Improve Bioethanol Production Research Articles

Salt stress improves thermotolerance and high-temperature bioethanol production of multi-stress-tolerant Pichia kudriavzevii by stimulating intracellular metabolism and inhibiting oxidative damage

BackgroundHigh-temperature bioethanol production benefits from yeast thermotolerance. Salt stress could induce obvious cross-protection against heat stress of Pichia kudriavzevii, contributing to the improvement of its thermotolerance and bioethanol fermentation. However, the underlying mechanisms of the cross-protection remain poorly understood.ResultsSalt stress showed obvious cross-protection for thermotolerance and high-temperature ethanol production of P. kudriavzevii observed by biomass, cell morphology and bioethanol production capacity. The biomass and ethanol production of P. kudriavzevii at 45 °C were, respectively, improved by 2.6 and 3.9 times by 300 mmol/L NaCl. Metabolic network map showed that salt stress obviously improved the key enzymes and intermediates in carbohydrate metabolism, contributing to the synthesis of bioethanol, ATP, amino acids, nucleotides, and unsaturated fatty acids, as well as subsequent intracellular metabolisms. The increasing trehalose, glycerol, HSPs, and ergosterol helped maintain the normal function of cell components. Heat stress induced serious oxidative stress that the ROS-positive cell rate and dead cell rate, respectively, rose from 0.5% and 2.4% to 28.2% and 69.2%, with the incubation temperature increasing from 30 to 45 °C. The heat-induced ROS outburst, oxidative damage, and cell death were obviously inhibited by salt stress, especially the dead cell rate which fell to only 20.3% at 300 mmol/L NaCl. The inhibiting oxidative damage mainly resulted from the abundant synthesis of GSH and GST, which, respectively, increased by 4.8 and 76.1 times after addition of 300 mmol/L NaCl. The improved bioethanol production was not only due to the improved thermotolerance, but resulted from the up-regulated alcohol dehydrogenases and down-regulated aldehyde dehydrogenases by salt stress.ConclusionThe results provide a first insight into the mechanisms of the improved thermotolerance and high-temperature bioethanol production of P. kudriavzevii by salt stress, and provide important information to construct genetic engineering yeasts for high-temperature bioethanol production.Graphical

Genetic Modification of KNAT7 Transcription Factor Expression Enhances Saccharification and Reduces Recalcitrance of Woody Biomass in Poplars.

The precise role of KNAT7 transcription factors (TFs) in regulating secondary cell wall (SCW) biosynthesis in poplars has remained unknown, while our understanding of KNAT7 functions in other plants is continuously evolving. To study the impact of genetic modifications of homologous and heterologous KNAT7 gene expression on SCW formation in transgenic poplars, we prepared poplar KNAT7 (PtKNAT7) overexpression (PtKNAT7-OE) and antisense suppression (PtKNAT7-AS) vector constructs for the generation of transgenic poplar lines via Agrobacterium-mediated transformation. Since the overexpression of homologous genes can sometimes result in co-suppression, we also overexpressed Arabidopsis KNAT7 (AtKNAT7-OE) in transgenic poplars. In all these constructs, the expression of KNAT7 transgenes was driven by developing xylem (DX)-specific promoter, DX15. Compared to wild-type (WT) controls, many SCW biosynthesis genes downstream of KNAT7 were highly expressed in poplar PtKNAT7-OE and AtKNAT7-OE lines. Yet, no significant increase in lignin content of woody biomass of these transgenic lines was observed. PtKNAT7-AS lines, however, showed reduced expression of many SCW biosynthesis genes downstream of KNAT7 accompanied by a reduction in lignin content of wood compared to WT controls. Syringyl to Guaiacyl lignin (S/G) ratios were significantly increased in all three KNAT7 knockdown and overexpression transgenic lines than WT controls. These transgenic lines were essentially indistinguishable from WT controls in terms of their growth phenotype. Saccharification efficiency of woody biomass was significantly increased in all transgenic lines than WT controls. Overall, our results demonstrated that developing xylem-specific alteration of KNAT7 expression affects the expression of SCW biosynthesis genes, impacting at least the lignification process and improving saccharification efficiency, hence providing one of the powerful tools for improving bioethanol production from woody biomass of bioenergy crops and trees.

Use of activated carbons for detoxification of a lignocellulosic hydrolysate: Statistical optimisation

In order to improve bioethanol production from hydrolysates from olive tree pruning residues the present study assessed a detoxification method using the same waste material. The objective was to develop optimal removal without diminishing fermentable sugars. A statistical optimisation study was performed for the production of activated carbons using the same olive tree pruning residue as raw material. The effect of the impregnation ratio, activation temperature and the time needed for maximum removal of the phenolic compounds was assayed. The conditions for maximum adsorption were as follows: 799.18 °C, IR 4.88 and 101.38 min of activation, removing 55%, 64% and 41% of total phenolic compounds, HMF and furfural, respectively. The activated carbon obtained under optimum conditions (Aop) was submitted to adsorption kinetics and equilibrium assays. Experimental kinetic data were well represented by a pseudo-first order kinetic and intraparticle diffusion model, which helped explain the adsorption mechanism. Experimental isotherms were analysed with Langmuir, Freundlich and Temkin models, and Langmuir best fitted the experimental data. Hydrolysate from olive tree pruning residues, detoxified with the optimum activated carbon, was fermented with a commercial Saccharomyces cerevisiae yeast, obtaining a 100% theoretical maximum ethanol yield and a 56% bioconversion of fermentable sugars.

Bioethanol Production by Pichia stipitis Immobilized on Water Hyacinth and Thin-Shell Silk Cocoon

Cell immobilization technique was applied in this study in order to examine effect of immobilized Pichia stipitis TISTR5806 on bioethanol production. Water hyacinth (WH) and thin-shell silk cocoon (CC) were used as cell carriers. Characteristics of the cell carriers were examined to explain the mechanism of bioethanol production. Carrier sizes and weights were optimized to improve bioethanol production. Moreover, stabilities of immobilized cells and carriers were evaluated. Because of high porosity, high surface area and good swelling ability of WH, cell immobilized on 1 g WH with 1 cm length produced the highest ethanol concentration at 13.3 g/L. Five cycles of a repeated batch of immobilized cell (IC) system on WH showed stable performance in ethanol production (8.2–10.4 g/L) with large numbers of the immobilized cells. The interaction between the immobilized cells and the WH surface were discovered.

Sequential fed batch extractive fermentation for enhanced bioethanol production using recycled Spathaspora passalidarum and mixed sugar composition

The simultaneous ethanol production and removal during sequential cell recycle fed batch fermentation provides a complementary route to produce this biofuel from sugar mixtures, which may greatly improve yields and productivity from lignocellulosic hydrolysates. Spathaspora passalidarum is a wild-type strain able to naturally convert glucose, fructose, xylose and arabinose into ethanol. Therefore, the present work has focused on 2G bioethanol production by S. passalidarum aiming at the consumption of all sugars released after pre-treatment and enzymatic hydrolysis of sugarcane bagasse in a single fermentation step. The fermentation strategy with sequential cell recycle, fed-batch mode and ethanol removal in situ was performed on a hemicellulosic hydrolysate medium supplemented with molasses. This strategy gave improved fermentation performance and enabled the co-fermention of all sugars under microaerobic conditions. The maximum ethanol yield and productivity was 0.482 g.g−1 and 9.5 g·L-1·h−1, respectively, showing a process efficiency of 94.3%. The selective ethanol removal enables the operation of the bioreactor at low levels of ethanol (20–30 g·L-1), even with high sugar concentration inputs, accelerating the fermentation performance and avoiding inhibitory effects on yeast metabolism. Applying the cell recycle strategy, S. passalidarum was able to increase its robustness, as shown by a 10-fold increase in ethanol productivity, and it was also able to tolerate a high acetic acid concentration (4.5 g·L-1) during long-term fermentations. These results demonstrate that the bioprocess strategy has a strong potential to improve bioethanol production of rich mixed sugar from lignocellulosic hydrolysates in a single fermentation step.

Feasibility of bioethanol production from rice bran

Rice bran is a by-product of rice production with a high carbohydrate and starch content and the potential for bioethanol production by alcoholic fermentation. This article describes bioethanol production by Saccharomyces cerevisiae from hydrolyzed defatted rice bran (DRB) a rice by-product applying ultrasonic treatment and protease addition, as well as a sequential strategy of experimental design (SEED). In the first Central Composite Rotatable Design (CCRD), the temperature (25-30 °C) and inoculum concentration (0.5-50 g L-1) had positive effects on bioethanol production, while the effect of pH (4.0-6.0) was not significant. In the second CCRD, the temperature (28-35 °C) and inoculum concentration (10-70 g L-1) had negative and positive effects on bioethanol production (p < 0.05). Protease addition (15 µL g-1) increased the conversion of substrate into bioethanol by 76%. The optimized conditions for the production of 40.7 g L-1 bioethanol were a temperature of 31.5 °C and an inoculum concentration of 70 g L-1. Validation in a benchtop bioreactor produced 40.0 g L-1 of bioethanol from hydrolyzed DRB, and the SEED was characterized as a useful tool to improve bioethanol production from DRB. Furthermore, the DRB proved to be a by-product with great potential for bioethanol production, derived from alternative sources not commonly used in human food.

Inhibition of Maize Caffeate 3-O-Methyltransferase by Nitecapone as a Possible Approach to Reduce Lignocellulosic Biomass Recalcitrance

Maize (Zea mays) caffeate 3-O-methyltransferase (ZmaysCOMT, EC 2.1.1.68), a key enzyme of the phenylpropanoid pathway, catalyzes the O-methylation of caffeic acid to ferulic acid, a precursor of lignin polymer and a crucial component of the cell wall structure. Plant cell wall recalcitrance is due to lignin, and the discovery of specific inhibitors of ZmaysCOMT could be useful to increase the digestibility of the lignocellulose biomass and improve the production of cellulosic biofuels. In this work, we have modeled the three-dimensional structure of ZmaysCOMT and prospected promising inhibitors by using virtual screening techniques. A set of 1668 putative candidates was screened from a virtual library and docked in the active site of the enzyme, and nitecapone was selected as one of the most promising enzyme inhibitors. Details of the mode of inhibition were assessed by in silico simulation and in vitro assays of nitecapone on the enzyme. In comparison with the nitecapone-free control, kinetics parameters showed different values of Vmax and KM, suggesting a kinetic profile such as mixed inhibition of the ZmaysCOMT. In brief, we suggest that the nitecapone-induced inhibition of ZmaysCOMT may serve as a non-transgenic strategy to explore the biosynthesis of ferulic acid and lignin, their relationships with the recalcitrance of lignocellulosic biomass, and, possibly, to improve bioethanol production.

The Improvement of Bioethanol Production by Pentose-Fermenting Yeasts Isolated from Herbal Preparations, the Gut of Dung Beetles, and Marula Wine.

Efficient conversion of pentose sugars to ethanol is important for an economically viable lignocellulosic bioethanol process. Ten yeasts fermenting both D-xylose and L-arabinose were subjected to an adaptation process with L-arabinose as carbon source in a medium containing acetic acid. Four Meyerozyma caribbica-adapted strains were able to ferment L-arabinose to ethanol in the presence of 3 g/L acetic acid at 35°C. Meyerozyma caribbica Mu 2.2f fermented L-arabinose to produce 3.0 g/L ethanol compared to the parental strain with 1.0 g/L ethanol in the absence of acetic acid. The adapted M. caribbica Mu 2.2f strain produced 3.6 and 0.8 g/L ethanol on L-arabinose and D-xylose, respectively, in the presence of acetic acid while the parental strain failed to grow. In a bioreactor, the adapted M. caribbica Mu 2.2f strain produced 5.7 g/L ethanol in the presence of 3 g/L acetic acid with an ethanol yield and productivity of 0.338 g/g and 0.158 g/L/h, respectively, at a KLa value of 3.3 h−1. The adapted strain produced 26.7 g/L L-arabitol with a yield of 0.900 g/g at a KLa value of 4.9 h−1.

Innovative hydrolysis of corn stover biowaste by modified magnetite laccase immobilized nanoparticles

Intensive studies have been performed on the improvement of bioethanol production by transformation of lignocellulose biomass. In this study, the digestibility of corn stover was dramatically improved by using laccase immobilized on Cu2+ modified recyclable magnetite nanoparticles, Fe3O4–NH2. After digestion, the laccase was efficiently separated from slurry. The degradation rate of lignin reached 40.76%, and the subsequent cellulose conversion rate 38.37% for 72 h at 35 °C with cellulase at 50 U g−1 of corn stover. Compared to those of free and inactivated mode, the immobilized laccase pre-treatment increased subsequent cellulose conversion rates by 23.98% and 23.34%, respectively. Moreover, the reusability of immobilized laccase activity remained 50% after 6 cycles. The storage and thermal stability of the fixed laccase enhanced by 70% and 24.1% compared to those of free laccase at 65 °C, pH 4.5, respectively. At pH 10.5, it exhibited 16.3% more activities than its free mode at 35 °C. Our study provides a new avenue for improving the production of bioethanol with immobilized laccase for delignification using corn stover as the starting material.

The Xylose Metabolizing Yeast Spathaspora passalidarum is a Promising Genetic Treasure for Improving Bioethanol Production

Currently, the fermentation technology for recycling agriculture waste for generation of alternative renewable biofuels is getting more and more attention because of the environmental merits of biofuels for decreasing the rapid rise of greenhouse gas effects compared to petrochemical, keeping in mind the increase of petrol cost and the exhaustion of limited petroleum resources. One of widely used biofuels is bioethanol, and the use of yeasts for commercial fermentation of cellulosic and hemicellulosic agricultural biomasses is one of the growing biotechnological trends for bioethanol production. Effective fermentation and assimilation of xylose, the major pentose sugar element of plant cell walls and the second most abundant carbohydrate, is a bottleneck step towards a robust biofuel production from agricultural waste materials. Hence, several attempts were implemented to engineer the conventional Saccharomyces cerevisiae yeast to transport and ferment xylose because naturally it does not use xylose, using genetic materials of Pichia stipitis, the pioneer native xylose fermenting yeast. Recently, the nonconventional yeast Spathaspora passalidarum appeared as a founder member of a new small group of yeasts that, like Pichia stipitis, can utilize and ferment xylose. Therefore, the understanding of the molecular mechanisms regulating the xylose assimilation in such pentose fermenting yeasts will enable us to eliminate the obstacles in the biofuels pipeline, and to develop industrial strains by means of genetic engineering to increase the availability of renewable biofuel products from agricultural biomass. In this review, we will highlight the recent advances in the field of native xylose metabolizing yeasts, with special emphasis on S. passalidarum for improving bioethanol production.

A method for concentration of monosaccharide and removal of inhibitors during hydrolysate pretreatment for improved bioethanol production

Due to its renewable and clean characteristics, bioethanol is an advantageous alternative energy source to fossil fuels. Although crop straw is a suitable material for bioethanol production, the low-concentration of fermentable sugars and the presence of inhibitors such as furfural and acetic acid in straw hydrolysate are not conducive to ethanol fermentation and pose significant limitations. This work aims to facilitate the removal of furfural and acetic acid during sugar concentration. A reverse osmosis (RO) membrane was chosen for the simultaneous concentration of sugar and removal of inhibitors. Additionally, an ethanol promotion method was developed, the results from which indicated that the retentions of furfural and acetic acid were reduced in the RO process. In different synthetic solutions with ethanol, the retention of furfural decreased by 48.40% when the ratio of ethanol/furfural concentration was 12.10, and the retention of acetic acid decreased by 22.30% when the ratio of ethanol/acetic acid concentration was 5.80. The findings of this study showed that the method for inhibitor removal proposed in this study could promote the removal of furfural and acetic acid from hydrolysate and has great potential for use in bioethanol production.

Improvement of bioethanol production in cornstalk fermentation through hydrolysis by fungi Trichoderma reesei exposed to gamma rays

The utilization of cornstalk as a raw material for bioethanol production is an alternative solution to increase the added value of lignocellulose from agricultural wastes. The glucose content of lignocellulose material can be increased through the hydrolysis process by cellulolytic microorganisms. This research aims to obtain fungi Trichoderma reesei which capable of optimizing glucose content on lignocellulose materials and influence to increase in bioethanol production by Saccharomyces cerevisiae. The experiments were conducted with 2 factorials consist of substrates pre-treatments and hydrolysis. Pre-treatment of substrates by aqua dest (S1), 1% H2SO4 (S2) and 1% NaOH (S3) solution. Treatments of hydrolysis consist of control (K) and fungi T.reesei 750 Gray (H). The fungi T.reesei 750 Gray was obtained from optimizing by gamma rays at a dose of 250, 500, 750 and 1000 Gray. The fungi T. reesei 750 Gy have the specific cellulase activity about 0.24 U/mg higher than the fungi T. reesei 0 Gy about 0.16 U/mg. Pre-treatment of substrates with 1% NaOH (1: 5 w/v) solution and hydrolysis by fungi T.reesei 750 Gray (S3H) is suitable for solid fermentation with S.cerevisiae. After 2 days of solid fermentation by Saccharomyces cerevisiae, the bioethanol contents in S3H medium about 387 ppm while S1K (control) about 196 ppm or increase about 98% (ppm/ppm). This result is expected to be an alternative solution for the utilization of cornstalk for bioethanol production.

Improvement of bioethanol production by Saccharomyces cerevisiae through the deletion of GLK1, MIG1 and MIG2 and overexpression of PGM2 using the red seaweed Gracilaria verrucosa

Gracilaria verrucosa can be used as a biomass source for bioethanol production with a high yield due to the high carbohydrate content. A monosaccharide concentration of 48.6 g/L was obtained from G. verrucosa using 0.2 M HNO3 at 150 °C for 10 min with a combined severity factor (Ro) of 0.47. The addition of Tween 20 with CTec2 increased the affinity of G. verrucosa slurry to the enzyme with a Km value of 1.14 g/L compared with the Km value of 1.82 g/L using CTec2 alone and reduced enzyme loading. To improve galactose consumption, the deletion of repressor genes including GLK1, MIG1, and MIG2 and the overexpression of PGM2 were investigated. The strain with the deletion of three repressor genes and overexpression of PGM2 produced 24.8 g/L ethanol (YEtOH = 0.46) and showed an enhanced galactose consumption rate (0.24 g/L/h) compared with the rate of the control strain (0.04 g/L/h). The deletion of three repressor genes and overexpression of PGM2 reduced the repression of GAL genes and increased the transcription level of GAL genes. However, the galactose consumption rate of this strain was lower than that of the strain adapted to a high concentration of galactose.

Improvement of Bioethanol Production from Sweet Sorghum Juice under Very High Gravity Fermentation: Effect of Nitrogen, Osmoprotectant, and Aeration

To improve ethanol production fermentation efficiency from sweet sorghum juice under a very high gravity (VHG, 280 g/L of total sugar) condition by Saccharomyces cerevisiae NP01, dried spent yeast (DSY), yeast extract, and glycine concentrations were optimized using an L9 (34) orthogonal array design. The results showed that the order of influence on the ethanol concentration (PE) was yeast extract > glycine > DSY. The optimal nutrient concentrations for ethanol production were determined as follows: yeast extract, 3; DSY, 4; and glycine, 5 g/L. When a verification experiment under the projected optimal conditions was done, the P, ethanol yield (Yp/s), and ethanol productivity (Qp) values were 120.1 g/L, 0.47, and 2.50 g/L·h, respectively. These values were similar to those of the positive control experiment with yeast extract supplementation at 9 g/L. The yeast viability under the optimal condition was higher than that of the control experiment. To improve sugar utilization and ethanol production, aeration at 2.5 vvm for 4 h was applied under the optimal nutrient supplementation. The P, Yp/s, and Qp values were significantly increased to 134.3 g/L, 0.50, and 2.80 g/L·h, respectively.

Improvement of bioethanol production from pomegranate peels via acidic pretreatment and enzymatic hydrolysis.

The aim of this study was to improve the ethanol production from pomegranate peels (PPs). Therefore, the effect of enzymatic hydrolysis and different pretreatments on ethanol production by yeasts was examined. There were three different enzyme concentrations (3.6, 7.2, 14.4 FPU/g substrate) tested for enzymatic hydrolysis, and four different PP media, such as WSPP (whole slurry of PP), LFPP (liquid fraction of PP), WSFPP (washed solid fraction of PP) and N-WSFPP (non-washed solid fraction of PP), were prepared. Bioethanol production was monitored for 96 h. Maximum ethanol concentrations were obtained at WSPP medium as 12.69 g/L, 14.35 g/L and 4.23 g/L in Saccharomyces cerevisiae, Kluyveromyces marxianus and Pichia stipitis, respectively. On the other hand, the washing step of biomass increased the kinetic parameters dramatically and the highest theoretical ethanol yields and YP/S values were obtained from WSFPP medium in all tested yeasts. Theoretical ethanol yields were 97.8%, 98.7% and 35.5% for S. cerevisiae, K. marxianus and P. stipitis, respectively. Qp values were observed as 0.98 g/L h, 0.99 g/L h and 0.04 g/L h for the same yeasts. The highest YP/S values were detected as 0.50 g/g for S. cerevisiae, 0.50 g/g for K. marxianus and 0.30 g/g for P. stipitis in the washed pomegranate peel biomass.

Co-cultivation of two engineered strains of Synechocystis sp. PCC 6803 results in improved bioethanol production

The Synechocystis sp. PCC 6803 is a promising host for ethanol biosynthesis from CO2. In this study, the production of ethanol was performed by utilizing two different engineered Synechocystis strains consisting of pdc-adh (pyruvate decarboxylase and alcohol dehydrogenase) overexpression (▲APX) and glgC-phaA (glucose-1-phosphate adenylyltransferase and PHA-specific β-ketothiolase) knockout (ΔGBK) pathways. This strategy involves destruction of glycogen and polyhydroxybutyrate (PHB) synthesis, both of which act as a storage polymer. The blocking of such biosynthetic pathways enabled Synechocystis to release various metabolites into the medium with an increase of chlorophyll a content and oxygen evolution due to the reduction of glycogen and PHB synthesis. In co-cultivation system, the extracellular metabolites released from knock out strain (ΔGBK) were utilized by the pdc-adh overexpressing strain (▲APX) and produced 4708 mg/L ethanol, which is comparatively higher than pdc-adh overexpressing strain containing glgC and PhaA knock out (ΔGBK-▲APX) (4103 mg/L). Although the concentration of ▲APX cells were lower in co-culture than in mono-culture, the overflow of metabolites from knock out Synechocystis and the consumption of those products by pdc-adh overexpressing Synechocystis in co-cultivation system improved the overall yield.

Co-culture strategy for improved 2G bioethanol production using a mixture of sugarcane molasses and bagasse hydrolysate as substrate

Mixed sugar co-fermentation represents an interesting fermentation strategy to improve bioethanol production from low-cost raw materials, such as sugarcane bagasse. The aim of this study was to evaluate a mixture of sugars (hexoses and pentoses) from sugarcane bagasse hemicellulosic hydrolysate diluted with sugarcane molasses as substrate to be converted either in monoculture or co-culture fermentations. A co-culture fermentation technique using hexoses-fermenting yeasts (Saccharomyces cerevisiae: from Santa Adélia Mill, an industrial strain (SA) and a respiratory-deficient mutant strain (ScP)) together with xylose-fermenting yeasts (Scheffersomyces stipitis (SS) and Spathaspora passalidarum (SP)) was carried out to enhance bioethanol production. Co-culture fermentation in a medium with a high mixed sugar concentration (100 g.L−1) resulted in a complete sugar consumption, ethanol titer of 30.2 g.L−1 and increased ethanol productivity (Qp =4.44 g.L−1. h−1) using wild type strains (SP + SA). In contrast, the maximum ethanol titer (49.2 g.L−1) was reached during co-culture performed with S. stipitis and the respiratory-deficient mutant strain of S. cerevisiae, however ethanol productivity (Qp =0.60 g.L−1. h−1) decreased due to the poor fermentation performance of the mutant strain. This work suggests that improved efficiency and good scalability of the mixed sugar fermentation process makes an important basis for the economic viability of bioethanol production from lignocelluloses.

A strategy for sequential fermentation by Saccharomyces cerevisiae and Pichia stipitis in bioethanol production from hardwoods

Climate change due to global warming has led to the expansion of deciduous forests. Increased product yields from hardwood provide an opportunity for the exploitation of biomass resources, such as bioethanol. In this study, the enzymatic hydrolysis of various hardwoods pretreated using the hydrogen peroxide–acetic acid (HPAC) method was evaluated. Glucose and xylose were fermented sequentially to improve the bioethanol production from hardwood. The HPAC pretreatment significantly reduced lignin and improved the hydrolysis efficiency compared with no treatment. The enhanced hydrolysis efficiency contributed to an increase in the bioethanol productivity, which was similar to that of simultaneous saccharification and fermentation, by reducing the processing time of separate hydrolysis and fermentation. The sequential fermentation of glucose and xylose via Saccharomyces cerevisiae and Pichia stipitis, respectively, improved ethanol production by approximately 12% over conventional fermentation used with glucose alone. The results suggest that sequential fermentation can improve bioethanol production from hardwood.

Detoxification of Hydrolysates of the Red Seaweed Gelidium amansii for Improved Bioethanol Production

In this study, bioethanol was produced from the seaweed Gelidium amansii as biomass through separate hydrolysis and fermentation (SHF) processes. The SHF processes examined in this study include thermal acid hydrolysis pretreatment, enzymatic saccharification, detoxification, and fermentation. Thermal acid hydrolysis pretreatment was conducted using H2SO4, with a slurry content of 8-16% and treatment time of 15-75min. The optimal conditions for thermal acid hydrolysis pretreatment were 12% (w/v) seaweed slurry content and 180mM H2SO4 at 121°C for 45min, at which 26.1g/L galactose and 6.8g/L glucose were produced. A monosaccharide (mainly glucose) was also obtained from the enzymatic saccharification of thermal acid hydrolysate using 16U/mL Celluclast 1.5L enzyme at 45°C for 36h. Detoxification was performed using the adsorption method with activated carbon, the overliming method with Ca (OH)2, and the ion exchange method with polyethyleneimine. Among those detoxification methods, activated carbon showed the best performance for hydroxymethylfurfural removal. Ethanol fermentation was performed using 12% (w/v) seaweed hydrolysate with Saccharomyces cerevisiae adapted to galactose as well as various detoxification treatments.

New technologies provide more metabolic engineering strategies for bioethanol production in Zymomonas mobilis.

Bioethanol has been considered as a potentially renewable energy source, and metabolic engineering plays an important role in the production of biofuels. As an efficient ethanol-producing bacterium, Zymomonas mobilis has garnered special attention due to its high sugar uptake, ethanol yield, and tolerance. Different metabolic engineering strategies have been used to establish new metabolic pathways for Z. mobilis to broaden its substrate range, remove competing pathways, and enhance its tolerance to ethanol and lignocellulosic hydrolysate inhibitors. Recent advances in omics technology, computational modeling and simulation, system biology, and synthetic biology contribute to the efficient re-design and manipulation of microbes via metabolic engineering at the whole-cell level. In this review, we summarize the progress of some new technologies used for metabolic engineering to improve bioethanol production and tolerance in Z. mobilis. Some successful examples of metabolic engineering used to develop strains for ethanol production are described in detail. Lastly, some important strategies for future metabolic engineering efforts are also highlighted.