Bio-ethanol--the fuel of tomorrow from the residues of today

Bioethanol Production from Azolla filiculoides by Saccharomyces cerevisiae, Pichia stipitis, Candida lusitaniae, and Kluyveromyces marxianus

Ethanol was produced by separate hydrolysis and fermentation using Azolla filiculoides as a biomass. Thermal acid hydrolysis and enzymatic saccharification were used as pretreatment methods to produce monosaccharides from Azolla. The optimal content for thermal acid hydrolysis of 14% (w/v) Azolla weed slurry produced 16.7-g/L monosaccharides by using 200 mM H₂SO₄ at 121 °C for 60 min. Enzymatic saccharification using 16 U/mL Viscozyme produced 61.6 g/L monosaccharide at 48 h. Ethanol productions with ethanol yield coefficients from Azolla weed hydrolysate using Kluyveromyces marxianus, Candida lusitaniae Saccharomyces cerevisiae, and Pichia stipitis were 26.8 g/L (Y_EtOH = 0.43), 23.2 g/L (Y_EtOH = 0.37), 18.2 g/L (Y_EtOH = 0.29), and 13.7 g/L (Y_EtOH = 0.22), respectively. Saccharomyces cerevisiae produces the lowest yield as it utilized only glucose. Bioethanol from Azolla weed hydrolysate can be successfully produced by using Kluyveromyces marxianus because it consumed the mixture of glucose and xylose completely within 60 h.

Comprehensive assessment of 2G bioethanol production

The advancements in second-generation bioethanol produced from lignocellulosic biomass, such as crops residues, woody crops or energy grasses are gaining momentum. Though, they are still representing less than 3% of total bioethanol production, the GHG reduction potential is higher than for 1G-bioethanol. The environmental impacts of bioethanol production are totally dependent on feedstock availability and conversion technology. The biochemical conversion route must overcome several technological and economical challenges such as pre-treatment, fermentation, hydrolysis process and separation. A completely mature technology is still to be developed and must adapted to the nature of the feedstock. Nevertheless, using process simulation software, Life Cycle Assessment and integrating the different steps of bioresource harvesting and treatment processes, including the energy balances and the water requirements, it is shown that 2G bioethanol production will reduce environmental impacts provided the evaluation addresses a long-time perspective, including all conversion steps and the regeneration of the bioresource.

Fermentation profiles of the yeast Brettanomyces bruxellensis in d-xylose and l-arabinose aiming its application as a second-generation ethanol producer

The yeast Brettanomyces bruxellensis is able to ferment the main sugars used in first-generation ethanol production. However, its employment in this industry is prohibitive because the ethanol productivity reached is significantly lower than the observed for Saccharomyces cerevisiae. On the other hand, a possible application of B. bruxellensis in the second-generation ethanol production has been suggested because this yeast is also able to use d-xylose and l-arabinose, the major pentoses released from lignocellulosic material. Although the latter application seems to be reasonable, it has been poorly explored. Therefore, we aimed to evaluate whether or not different industrial strains of B. bruxellensis are able to ferment d-xylose and l-arabinose, both in aerobiosis and oxygen-limited conditions. Three out of nine tested strains were able to assimilate those sugars. When in aerobiosis, B. bruxellensis cells exclusively used them to support biomass formation, and no ethanol was produced. Moreover, whereas l-arabinose was not consumed under oxygen limitation, d-xylose was only slightly used, which resulted in low ethanol yield and productivity. In conclusion, our results showed that d-xylose and l-arabinose are not efficiently converted to ethanol by B. bruxellensis, most likely due to a redox imbalance in the assimilatory pathways of these sugars. Therefore, despite presenting other industrially relevant traits, the employment of B. bruxellensis in second-generation ethanol production depends on the development of genetic engineering strategies to overcome this metabolic bottleneck.

Zmo0994, a novel LEA-like protein from Zymomonas mobilis, increases multi-abiotic stress tolerance in Escherichia coli

BACKGROUND

Pretreatment processes and subsequent enzymatic hydrolysis are prerequisites to utilize lignocellulosic sugar for fermentation. However, the resulting hydrolysate frequently hinders fermentation processes due to the presence of inhibitors and toxic products (e.g., ethanol). Thus, it is crucial to develop robust microbes conferring multi-stress tolerance.

RESULTS

Zmo0994, a functionally uncharacterized protein from Zymomonas mobilis, was identified and characterized for the first time. A major effect of Zmo0994 was a significant enhancement in the tolerance to abiotic stresses such as ethanol, furfural, 5'-hydroxymethylfurfural and high temperature, when expressed in Escherichia coli. Through transcriptome analysis and in vivo experiments, the cellular mechanism of this protein was revealed as due to its ability to trigger genes, involved in aerobic respiration for ATP synthesis.

CONCLUSIONS

These findings have significant implications that might lead to the development of robust microbes for the highly efficient industrial fermentation processes.

Production of bioethanol and xylitol from non-detoxified corn cob via a two-stage fermentation strategy

A novel two-stage fermentation strategy was applied to produce xylitol and ethanol from the whole acid-pretreated corn cob slurry. The acid-pretreated corn cob was used without filtration and detoxification by the two-stage fermentation with the robust Kluyveromyces marxianus CICC 1727-5. In the first stage, xylose in the slurry after dilute acid pretreatment of lignocellulosic biomass was used to produce xylitol under micro-aeration conditions. In the second stage, simultaneous saccharification fermentation was carried out, and the ethanol was produced from glucose releasing from the solid. Important parameters, such as aeration rate, cellulase loading during xylose utilization and SSF fermentation were studied for best performance. The two-stage fermentation strategy removed the inhibition of glucose on xylose, and little xylose was left in the fermentation broth. Under the optimized condition, the maximum ethanol and xylitol concentration were 52 g/L and 24.2 g/L corresponding to the yield of 0.41 g/g and 0.82 g/g, respectively.

Current advances on waste biomass transformation into value-added products

Ceaseless growth in human population led to high demand in everything. Currently, the world largely depends on petroleum-based "all material synthesis" scheme. On the other hand, depletion of fossil-based resources and their huge impact on environmental pollution have forced us to search for sustainable and eco-friendly alternative resources. In this context, the notion to utilize waste biomass could possibly provide environmental and economic benefits. This study was carefully designed to critically review state of the art in the transformation of waste biomass into value-added products. Even though extensive reviews on biomass utilization have been published in the past few years, the current study basically focused on new trends and prospective in this area. Here, global biomass potential, research developments and practices, novel biomass transformation approaches, and future perspectives were broadly discussed. More importantly, in addition to revising published researches, already implemented and ongoing large-scale projects on valorization of waste biomass have been assessed. Therefore, this study is believed to give crucial information on the current status and future direction of waste biomass utilization so as to accomplish the quest towards green economy.Key Points • Huge biomass potential and dramatically increase in R&D trends on waste biomass.• Selection of appropriate waste biomass valorization techniques. • Development of efficient and feasible waste biomass transformation technology. • Coproduction of low-value, high-volume and high-value, low volume products.

The Sustainability and Life Cycle Assessments of Industrial Biotechnology: An Introduction

Industrial biotechnology (IB) uses biological and biochemical processes in industrial production and is often regarded as an emerging key technology revolutionizing the production of many products while protecting resources and the environment and fostering economic development. This contribution describes the background and sketches the content of the volume 'Sustainability and Life Cycle Assessment of Industrial Biotechnology' in the Springer series 'Advances in Biochemical Engineering/Biotechnology'. The field of IB is introduced from different perspectives (milestones in IB history, economics of biotechnology industry, environmental and social as well as ethical issues and impacts, green chemistry) and in several applications fields (production of chemicals, geobiotechnology in mining).

Enhancement of Thermal Resistance by Metal Ions in Thermotolerant Zymomonas mobilis TISTR 548

The thermal resistance of fermenting microbes is a key characteristic of stable fermentation at high temperatures. Therefore, the effects of various metal ions on the growth of Zymomonas mobilis TISTR 548, a thermotolerant ethanologenic bacterium, at a critical high temperature (CHT) were examined. Addition of Mg2+ and K+ increased CHT by 1°C, but the effects of the addition of Mn2+, Ni2+, Co2+, Al3+, Fe3+, and Zn2+ on CHT were negligible. To understand the physiological functions associated with the addition of Mg2+ or K+, cell morphology, intracellular reactive oxygen species (ROS) level, and ethanol productivity were investigated at 39°C (i.e., above CHT). Cell elongation was repressed by Mg2+, but not by K+. Addition of both metals reduced intracellular ROS level, with only K+ showing the highest reduction strength, followed by both metals and only Mg2+. Additionally, ethanol productivity was recovered with the addition of both metals. Moreover, the addition of Mg2+ or K+ at a non-permissive temperature in 26 thermosensitive, single gene-disrupted mutants of Z. mobilis TISTR 548 revealed that several mutants showed metal ion-specific growth improvement. Remarkably, K+ repressed growth of two mutants. These results suggest that K+ and Mg2+ enhance cell growth at CHT via different mechanisms, which involve the maintenance of low intracellular ROS levels.

Briquettes Production from Olive Mill Waste under Optimal Temperature and Pressure Conditions: Physico-Chemical and Mechanical Characterizations

This paper aims at investigating the production of high quality briquettes from olive mill solid waste (OMSW) mixed with corn starch as a binder for energy production. For this purpose, different mass percentages of OMSW and binder were considered; 100%-0%, 90%-10%, 85%-15%, and 70%-30%, respectively. The briquetting process of the raw mixtures was carried out based on high pressures. Physico-chemical and mechanical characterizations were performed in order to select the best conditions for the briquettes production. It was observed that during the densification process, the optimal applied pressure increases notably the unit density, the bulk density, and the compressive strength. Mechanical characterization shows that the prepared sample with 15% of corn starch shows the best mechanical properties. Moreover, the corn starch binder affects quietly the high heating value (HHV) which increases from 16.36 MJ/Kg for the 100%-0% sample to 16.92 MJ/Kg for the 85%-15% sample. In addition, the kinetic study shows that the binder agent does not affect negatively the thermal degradation of the briquettes. Finally, the briquettes characterization shows that the studied samples with particles size less than 100 μm and blended with 15% of corn starch binder are promising biofuels either for household or industrial plants use.

Evaluation of pre-treatment methods for Lantana camara stem for enhanced enzymatic saccharification

This study evaluates certain pre-treatment methods for Lantana camara stem for efficient conversion to fermentable sugars. The composition analysis of L. camara stem showed 66.8% (w/w) holocellulose, 34.9% (w/w) cellulose and 17% (w/w) hemicellulose. Comparative analysis of various chemical, physical or physico-chemical pre-treatments on L. camara stem was performed. Of all pretreatment methods used, pre-treatment with 1% (v/v) H₂SO₄ assisted autoclaving gave maximum total reducing sugar yield 132.7 mg/g (13.2 g/L) of raw biomass in pretreated hydrolysate. Major contribution to total reducing sugar was from hemicellulosic fraction, because total pentose sugar yield was 119.4 mg/g of raw biomass whereas, glucose released was only 10 mg/g of untreated biomass. The enzymatic saccharification of pre-treated L. camara stem by 1% (v/v) H₂SO₄ assisted autoclaving was performed with partially purified carboxymethylcellulase from Bacillus amyloliquefaciens SS35. Enzymatic saccharification at 30 °C for 48 h gave total reducing sugar yield, 63.3 mg/g of pre-treated biomass in the hydrolysate, while untreated biomass gave 43.3 mg/g of untreated biomass. The total sugar yield i.e. the sum of pre-treated biomass hydrolysate total reducing sugar (132.7 mg/g of raw biomass) and enzymatic hydrolysate total reducing sugar (63.3 mg/g of pre-treated biomass) was 196.0 mg/g of raw biomass, indicating the effectiveness of pre-treatment method. Field emission scanning electron microscopy, Fourier transform infrared and X-ray diffraction analyses displayed enhanced porosity, removal of non-cellulosic sugars and increased cellulose crystallinity, respectively, in pre-treated L. camara stem, showing the effectiveness of acid assisted autoclaving pre-treatment.

Ethanol Production and Other Bioproducts by Galactomyces geotrichum from Sugarcane Bagasse Hydrolysate

This study investigated the enzymatic saccharification of alkaline-pretreated sugarcane bagasse (PSB) and the bioconversion of simple sugars from hydrolysates to ethanol and other bioproducts by the yeast Galactomyces geotrichum. The effects of percentage of dry substrate (3 and 10% w/v) and time of hydrolysis (24 and 72 h) in the content of released sugars were evaluated. The concentrations of monosaccharides and total reducing sugars (TRS) were calculated by high-performance liquid chromatography (HPLC) and by 3.5-dinitrosalicylic acid (DNS) method, respectively. The highest concentrations of TRS, glucose and xylose (73.96, 31.78 and 10.85 g/L, respectively) were obtained after the saccharification of 10% of PSB with Cellic CTec3 multi-enzyme cocktail (10 FPU/g cellulose) during 72 h (hydrolysate IV). G. geotrichum UFVJM-R150 fermented both glucose and xylose from the hydrolysates. The most efficient ethanol production was obtained after the fermentation of hydrolysate IV (9.99 g/L of ethanol, volumetric productivity-Q_P of 0.42 g/L.h and yield of ethanol as a function of the substrate-Y_P/S of 0.27 g_ethanol/g_sugar). Besides ethanol, G. geotrichum was also able to produce other high-value chemicals such as isoamyl alcohol and galacturonic acid. This is the first report of the potential of the yeast G. geotrichum to fermentate sugarcane bagasse hydrolysates with the production of important bioproducts to further use by biorefineries.

Insertional tagging of the Scheffersomyces stipitis gene HEM25 involved in regulation of glucose and xylose alcoholic fermentation

Amid known microbial bioethanol producers, the yeast Scheffersomyces (Pichia) stipitis is particularly promising in terms of alcoholic fermentation of both glucose and xylose, the main constituents of lignocellulosic biomass hydrolysates. However, the ethanol yield and productivity, especially from xylose, are still insufficient to meet the requirements of a feasible industrial technology; therefore, the construction of more efficient S. stipitis ethanol producers is of great significance. The aim of this study was to isolate the insertional mutants of S. stipitis with altered ethanol production from glucose and xylose and to identify the disrupted gene(s). Mutants obtained by random insertional mutagenesis were screened for their growth abilities on solid media with different sugars and for resistance to 3-bromopyruvate. Of more than 1,300 screened mutants, 17 were identified to have significantly changed ethanol yields during the fermentation. In one of the best fermenting strains (strain 4.6), insertion was found to occur within the ORF of a homolog to the Saccharomyces cerevisiae gene HEM25 (YDL119C), encoding a mitochondrial glycine transporter required for heme synthesis. The role of HEM25 in heme accumulation, respiration, and alcoholic fermentation in the yeast S. stipitis was studied using strain 4.6, the complementation strain Comp-a derivative from the 4.6 strain with expression of the WT HEM25 allele and the deletion strain hem25Δ. As hem25Δ produced lower amounts of ethanol than strain 4.6, we assume that the phenotype of strain 4.6 may be caused not only by HEM25 disruption but additionally by some point mutation.

Domino lignin depolymerization and reconnection to complex molecules mediated by boryl radicals

Lignin has been demonstrated as a source of complex molecules via a boryl-mediated domino degradation/reconnection process.

Development and characterization of acidic-pH-tolerant mutants of Zymomonas mobilis through adaptation and next-generation sequencing-based genome resequencing and RNA-Seq

BACKGROUND

Acid pretreatment is a common strategy used to break down the hemicellulose component of the lignocellulosic biomass to release pentoses, and a subsequent enzymatic hydrolysis step is usually applied to release hexoses from the cellulose. The hydrolysate after pretreatment and enzymatic hydrolysis containing both hexoses and pentoses can then be used as substrates for biochemical production. However, the acid-pretreated liquor can also be directly used as the substrate for microbial fermentation, which has an acidic pH and contains inhibitory compounds generated during pretreatment. Although the natural ethanologenic bacterium Zymomonas mobilis can grow in a broad range of pH 3.5 ~ 7.5, cell growth and ethanol fermentation are still affected under acidic-pH conditions below pH 4.0.

RESULTS

In this study, adaptive laboratory evolution (ALE) strategy was applied to adapt Z. mobilis under acidic-pH conditions. Two mutant strains named 3.6M and 3.5M with enhanced acidic pH tolerance were selected and confirmed, of which 3.5M grew better than ZM4 but worse than 3.6M in acidic-pH conditions that is served as a reference strain between 3.6M and ZM4 to help unravel the acidic-pH tolerance mechanism. Mutant strains 3.5M and 3.6M exhibited 50 ~ 130% enhancement on growth rate, 4 ~ 9 h reduction on fermentation time to consume glucose, and 20 ~ 63% improvement on ethanol productivity than wild-type ZM4 at pH 3.8. Next-generation sequencing (NGS)-based whole-genome resequencing (WGR) and RNA-Seq technologies were applied to unravel the acidic-pH tolerance mechanism of mutant strains. WGR result indicated that compared to wild-type ZM4, 3.5M and 3.6M have seven and five single nucleotide polymorphisms (SNPs), respectively, among which four are shared in common. Additionally, RNA-Seq result showed that the upregulation of genes involved in glycolysis and the downregulation of flagellar and mobility related genes would help generate and redistribute cellular energy to resist acidic pH while keeping normal biological processes in Z. mobilis. Moreover, genes involved in RND efflux pump, ATP-binding cassette (ABC) transporter, proton consumption, and alkaline metabolite production were significantly upregulated in mutants under the acidic-pH condition compared with ZM4, which could help maintain the pH homeostasis in mutant strains for acidic-pH resistance. Furthermore, our results demonstrated that in mutant 3.6M, genes encoding F₁F₀ ATPase to pump excess protons out of cells were upregulated under pH 3.8 compared to pH 6.2. This difference might help mutant 3.6M manage acidic conditions better than ZM4 and 3.5M. A few gene targets were then selected for genetics study to explore their role in acidic pH tolerance, and our results demonstrated that the expression of two operons in the shuttle plasmids, ZMO0956-ZMO0958 encoding cytochrome bc1 complex and ZMO1428-ZMO1432 encoding RND efflux pump, could help Z. mobilis tolerate acidic-pH conditions.

CONCLUSION

An acidic-pH-tolerant mutant 3.6M obtained through this study can be used for commercial bioethanol production under acidic fermentation conditions. In addition, the molecular mechanism of acidic pH tolerance of Z. mobilis was further proposed, which can facilitate future research on rational design of synthetic microorganisms with enhanced tolerance against acidic-pH conditions. Moreover, the strategy developed in this study combining approaches of ALE, genome resequencing, RNA-Seq, and classical genetics study for mutant evolution and characterization can be applied in other industrial microorganisms.

Isolation of xylose-assimilating yeasts and optimization of xylitol production by a new Meyerozyma guilliermondii strain

Production of xylitol from lignocellulosic biomass is of interest to modern biorefineries, because this biomass should be processed into a spectrum of chemicals (bio-based products) and not only energy. The isolation of new yeast strains capable of efficiently converting xylose into xylitol and withstanding inhibitors released from biomass hydrolysis can contribute to making its production feasible in biorefineries. Forty-three out of 128 yeast strains isolated from the gut of Passalidae beetles were capable of assimilating xylose as the sole carbon source. Meyerozyma guilliermondii UFV-1 was selected due to its ability to grow and ferment D-xylose in a synthetic medium. This yeast assimilated the broad range of sugars present in lignocellulosic biomass hydrolysates, such as xylose, raffinose, cellobiose, rhamnose, arabinose, and glucose. Its optimum growth conditions were pH 8.0 and a temperature of 30 °C. In concentrations of 0.07 mol/L acetic acid, 0.05 mol/L 5-hydroximethylfurfural, and 0.04 mol/L furfural, M. guilliermondii UFV-1 did not grow. Maximum xylitol production in aerobiosis and hypoxia were 51.88 and 27.73 g/L, respectively. Under aerobic condition, xylose concentration and agitation rate were the factors which were statistically significant, while only the agitation rate was significant in hypoxia. We fitted a response surface (RS) that estimated the best agitation rate (113.33 rpm) and xylose concentration (90 g/L) for maximum xylitol production in aerobiosis. Therefore, M. guilliermondii UFV-1 displays potential for being used for xylitol production in biorefineries.

Heteropoly acids enhanced neutral deep eutectic solvent pretreatment for enzymatic hydrolysis and ethanol fermentation of Miscanthus x giganteus under mild conditions

To improve the neutral DES (choline chloride/glycerol) pretreatment performance, three environmentally friendly heteropoly acids (phosphotungstic, phosphomolybdic and silicotungstic acids) were used as catalysts. Pretreatment with silicotungstic acid at 120 °C for 3 h resulted in 97.3% of enzymatic digestibility at an enzyme loading of 15FPU/g substrate, which was approximately eight times more than that of raw samples. More importantly, 80% of glucose yield was obtained within 12 h. Simultaneously, 81.8% of ethanol yield was achieved in the SSSF process. The efficient conversion was ascribed to the significant delignification (89.5%), which resulted in the exposure of more accessible specific surface area. This was attributed to that the proton (H⁺) from heteropoly acids could significantly contribute to the lignin degradation. Intriguingly, trace acetic acid (0.39 g/L) and HMF (0.21-0.95 g/L) in the pretreatment liquor were produced without any significant deleterious effects. These discoveries provide new insights for efficient biomass conversion under mild conditions.

β-Xylosidases: Structural Diversity, Catalytic Mechanism, and Inhibition by Monosaccharides

Xylan, a prominent component of cellulosic biomass, has a high potential for degradation into reducing sugars, and subsequent conversion into bioethanol. This process requires a range of xylanolytic enzymes. Among them, β-xylosidases are crucial, because they hydrolyze more glycosidic bonds than any of the other xylanolytic enzymes. They also enhance the efficiency of the process by degrading xylooligosaccharides, which are potent inhibitors of other hemicellulose-/xylan-converting enzymes. On the other hand, the β-xylosidase itself is also inhibited by monosaccharides that may be generated in high concentrations during the saccharification process. Structurally, β-xylosidases are diverse enzymes with different substrate specificities and enzyme mechanisms. Here, we review the structural diversity and catalytic mechanisms of β-xylosidases, and discuss their inhibition by monosaccharides.

Construction of advanced producers of first- and second-generation ethanol in Saccharomyces cerevisiae and selected species of non-conventional yeasts (Scheffersomyces stipitis, Ogataea polymorpha)

This review summarizes progress in the construction of efficient yeast ethanol producers from glucose/sucrose and lignocellulose. Saccharomyces cerevisiae is the major industrial producer of first-generation ethanol. The different approaches to increase ethanol yield and productivity from glucose in S. cerevisiae are described. Construction of the producers of second-generation ethanol is described for S. cerevisiae, one of the best natural xylose fermenters, Scheffersomyces stipitis and the most thermotolerant yeast known Ogataea polymorpha. Each of these organisms has some advantages and drawbacks. S. cerevisiae is the primary industrial ethanol producer and is the most ethanol tolerant natural yeast known and, however, cannot metabolize xylose. S. stipitis can effectively ferment both glucose and xylose and, however, has low ethanol tolerance and requires oxygen for growth. O. polymorpha grows and ferments at high temperatures and, however, produces very low amounts of ethanol from xylose. Review describes how the mentioned drawbacks could be overcome.

Regulation of gene expression by the action of a fungal lncRNA on a transactivator

Long non-coding RNAs (lncRNAs) are crucial factors acting on regulatory processes in eukaryotes. Recently, for the first time in a filamentous fungus, the lncRNA HAX1 was characterized in the ascomycete Trichoderma reesei. In industry, this fungus is widely applied for the high-yield production of cellulases. The lncRNA HAX1 was reported to influence the expression of cellulase-encoding genes; interestingly, this effect is dependent on the presence of its most abundant length. Clearly, HAX1 acts in association with a set of well-described transcription factors to regulate gene expression. In this study, we attempted to elucidate the regulatory strategy of HAX1 and its interactions with the major transcriptional activator Xylanase regulator 1 (Xyr1). We demonstrated that HAX1 interferes with the negative feedback regulatory loop of Xyr1 in a sophisticated manner and thus ultimately has a positive effect on gene expression.

The Impact of Substrate–Enzyme Proportion for Efficient Hydrolysis of Hay

Fuel alcohol production yields can be influenced by lignocellulosic biomass loading. High solid loadings (>20 wt%) are suggested to have the potential to produce more products. However, most often, low substrate loadings (<5% solids, w/w) are used to ensure good wetting and enzyme accessibility, and to minimize any inhibitory effect on the hydrolysis products. Here, we analyzed the effect of substrate loading on the enzymatic hydrolysis of hay with non-commercial enzyme products obtained from white-rot fungi. A significant negative effect on hydrolysis was observed when 10 wt% hay loading was used with the commercial enzyme, however, non-commercial enzyme products from white-rot fungi had no impact on hydrolysis in biomass loading rates from 1 to 10 wt%. Moreover, it was estimated that enzymes extracted from white-rot fungi could be used at a concentration of 0.2 FPU/mL at a biomass loading from 1–10 wt%, resulting in 0.17–0.24 g of released reducing carbohydrates per gram of biomass. Higher concentrations did not result in any significant conversion increase. A mixing impact was only observed in test runs at a substrate loading of 10 wt%. The apparently positive features of the non-commercial enzyme mixes give rise to their future use. The combination and upgrade of existing technologies, e.g., efficient pre-treatment, membrane purification, and concentration and efficient product recovery, should result in even higher conversion yields.

Bioethanol production from waste lignocelluloses: A review on microbial degradation potential

Tremendous explosion of population has led to about 200% increment of total energy consumptions in last twenty-five years. Apart from conventional fossil fuel as limited energy source, alternative non-conventional sources are being explored worldwide to cater the energy requirement. Lignocellulosic biomass conversion for biofuel production is an important alternative energy source due to its abundance in nature and creating less harmful impacts on the environment in comparison to the coal or petroleum-based sources. However, lignocellulose biopolymer, the building block of plants, is a recalcitrant substance and difficult to break into desirable products. Commonly used chemical and physical methods for pretreating the substrate are having several limitations. Whereas, utilizing microbial potential to hydrolyse the biomass is an interesting area of research. Because of the complexity of substrate, several enzymes are required that can act synergistically to hydrolyse the biopolymer producing components like bioethanol or other energy substances. Exploring a range of microorganisms, like bacteria, fungi, yeast etc. that utilizes lignocelluloses for their energy through enzymatic breaking down the biomass, is one of the options. Scientists are working upon designing organisms through genetic engineering tools to integrate desired enzymes into a single organism (like bacterial cell). Studies on designer cellulosomes and bacteria consortia development relating consolidated bioprocessing are exciting to overcome the issue of appropriate lignocellulose digestions. This review encompasses up to date information on recent developments for effective microbial degradation processes of lignocelluloses for improved utilization to produce biofuel (bioethanol in particular) from the most plentiful substances of our planet.

Chemical and bio-chemical reactions assisted by pervaporation technology

Since several decades ago, the application of pervaporation (PV) technology has been mainly aimed at the separation of different types of water-organic, organic-water and organic-organic mixtures, reaching its large-scale application in industry for the dehydration of organics. Today, the versatility and high selectivity toward specific compounds have led its consideration to other types of application such as the assisted chemical and bio-chemical reactions. The focus of this review is to provide a compelling overview on the recent developments of PV combined with chemical and bio-chemical reactions. After a general introduction of PV and its theoretical background, particular emphasis is given to the results obtained in the field for different reactions considered, identifying the key features and weak points of PV in such particular applications. Furthermore, future trends and perspectives are also addressed according to the latest literature reports.

Aromatics from Beechwood Organosolv Lignin through Thermal and Catalytic Pyrolysis

Biomass fractionation, as an alternative to biomass pretreatment, has gained increasing research attention over the past few years as it provides separate streams of cellulose, hemicellulose, and lignin. These streams can be used separately and can provide a solution for improving the economics of emerging biorefinery technologies. The sugar streams are commonly used in microbial conversions, whereas during recent years lignin has been recognized as a valuable compound as it is the only renewable and abundant source of aromatic chemicals. Successfully converting lignin into valuable chemicals and products is key in achieving both environmental and economic sustainability of future biorefineries. In this work, lignin retrieved from beechwood sawdust delignification pretreatment via an organosolv process was depolymerized with thermal and catalytic pyrolysis. ZSM-5 commercial catalyst was used in situ to upgrade the lignin bio-oil vapors. Lignins retrieved from different modes of organosolv pretreatment were tested in order to evaluate the effect that upstream pretreatment has on the lignin fraction. Both thermal and catalytic pyrolysis yielded oils rich in phenols and aromatic hydrocarbons. Use of ZSM-5 catalyst assisted in overall deoxygenation of the bio-oils and enhanced aromatic hydrocarbons production. The oxygen content of the bio-oils was reduced at the expense of their yield. Organosolv lignins were successfully depolymerized towards phenols and aromatic hydrocarbons via thermal and catalytic pyrolysis. Hence, lignin pyrolysis can be an effective manner for lignin upgrading towards high added value products.

Nanocellulose production from recycled paper mill sludge using ozonation pretreatment followed by recyclable maleic acid hydrolysis

Nanocellulose (NC) have garnered much interest worldwide due to its physical and chemical properties. Nanocellulose is produced from biomass materials by bleaching pretreatment, followed by acid hydrolysis. This work demonstrated the production of NC from recycled paper sludge (RPS), a crystalline cellulose rich waste, by ozonation pretreatment, followed by maleic acid hydrolysis. Ozonation resulted in removal of lignin (as evident by TGA analysis), negative zeta potential of RPS and enhanced NC production, from 60 mg/L to over 80 mg/L after 60-120 min of ozone treatment. Maleic acid was successfully recovered, although longer ozonation times reduced the amount of acid available for recovery. These results demonstrate that ozonation can be used as an effective pretreatment for NC production.

Overexpression of RCK1 improves acetic acid tolerance in Saccharomyces cerevisiae

Mixed sugars derived from lignocellulosic biomass can be converted into biofuels and chemicals by engineered microorganisms, but toxic fermentation inhibitors produced from harsh depolymerization processes of lignocellulosic biomass pose a critical challenge for economic production of biofuels and chemicals. Unlike other fermentation inhibitors generated from sugar degradation, acetic acid is inevitably produced from acetylated hemicellulose, and its concentrations in cellulosic hydrolysates are substantially higher than other fermentation inhibitors. The aim of this study was to identify novel genetic perturbations for improved acetic acid tolerance in Saccharomyces cerevisiae. Through a genomic library-based approach, we identified an overexpression gene target RCK1 coding for a protein kinase involved in oxidative stress. Overexpression of RCK1 significantly improved glucose and xylose fermentation under acetic acid stress conditions. Specifically, the RCK1-overexpressing strain exhibited a two-fold higher specific ethanol productivity than the control strain in glucose fermentation under the presence of acetic acid. Interestingly, the engineered S. cerevisiae overexpressing RCK1 showed 40% lower intracellular reactive oxygen species (ROS) levels as compared to the parental strain when the strains were exposed to acetic acid, suggesting that RCK1 overexpression might play a role in reducing the oxidative stress caused by acetic acid.

Construction of industrial Saccharomyces cerevisiae strains for the efficient consolidated bioprocessing of raw starch

BACKGROUND

Consolidated bioprocessing (CBP) combines enzyme production, saccharification and fermentation into a one-step process. This strategy represents a promising alternative for economic ethanol production from starchy biomass with the use of amylolytic industrial yeast strains.

RESULTS

Recombinant Saccharomyces cerevisiae Y294 laboratory strains simultaneously expressing an α-amylase and glucoamylase gene were screened to identify the best enzyme combination for raw starch hydrolysis. The codon optimised Talaromyces emersonii glucoamylase encoding gene (temG_Opt) and the native T. emersonii α-amylase encoding gene (temA) were selected for expression in two industrial S. cerevisiae yeast strains, namely Ethanol Red™ (hereafter referred to as the ER) and M2n. Two δ-integration gene cassettes were constructed to allow for the simultaneous multiple integrations of the temG_Opt and temA genes into the yeasts' genomes. During the fermentation of 200 g l^-1 raw corn starch, the amylolytic industrial strains were able to ferment raw corn starch to ethanol in a single step with high ethanol yields. After 192 h at 30 °C, the S. cerevisiae ER T12 and M2n T1 strains (containing integrated temA and temG_Opt gene cassettes) produced 89.35 and 98.13 g l^-1 ethanol, respectively, corresponding to estimated carbon conversions of 87 and 94%, respectively. The addition of a commercial granular starch enzyme cocktail in combination with the amylolytic yeast allowed for a 90% reduction in exogenous enzyme dosage, compared to the conventional simultaneous saccharification and fermentation (SSF) control experiment with the parental industrial host strains.

CONCLUSIONS

A novel amylolytic enzyme combination has been produced by two industrial S. cerevisiae strains. These recombinant strains represent potential drop-in CBP yeast substitutes for the existing conventional and raw starch fermentation processes.

Direct conversion of cellulose into ethanol catalysed by a combination of tungstic acid and zirconia-supported Pt nanoparticles

Direct conversion of cellulose to ethanol is achieved by using a combination of H₂WO₄ and Pt/ZrO₂ catalysts in an aqueous medium.

Lipid metabolism of phenol-tolerant Rhodococcus opacus strains for lignin bioconversion

BACKGROUND

Lignin is a recalcitrant aromatic polymer that is a potential feedstock for renewable fuel and chemical production. Rhodococcus opacus PD630 is a promising strain for the biological upgrading of lignin due to its ability to tolerate and utilize lignin-derived aromatic compounds. To enhance its aromatic tolerance, we recently applied adaptive evolution using phenol as a sole carbon source and characterized a phenol-adapted R. opacus strain (evol40) and the wild-type (WT) strain by whole genome and RNA sequencing. While this effort increased our understanding of the aromatic tolerance, the tolerance mechanisms were not completely elucidated.

RESULTS

We hypothesize that the composition of lipids plays an important role in phenol tolerance. To test this hypothesis, we applied high-resolution mass spectrometry analysis to lipid samples obtained from the WT and evol40 strains grown in 1 g/L glucose (glucose), 0.75 g/L phenol (low phenol), or 1.5 g/L phenol (high phenol, evol40 only) as a sole carbon source. This analysis identified > 100 lipid species of mycolic acids, phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), and triacylglycerols. In both strains, mycolic acids had fewer double bond numbers in phenol conditions than the glucose condition, and evol40 had significantly shorter mycolic acid chain lengths than the WT strain in phenol conditions. These results indicate that phenol adaptation affected mycolic acid membrane composition. In addition, the percentage of unsaturated phospholipids decreased for both strains in phenol conditions compared to the glucose condition. Moreover, the PI content increased for both strains in the low phenol condition compared to the glucose condition, and the PI content increased further for evol40 in the high phenol condition relative to the low phenol condition.

CONCLUSIONS

This work represents the first comprehensive lipidomic study on the membrane of R. opacus grown using phenol as a sole carbon source. Our results suggest that the alteration of the mycolic acid and phospholipid membrane composition may be a strategy of R. opacus for phenol tolerance.

Insights into the economic viability of cellulases recycling on bioethanol production from recycled paper sludge

The economics of Recycled Paper Sludge conversion into ethanol was here assessed with emphasis on integrating a cellulase recycling system. Without cellulases recycling this process presented positive economic outputs (payback period of 7.85 years; 10.90 Million US$ of accumulated NPV) despite the modest ethanol titers. Recycling both free and solid-bound enzymes allowed considerable savings of enzyme but also an increase on annual costs (0.88%), resulting on a superior economic output: payback period decreased to 7.25 years; accumulated NPV increased to 14.44 Million US$. Recycling exclusively the liquid fraction enabled a clear costs reduction, however, also total ethanol decreased, attenuating the abovementioned benefits. Targeting higher ethanol concentrations, superior solids consistencies were also evaluated. Despite a costs reduction, total ethanol decreased due to a higher ethanol retention on the solid. A sensitivity analysis further revealed that the cost of enzymes and ultrafiltration membrane may be critical on enzyme recycling economic feasibility.