqueous ammonia for bioethanol production

The methods of pretreatment of lignocellulosic biomass using aqueous ammonia are described. The main effect of ammonia treatment of biomass is delignification without significantly affecting the carbohydrate contents. It is a very effective pretreatment method especially for substrates that have low lignin contents such as agricultural residues and herbaceous feedstock. The ammonia-based pretreatment is well suited for simultaneous saccharification and co-fermentation (SSCF) because the treated biomass retains cellulose as well as hemicellulose. It has been demonstrated that overall ethanol yield above 75% of the theoretical maximum on the basis of total carbohydrate is achievable from corn stover pretreated with aqueous ammonia by way of SSCF. There are two different types of pretreatment methods based on aqueous ammonia: (1) high severity, low contact time process (ammonia recycle percolation; ARP), (2) low severity, high treatment time process (soaking in aqueous ammonia; SAA). Both of these methods are described and discussed for their features and effectiveness.

Bioethanol production from barley hull using SAA (soaking in aqueous ammonia) pretreatment

Barley hull, a lignocellulosic biomass, was pretreated using aqueous ammonia, to be converted into ethanol. Barley hull was soaked in 15 and 30 wt.% aqueous ammonia at 30, 60, and 75 degrees C for between 12 h and 11 weeks. This pretreatment method has been known as "soaking in aqueous ammonia" (SAA). Among the tested conditions, the best pretreatment conditions observed were 75 degrees C, 48 h, 15 wt.% aqueous ammonia and 1:12 of solid:liquid ratio resulting in saccharification yields of 83% for glucan and 63% for xylan with 15 FPU/g-glucan enzyme loading. Pretreatment using 15 wt.% ammonia for 24-72 h at 75 degrees C removed 50-66% of the original lignin from the solids while it retained 65-76% of the xylan without any glucan loss. Addition of xylanase along with cellulase resulted in synergetic effect on ethanol production in SSCF (simultaneous saccharification and co-fermentation) using SAA-treated barley hull and recombinant E. coli (KO11). With 3% w/v glucan loading and 4 mL of xylanase enzyme loadings, the SSCF of the SAA treated barley hull resulted 24.1g/L ethanol concentration at 15 FPU cellulase/g-glucan loading, which corresponds to 89.4% of the maximum theoretical yield based on glucan and xylan. SEM results indicated that SAA treatment increased surface area and the pore size. It is postulated that these physical changes enhance the enzymatic digestibility in the SAA treated barley hull.

Bioconversion of water-hyacinth (Eichhornia crassipes) hemicellulose acid hydrolysate to motor fuel ethanol by xylose-fermenting yeast

Water-hyacinth (Eichhornia crassipes) hemicellulose acid hydrolysate has been utilized as a substrate for ethanol production using Pichia stipitis NRRL Y-7124. Hydrolysate fermentability was considerable improved by boiling, and overliming up to pH 10.0 with solid Ca(OH)(2) in combination with sodium sulfite. The percent total sugar utilized and ethanol yield (Y(p/s)) for the untreated hydrolysate were 20.15+/-0.17% and 0.19+/-0.003 g(p) g(s)(-1), respectively, compared with 76.0+/-0.32% and 0.35 g(p) g(s)(-1), respectively for the treated material. The fermentation was very effective at an aeration rate of 0.02 v/v/m, temperature 30+/-0.2 degrees C and pH 6.0+/-0.2. However, the volumetric productivity (Q(p)) was still considerably less than observed in a simulated synthetic hydrolysate medium with a sugar composition similar to the hemicellulose acid hydrolysate. L-Arabinose was not fermented but assimilated. The presence of acetic acid in the hydrolysate decreased the ethanol yield and productivity considerably.

Ethanol production from wheat straw hemicellulose hydrolysate by Pichia stipitis

Ethanol production was evaluated from wheat straw (WS) hemicellulose acid hydrolysate using an adapted and parent strain of Pichia stipitis. NRRL Y-7124. The treatment by boiling and overliming with Ca(OH)(2) significantly improved the fermentability of the hydrolysate. Ethanol yield (Yp/s) and productivity (Qp av) were increased 2.4+/-0.10 and 5.7+/-0.24 folds, respectively, compared to neutralized hydrolysate. Adaptation of the yeast to the hydrolysate resulted further improvement in yield and productivity. The maximum yield was 0.41+/-0.01 g(p) g(s)(-1), equivalent to 80.4+/-0.55% theoretical conversion efficiency. Acetic acid, furfurals and lignins present in the hydrolysate were inhibitory to microbial growth and ethanol production. The addition of these inhibitory components individually or in various combinations at a concentrations similar to that found in hydrolysate to simulated medium resulted a reduction in ethanol yield (Yp/s) and productivity (Qp av). The hydrolysate used had the following composition (expressed in g x l(-1)): xylose 12.8+/-0.25; glucose 1.7+/-0.3; arabinose 2.6+/-0.21 and acetic acid 2.7+/-0.33.

Development of xylose-fermenting yeast Pichia stipitis for ethanol production through adaptation on hardwood hemicellulose acid prehydrolysate

AIMS

The objective of this study was to develop a mutant from Pichia stipitis NRRL Y-7124, tolerant of high concentrations of acetic acid and other inhibitory components present in acid hydrolysates, to improve ethanol yield and productivity.

METHODS AND RESULTS

The mutant was developed through adaptation in acid hydrolysate supplemented with nutrients and minerals at 30 +/- 0.5 degrees C. When it was tested for its ability to ferment acid hydrolysate, it showed shorter fermentation time, better tolerance to acid and could ferment at lower pH. The ethanol yield (Yp/s) and productivity (Qp) were increased 1.6- and 2.1-fold, respectively.

CONCLUSION

The development of a mutant and its tolerance to acetic acid present in hydrolysates is described. The selected mutant is capable of fermenting both hexoses and pentoses present in hydrolysate at lower pH in comparison with the parent strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

The mutant could play a significant role in reducing environmental pollution by using sugars present in pulp mill effluent and, at the same time, could produce a marketable liquid fuel ethanol.

Metabolic engineering of Saccharomyces cerevisiae

Comprehensive knowledge regarding Saccharomyces cerevisiae has accumulated over time, and today S. cerevisiae serves as a widley used biotechnological production organism as well as a eukaryotic model system. The high transformation efficiency, in addition to the availability of the complete yeast genome sequence, has facilitated genetic manipulation of this microorganism, and new approaches are constantly being taken to metabolicially engineer this organism in order to suit specific needs. In this paper, strategies and concepts for metabolic engineering are discussed and several examples based upon selected studies involving S. cerevisiae are reviewed. The many different studies of metabolic engineering using this organism illustrate all the categories of this multidisciplinary field: extension of substrate range, improvements of producitivity and yield, elimination of byproduct formation, improvement of process performance, improvements of cellular properties, and extension of product range including heterologous protein production.

Cloning and characterization of two pyruvate decarboxylase genes from Pichia stipitis CBS 6054

In Pichia stipitis, fermentative and pyruvate decarboxylase (PDC) activities increase with diminished oxygen rather than in response to fermentable sugars. To better characterize PDC expression and regulation, two genes for PDC (PsPDC1 and PsPDC2) were cloned and sequenced from P. stipitis CBS 6054. Aside from Saccharomyces cerevisiae, from which three PDC genes have been characterized, P. stipitis is the only organism from which multiple genes for PDC have been identified and characterized. PsPDC1 and PsPDC2 have diverged almost as far from one another as they have from the next most closely related known yeast gene. PsPDC1 contains an open reading frame of 1,791 nucleotides encoding 597 amino acids. PsPDC2 contains a reading frame of 1,710 nucleotides encoding 570 amino acids. An 81-nucleotide segment in the middle of the beta domain of PsPDC1 codes for a unique segment of 27 amino acids, which may play a role in allosteric regulation. The 5' regions of both P. stipitis genes include two putative TATA elements that make them similar to the PDC genes from S. cerevisiae, Kluyveromyces marxianus, and Hanseniaspora uvarum.

Phosphorus-31 and carbon-13 nuclear magnetic resonance studies of glucose and xylose metabolism in Candida tropicalis cell suspensions

The metabolism of glucose and xylose was studied as a function of oxygenation in suspensions of Candida tropicalis by 31P and 13C nuclear magnetic resonance spectroscopy. Both the rate of carbohydrate metabolism and the cytoplasmic pH were independent of the rate of oxygenation in cells metabolizing glucose. However, these two parameters were markedly dependent on the rate of oxygenation in C. tropicalis cells metabolizing xylose. For example, the cytoplasmic pH in fully oxygenated xylose-metabolizing cells was 7.8 but decreased to 6.3 in anoxic cells. In general, suspensions of cells consuming xylose had a lower rate of sugar uptake, a more acidic cytoplasmic pH, lower levels of sugarphosphomonoesters (SP) and ATP, higher levels of intracellular Pi, a more alkaline vacuolar pH, and a lower rate of extracellular Pi assimilation and polyphosphate synthesis than cells consuming glucose. These observations indicate that C. tropicalis metabolizing xylose is less energized than glucose-metabolizing cells. On both carbon sources, however, an inverse correlation between intracellular levels of SP and Pi was observed. Also, uptake of extracellular Pi correlated with the synthesis of polyphosphates within the cells. During anoxia, Pi was not taken up, and polyphosphates were hydrolyzed instead to fulfill the cells' requirements for phosphate.

High-efficiency transformation of Pichia stipitis based on its URA3 gene and a homologous autonomous replication sequence, ARS2

This paper describes the first high-efficiency transformation system for the xylose-fermenting yeast Pichia stipitis. The system includes integrating and autonomously replicating plasmids based on the gene for orotidine-5'-phosphate decarboxylase (URA3) and an autonomous replicating sequence (ARS) element (ARS2) isolated from P. stipitis CBS 6054. Ura- auxotrophs were obtained by selecting for resistance to 5-fluoroorotic acid and were identified as ura3 mutants by transformation with P. stipitis URA3. P. stipitis URA3 was cloned by its homology to Saccharomyces cerevisiae URA3, with which it is 69% identical in the coding region. P. stipitis ARS elements were cloned functionally through plasmid rescue. These sequences confer autonomous replication when cloned into vectors bearing the P. stipitis URA3 gene. P. stipitis ARS2 has features similar to those of the consensus ARS of S. cerevisiae and other ARS elements. Circular plasmids bearing the P. stipitis URA3 gene with various amounts of flanking sequences produced 600 to 8,600 Ura+ transformants per micrograms of DNA by electroporation. Most transformants obtained with circular vectors arose without integration of vector sequences. One vector yielded 5,200 to 12,500 Ura+ transformants per micrograms of DNA after it was linearized at various restriction enzyme sites within the P. stipitis URA3 insert. Transformants arising from linearized vectors produced stable integrants, and integration events were site specific for the genomic ura3 in 20% of the transformants examined. Plasmids bearing the P. stipitis URA3 gene and ARS2 element produced more than 30,000 transformants per micrograms of plasmid DNA. Autonomously replicating plasmids were stable for at least 50 generations in selection medium and were present at an average of 10 copies per nucleus.

Effects of pH and acetic acid on glucose and xylose metabolism by a genetically engineered ethanologenic Escherichia coli

Efficient utilization of the pentosan fraction of hemicellulose from lignocellulosic feedstocks offers an opportunity to increase the yield and to reduce the cost of producing fuel ethanol. The patented, genetically engineered, ethanologen Escherichia coli B (pLOI297) exhibits high-performance characteristics with respect to both yield and productivity in xylose-rich lab media. In addition to producing monomer sugar residues, thermochemical processing of biomass is known to produce substances that are inhibitory to both yeast and bacteria. During prehydrolysis, acetic acid is formed as a consequence of the deacetylation of the acetylated pentosan. Our investigations have shown that the acetic acid content of hemicellulose hydrolysates from a variety of biomass/waste materials was in the range 2-10 g/L (33-166 mM). Increasing the reducing sugar concentration by evaporation did not alter the acetic acid concentration. Acetic acid toxicity is pH dependent. By virtue of its ability to traverse the cell membrane freely, the undissociated (protonated) form of acetic acid (HAc) acts as a membrane protonophore and causes its inhibitory effect by bringing about the acidification of the cytoplasm. With recombinant E. coli B, the pH range for optimal growth with glucose and xylose was 6.4-6.8. With glucose, the pH optimum for ethanol yield and volumetric productivity was 6.5, and for xylose it was 6.0 and 6.5, respectively. However, the decrease in growth and fermentation efficiency at pH 7 is not significant. At pH 7, only 0.56% of acetic acid is undissociated, and at 10 g/L, neither the ethanol yield nor the maximum volumetric productivity, with glucose or xylose, is significantly decreased. The "uncoupling" effect of HAc is more pronounced with xylose and the potency of HAc is potentiated in a minimal salts medium. Controlling the pH at 7 provided an effective means of circumventing acetic acid toxicity without significant loss in fermentation performance of the recombinant biocatalyst.

Production of ethanol from pulp mill hardwood and softwood spent sulfite liquors by genetically engineered E. coli

Although lignocellulosic biomass and wastes are targeted as an attractive alternative fermentation feedstock for the production of fuel ethanol, cellulosic ethanol is not yet an industrial reality because of problems in bioconversion technologies relating both to depolymerization and fermentation. In the production of wood pulp by the sulfite process, about 50% of the wood (hemicellulose and lignin) is dissolved to produce cellulose pulp, and the pulp mill effluent ("spent sulfite liquor" SSL) represents the only lignocellulosic hydrolysate available today in large quantities (about 90 billion liters annually worldwide). Although softwoods have been the traditional feedstock for pulping operations, hardwood pulping is becoming more popular, and the pentose sugars in hardwood SSL (principally xylose) are not fermented by the yeasts currently being used in the production of ethanol from softwood SSL. This study assessed the fermentation performance characteristics of a patented (US Pat. 5,000,000), recombinant Escherichia coli B (ATCC 11303 pLOI297) in anaerobic batch fermentations of both nutrient-supplemented soft and hardwood SSL (30-35 g/L total reducing sugars). The pH was controlled at 7.0 to maximize tolerance to acetic acid. In contrast to the high-performance characteristics exhibited in synthetic media, formulated to mimic the composition of softwood and hardwood SSL (yield approaching theoretical maximum), performance in SSL media was variable with conversion efficiencies in the range of 67-84% for hardwood SSL and 53-76% for softwood SSL. Overlimiting treatment of HSSL, using Ca(OH)2, improved overall volumetric productivity two- to sevenfold to a max of 0.42 g/L/h at an initial cell loading of 0.5 g dry wt/L. A conversion efficiency of 92% (6.1 g/L ethanol) was achieved using diluted Ca(OH)2-treated hardwood SSL. The variable behavior of this particular genetic construct is viewed as a major detractant regarding its candidacy as a biocatalyst for SSL fermentations.

Isolation and characterization of acetic acid-tolerant galactose-fermenting strains of Saccharomyces cerevisiae from a spent sulfite liquor fermentation plant

From a continuous spent sulfite liquor fermentation plant, two species of yeast were isolated, Saccharomyces cerevisiae and Pichia membranaefaciens. One of the isolates of S. cerevisiae, no. 3, was heavily flocculating and produced a higher ethanol yield from spent sulfite liquor than did commercial baker's yeast. The greatest difference between isolate 3 and baker's yeast was that of galactose fermentation, even when galactose utilization was induced, i.e., when they were grown in the presence of galactose, prior to fermentation. Without acetic acid present, both baker's yeast and isolate 3 fermented glucose and galactose sequentially. Galactose fermentation with baker's yeast was strongly inhibited by acetic acid at pH values below 6. Isolate 3 fermented galactose, glucose, and mannose without catabolite repression in the presence of acetic acid, even at pH 4.5. The xylose reductase (EC 1.1.1.21) and xylitol dehydrogenase (EC 1.1.1.9) activities were determined in some of the isolates as well as in two strains of S. cerevisiae (ATCC 24860 and baker's yeast) and Pichia stipitis CBS 6054. The S. cerevisiae strains manifested xylose reductase activity that was 2 orders of magnitude less than the corresponding P. stipitis value of 890 nmol/min/mg of protein. The xylose dehydrogenase activity was 1 order of magnitude less than the corresponding activity of P. stipitis (330 nmol/min/mg of protein).

Ethanolic fermentation of pentoses in lignocellulose hydrolysates

In the fermentation of lignocellulose hydrolysates to ethanol, two major problems are encountered: the fermentation of the pentose sugar xylose, and the presence of microbial inhibitors. Xylose can be directly fermented with yeasts, such as Pachysolen tannophilus, Candida shehatae, and Pichia stipis, or by isomerization of xylose to xylulose with the enzyme glucose (xylose) isomerase (XI; EC 5.3.1.5), and subsequent fermentation with bakers' yeast, Saccharomyces cerevisiae. The direct fermentation requires low, carefully controlled oxygenation, as well as the removal of inhibitors. Also, the xylose-fermenting yeasts have a limited ethanol tolerance. The combined isomerization and fermentation with XI and S. cerevisiae gives yields and productivities comparable to those obtained in hexose fermentations without oxygenation and removal of inhibitors. However, the enzyme is not very stable in a lignocellulose hydrolysate, and S. cerevisiae has a poorly developed pentose phosphate shunt. Different strategies involving strain adaptation, and protein and genetic engineering adopted to overcome these different obstacles, are discussed.