Effects of Forage Quality and Cell Wall Constituents of Bermuda Grass on Biochemical Conversion to Ethanol

Bermuda grass is an attractive candidate as a feedstock for biofuel production because over four million hectares of Bermuda grass are already grown for forage in the Southern USA. Because both rumen digestion and biochemical conversion to ethanol depend upon enzymatic conversion of the cell wall polysaccharides into fermentable sugars, it is probable that grasses bred for increased forage quality would be more amenable for ethanol production. However, it is not known how variation in rumen digestibility and cell wall/fiber components correlates with efficiency of conversion to ethanol via fermentation. The objective of this research was to determine relationships between ethanol production evaluated by simultaneous saccharification and fermentation (SSF), 72-h in vitro ruminal dry matter digestibility (IVDMD), in vitro ruminal gas production after 24 and 96 h, and biomass composition for 50 genetically diverse Bermuda grass accessions. The Bermuda grass samples were subjected to standard 72-h IVDMD and forage fiber analyses. Also, in separate labs, gas production was measured in sealed volume-calibrated vials after 24 (NNG24) and 96 h (NNG96) of in vitro fermentation by ruminal fluid; ethanol and pentose sugar productions were measured from a bench-top SSF procedure; cell wall constituents were determined by the Uppsala Dietary Fiber Method; and total nitrogen, carbon, and ash concentrations were determined by using the LECO combustion method. Ethanol production was moderately correlated with IVDMD (r?=?0.55) and NNG96 (r?=?0.63) but highly correlated with NNG24 (r?=?0.93). Ethanol was negatively correlated with neutral detergent fiber (NDF; r?=??0.53) and pentose sugars (r?=??0.60), but not correlated with glucose content. Regression models indicated that NDF and cell wall pentose sugar concentrations had significant negative effects on ethanol production. Variation among entries for IVDMD was affected by variability of NDF, pentose sugar concentrations, and biomass nitrogen content. Variation in Klason lignin content had only minor negative impacts on ethanol production and IVDMD. Biochemical conversion efficiency of Bermuda grass by SSF can be best estimated by NNG24 but not by IVDMD.     

Updated model of the batch acetone-butanol fermentation

Summary The recent models of the Acetone-Butanol fermentation did not adequately describe the culture inhibition by the accumulating metabolites and were unable to simulate the acidogenic culture dynamics at elevated pH levels. The present updated modification of the model features a generalised inhibition term and a pH dependent terms for intracellular conversion of undissociated acids into solvent products. The culture dynamics predictions by the developed model compared well with experimental results from an unconventional acidogenic fermentation ofC. acetobutylicum.Nomenclature A acetone concentration in the fermentation broth, [g/L] - AA total concentration of dissociated and undissociated acetic acid, [g/L] - AA _undiss concentration of undissociated acetic acid, [g/L] - APS Absolute Parameter Sensitivity - AT acetoin concentration in the fermentation broth, [g/L] - B butanol concentration in the fermentation broth, [g/L] - BA total concentration of dissociated and undissociated butyric acid, [g/L] - BA _undiss concentration of undissociated butyric acid, [g/L] - E ethanol concentration in the fermentation broth, [g/L] - f(T) inhibition function as defined in Equation (2) - k ₁ constant in Equation (4), [g substrate/g biomass] - k ₂ constant in Equation (4), [g substrate/(g biomass.h)] - k ₁ constant in Equation (5), [g substrate/(g biomass] - k ₂ constant in Equation (5), [g substrate/(g biomass.h)] - k ₃ constant in Equation (6), [g butyric acid/g substrate] - k ₄ constant in Equation (6), [g butyric acid/(g biomass.h)] - k ₅ constant in Equation (7), [g butanol/g substrate] - k ₆ constant in Equation (8), [g acetic acid/g substrate] - k ₇ constant in Equation (8), [g acetic acid/(g biomass.h)] - k ₈ constant in Equation (9), [g acetone/g substrate] - k ₉ constant in Equation (10), [g ethanol/g substrate] - k ₁₀ constant in Equation (11), [g acetoin/g substrate] - k ₁₁ constant in Equation (12), [g lactic acid/g substrate] - K _I Inhibition constant, [g inhibitory products/L] - ke maintenance energy requirement for the cell, [g substrate/(g biomass.h)] - K _AA acetic acid saturation constant, [g acetic acid/L] - K _BA butyric acid saturation constant, [g butyric acid/L] - K _S Monod's saturation constant, [g substrate/L] - LA lactic acid concentration in the fermentation broth, [g/L] - m _i ,n _i constants in Equation (14) - n empirical constant, dependent on degree of inhibition. - P concentration of inhibitory products (B+BA+AA), [g/L] - P _max maximum value of product concentration to inhibit the fermentation, [g/L] - pK_a equilibrium constant - r _A rate of acetone production, [g acetone/L.h] - r _AA rate of acetic acid production, [g acetic acid/L.h] - r _AT rate of acetoin production, [g acetoin/L.h] - r _B rate of butanol production, [g butanol/L.h] - r _BA rate of butyric acid production, [g butyric acid/L.h] - r _E rate of ethanol production, [g ethanol/L.h] - RPS Relative Parameter Sensitivity - r _LA rate of lactic acid production, [g lactic acid/L.h] - r _S dS/dt=total substrate consumption rate, [g substrate/L.h] - r _S substrate utilization rate, [g substrate/L.h] - S substrate concentration in the fermentation broth, [g substrate/L] - S ₀ initial substrate concentration, [substrate/L] - t time, [h] - X biomass concentration, [g/L] - Y _X yield of biomass with respect to substrate, [g biomass/g substrate] - Y _{P i} yield of metabolic product with respect to substrate, [g product/g substrate] Derivatives dX/dt rate of biomass production, [g biomass/L.h] - dP _i /dt rate of product formation, [g product/L.h] Greek letters specific growth rate of the culture, [h^–1] - I specific growth rate of the culture in the presence of the inhibitory products, [h^–1] - µ_max maximum specific growth rate of the culture, [h^–1]     

Ethanol fermentation by flocculating yeast: Performance and stability dependence on a critical fermentation rate

Summary A system coupling fermentor and decantor permitted strong accumulation of yeast flocs that were homogeneously suspended in the reactional volume. At 100–190 g/l glucose feed practically total substrate conversion was attained. At 130 g/l glucose feed the highest productivity (18.4 g.l.h) and the highest ethanol yield (90.6%) were reached with biomass levels of 80–90 g/l. We observed that the stability of this system is limited when a critical fermentation rate (D.S_o) close to 39–40 g/l.h (with corresponding ethanol productivities of 19–20 g/l.h) is reached. Higher fermentation rates provoked de-flocculation and lost of biomass.Symbols D dilution rate (h^–1) - E ethanol (g/l) - S_r residual substrate (g/l) - S_o substrate in the feed (g/l) - X biomass (g/l) - ethanol yield (%) - DS_o fermentation rate (g/l.h) (for S_r0) - P_E ethanol productivity (g/l.h)     

Application of a membrane recycle bioreactor for continuous ethanol production

Summary A series of continuous fermentations were carried out with a production strain of the yeast Saccharomyces cerevisiae in a membrane bioreactor. A membrane separation module composed of ultrafiltration tubular membranes retained all biomass in a fermentation zone of the bioreactor and allowed continuous removal of fermentation products into a cell-free permeate. In a system with total (100%) cell recycle the impact of fermentation conditions [dilution rate (0.03–0.3 h^–1); substrate concentration in the feed (50–300 g·1^–1); biomass concentration (depending on the experimental conditions)] was studied on the behaviour of the immobilized cell population and on ethanol formation. Maximum ethanol productivity (15 g·1^–1·h^–1) was attained at an ethanol concentration of 81 g·1^–1. The highest demands of cells for maintenance energy were found at the maximum feed substrate concentration (300 g·1^–1) and at very low concentrations of cells in the broth.     

Seasonal variation in Laminaria digitata and its impact on biochemical conversion routes to biofuels

Laminaria digitata is a highly prevalent kelp growing off the coast of the UK but has rarely been considered as a source of biomass to date. This study shows it can be used as a feedstock in both ethanol fermentation and anaerobic digestion for methane production. The study optimised several parameters in the fermentation of L. digitata and investigated the suitability of the macroalgae through the year using samples harvested every month. For both methane and ethanol production, minimum yields were seen in material harvested in March when the carbohydrates laminarin and mannitol were lowest. July material contained the highest combined laminarin and mannitol content and maximum yields of 167 mL ethanol and 0.219 m³ kg⁻¹L. digitata.     

Large‐scale screening and characterisation of Lemna aequinoctialis and Spirodela polyrhiza strains for starch production

• Duckweed is considered a promising feedstock for bioethanol production due to its high biomass and starch production. Selection of duckweed strains with high starch accumulation is essential for application of duckweeds to bioethanol production. Geographic differentiation had a large influence on genetic diversity of duckweeds.
• Biomass production, starch content and starch amount in geographically isolated strains of 20 Lemna aequinoctialis and Spirodela polyrhiza were calculated to evaluate their potential for bioethanol production. The influence of different collection time, culture medium and NaCl concentration on starch accumulation of the best strains were analysed.
• The results showed that biomass production, starch content and starch production of duckweeds demonstrated clonal dependency. The best strain was L. aequinoctialis 6000, with biomass production of 15.38 ± 1.47 g m^?2, starch content of 28.68 ± 1.10% and starch production of 4.39 ± 0.25 g m^?2. Furthermore, starch content of L. aequinoctialis 6000 was highest after 8 h of light, tap water was the best medium for starch induction, and NaCl did not induce starch accumulation.
• This study suggests duckweed biomass production and starch production demonstrate clonal dependency, indicating that extensive clonal comparisons will be required to identify the most suitable isolates for duckweed selective breeding for bioethanol.

     

Improved glycerol to ethanol conversion by <Emphasis Type="Italic">E. coli</Emphasis> using a metagenomic fragment isolated from an anaerobic reactor

Crude glycerol obtained as a by-product of biodiesel production is a reliable feedstock with the potential to be converted into reduced chemicals with high yields. It has been previously shown that ethanol is the primary product of glycerol fermentation by Escherichia coli. However, few efforts were made to enhance this conversion by means of the expression of heterologous genes with the potential to improve glycerol transport or metabolism. In this study, a fosmid-based metagenomic library constructed from an anaerobic reactor purge sludge was screened for genetic elements that promote the use and fermentation of crude glycerol by E. coli. One clone was selected based on its improved growth rate on this feedstock. The corresponding fosmid, named G1, was fully sequenced (41 kbp long) and the gene responsible for the observed phenotype was pinpointed by in vitro insertion mutagenesis. Ethanol production from both pure and crude glycerol was evaluated using the parental G1 clone harboring the ethanologenic plasmid pLOI297 or the industrial strain LY180 complemented with G1. In mineral salts media containing 50 % (v/v) pure glycerol, ethanol concentrations increased two-fold on average when G1 was present in the cells reaching up to 20 g/L after 24 h fermentation. Similar fermentation experiments were done using crude instead of pure glycerol. With an initial OD₆₂₀ of 8.0, final ethanol concentrations after 24 h were much higher reaching 67 and 75 g/L with LY180 cells carrying the control fosmid or the G1 fosmid, respectively. This translates into a specific ethanol production rate of 0.39 g h^?1 OD^?1 L^?1.     

Improving biomass production and saccharification in <Emphasis Type="Italic">Brachypodium distachyon</Emphasis> through overexpression of a sucrose-phosphate synthase from sugarcane

The substitution of fossil by renewable energy sources is a major strategy in reducing CO₂ emission and mitigating climate change. In the transport sector, which is still mainly dependent on liquid fuels, the production of second generation ethanol from lignocellulosic feedstock is a promising strategy to substitute fossil fuels. The main prerequisites on designated crops for increased biomass production are high biomass yield and optimized saccharification for subsequent use in fermentation processes. We tried to address these traits by the overexpression of a sucrose-phosphate synthase gene (SoSPS) from sugarcane (Saccharum officinarum) in the model grass Brachypodium distachyon. The resulting transgenic B. distachyon lines not only revealed increased plant height at early growth stages but also higher biomass yield from fully senesced plants, which was increased up to 52 % compared to wild-type. Additionally, we determined higher sucrose content in senesced leaf biomass from the transgenic lines, which correlated with improved biomass saccharification after conventional thermo-chemical pretreatment and enzymatic hydrolysis. Combining increased biomass production and saccharification efficiency in the generated B. distachyon SoSPS overexpression lines, we obtained a maximum of 74 % increase in glucose release per plant compared to wild-type. Therefore, we consider SoSPS overexpression as a promising approach in molecular breeding of energy crops for optimizing yields of biomass and its utilization in second generation biofuel production.     

A semi-continuous culture system for production of cellulolytic and xylanolytic enzymes by the anaerobic fungus Piromyces sp. strain E2

Summary A system was developed for the semi-continuous cultivation of an anaerobic fungus, Piromyces sp. strain E2 (isolated from an Indian elephant), on Avicel (microcrystalline cellulose). The fungus was grown in a semi-continuous culture system: solids and fungal biomass was retained by means of a simple filter construction whereas the culture fluid was removed continuously. The production of fermentation products (acetate, ethanol, formate, lactate, hydrogen or methane), cellulolytic and xylanolytic enzymes, and protein by the fungus in monoculture or co-culture with Methanobacterium formicicum during growth on Avicel was monitored up to 45 days. These productions stabilized after an adaptation period of 24 and 30 days in the semi-continuous co-culture and monoculture, respectively. After this period the average (±SD) avicelase, -glucosidase, endoglucanase, and xylanase production in the semi-continuous monoculture were 27±6, 140±16, 1057±120 and 5012±583 IU.l^–1.dya^–1, respectively. Co-culture with the methanogen caused a shift in fermentation products to more acetate, and less ethanol and lactate. Furthermore, the production of all cellulolytic enzymes increased (40%) and xylanolytic enzyme production decreased (35%).Correspondence to: H. J. M. Op den Camp     

Fermentability of eastern gamagrass, big bluestem and sand bluestem grown across a wide variety of environments

Plant biomass has attracted interest as a feedstock for biofuels production, but much of this work has been focused on relatively few plant species. In this study, three relatively-unstudied species of warm-season perennial grasses, grown at multiple locations in the eastern and central US and harvested over a three year period, were examined for fermentability via in vitro ruminal gas production and dry matter digestibility assays, and near-infrared reflectance calibrations were developed for these fermentation parameters. Big bluestem (Andropogon gerardii Vitman) displayed greater fermentability than did sand bluestem (Andropogon hallii Hack) or eastern gamagrass [Tripsacum dactyloides (L.) L.], but displayed lower biomass yields. The bluestems also displayed lower N contents and less variation in fermentability over different growth environments (geographic locations and harvest years), suggesting a more consistent biomass quality than for eastern gamagrass. Thus, in addition to their use as forage for ruminant animals, bluestems may be of particular interest as feedstocks for bioconversion to ethanol and other products via direct microbial fermentation (consolidated bioprocessing) schemes, and thus merit additional efforts to enhance biomass yield potential.     

Isolation,characterization, and quantification of <Emphasis Type="Italic">Clostridium kluyveri</Emphasis> from the bovine rumen

A strain of Clostridium kluyveri was isolated from the bovine rumen in a medium containing ethanol as an electron donor and acetate and succinate (common products of rumen fermentation) as electron acceptors. The isolate displayed a narrow substrate range but wide temperature and pH ranges atypical of ruminal bacteria and a maximum specific growth rate near the typical liquid dilution rate of the rumen. Quantitative real-time PCR revealed that C. kluyveri was widespread among bovine ruminal samples but was present at only very low levels (0.00002% to 0.0002% of bacterial 16S rRNA gene copy number). However, the species was present in much higher levels (0.26% of bacterial 16S rRNA gene copy number) in lucerne silage (but not maize silage) that comprised much of the cows’ diet. While C. kluyveri may account for several observations regarding ethanol utilization and volatile fatty acid production in the rumen, its population size and growth characteristics suggest that it is not a significant contributor to ruminal metabolism in typical dairy cattle, although it may be a significant contributor to silage fermentation. The ability of unadapted cultures to produce substantial levels (12.8 g L⁻¹) of caproic (hexanoic) acid in vitro suggests that this strain may have potential for industrial production of caproic acid.     

Formal kinetics of poly-β-hydroxybutyric acid (PHB) production in Alcaligenes eutrophus H 16 and Mycoplana rubra R 14 with respect to the dissolved oxygen tension in ammonium-limited batch cultures

Summary Under chemolithoautotrophic growth conditions with the organism Alcaligenes eutrophus H16 the exponential growth phase is characterized by two different growth rates, each associated with different specific rates of ammonium consumption. On the basis of the analytical determination of Poly--hydroxybutyric acid (PHB), it can be conclusively shown that PHB is synthesized even during the exponential growth phase at a specific rate proportional to the specific growth rates of total biomass. After complete consumption of ammonium, the increase of biomass is exclusively due to PHB synthesis, whereas protein and rest biomass (cell dry weight minus PHB) remain constant. After an extended period of fermentation, the PHB content reaches a saturation value. The transient phase between the growth and the storage phase is very short in comparison to the duration of the whole fermentation. In the case of Alcaligenes eutrophus, strain H 16, high concentrations of dissolved oxygen strongly influence growth as well as PHB synthesis.Abbrevations ^cO₂,L concentration of oxygen in the liquid phase (dissolved oxygen tension: d.o.t) - ^cH₂,L concentration of hydrogen in the liquid phase - ^cCO₂,L concentration of carbon dioxide in the liquid phase - S limiting substrate, concentration of - X total biomass, concentration of; total cell dry weight - P product; PHB, concentration of - R rest biomass: X-P, concentration of - ^rX dX/dt growth rate - ^rP dP/dt rate of PHB synthesis - ^rR dR/dt rate of rest biomass production - ^r0 ^dcO₂,L/dt rate of oxygen consumption - X dX/dt·1/X=r_X·1/X specific growth rate - P dP/dt·1/P=r_P·1/P specific rate of product formation - R dR/dt·1/R=r_R·1/R specific rate of rest biomass formation - r₀/R specific respiration rate     

Quantifying Actual and Theoretical Ethanol Yields for Switchgrass Strains Using NIRS Analyses

Quantifying actual and theoretical ethanol yields from biomass conversion processes such as simultanteous saccharification and fermentation (SSF) requires expensive, complex fermentation assays, and extensive compositional analyses of the biomass sample. Near-infrared reflectance spectroscopy (NIRS) is a non-destructive technology that can be used to obtain rapid, low-cost, high-throughput, and accurate estimates of agricultural product composition. In this study, broad-based NIRS calibrations were developed for switchgrass biomass that can be used to estimate over 20 components including cell wall and soluble sugars and also ethanol production and pentose sugars released as measured using a laboratory SSF procedure. With this information, an additional 13 complex feedstock traits can be determined including theoretical and actual ethanol yields from hexose fermentation. The NIRS calibrations were used to estimate feedstock composition and conversion information for biomass samples from a multi-year switchgrass (Panicum virgatum L.) biomass cultivar evaluation trial. There were significant differences among switchgrass strains for all biomass conversion and composition traits including actual ethanol yields, ETOHL (L Mg^?1) and theoretical ethanol yields, ETOHTL (L Mg^?1), based on cell wall and non-cell wall composition NIRS analyses. ETOHL means ranged from 98 to 115 L Mg^?1 while ETOHTL means ranged from 203 to 222 L Mg^?1. Because of differences in both biomass yields and conversion efficiency, there were significant differences among strains for both actual (2,534?C3,720 L ha^?1) and theoretical (4,878?C7,888 L ha^?1) ethanol production per hectare. It should be feasible to improve ethanol yields per hectare by improving both biomass yield and conversion efficiency by using NIRS analyses to quantify differences among cultivars and management practices.     

A segmented gas/liquid delivery system for continuous culture of microorganisms on insoluble substrates and its use for growth of Ruminococcus flavefaciens on cellulose

Summary An anaerobic continuous culture device was constructed that permits accurate delivery of media containing insoluble substrates, even at very low volumetric flow rates (<3 ml/h). The system consisted of: (1) a reservoir in which the medium slurry was mixed well by the combined use of stirring and diffusive gas sparging to suspend a cellulose substrate of small (< 45 m) particle size; (2) a delivery system that segmented the slurry into small (~ 20l) discrete liquid segments separated by intervening bubbles of CO₂ gas; and (3) a stirred, temperature-controlled 2-l fermentation vessel. The device was used to examine substrate consumption, product formation, and cell yield by the anaerobic ruminal cellulolytic bacterium Ruminococcus flavefaciens FD-1 under steady-state, cellulose-limited conditions at six different dilution rates (D) ranging from 0.017 to 0.101 h^–1 (pH 6.4–6.6). Cellulose consumption decreased from 4.00 g/1 (at D=0.017 h^–1) to 2.56 g/1 (at D=0.101 h^–1). Increases in D resulted in a progressive shift toward production of more acetate and formate, and less succinate. Redox balance calculations revealed a deficiency in reduced products, probably due to the production of H₂, which was not directly measured. Reducing sugar values remained low (0.05–0.10 g/1, as glucose) at all D values. The cellulose fermentation was characterized by a low maintenance coefficient (0.07 g cellulose/g cells per hour) and a high true growth yield (Y_G = 0.24 g cells/g cellulose, corrected for maintenance). Comparison of the data with literature values suggests that the fermentation of cellulose by this organism gives cell yields at least as great as the yields obtained from the fermentation of soluble sugars.Mention of specific products is for the benefit of the reader and does not constitute an endorsement of said products over other products not mentionedOffprint requests to: P. J. Weimer     

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

Background

While advantages of biofuel have been widely reported, studies also highlight the challenges in large scale production of biofuel. Cost of ethanol and process energy use in cellulosic ethanol plants are dependent on technologies used for conversion of feedstock. Process modeling can aid in identifying techno-economic bottlenecks in a production process. A comprehensive techno-economic analysis was performed for conversion of cellulosic feedstock to ethanol using some of the common pretreatment technologies: dilute acid, dilute alkali, hot water and steam explosion. Detailed process models incorporating feedstock handling, pretreatment, simultaneous saccharification and co-fermentation, ethanol recovery and downstream processing were developed using SuperPro Designer. Tall Fescue (Festuca arundinacea Schreb) was used as a model feedstock.

Results

Projected ethanol yields were 252.62, 255.80, 255.27 and 230.23 L/dry metric ton biomass for conversion process using dilute acid, dilute alkali, hot water and steam explosion pretreatment technologies respectively. Price of feedstock and cellulose enzymes were assumed as $50/metric ton and 0.517/kg broth (10% protein in broth, 600 FPU/g protein) respectively. Capital cost of ethanol plants processing 250,000 metric tons of feedstock/year was $1.92, $1.73, $1.72 and $1.70/L ethanol for process using dilute acid, dilute alkali, hot water and steam explosion pretreatment respectively. Ethanol production cost of $0.83, $0.88, $0.81 and $0.85/L ethanol was estimated for production process using dilute acid, dilute alkali, hot water and steam explosion pretreatment respectively. Water use in the production process using dilute acid, dilute alkali, hot water and steam explosion pretreatment was estimated 5.96, 6.07, 5.84 and 4.36 kg/L ethanol respectively.

Conclusions

Ethanol price and energy use were highly dependent on process conditions used in the ethanol production plant. Potential for significant ethanol cost reductions exist in increasing pentose fermentation efficiency and reducing biomass and enzyme costs. The results demonstrated the importance of addressing the tradeoffs in capital costs, pretreatment and downstream processing technologies.     

Modelling of alcohol fermentation in a tubular reactor with high biomass recycle

Fermentation in tubular recycle reactors with high biomass concentrations is a way to boost productivity in alcohol production. A computer model has been developed to investigate the potential as well as to establish the limits of this process from a chemical engineering point of view. The model takes into account the kinetics of the reaction, the nonideality of flow and the segregation in the bioreactor. In accordance with literature, it is shown that tubular reactors with biomass recycle can improve productivity of alcohol fermentation substantially.With the help of the computer based reactor model it was also possible to estimate the detrimental effects of cell damage due to pumping. These effects are shown to play a major role, if the biomass separation is performed by filtration units which need high flow rates, e.g. tangential flow filters.List of Symbols Bo d Bodenstein number - c kg/m³ concentration of any component - CPFR continuous plug flow reactor - CSTR continuous stirred tank reactor - d _h m hydraulic diameter - D _eff m²/s dispersion coefficient - f residence time distribution function - K _s kg/m³ monod constant for biomass production - K _s kg/m³ monod constant for alcohol production - p kg/m³ product concentration - P _i kg/m³ lower inhibition limit concentration for biomass production - p _i kg/m³ lower inhibition limit concentration for alcohol production - p _m kg/m³ maximum inhibition limit concentration for biomass production - p _m kg/m³ maximum inhibition limit concentration for alcohol production - q _p h^–1 specific production rate - q _p,max h^–1 maximum specific production rate for alcohol production - q _s h^–1 specific substrate consumption rate - Q _L m _gas ³ /m³h specific gas rate - r _p , r _s , r _x kg/(m³ · h) reaction rate for ethanol production substrate consumption and cell growth, respectively - S _F kg/m³ substrate concentration in feed stream - s kg/m³ substrate concentration - t h time - x kg/m³ biomass concentration - x _max kg/m³ maximum biomass concentration for biomass production - Y _p/s yield coefficient - h^–1 specific growth rate - _max h^–1 maximum specific growth rate - dimensionless time (t/) - h mean residence time - _s glucose conversion     

The effect of pH on kinetic and yield parameters during the ethanolic fermentation of D-xylose with Pachysolen tannophilus

We have studied the ethanolic fermentation of D-xylose with Pachysolen tannophilus in batch cultures. We propose a model to predict variations in D-xylose consumed, and biomass and ethanol produced, in which we include parameters for the specific growth rate, for the consumption of D-xylose and production of ethanol either related or not to growth.The ideal initial pH for ethanol production turned out to be 4.5. At this pH value the net specific growth rate was 0.26 h^–1, biomass yield was 0.16 g.g^–1, the cell-maintenance coefficient was 0.073 g.g^–1.h^–1, the parameter for ethanol production non-related to growth was 0.064 g.g^–1,h^–1 and the maximum ethanol yield was 0.32 g.g^–1.List of Symbols A _c Carbon atomic weight - a _d1/h Specific cell-maintenance rate defined in Eq. (8) - c Mass fraction of carbon in the biomass - E g/l Ethanol concentration - f _x Correction factor defined in Eq. (13) - f _x Correction factor defined in Eq. (13) - f _xi Correction factor defined in Eq. (14) - k _d1/h Death constant - M _E Ethanol molecular weight - M _s Xylose molecular weight - M _xi Xylitol molecular weight - m g xylose/g biomass Maintenance coefficient for substrate - m _dg xylose/g biomass Maintenance coefficient when k _d - q _Eg ethanol/g biomass. Specific ethanol production rate - s g/l Residual xylose concentration - s ₀ g/l Initial xylose concentration - t h Time - x g/l Biomass concentration - x ₀ g/l Initial biomass concentration - Y _E/sg ethanol/g xylose Instantaneous ethanol yield - ¯Y _E/sg ethanol/g xylose Mean ethanol yield - Y _E ^s/T g ethanol/g xylose Theoretical ethanol yield - Y _E ^s/* g ethanol/g xylose Corrected instantaneous ethanol yield - ¯Y _E ^s/* g ethanol/g xylose Corrected mean ethanol yield - Y _x/sg biomass/g xylose Biomass yield - ¯Y _xi/sg xylitol/g xylose Mean xylitol yield Greek Letters g ethanol/g biomass Growth-associated product formation parameter - g ethanol/g biomass.h Non-growth-associated product formation parameter - _dg ethanol/g biomass.h Non-growth-associated product formation parameter when k _d0 - h Variable defined in Eq. (6) or Eq. (7) - 1/h Specific growth rate - _m1/h Maximum specific growth rate     

The impacts of deacetylation prior to dilute acid pretreatment on the bioethanol process

Background

There is currently considerable interest in developing renewable sources of energy. One strategy is the biological conversion of plant biomass to liquid transportation fuel. Several technical hurdles impinge upon the economic feasibility of this strategy, including the development of energy crops amenable to facile deconstruction. Reliable assays to characterize feedstock quality are needed to measure the effects of pre-treatment and processing and of the plant and microbial genetic diversity that influence bioconversion efficiency.

Results

We used the anaerobic bacterium Clostridium phytofermentans to develop a robust assay for biomass digestibility and conversion to biofuels. The assay utilizes the ability of the microbe to convert biomass directly into ethanol with little or no pre-treatment. Plant samples were added to an anaerobic minimal medium and inoculated with C. phytofermentans, incubated for 3 days, after which the culture supernatant was analyzed for ethanol concentration. The assay detected significant differences in the supernatant ethanol from wild-type sorghum compared with brown midrib sorghum mutants previously shown to be highly digestible. Compositional analysis of the biomass before and after inoculation suggested that differences in xylan metabolism were partly responsible for the differences in ethanol yields. Additionally, we characterized the natural genetic variation for conversion efficiency in Brachypodium distachyon and shrub willow (Salix spp.).

Conclusion

Our results agree with those from previous studies of lignin mutants using enzymatic saccharification-based approaches. However, the use of C. phytofermentans takes into consideration specific organismal interactions, which will be crucial for simultaneous saccharification fermentation or consolidated bioprocessing. The ability to detect such phenotypic variation facilitates the genetic analysis of mechanisms underlying plant feedstock quality.     

Batch fermentation kinetics of ethanol production by Zymomonas mobilis on cellulose hydrolysate

Summary Growth and ethanol production by three strains (MSN77, thermotolerant, SBE15, osmotolerant and wild type ZM4) of the bacterium Zymomonas mobilis were tested in a rich medium containing the hexose fraction from a cellulose hydrolysate (Aspen wood). The variations of yield and kinetic parameters with fermentation time revealed an inhibition of growth by the ethanol produced. This inhibition may result from the increase in medium osmolality due to ethanol formation from glucose.Nomenclature S glucose concentration (g/L) - C conversion of glucose (%) - t fermentation time (h) - q_S specific glucose uptake rate (g/g.h) - q_p specific ethanol productivity (g/g.h) - Q_p volumetric ethanol productivity (g/L.h) - Q_X volumetric biomass productivity (g/L.h) - Y_X/S biomass yield (g/g) - Y_p/S ethanol yield (g/g) - specific growth rate (h^-1)     

Agave for tequila and biofuels: an economic assessment and potential opportunities

This paper explores the economic viability of producing biofuels from Agave in Mexico and the potential for it to complement the production of tequila or mescal. We focus on Agave varieties currently being used by the tequila industry to produce two beverages, tequila and mescal, and explore the potential for biofuel production from these plants. Without competing directly with beverage production, we discuss the economic costs and benefits of converting Agave by‐products to liquid fuel as an additional value‐added product and expanding cultivation of Agave on available land. We find that the feedstock cost for biofuel from the Agave piña alone could be more than US$3 L^?1 on average. This is considerably higher than the feedstock costs of corn ethanol and sugarcane ethanol. However, there may be potential to reduce these costs with higher conversion efficiencies or by using sugar present in other parts of the plant. The costs of cellulosic biofuels using the biomass from the entire plant could be lower depending on the conversion efficiency of biomass to fuel and the additional costs of harvesting, collecting and transporting that biomass.