Ethanol production from wheat straw by Saccharomyces cerevisiae and Scheffersomyces stipitis co-culture in batch and continuous system

Optimizing the Conditions of Pretreatment and Enzymatic Hydrolysis of Sugarcane Bagasse for Bioethanol Production

The agricultural waste sugarcane bagasse (SCB) is a kind of plentiful biomass resource. In this study, different pretreatment methods (NaOH, H2SO4, and sodium percarbonate/glycerol) were utilized and compared. Among the three pretreatment methods, NaOH pretreatment was the most optimal method. Response surface methodology (RSM) was utilized to optimize NaOH pretreatment conditions. After optimization by RSM, the solid yield and lignin removal were 54.60 and 82.30% under the treatment of 1% NaOH, a time of 60 min, and a solid-to-liquid ratio of 1:15, respectively. Then, the enzymolysis conditions of cellulase for NaOH-treated SCB were optimized by RSM. Under the optimal enzymatic hydrolysis conditions (an enzyme dose of 18 FPU/g, a time of 64 h, and a solid-to-liquid ratio of 1:30), the actual yield of reducing sugar in the enzyme-treated hydrolysate was 443.52 mg/g SCB with a cellulose conversion rate of 85.33%. A bacterium, namely, Bacillus sp. EtOH, which produced ethanol and Baijiu aroma substances, was isolated from the high-temperature Daqu of Danquan Baijiu in our previous study. At last, when the strain EtOH was cultured for 36 h in a fermentation medium (reducing sugar from cellulase-treated SCB hydrolysate, yeast extract, and peptone), ethanol concentration reached 2.769 g/L (0.353%, v/v). The sugar-to-ethanol and SCB-to-ethanol yields were 13.85 and 11.81% in this study, respectively. In brief, after NaOH pretreatment, 1 g of original SCB produced 0.5460 g of NaOH-treated SCB. Then, after the enzymatic hydrolysis, reducing sugar yield (443.52 mg/g SCB) was obtained. Our study provided a suitable method for bioethanol production from SCB, which achieved efficient resource utilization of agricultural waste SCB.

An overview on fermentation strategies to overcome lignocellulosic inhibitors in second-generation ethanol production using cell immobilization

The development of technologies to ferment carbohydrates (mainly glucose and xylose) obtained from the hydrolysis of lignocellulosic biomass for the production of second-generation ethanol (2G ethanol) has many economic and environmental advantages. The pretreatment step of this biomass is industrially performed mainly by steam explosion with diluted sulfuric acid and generates hydrolysates that contain inhibitory compounds for the metabolism of microorganisms, harming the next step of ethanol production. The main inhibitors are: organic acids, furan, and phenolics. Several strategies can be applied to decrease the action of these compounds in microorganisms, such as cell immobilization. Based on data published in the literature, this overview will address the relevant aspects of cell immobilization for the production of 2G ethanol, aiming to evaluate this method as a strategy for protecting microorganisms against inhibitors in different modes of operation for fermentation. This is the first overview to date that shows the relation between inhibitors, cells immobilization, and fermentation operation modes for 2G ethanol. In this sense, the state of the art regarding the main inhibitors in 2G ethanol and the most applied techniques for cell immobilization, besides batch, repeated batch and continuous fermentation using immobilized cells, in addition to co-culture immobilization and co-immobilization of enzymes, are presented in this work.

Microbial pathways for advanced biofuel production

Decarbonisation of the transport sector is essential to mitigate anthropogenic climate change. Microbial metabolisms are already integral to the production of renewable, sustainable fuels and, building on that foundation, are being re-engineered to generate the advanced biofuels that will maintain mobility of people and goods during the energy transition. This review surveys the range of natural and engineered microbial systems for advanced biofuels production and summarises some of the techno-economic challenges associated with their implementation at industrial scales.

Cellulosic ethanol production by consortia of Scheffersomyces stipitis and engineered Zymomonas mobilis

BACKGROUND

As one of the clean and sustainable energies, lignocellulosic ethanol has achieved much attention around the world. The production of lignocellulosic ethanol does not compete with people for food, while the consumption of ethanol could contribute to the carbon dioxide emission reduction. However, the simultaneous transformation of glucose and xylose to ethanol is one of the key technologies for attaining cost-efficient lignocellulosic ethanol production at an industrial scale. Genetic modification of strains and constructing consortia were two approaches to resolve this issue. Compared with strain improvement, the synergistic interaction of consortia in metabolic pathways should be more useful than using each one separately.

RESULTS

In this study, the consortia consisting of suspended Scheffersomyces stipitis CICC1960 and Zymomonas mobilis 8b were cultivated to successfully depress carbon catabolite repression (CCR) in artificially simulated 80G40XRM. With this strategy, a 5.52% more xylose consumption and a 6.52% higher ethanol titer were achieved by the consortium, in which the inoculation ratio between S. stipitis and Z. mobilis was 1:3, compared with the Z. mobilis 8b mono-fermentation. Subsequently, one copy of the xylose metabolic genes was inserted into the Z. mobilis 8b genome to construct Z. mobilis FR2, leading to the xylose final-consumption amount and ethanol titer improvement by 15.36% and 6.81%, respectively. Finally, various corn stover hydrolysates with different sugar concentrations (glucose and xylose 60, 90, 120 g/L), were used to evaluate the fermentation performance of the consortium consisting of S. stipitis CICC1960 and Z. mobilis FR2. Fermentation results showed that a 1.56-4.59% higher ethanol titer was achieved by the consortium compared with the Z. mobilis FR2 mono-fermentation, and a 46.12-102.14% higher ethanol titer was observed in the consortium fermentation when compared with the S. stipitis CICC1960 mono-fermentation. Furthermore, qRT-PCR analysis of xylose/glucose transporter and other genes responsible for CCR explained the reason why the initial ratio inoculation of 1:3 in artificially simulated 80G40XRM had the best fermentation performance in the consortium.

CONCLUSIONS

The fermentation strategy used in this study, i.e., using a genetically modified consortium, had a superior performance in ethanol production, as compared with the S. stipitis CICC1960 mono-fermentation and the Z. mobilis FR2 mono-fermentation alone. This result showed that this strategy has potential for future lignocellulosic ethanol production.

Deciphering the genetic diversity and population structure of Turkish bread wheat germplasm using iPBS-retrotransposons markers

BACKGROUND

Research activities aiming to investigate the genetic diversity are very crucial because they provide information for the breeding and germplasm conservation activities. Wheat is one of the most important cereal crops globally by feeding more than a third of the human population around the world.

METHODS AND RESULTS

During present investigation, a total of 74 Turkish bread wheat accessions (54 landraces and 20 cultivars) were used as plant material and iPBS-retrotransposons marker system was used for the molecular characterization. 13 polymorphic primers used for molecular characterization resulted a total of 152 bands. Range of calculated diversity indices like polymorphism information content (0.11-0.702), effective numbers of alleles (1.026-1.526), Shannon's information index (0.101-0.247) and gene diversity (0.098-0.443) confirmed higher genetic variations in studied germplasm. Bread wheat landraces reflected higher genetic variations compared to commercial cultivars. Analysis of molecular variance resulted that higher (98%) genetic variations are present within populations. The model-based structure algorithm separated 74 bread wheat accessions in to two populations. Diversity indices based on structure evaluated population's revealed population B as a more diverse population. The principal coordinate analysis and neighbor-joining analysis separated 74 bread wheat accessions according to their collection points. Genetic distance for 74 Turkish bread wheat accessions explored Bingol and Asure accessions as genetically diverse that can be used as parents for breeding activities.

CONCLUSIONS

The extensive diversity of bread wheat in Turkish germplasm might be used as genetic resource for the exhaustive wheat breeding program. For instance, accessions Bingol and Asure were found genetically diverse and can be used as parents for future breeding activities.

Bioethanolic yeasts from dung beetles: tapping the potential of extremophilic yeasts for improvement of lignocellulolytic feedstock fermentation

Bioethanol from abundant and inexpensive agricultural and industrial wastes possesses the potential to reduce greenhouse gas emissions. Bioethanol as renewable fuel addresses elevated production costs, as well as food security concerns. Although technical advancements in simultaneous saccharification and fermentation have reduced the cost of production, one major drawback of this technology is that the pre-treatment process creates environmental stressors inhibitory to fermentative yeasts subsequently reducing bioethanol productivity. Robust fermentative yeasts with extreme stress tolerance remain limited. This review presents the potential of dung beetles from pristine and unexplored environments as an attractive source of extremophilic bioethanolic yeasts. Dung beetles survive on a recalcitrant lignocellulose-rich diet suggesting the presence of symbiotic yeasts with a cellulolytic potential. Dung beetles inhabiting extreme stress environments have the potential to harbour yeasts with the ability to withstand inhibitory environmental stresses typically associated with bioethanol production. The review further discusses established methods used to isolate bioethanolic yeasts, from dung beetles.

Current state-of-the-art in ethanol production from lignocellulosic feedstocks

The renewable lignocellulosic biomass is a sustainable feedstock for the production of bioethanol, which shows the potential to replace fossil fuels. Due to the recalcitrant structure of plant cell wall made of cellulose, hemicellulose, and lignin, the biomass conversion process requires the use of efficient pretreatment process before enzymatic hydrolysis and fermentation to degrade the crystallinity of cellulose fibres and to remove lignin from biomass. Proper pretreatment techniques, economical production of cellulolytic enzymes, and effective fermentation of glucose and xylose in the presence of inhibitors are key challenges for the viable production of bioethanol. Although new strains capable of fermenting xylose are being designed, they are often not resistant to toxic compounds in hydrolysates. This paper provides an in-depth review of lignocellulosic bioethanol production via biochemical route, focusing on the most widely used pretreatment technologies and key operational conditions of enzymatic hydrolysis and fermentation considering sugar/ethanol yields. In addition, this review examines the relevant detoxification strategies for the removal of toxic substances and the importance of immobilization. The review also indicates potential usage of engineered microorganisms to improve glucose and xylose fermentation, cellulolytic enzymes production, and response to stress conditions.

Intensification of lignocellulosic bioethanol production process using continuous double-staged immersed membrane bioreactors

Processing complexities associated with different lignocellulosic bioethanol production stages have hindered reaching full commercial capacity. Therefore, in this study efforts were made to remediate some issues associated with hydrolysis and fermentation, by the integration of immersed membrane bioreactors (iMBRs) into lignocellulosic bioethanol production process. In this regards, double-staged continuous saccharification-filtration and co-fermentation-filtration of wheat straw slurry was conducted using iMBRs at filtration fluxes up to 51.0 l.m^-2.h^-1 (LMH). The results showed a stable long-term (264 h) continuous hydrolysis-filtration and fermentation-filtration with effective separation of lignin-rich solids (up to 70% lignin) from hydrolyzed sugars, and separation of yeast cells from bioethanol stream at an exceptional filtration performance at 21.9 LMH. Moreover, the effect of factors such as filtration flux, medium quality and backwashing on fouling and cake-layer formation was studied. The results confirmed the process intensification potentials of iMBRs in tackling commonly faced technical obstacles in lignocellulosic bioethanol production.

Improved ethanol production from the slurry of pretreated brewers' spent grain through different co-fermentation strategies

The aim of this work was to bioconvert all sugars in BSG into ethanol using a process scheme that includes the enzymatic hydrolysis of the whole slurry resulting from the pretreatment of BSG with phosphoric and sulfuric acid using previously optimised conditions, followed by the co-fermentation of the mixed sugars. More than 90% of the sugars in raw BSG were recovered in the pretreatment and the subsequent enzymatic hydrolysis of the whole slurry. The co-fermentation of the enzymatic hydrolysates with Escherichia coli was then compared with that the co-culture of Scheffersomyces stipitis and Saccharomyces cerevisiae, which resulted in lower ethanol production. The co-fermentation strategy with a single microorganism (E. coli) when BSG was pretreated with phosphoric acid resulted into the highest ethanol concentration, 39 g/L, which means that 222 L of ethanol can be obtained from a ton of BSG without detoxification requirements.

Effect of dilute acid pretreatment on the saccharification and fermentation of rye straw

This research shows the effect of dilute acid pretreatment with various sulfuric acid concentrations (0.5-2.0% [wt/vol]) on enzymatic saccharification and fermentation yield of rye straw. After pretreatment, solids of rye straw were suspended in Na citrate buffer or post-pretreatment liquids (prehydrolysates) containing sugars liberated after hemicellulose hydrolysis. Saccharification was conducted using enzymes dosage of 15 or 25 FPU/g cellulose. Cellulose saccharification rate after rye straw pretreatment was enhanced by performing enzymatic hydrolysis in sodium citrate buffer in comparison with hemicellulose prehydrolysate. The maximum cellulose saccharification rate (69%) was reached in sodium citrate buffer (biomass pretreated with 2.0% [wt/vol] H₂ SO₄ ). Lignocellulosic complex of rye straw after pretreatment was subjected to separate hydrolysis and fermentation (SHF) or separate hydrolysis and co-fermentation (SHCF). The SHF processes conducted in the sodium citrate buffer using monoculture of Saccharomyces cerevisiae (Ethanol Red) were more efficient compared to hemicellulose prehydrolysate in respect with ethanol yields. Maximum fermentation efficiency of SHF processes obtained after rye straw pretreatment at 1.5% [wt/vol] H₂ SO₄ and saccharification using enzymes dosage of 25 FPU/g in sodium citrate buffer, achieving 40.6% of theoretical yield. However, SHCF process using cocultures of pentose-fermenting yeast, after pretreatment of raw material at 1.5% [wt/vol] H₂ SO₄ and hydrolysis using enzymes dosage of 25 FPU/g, resulted in the highest ethanol yield among studied methods, achieving 9.4 g/L of ethanol, corresponding to 55% of theoretical yield.

Yeasts in sustainable bioethanol production: A review

Bioethanol has been identified as the mostly used biofuel worldwide since it significantly contributes to the reduction of crude oil consumption and environmental pollution. It can be produced from various types of feedstocks such as sucrose, starch, lignocellulosic and algal biomass through fermentation process by microorganisms. Compared to other types of microoganisms, yeasts especially Saccharomyces cerevisiae is the common microbes employed in ethanol production due to its high ethanol productivity, high ethanol tolerance and ability of fermenting wide range of sugars. However, there are some challenges in yeast fermentation which inhibit ethanol production such as high temperature, high ethanol concentration and the ability to ferment pentose sugars. Various types of yeast strains have been used in fermentation for ethanol production including hybrid, recombinant and wild-type yeasts. Yeasts can directly ferment simple sugars into ethanol while other type of feedstocks must be converted to fermentable sugars before it can be fermented to ethanol. The common processes involves in ethanol production are pretreatment, hydrolysis and fermentation. Production of bioethanol during fermentation depends on several factors such as temperature, sugar concentration, pH, fermentation time, agitation rate, and inoculum size. The efficiency and productivity of ethanol can be enhanced by immobilizing the yeast cells. This review highlights the different types of yeast strains, fermentation process, factors affecting bioethanol production and immobilization of yeasts for better bioethanol production.

Effective ethanol production from whey powder through immobilized E. coli expressing Vitreoscilla hemoglobin

Ethanol production from whey powder was investigated by using free as well as alginate immobilized E. coli and E. coli expressing Vitreoscilla hemoglobin (VHb) in both shake flask and fermenter cultures. Media with varying levels of whey (lactose contents of 3%, 5%, 8% or 15%) and yeast extract (0.3% or 0.5%) were evaluated with fermentation times of 48-96 h. Immobilization and VHb expression resulted in higher ethanol production with all media; the increases ranged from 2% to 89% for immobilization and from 2% to 182% for VHb expression. It was determined that growth medium containing 8% lactose with 0.5% yeast extract yielded the highest ethanol production for free or immobilized strains, with or without VHb expression, in both shake flask and fermenter cultures. Immobilization with alginate was found to be a promising process for ethanol production by VHb-expressing ethanologenic E. coli.

Direct bioethanol production from wheat straw using xylose/glucose co-fermentation by co-culture of two recombinant yeasts

To achieve a cost-effective bioconversion of lignocellulosic materials, a novel xylose/glucose co-fermentation process by co-culture of cellulose-utilizing recombinant Saccharomyces cerevisiae (S. cerevisiae) and xylan-utilizing recombinant Pichia pastoris (P. pastoris) was developed, in which ethanol was produced directly from wheat straw without additional hydrolytic enzymes. Recombinant S. cerevisiae coexpressing three types of cellulase and recombinant P. pastoris coexpressing two types of xylanase were constructed, respectively. All cellulases and xylanases were successfully expressed and similar extracellular activity was demonstrated. The maximum ethanol concentration of 32.6 g L−1 with the yield 0.42 g g−1 was achieved from wheat straw corresponding to 100 g L−1 of total sugar after 80 h co-fermentation, which corresponds to 82.6% of the theoretical yield. These results demonstrate that the direct and efficient ethanol production from lignocellulosic materials is accomplished by simultaneous saccharification (cellulose and hemicellulose) and co-fermentation (glucose and xylose) with the co-culture of the two recombinant yeasts.

Long-term production of bioethanol in repeated-batch fermentation of microalgal biomass using immobilized Saccharomyces cerevisiae

Separate hydrolysis fermentation (SHF) and simultaneous saccharification fermentation (SSF) processes were studied for bioethanol production from microalgal biomass. SSF was selected as an efficient process to enhance the bioethanol yield through repeated-batches using immobilized yeast cells. Combined sonication and enzymatic hydrolysis of Chlamydomonas mexicana generated 10.5 and 8.48g/L of ethanol in SSF and SHF, respectively. Yeast utilized maximum portion of total reducing sugar (TRS) reaching a consumption efficiency of 91-98%. A bioethanol yield of 0.5g/g (88.2% of theoretical yield) and volumetric productivity of 0.22g/L/h was obtained after 48h of SSF. Immobilized yeast cells enabled repetitive production of ethanol for 7 cycles displaying a fermentation efficiency up to 79% for five consecutive cycles. The maximum ethanol production was 9.7g/L in 2nd-4th cycles. A total energy recovery of 85.81% was achieved from microalgal biomass in the form of bioethanol. Repeated-batch SSF demonstrated the possibility of cost-effective bioethanol production.

In situ detoxification of dry dilute acid pretreated corn stover by co-culture of xylose-utilizing and inhibitor-tolerant Saccharomyces cerevisiae increases ethanol production

Co-culture of xylose-utilizing and inhibitor-tolerant Saccharomyces cerevisiae was developed for bioethanol production from undetoxified pretreated biomass in simultaneously saccharification and co-fermentation (SSCF) process. Glucose accumulation during late fermentation phase in SSCF using xylose-utilizing strain can be eliminated by the introduction of inhibitor-tolerant strain. Effect of different ratios of two strains was investigated and xylose-utilizing strain to inhibitor-tolerant strain ratio of 10:1 (w/w) showed the best xylose consumption and the highest ethanol yield. Inoculating of xylose-utilizing strain at the later stage of SSCF (24-48h) exhibited lower ethanol yield than inoculating at early stage (the beginning 0-12h), probably due to the reduced enzymatic efficiency caused by the unconsumed xylose and oligomeric sugars. Co-culture SSCF increased ethanol concentration by 21.2% and 41.0% comparing to SSCF using individual inhibitor-tolerant and xylose-utilizing strain (increased from 48.5 and 41.7g/L to 58.8g/L), respectively, which suggest this co-culture system was very promising.

Recent trends in bioethanol production from food processing byproducts

The widespread use of corn starch and sugarcane as sources of sugar for the production of ethanol via fermentation may negatively impact the use of farmland for production of food. Thus, alternative sources of fermentable sugars, particularly from lignocellulosic sources, have been extensively investigated. Another source of fermentable sugars with substantial potential for ethanol production is the waste from the food growing and processing industry. Reviewed here is the use of waste from potato processing, molasses from processing of sugar beets into sugar, whey from cheese production, byproducts of rice and coffee bean processing, and other food processing wastes as sugar sources for fermentation to ethanol. Specific topics discussed include the organisms used for fermentation, strategies, such as co-culturing and cell immobilization, used to improve the fermentation process, and the use of genetic engineering to improve the performance of ethanol producing fermenters.

The Influence of Dopants on the Effectiveness of Alginate Beads in Immobilized Cell Reactors

Zymomonas mobilis immobilized in doped calcium alginate (Ca-alginate) was successfully employed for the production of ethanol in an immobilized cell reactor. Polyethylene oxide and F127 dimethacrylate were evaluated as potential dopants for Ca-alginate beads to decrease lag time and increase initial ethanol yield. The influence of the type and concentration of the dopant on the effectiveness of the microbe immobilized in Ca-alginate beads to produce ethanol was studied, and results were compared to the widely used 2 % Ca-alginate with no dopants, which acted as control. Immobilized cell reactors that were operated using beads doped with 0.25 % polyethylene oxide (PEO) reached an ethanol yield of ∼70 % in 24 h, which was significantly higher than an ethanol yield of 25 % obtained for the control reactor operated using undoped Ca-alginate beads. This study shows that the use of water-soluble dopants can potentially reduce the lag phase and thus improve the initial production yield of immobilized cell reactors, likely due to an increase in porosity and diffusion rate of the doped beads.

Influence of operational parameters on the fluid-side mass transfer resistance observed in a packed bed bioreactor

The influence of mass transfer on productivity as well as the performance of packed bed bioreactor was determined by varying a number of parameters; flow rate, glucose concentration and polymers (chitosan). Saccharomyces cerevisiae cells were immobilized in chitosan and non-chitosan coated alginate beads to demonstrate the effect on external mass transfer by substrate consumption time, lag phase and ethanol production. The results indicate that coating has a significant effect on the lag phase duration, being 30-40 min higher than non-coated beads. After lag phase, no significant change was observed in both types of beads on consumption of glucose with the same flow rate. It was observed that by increasing flow rates; lag phase and glucose consumption time decreased. The reason is due to the reduction of external mass transfer as a result of increase in flow rate as glucose is easily transported to and from the beads surface by diffusion. It is observed that chitosan acts as barrier for transfer of substrate and products, in and out of beads, at initial time of fermentation as it shows longer lag phase for chitosan coated beads than non-coated. Glucose consumption at low flow rate was lower as compared to higher flow rates. The optimum combination of parameters consisting of higher flow rates 30-90 ml/min and between 10 and 20 g/l of glucose was found for maximum production of ethanol.

Current Trends in Bioethanol Production by Saccharomyces cerevisiae: Substrate, Inhibitor Reduction, Growth Variables, Coculture, and Immobilization

Bioethanol is one of the most commonly used biofuels in transportation sector to reduce greenhouse gases. S. cerevisiae is the most employed yeast for ethanol production at industrial level though ethanol is produced by an array of other yeasts, bacteria, and fungi. This paper reviews the current and nonmolecular trends in ethanol production using S. cerevisiae. Ethanol has been produced from wide range of substrates such as molasses, starch based substrate, sweet sorghum cane extract, lignocellulose, and other wastes. The inhibitors in lignocellulosic hydrolysates can be reduced by repeated sequential fermentation, treatment with reducing agents and activated charcoal, overliming, anion exchanger, evaporation, enzymatic treatment with peroxidase and laccase, in situ detoxification by fermenting microbes, and different extraction methods. Coculturing S. cerevisiae with other yeasts or microbes is targeted to optimize ethanol production, shorten fermentation time, and reduce process cost. Immobilization of yeast cells has been considered as potential alternative for enhancing ethanol productivity, because immobilizing yeasts reduce risk of contamination, make the separation of cell mass from the bulk liquid easy, retain stability of cell activities, minimize production costs, enable biocatalyst recycling, reduce fermentation time, and protect the cells from inhibitors. The effects of growth variables of the yeast and supplementation of external nitrogen sources on ethanol optimization are also reviewed.