Dilute sulfuric acid pretreatment of agricultural and agro-industrial residues for ethanol production

Biomass Valorization of Walnut Shell for Liquefaction Efficiency

Globally, lignocellulosic biomass has great potential for industrial production of materials and products, but this resource must be used in an environmentally friendly, socially acceptable and sustainable manner. Wood and agricultural residues such as walnut shells as lignocellulosic biomass are one of the most affordable and important renewable resources in the world, which can partially replace fossil resources. The overall objective of the research is to provide background information that supports new applications of walnut shells in a biorefinery context and to increase the economic value of these non-wood forest products. This paper presents the properties characterization of liquefied biomass according to their chemical composition. All results were compared to liquefied wood. In this study, the liquefaction properties of five different walnut shell particle sizes were determined using glycerol as the liquefaction reagent under defined reaction conditions. The liquefied biomass was characterized for properties such as percentage residue, degree of liquefaction, and hydroxyl OH numbers. The chemical composition of the same biomass was investigated for its influence on the liquefaction properties. Accordingly, the main objective of this study was to determine the liquefaction properties of different particle sizes as a function of their chemical composition, also in comparison with the chemical composition of wood. The study revealed that walnut shell biomass can be effectively liquefied into glycerol using H2SO4 as the catalyst, with liquefaction efficiency ranging from 89.21 to 90.98%.

Transcriptomic analysis of formic acid stress response in Saccharomyces cerevisiae

Formic acid is a representative small molecule acid in lignocellulosic hydrolysate that can inhibit the growth of Saccharomyces cerevisiae cells during alcohol fermentation. However, the mechanism of formic acid cytotoxicity remains largely unknown. In this study, RNA-Seq technology was used to study the response of S. cerevisiae to formic acid stress at the transcriptional level. Scanning electron microscopy and Fourier transform infrared spectroscopy were conducted to observe the surface morphology of yeast cells. A total of 1504 genes were identified as being differentially expressed, with 797 upregulated and 707 downregulated genes. Transcriptomic analysis showed that most genes related to glycolysis, glycogen synthesis, protein degradation, the cell cycle, the MAPK signaling pathway, and redox regulation were significantly induced under formic acid stress and were involved in protein translation and synthesis amino acid synthesis genes were significantly suppressed. Formic acid stress can induce oxidative stress, inhibit protein biosynthesis, cause cells to undergo autophagy, and activate the intracellular metabolic pathways of energy production. The increase of glycogen and the decrease of energy consumption metabolism may be important in the adaptation of S. cerevisiae to formic acid. In addition, formic acid can also induce sexual reproduction and spore formation. This study through transcriptome analysis has preliminarily reveal the molecular response mechanism of S. cerevisiae to formic acid stress and has provided a basis for further research on methods used to improve the tolerance to cell inhibitors in lignocellulose hydrolysate.

Acetic acid acting as a signaling molecule in the quorum sensing system increases 2,3-butanediol production in Saccharomyces cerevisiae

2,3-Butanediol (2,3-BD) has been extensively used in chemical syntheses. This study aimed to explore acetic acid as a signaling molecule that activates a quorum sensing (QS) system to promote the production of 2,3-BD. The yield of 2,3-BD is proportional to the cell density. Saccharomyces cerevisiae W141 does not produce 2,3-BD when the cell density is lower than the threshold concentration (OD_600 nm = 10 or cell density 4.4 × 10⁸ CFU/mL). When 1.5 g/L acetic acid is added, the yield of 2,3-BD is 3.01 ± 0.04 g/L. Subsequently, S. cerevisiae W141 was cocultured with Acetobacter pasteurianus Huniang 1.01 under the optimal conditions, the acetic acid production was increased by 76.7% and 30.6% compared with the original strain and the strain cultivated with 1.5 g/L acetic acid, and the yield of 2,3-BD was increased by 81.9% and 3.3%, respectively. This difference is due to the activity of acetyl lactic acid synthase (ILV2) and 2,3-BD dehydrogenase (BDH1), as the relative expression of the ilv2 and bdh1 genes is increased. The results showed that the biosynthesis of 2,3-BD was regulated by acetic acid as a signaling molecule. S. cerevisiae is a promising host for producing 2,3-BD for industrial applications.

Production of bioethanol as useful biofuel through the bioconversion of water hyacinth (Eichhornia crassipes)

Water hyacinth (Eichhornia crassipes) represents a promising candidate for fuel ethanol production in tropical countries because of their high availability and high biomass yield. Bioconversion of such biomass to bioethanol could be wisely managed through proper technological approach. In this work, pretreatment of water hyacinth (10 %, w/v) with dilute sulfuric acid (2 %, v/v) at high temperature and pressure was integrated in the simulation and economic assessment of the process for further enzymatic saccharification was studied. The maximum sugar yield (425.6 mg/g) through enzymatic saccharification was greatly influenced by the solid content (5 %), cellulase load (30 FPU), incubation time (24 h), temperature (50 °C), and pH (5.5) of the saccharifying medium. Central composite design optimized an ethanol production of 13.6 mg/ml though a mixed fermentation by Saccharomyces cerevisiae (MTCC 173) and Zymomonas mobilis (MTCC 2428). Thus the experiment imparts an economic value to water hyacinths that are cleared from choking waterways.

Optimization of Two-Step Acid-Catalyzed Hydrolysis of Oil Palm Empty Fruit Bunch for High Sugar Concentration in Hydrolysate

Getting high sugar concentrations in lignocellulosic biomass hydrolysate with reasonable yields of sugars is commercially attractive but very challenging. Two-step acid-catalyzed hydrolysis of oil palm empty fruit bunch (EFB) was conducted to get high sugar concentrations in the hydrolysate. The biphasic kinetic model was used to guide the optimization of the first step dilute acid-catalyzed hydrolysis of EFB. A total sugar concentration of 83.0 g/L with a xylose concentration of 69.5 g/L and a xylose yield of 84.0% was experimentally achieved, which is in well agreement with the model predictions under optimal conditions (3% H2SO4and 1.2% H3PO4, w/v, liquid to solid ratio 3 mL/g, 130°C, and 36 min). To further increase total sugar and xylose concentrations in hydrolysate, a second step hydrolysis was performed by adding fresh EFB to the hydrolysate at 130°C for 30 min, giving a total sugar concentration of 114.4 g/L with a xylose concentration of 93.5 g/L and a xylose yield of 56.5%. To the best of our knowledge, the total sugar and xylose concentrations are the highest among those ever reported for acid-catalyzed hydrolysis of lignocellulose.

Hydrolysis of various thai agricultural biomasses using the crude enzyme from Aspergillus aculeatus iizuka FR60 isolated from soil

In this study, forty-two fungi from soil were isolated and tested for their carboxymethyl cellulase (CMCase) and xylanase activities. From all isolates, the fungal isolate FR60, which was identified as Aspergillus aculeatus Iizuka, showed high activities in both CMCase and xylanase with 517 mU/mg protein and 550 mU/mg protein, respectively. The crude enzyme from A. aculeatus Iizuka FR60 could hydrolyze several agricultural residues such as corncob, and sweet sorghum leaf and stalk at comparable rates with respect to the tested commercial enzymes and with a maximum rate in rice hull hydrolysis (29 μg sugar g(-1) dry weight substrate mg(-1) enzyme hr(-1)). The highest amount of glucose was obtained from corncob by using the crude enzyme from A. aculeatus Iizuka FR60 (10.1 g/100 g dry substrate). From overall enzymatic treatment results, the lowest sugar yield was from rice hulls treatment (1.6 g/100 g dry weight) and the highest amount of reducing sugar was obtained from rice straw treatment (15.3 g/100 g dry weight). Among tested agricultural wastes, rice hull could not be effectively hydrolyzed by enzymes, whereas sugarcane leaf and stalk, and peanut shell could be effectively hydrolyzed (30-31% total sugar comparing with total sugar yield from acid treatment).

Metabolic engineering of Saccharomyces cerevisiae for increased bioconversion of lignocellulose to ethanol

The absence of pentose-utilizing enzymes in Saccharomyces cerevisiae is an obstacle for efficiently converting lignocellulosic materials to ethanol. In the present study, the genes coding xylose reductase (XYL1) and xylitol dehydrogenase (XYL2) from Pichia stipitis were successfully engineered into S. cerevisae. As compared to the control transformant, engineering of XYL1 and XYL2 into yeasts significantly increased the microbial biomass (8.1 vs. 3.4 g/L), xylose consumption rate (0.15 vs. 0.02 g/h) and ethanol yield (6.8 vs. 3.5 g/L) after 72 h fermentation using a xylose-based medium. Interestingly, engineering of XYL1 and XYL2 into yeasts also elevated the ethanol yield from sugarcane bagasse hydrolysate (SUBH). This study not only provides an effective approach to increase the xylose utilization by yeasts, but the results also suggest that production of ethanol by this recombinant yeasts using unconventional nutrient sources, such as components in SUBH deserves further attention in the future.

Soybean hulls pretreated using thermo-mechanical extrusion--hydrolysis efficiency, fermentation inhibitors, and ethanol yield

Soybean hulls were subjected to thermo-mechanical extrusion pretreatment at various in-barrel moisture contents and screw speeds. Extrusion degraded the lignocellulosic structure and enhanced enzymatic hydrolysis of soybean hulls, with up to 155% increase in glucose yield as compared to untreated substrate. Greater glucose yields were observed at higher in-barrel moistures (45% and 50%) and lower screw speed (280 and 350 rpm). Maximum 74% cellulose to glucose conversion resulted from using a two-enzyme cocktail consisting of cellulase and β-glucosidase. Conversion increased to 87% when a three-enzyme cocktail having a cell wall degrading enzyme complex was used for hydrolysis. Fermentation inhibitors, such as furfural, 5-(hydroxymethyl)-2-furaldehyde (HMF), and acetic acid, were found in the extrusion pretreated soybean hulls and hydrolysate. However, their concentrations were below the known thresholds for inhibition. Fermentation of hydrolysate by Saccharomyces cerevisiae led to high yields of ethanol, with concentration ranging from 13.04 to 15.44 g/L.

Dilute acid pretreatment and enzymatic saccharification of sugarcane tops for bioethanol production

The aim of this work was to study the feasibility of using sugarcane tops as feedstock for the production of bioethanol. The process involved the pretreatment using acid followed by enzymatic saccharification using cellulases and the process was optimized for various parameters such as biomass loading, enzyme loading, surfactant concentration and incubation time using Box-Behnken design. Under optimum hydrolysis conditions, 0.685 g/g of reducing sugar was produced per gram of pretreated biomass. The fermentation of the hydrolyzate using Saccharomyces cerevisae produced 11.365 g/L of bioethanol with an efficiency of about 50%. This is the first report on utilization of sugarcane tops for bioethanol production.

Metabolic pathway engineering based on metabolomics confers acetic and formic acid tolerance to a recombinant xylose-fermenting strain of Saccharomyces cerevisiae

Background

The development of novel yeast strains with increased tolerance toward inhibitors in lignocellulosic hydrolysates is highly desirable for the production of bio-ethanol. Weak organic acids such as acetic and formic acids are necessarily released during the pretreatment (i.e. solubilization and hydrolysis) of lignocelluloses, which negatively affect microbial growth and ethanol production. However, since the mode of toxicity is complicated, genetic engineering strategies addressing yeast tolerance to weak organic acids have been rare. Thus, enhanced basic research is expected to identify target genes for improved weak acid tolerance.

Results

In this study, the effect of acetic acid on xylose fermentation was analyzed by examining metabolite profiles in a recombinant xylose-fermenting strain of Saccharomyces cerevisiae. Metabolome analysis revealed that metabolites involved in the non-oxidative pentose phosphate pathway (PPP) [e.g. sedoheptulose-7-phosphate, ribulose-5-phosphate, ribose-5-phosphate and erythrose-4-phosphate] were significantly accumulated by the addition of acetate, indicating the possibility that acetic acid slows down the flux of the pathway. Accordingly, a gene encoding a PPP-related enzyme, transaldolase or transketolase, was overexpressed in the xylose-fermenting yeast, which successfully conferred increased ethanol productivity in the presence of acetic and formic acid.

Conclusions

Our metabolomic approach revealed one of the molecular events underlying the response to acetic acid and focuses attention on the non-oxidative PPP as a target for metabolic engineering. An important challenge for metabolic engineering is identification of gene targets that have material importance. This study has demonstrated that metabolomics is a powerful tool to develop rational strategies to confer tolerance to stress through genetic engineering.

Green processing of tropical banagrass into biofuel and biobased products: an innovative biorefinery approach

Banagrass (Pennisetum purpureum) approximately 4 months old was hand-harvested and shredded. Half of the sample was dewatered using a screw-press, whereby the extracted juice was used for cultivating an edible fungus, Rhizopus microsporus, for aquaculture feed supplementation. The remaining biomass was divided into four separate streams: (1) wet, juiced; (2) dry, juiced; (3) wet, unjuiced; and (4) dry, unjuiced. Each stream was pretreated with dilute sulfuric acid and compared on the basis of sugar release at varying acid concentrations, temperatures, and residence times. Wet, juiced banagrass released the most soluble sugars (theoretical xylose and ∼85% glucose). Ultrasonication (20 kHz) was applied to further increase monomeric sugar release but demonstrated little improvement on total sugar yields. Fungal biomass generated from banagrass juice exhibited potential as a fungal-protein production medium producing 1.16±0.34 g biomass increase/g initial biomass.

Elucidating acetate tolerance in E. coli using a genome-wide approach

Engineering organisms for improved performance using lignocellulose feedstocks is an important step towards a sustainable fuel and chemical industry. Cellulosic feedstocks contain carbon and energy in the form of cellulosic and hemicellulosic sugars that are not metabolized by most industrial microorganisms. Pretreatment processes that hydrolyze these polysaccharides often also result in the accumulation of growth inhibitory compounds, such as acetate and furfural among others. Here, we have applied a recently reported strategy for engineering tolerance towards the goal of increasing Escherichia coli growth in the presence of elevated acetate concentrations (Lynch et al., 2007). We performed growth selections upon an E. coli genome library developed using a moderate selection pressure to identify genomic regions implicated in acetate toxicity and tolerance. These studies identified a range of high-fitness genes that are normally involved in membrane and extracellular processes, are key regulated steps in pathways, and are involved in pathways that yield specific amino acids and nucleotides. Supplementation of the products and metabolically related metabolites of these pathways significantly increased growth rate (a 130% increase in specific growth) at inhibitory acetate concentrations. Our results suggest that acetate tolerance will not involve engineering of a single pathway; rather we observe a range of potential mechanisms for overcoming acetate based inhibition.

Cellulosic hydrolysate toxicity and tolerance mechanisms in Escherichia coli

The sustainable production of biofuels will require the efficient utilization of lignocellulosic biomass. A key barrier involves the creation of growth-inhibitory compounds by chemical pretreatment steps, which ultimately reduce the efficiency of fermentative microbial biocatalysts. The primary toxins include organic acids, furan derivatives, and phenolic compounds. Weak acids enter the cell and dissociate, resulting in a drop in intracellular pH as well as various anion-specific effects on metabolism. Furan derivatives, dehydration products of hexose and pentose sugars, have been shown to hinder fermentative enzyme function. Phenolic compounds, formed from lignin, can disrupt membranes and are hypothesized to interfere with the function of intracellular hydrophobic targets. This review covers mechanisms of toxicity and tolerance for these compounds with a specific focus on the important industrial organism Escherichia coli. Recent efforts to engineer E. coli for improved tolerance to these toxins are also discussed.

Cellulase production from spent lignocellulose hydrolysates by recombinant Aspergillus niger

A recombinant Aspergillus niger strain expressing the Hypocrea jecorina endoglucanase Cel7B was grown on spent hydrolysates (stillage) from sugarcane bagasse and spruce wood. The spent hydrolysates served as excellent growth media for the Cel7B-producing strain, A. niger D15[egI], which displayed higher endoglucanase activities in the spent hydrolysates than in standard medium with a comparable monosaccharide content (e.g., 2,100 nkat/ml in spent bagasse hydrolysate compared to 480 nkat/ml in standard glucose-based medium). In addition, A. niger D15[egI] was also able to consume or convert other lignocellulose-derived compounds, such as acetic acid, furan aldehydes, and phenolic compounds, which are recognized as inhibitors of yeast during ethanolic fermentation. The results indicate that enzymes can be produced from the stillage stream as a high-value coproduct in second-generation bioethanol plants in a way that also facilitates recirculation of process water.