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Applied Biochemistry and Biotechnology: Part A: Enzyme Engineering and Biotechnology (v.105, #1-3)
Feedstock production, genetic modification, and processing
by Sharon P. Shoemaker; Lynn L. Wright (pp. 3-4).
Rapid biomass analysis by Bonnie R. Hames; Steven R. Thomas; Amie D. Sluiter; Christine J. Roth; David W. Templeton (pp. 5-16).
New, rapid, and inexpensive methods that monitor the chemical composition of corn stover and corn stover-derived samples are a key element to enabling the commercialization of processes that convert stover to fuels and chemicals. These new techniques combine near infrared (NIR) spectroscopy and projection to latent structures (PLS) multivariate analysis to allow the compositional analysis of hundreds of samples in 1 d at a cost of about $10 each. The new NIR/PLS rapid analysis methods can also be used to support a variety of research projects that would have been too costly to pursue by traditional methods.
Keywords: Corn stover; near infrared spectroscopy; projection to latent structures; feedstock; multivariate analysis
Xylem-specific and tension stress-responsive expression of cellulose synthase genes from aspen trees by Chandrashekhar P. Joshi (pp. 17-25).
Genetic improvement of cellulose biosynthesis in woody trees is one of the major goals of tree biotechnology research. Yet, progress in this field has been slow owing to (1) unavailability of key genes from tree genomes, (2) the inability to isolate active and intact cellulose synthase complexes and, (3) the limited understanding of the mechanistic processes involved in the wood cellulose development. Here I report on the recent advances in molecular genetics of cellulose synthases (CesA) from aspen trees. Two different types of cellulose synthases appear to be involved in cellulose deposition in primary and secondary walls in aspen xylem. The three distinct secondary CesAs from aspen—PtrCesA1, PtrCesA2, and PtrCesA3—appear to be aspen homologs of Arabidopsis secondary CesAs AtCesA8, AtCesA7, and AtCesA4, respectively, based on their high identity/similarity (>80%). These aspen CesA proteins share the transmembrane domain (TMD) structure that is typical of all known “true” CesA proteins: two TMDs toward the N-terminal and six TMDs toward the C-terminal. The putative catalytic domain is present between TMDs 2 and 3. All signature motifs of processive glycosyltransferases are also present in this catalytic domain. In a phylogenetic tree based on various predicted CesA proteins from Arabidopsis and aspen, aspen CesAs fall into families similar to those seen with Arabidopsis CesAs, suggesting their functional similarity. The coordinate expression of three aspen secondary CesAs in xylem and phloem fibers, along with their simultaneous tension stress-responsive upregulation, suggests that these three CesAs may play a pivotal role in biosynthesis of better-quality cellulose in secondary cell walls of plants. These results are likely to have a direct impact on genetic manipulation of trees in the future.
Keywords: Aspen; cellulose biosynthesis; cellulose synthase; trees; wood development
Microbial pretreatment of biomass by Fred A. Keller; Jenny E. Hamilton; Quang A. Nguyen (pp. 27-41).
Typical pretreatment requires high-energy (steam and electricity) and corrosion-resistant, high-pressure reactors. A review of the literature suggests that fungal pretreatment could potentially lower the severity requirements of acid, temperature and time. These reductions in severity are also expected to result in less biomass degradation and consequently lower inhibitor concentrations compared to conventional thermochemical pretreatment. Furthermore, potential advantages of fungal pretreatment of agricultural residues, such as corn stover, are suggested by its effectiveness in improving the cellulose digestibility of many types of forage fiber and agricultural wastes. Our preliminary tests show a three- to five-fold improvement in enzymatic cellulose digestibility of corn stover after pretreatment with Cyathus stercoreus; and a ten- to 100-fold reduction in shear force needed to obtain the same shear rate of 3.2 to 7 rev/s, respectively, after pretreatment with Phanerochaete chrysosporium.
Keywords: Microbial pretreatment; fungal pretreatment; corn stover; enzymatic hydrolysis
Physical separation of straw stem components to reduce silica by J. Richard Hess; David N. Thompson; Reed L. Hoskinson; Peter G. Shaw; Duane R. Grant (pp. 43-51).
In this paper, we describe ongoing efforts to solve challenges to using straw for bioenergy and bioproducts. Among these, silica in straw forms a low-melting eutectic with potassium, causing slag deposits, and chlorides cause corrosion beneath the deposits. Straw consists principally of stems, leaves, sheaths, nodes, awns, and chaff. Leaves and sheaths are higher in silica, while chaff, leaves, and nodes are the primary sources of fines. Our approach to reducing silica is to selectively harvest the straw stems using an in-field physical separation, leaving the remaining components in the field to build soil organic matter and contribute soil nutrients.
Keywords: Wheat straw; silica; selective harvest; bioenergy; combustion; whole crop utilization
Application of a depolymerization model for predicting thermochemical hydrolysis of hemicellulose by Todd Lloyd; Charles E. Wyman (pp. 53-67).
Literature data were collected and analyzed to guide selection of conditions for pretreatment by dilute acid and water-only hemicellulose hydrolysis, and the severity parameter was used to relate performance of different studies on a consistent basis and define attractive operating conditions. Experiments were then run to confirm performance with corn stover. Although substantially better hemicellulose sugar yields are observed when acid is added, costs would be reduced and processing operations simplified if less acid could be used while maintaining good yields, and understanding the relationship between operating conditions and yields would be invaluable to realizing this goal. However, existing models seldom include the oligomeric intermediates prevalent at lower acid levels, and the few studies that include such species do not account for the distribution of chain lengths during reaction. Therefore, the polymeric nature of hemicellulose was integrated into a kinetic model often used to describe the decomposition of synthetic polymers with the assumption that hemicellulose linkages are randomly broken during hydrolysis. Predictions of monomer yields were generally consistent with our pretreatment data, data reported in the literature, and predictions of other models, but the model tended to overpredict oligomer yields. These differences need to be resolved by gathering additional data and improving the model.
Keywords: Hemicellulose; hydrolysis; kinetic model; dilute acid; depolymerization
Dilute-sulfuric acid pretreatment of corn stover in pilot-scale reactor by Daniel J. Schell; Jody Farmer; Millie Newman; James D. McMillan (pp. 69-85).
Corn stover is a domestic feedstock that has potential to produce significant quantities of fuel ethanol and other bioenergy and biobased products. However, comprehensive yield and carbon mass balance information and validated kinetic models for dilute-sulfuric acid (H2SO4) pretreatment of corn stover have not been available. This has hindered the estimation of process economics and also limited the ability to perform technoeconomic modeling to guide research. To better characterize pretreatment and assess its kinetics, we pretreated corn stover in a continuous 1 t/d reactor. Corn stover was pretreated at 20% (w/w) solids concentration over a range of conditions encompassing residence times of 3–12 min, temperatures of 165–195°C, and H2SO4 concentrations of 0.5–1.4% (w/w). Xylan conversion yield and carbon mass balance data were collected at each run condition. Performance results were used to estimate kinetic model parameters assuming biphasic hemicellulose hydrolysis and a hydrolysis mechanism incorporating formation of intermediate xylo-oligomers. In addition, some of the pretreated solids were tested in a simultaneous saccharification and fermentation (SSF) process to measure the reactivity of their cellulose component to enzymatic digestion by cellulase enzymes. Monomeric xylose yields of 69–71% and total xylose yields (monomers and oligomers) of 70–77% were achieved with performance level depending on pretreatment severity. Cellulose conversion yields in SSF of 80–87% were obtained for some of the most digestible pretreated solids.
Keywords: Pretreatment; dilute-sulfuric acid; enzymatic conversion; corn stover; xylan conversion; kinetics; pilot scale
Hydrothermal pretreatment conditions to enhance ethanol production from poplar biomass by Maria José Negro; Paloma Manzanares; Ignacio Ballesteros; Jose Miguel Oliva; Araceli Cabañas; Mercedes Ballesteros (pp. 87-100).
Pretreatment has been recognized as a key step in enzyme-based conversion processes of lignocellulose biomass to ethanol. The aim of this study is to evaluate two hydrothermal pretreatments (steam explosion and liquid hot water) to enhance ethanol production from poplar (Populus nigra) biomass by a simultaneous saccharification and fermentation (SSF) process. The composition of liquid and solid fractions obtained after pretreatment, enzymatic digestibility, and ethanol production of poplar biomass pretreated at different experimental conditions was analyzed. The best results were obtained in steam explosion pretreatment at 210°C and 4 min, taking into account cellulose recovery above 95%, enzymatic hydrolysis yield of about 60%, SSF yield of 60% of theoretical, and 41% xylose recovery in the liquid fraction. Large particles can be used for poplar biomass in both pretreatments, since no significant effect of particle size on enzymatic hydrolysis and SSF was obtained.
Keywords: Poplar; ethanol; pretreatment; steam explosion; liquid hot water
Estimation of temperature transients for biomass pretreatment in tubular batch reactors and impact on xylan hydrolysis kinetics by Suzanne L. Stuhler; Charles E. Wyman (pp. 101-114).
A combined heat transfer/kinetic model was developed to quantify temperature variations in small tubular batch reactors and estimate the effect of deviations from isothermal operation on the kinetics of biomass pretreatment. Assuming that heat transfer was dominated by conduction in the radial direction, a classic parabolic time-dependent partial differential equation was applied to describe the temperature in the system and dedimensionalized to provide a single solution for application to all situations. A dimensionless expression for the reaction kinetics for xylan hydrolysis was then developed, and a single parameter expressed as the dimensionless ratio of the first-order rate constant times the tube radius squared divided by the thermal diffusivity was found to control the reaction rate. Three different characterizations of the deviation between the concentration profile predicted for isothermal xylan hydrolysis and that based on the transient temperature were directly related to this dimensionless rate constant parameter for both catalyzed and uncatalyzed hydrolysis kinetics. These results were then used to project the relationship between deviations in yield from isothermal results and the tube radius and reaction time.
Keywords: Reactor design; heat transfer; kinetics; hydrolysis; pretreatment
Effect of sulfuric and phosphoric acid pretreatments on enzymatic hydrolysis of corn stover by Byung-Hwan Um; M. Nazmul Karim; Linda L. Henk (pp. 115-125).
The pretreatment of corn stover with H2SO4 and H3PO4 was investigated. Pretreatments were carried out from 30 to 120 min in a batch reactor at 121°C, with acid concentrations ranging from 0 to 2% (w/v) at a solid concentration of 5% (w/v). Pretreated corn stover was washed with distilled water until the filtrate was adjusted to pH 7.0, followed by surfactant swelling of the cellulosic fraction in a 0–10% (w/v) solution of Tween-80 at room temperature for 12 h. The dilute acid treatment proved to be a very effective method in terms of hemicellulose recovery and cellulose digetibility. Hemicellulose recovery was 62–90%, and enzymatic digestibility of the cellulose that remained in the solid was >80% with 2% (w/v) acid. In all cases studied, the performance of H2SO4 pretreatment (hemicellulose recovery and cellulose digestibility) was significantly better than obtained with H3PO4. Enzymatic hydrolysis was more effective using surfactant than without it, producing 10–20% more sugar. Furthermore, digestibility was investigated as a function of hemicellulose removal. It was found that digestibility was more directly related to hemicellulose removal than to delignification.
Keywords: Corn stover; enzymatic hydrolysis; H3PO4 ; pretreatment; H2SO4
Combined use of H2SO4 and SO2 impregnation for steam pretreatment of spruce in ethanol production by Johanna Söderström; Linda Pilcher; Mats Galbe; Guido Zacchi (pp. 127-140).
Fuel ethanol can be produced from softwood through hydrolysis in an enzymatic process. Prior to enzymatic hydrolysis of the softwood, pretreatment is necessary. In this study, two-step steam pretreatment employing dilute H2SO4 impregnation in the first step and SO2 impregnation in the second step, to improve the overall sugar and ethanol yield, was investigated. The first pretreatment step was performed under conditions of low severity (180°C, 10 min, 0.5% H2SO4) to optimize the amount of hydrolyzed hemicellulose. In the second step, the washed solid material from the first pretreatment step was impregnated with SO2 and pretreated under conditions of higher severity to make the cellulose more accessible to enzymatic attack, as well as to hydrolyze a portion of the cellulose. A wide range of conditions was used in the second step to determine the most favorable combination. The temperatures investigated were between 190 and 230°C, the residence times were 2, 5, and 10 min; and the SO2 concentration was 3%. The effect of pretreatment was assessed by both enzymatic hydrolysis of the solids and by simultaneous saccharification and fermentation (SSF) of the whole slurry, after the second pretreatment step. For each set of pretreatment conditions, the liquid fraction was also fermented to determine any inhibitory effects. Ethanol yield using the SSF configuration reached 66% of the theoretical value for pretreatment conditions in the second step of 210°C and 5 min. The sugar yield using the separate hydrolysis and fermentation configuration reached 71% for pretreatment conditions of 220°C and 5 min.
Keywords: Enzymatic hydrolysis; softwood; simultaneous saccharification and fermentation; separate hydrolysis and fermentation
Effect of lignocellulosic degradation compounds from steam explosion pretreatment on ethanol fermentation by thermotolerant yeast Kluyveromyces marxianus by Jose Miguel Oliva; Felicia Sáez; Ignacio Ballesteros; Alberto González; Maria José Negro; Paloma Manzanares; Mercedes Ballesteros (pp. 141-153).
The filtrate from steam-pretreated poplar was analyzed to identify degradation compounds. The effect of selected compounds on growth and ethanolic fermentation of the thermotolerant yeast strain Kluyveromyces marxianus CECT 10875 was tested. Several fermentations on glucose medium, containing individual inhibitory compounds found in the hydrolysate, were carried out. The degree of inhibition on yeast strain growth and ethanolic fermentation was determined. At concentrations found in the prehy-drolysate, none of the individual compounds significantly affected the fermentation. For all tested compounds, growth was inhibited to a lesser extent than ethanol production. Lower concentrations of catechol (0.96 g/L) and 4-hydroxybenzaldehyde (1.02 g/L) were required to produce the 50% reduction in cell mass in comparison to other tested compounds.
Keywords: Ethanol production; Kluyveromyces marxianus ; poplar biomass; inhibitors; fermentation
Enzymatic hydrolysis of ammonia-treated rice straw by Betzabé Sulbarán-de-Ferrer; Marielena Aristiguieta; Bruce E. Dale; Alexis Ferrer; Graciela Ojeda-de-Rodriguez (pp. 155-164).
Rice straw pretreated with liquid anhydrous ammonia was hydrolyzed with cellulase, cellobiase, and hemicellulase. Ammonia-processing conditions were 1.5 g of NH3/g of dry matter, 85°C, and several sample moisture contents. There were four ammonia addition time (min)-processing time (min) combinations. Sugars produced were analyzed as reducing sugars (dinitrosalicylic acid method) and by high-performance liquid chromatography. Monomeric sugars increased from 11% in the nontreated rice straw to 61% of theoretical in treated rice straw (79.2% conversion as reducing sugars). Production of monosaccharides was greater at higher moisture content and was processing time dependent. Glucose was the monosaccharide produced in greater amounts, 56.0%, followed by xylose, arabinose, and fructose, with 35.8, 6.6, and 1.4%, respectively.
Keywords: Sugars; rice straw; ammonia treatment
Effects of temperature and moisture on dilute-acid steam explosion pretreatment of corn stover and cellulase enzyme digestibility by Melvin P. Tucker; Kyoung H. Kim; Mildred M. Newman; Quang A. Nguyen (pp. 165-177).
Corn stover is emerging as a viable feedstock for producing bioethanol from renewable resources. Dilute-acid pretreatment of corn stover can solubilize a significant portion of the hemicellulosic component and enhance the enzymatic digestibility of the remaining cellulose for fermentation into ethanol. In this study, dilute H2SO4 pretreatment of corn stover was performed in a steam explosion reactor at 160°C, 180°C, and 190°C, approx 1 wt% H2SO4, and 70-s to 840-s residence times. The combined severity (Log10 [R o ] - pH), an expression relating pH, temperature, and residence time of pretreatment, ranged from 1.8 to 2.4. Soluble xylose yields varied from 63 to 77% of theoretical from pretreatments of corn stover at 160 and 180°C. However, yields >90% of theoretical were found with dilute-acid pretreatments at 190°C. A narrower range of higher combined severities was required for pretreatment to obtain high soluble xylose yields when the moisture content of the acid-impregnated feedstock was increased from 55 to 63 wt%. Simultaneous saccharification and fermentation (SSF) of washed solids from corn stover pretreated at 190°C, using an enzyme loading of 15 filter paper units (FPU)/g of cellulose, gave ethanol yields in excess of 85%. Similar SSF ethanol yields were found using washed solid residues from 160 and 180°C pretreatments at similar combined severities but required a higher enzyme loading of approx 25 FPU/g of cellulose.
Keywords: Pretreatment; dilute-acid; acid hydrolysis; corn stover; enzymatic hydrolysis
Sugar monomer and oligomer solubility by Matthew C. Gray; Alvin O. Converse; Charles E. Wyman (pp. 179-193).
Oligomer solubility could potentially play an important role in controlling the rates and yields in the thermochemical hydrolysis of hemicellulose as a pretreatment for subsequent enzymatic conversion of cellulose. However, limited data or models are available to describe the aqueous solubility of sugar monomers and oligomers. In this work, we measured the solubilities of sugars common to many biomass feedstocks in the temperature range of 25–30°C. Then we reviewed solubility models for sugars from the open literature. Finally, we applied models to test their ability to describe this and other data reported in the literature. It was found that the solubility of sugar monomers was not well described by the ideal solubility law or other more complex models. However, with an empirical adjustment to the enthalpy of fusion, the ideal solubility law was able to approximately predict the solubility of cello-oligomers. Based on these results, solubilities for low molecular weight xylo-oligomers are predicted to investigate their possible importance in pretreatment and define further experimental measurements needed to improve our understanding of sugar and oligomer solubility.
Keywords: Hydrolysis; oligomers; pretreatment; solubility; sugars
Influence of pressure in ethanol/water pulping of sugarcane bagasse by Adilson R. Gonçalves; Denise S. Ruzene (pp. 195-204).
The influence of the pressure in the ethanol/water pulping of sugarcane bagasse was studied using argon pressure varying from 0.5 to 1.5 MPa. The reaction volume and activation volume were studied. For the reaction volume, temperature and time were constant and pressure was varied, and for the activation volume, temperature was constant and pressure and time were varied. The degradation of cellulose was not promoted by the pressure with positive reaction volume (4100 cm3/mol). On the other hand, degradation of xylan (polyoses) and lignin was strongly favored by the pressure and reaction volume ranged from −1000 to −3000 cm3/mol.
Keywords: Ethanol/water pulping; sugarcane bagasse; pressure; reaction volume; activation volume
Post-harvest processing methods for reduction of silica and alkali metals in wheat straw by David N. Thompson; Peter G. Shaw; Jeffrey A. Lacey (pp. 205-218).
Silica and alkali metals in wheat straw limit its use for bioenergy and gasification. Slag deposits occur via the eutectic melting of SiO2 with K2O, trapping chlorides at surfaces and causing corrosion. A minimum melting point of 950°C is desirable, corresponding to an SiO2:K2O weight ratio of about 3:1. Mild chemical treatments were used to reduce Si, K, and Cl, while varying temperature, concentration, % solids, and time. Dilute acid was more effective at removing K and Cl, while dilute alkali was more effective for Si. Reduction of minerals in this manner may prove economical for increasing utilization of the straw for combustion or gasification.
Keywords: Bioenergy; combustion; gasification; fluidized bed; silica; potassium; chloride; slagging
Composition and ethanol production potential of cotton gin residues by Foster A. Agblevor; Sandra Batz; Jessica Trumbo (pp. 219-230).
Cotton gin residue (CGR) collected from five cotton gins was fractionated and characterized for summative composition. The major fractions of the CGR varied widely between cotton gins and consisted of clean lint (5–12%), hulls (16–48%), seeds (6–24%), motes (16–24%), and leaves (14–30%). The summative composition varied within and between cotton gins and consisted of ash (7.9–14.6%), acid-insoluble material (18–26%), xylan (4–15%), and cellulose (20–38%). Overlimed steam-exploded cotton gin waste was readily fermented to ethanol by Escherichia coli KO11. Ethanol yields were feedstock and severity dependent and ranged from 58 to 92.5% of the theoretical yields. The highest ethanol yield was 191 L (50 gal)/t, and the lowest was 120 L (32 gal)/t.
Keywords: Cotton gin waste; steam explosion; characterization; summative composition
Wood-ethanol for climate change mitigation in Canada by Peter J. Graham; David J. Gregg; John N. Saddler (pp. 231-242).
The impetus for this paper is Canada’s commitment under the United Nations Framework Convention on Climate Change to reduce national greenhouse gas emissions as well as reducing our dependency on fossil fuels. Wood-based ethanol offers an excellent opportunity for greenhouse gas mitigation due to market potential, an ability to offset significant emissions from the transportation sector, a reduction of emissions from CO2-intensive waste-management systems, and carbon sequestration in afforested plantations. While there are technological and economic barriers to overcome, using wood-biomass as a source of ethanol can be an economically viable tool for reducing greenhouse gas levels in the atmosphere. This paper examines the costs and mitigation potential of the production of ethanol from biomass supplied from industrial wood waste as well as from trees harvested from afforested land.
Keywords: Ethanol; greenhouse gas; afforestation; wood waste; economics
Microbial catalysis and metabolic engineering
by Nancy W. Y. Ho; Francisco F. Roberto (pp. 245-246).
Saccharification of marine microalgae using marine bacteria for ethanol production by Mitsufumi Matsumoto; Hiroko Yokouchi; Nobukazu Suzuki; Hiroshi Ohata; Tadashi Matsunaga (pp. 247-254).
The saccharification of marine microalgae using amylase from marine bacteria in saline conditions was investigated. An amylase-producing bacterium, Pseudoalterimonas undina NKMB 0074 was isolated and identified. The green microalga NKG 120701 was determined to have the highest concentration of intracellular carbohydrate and was found from our algal culture stocks. P. undina NKMB 0074 was inoculated into suspensions containing NKG 120701 cells and increasingly reduced suspended sugars with incubation time. Terrestrial amylase and glucoamylase were inactive in saline suspension. Therefore, marine amylase is necessary in saline conditions for successful saccharification of marine microalgae.
Keywords: Saccharification; marine algae; marine bacteria; amylase; ethanol; biomass