Ethanol Production Rate Research Articles

Enhanced xylose fermentation and ethanol production by engineered Saccharomyces cerevisiae strain.

We have recently demonstrated that heterologous expression of a bacterial xylose isomerase gene (xylA) of Burkholderia cenocepacia enabled a laboratorial Saccharomyces cerevisiae strain to ferment xylose anaerobically, without xylitol accumulation. However, the recombinant yeast fermented xylose slowly. In this study, an evolutionary engineering strategy was applied to improve xylose fermentation by the xylA-expressing yeast strain, which involved sequential batch cultivation on xylose. The resulting yeast strain co-fermented glucose and xylose rapidly and almost simultaneously, exhibiting improved ethanol production and productivity. It was also observed that when cells were grown in a medium containing higher glucose concentrations before being transferred to fermentation medium, higher rates of xylose consumption and ethanol production were obtained, demonstrating that xylose utilization was not regulated by catabolic repression. Results obtained by qPCR demonstrate that the efficiency in xylose fermentation showed by the evolved strain is associated, to the increase in the expression of genes HXT2 and TAL1, which code for a low-affinity hexose transporter and transaldolase, respectively. The ethanol productivity obtained after the introduction of only one genetic modification and the submission to a one-stage process of evolutionary engineering was equivalent to those of strains submitted to extensive metabolic and evolutionary engineering, providing solid basis for future applications of this strategy in industrial strains.Electronic supplementary materialThe online version of this article (doi:10.1186/s13568-015-0102-y) contains supplementary material, which is available to authorized users.

Design, development and comparison of ethanol production from free and immobilizedsaccharomyces cerevisiaeunder stationary culture

Background: The aim of the present study is to design, develop and compare the production capacity and the difference in the yield of the ethanol by fermentation technology. The utility of the alcohol per annum is increasing day by day as the number of industries using ethanol as a solvent, pharmaceutical aid. Hence the production of ethanol by fermentation technology has triggered us to investigate the best way and method to produce ethanol of benefit of Pharmaceutical Industry. Method: The conventional method of production of alcohol was performed by aerobic batch type of fermentation: the yield was noted. The same production was performed by using immobilized yeast cells by gel entrapment technique. We produced ethanol by free and immobilized 'saccharomyces cerevisiae' under stationary culture. Cane molasses was used as a sugar source for about (12 - 21%), medium pH (4-5.5). incubation temperature (25 degreesc-30 degreesc), volume of fermentation medium (200 - 350ml) and reuse of immobilized yeast cells. Result: Rate of ethanol production was maximal with immobilized yeast cells. The results indicate that 2gm vegetative cells of yeast on utilizing molasses at 15% sugar concentration with medium pH 4.5 at 30oC. Conclusion: Hence we conclude that the fermentation optimization and the yield value of popular organic solvent (ethanol) was studied and reported. The specific gravity of the distillated alcohol by free and immobilized yeast cells were found to be 0.9730, 0.9705 at room temperature and the alcohol content was found to be 85%, 93.52% as per International Pharmacopeia respectively. To conclude the immobilized yeast cells were found to yield more ethanol than free yeast cells.

Xylose fermentation as a challenge for commercialization of lignocellulosic fuels and chemicals.

Fuel ethanol production from lignocellulosic materials is at a level where commercial biofuel production is becoming a reality. The solubilization of the hemicellulose fraction in lignocellulosic-based feedstocks results in a large variety of sugar mixtures including xylose. However, allowing xylose fermentation in yeast that normally is used for fuel ethanol production requires genetic engineering. Moreover, the efficiency of lignocellulosic pretreatment, together with the release and generation of inhibitory compounds in this step, are some of the new challenges faced during second generation ethanol production. Successful advances in all these aspects will improve ethanol yield, productivity and titer, which will reduce the impact on capital and operating costs, leading to the consolidation of the fermentation of lignocellulosic biomass as an economically feasible option for the production of renewable fuels. Therefore the development of yeast strains capable of fermenting a wide variety of sugars in a highly inhibitory environment, while maintaining a high ethanol yield and production rate, is required. This review provides an overview of the current status in the use of xylose-engineered yeast strains and describes the remaining challenges to achieve an efficient deployment of lignocellulosic-based ethanol production.

Coffee extract residue for production of ethanol and activated carbons

Biomass as a waste product in the form coffee extract residue (CER) has been shown to have potential for the dual purpose of ethanol production and preparation of activated carbons. A straightforward method of direct hydrolysis and fermentation (DHF) is considered as the main option utilized in this study for the generation of fuel ethanol from the biomass waste. Factors such as loadings of saccharomyces cerevisiae, temperatures (21 and 30 °C) and substrate content were investigated to maximize the yield of ethanol. Ethanol production rates between 1.1 g and 0.70 g h−1 kg−1 without pretreatment and 2.7 and 2.3 g h−1 kg−1 dry substance with mild treatment were obtained, respectively. The CER was also used to prepare activated carbons using both chemical and physical activation methods. The effects of process parameters such as temperatures and concentrations of acid were varied and determined as to the yield, BET-surface areas and porosities of the final product. H3PO4 treatment at 600 °C and steam treatment at 700 °C show maximum surface area of >640 m2 g−1 with increased total pore and micropore volumes.

Utilization of Cheese Whey for Bio-ethanol Production

This study was aimed to investigate the feasibility of bio- ethanol production by batch fermentation of kareish cheese whey. Two forms of whey; untreated (crude) whey containing 5% lactose and treated whey (deproteinized and concentrated to 14% lactose) were utilized. Fermentation processes were performed by two strains of Kluyveromyces marxianus and four strains of Saccharomyces cerevisiae which were previously recognized as ethanol- producing strains. Effects of different initial pH values, as well as, external supplementation of treated whey by four different nitrogen sources on the rate of ethanol production by two of the highest producing strains were also investigated. All the studied yeast strains were able to grow and produce ethanol from both crude and treated whey. Levels of ethanol production ranged between 3.4- 18.5g/l and 24.11-57.66 g/l from crude and treated whey, respectively. The most suitable initial pH maximizing ethanol yield was 5.5 and the best added nitrogen source was yeast extract. Maximum ethanol levels produced by K. marxianus ZMS3GU133329 and S.cerevisiae EC1118 from treated whey adjusted to pH 5.5 and supplemented by 0.3% yeast extract reached to 69.85 and 65.36 g/L, corresponding to 97.8 and 91.4% of the theoretical values, respectively. The kinetic parameters and productivity were calculated and discussed for all experiments.

Enhanced hexose fermentation by Saccharomyces cerevisiae through integration of stoichiometric modeling and genetic screening

In order to determine beneficial gene deletions for ethanol production by the yeast Saccharomyces cerevisiae, we performed an in silico gene deletion experiment based on a genome-scale metabolic model. Genes coding for two oxidative phosphorylation reactions (cytochrome c oxidase and ubiquinol cytochrome c reductase) were identified by the model-based simulation as potential deletion targets for enhancing ethanol production and maintaining acceptable overall growth rate in oxygen-limited conditions. Since the two target enzymes are composed of multiple subunits, we conducted a genetic screening study to evaluate the in silico results and compare the effect of deleting various portions of the respiratory enzyme complexes. Over two-thirds of the knockout mutants identified by the in silico study did exhibit experimental behavior in qualitative agreement with model predictions, but the exceptions illustrate the limitation of using a purely stoichiometric model-based approach. Furthermore, there was a substantial quantitative variation in phenotype among the various respiration-deficient mutants that were screened in this study, and three genes encoding respiratory enzyme subunits were identified as the best knockout targets for improving hexose fermentation in microaerobic conditions. Specifically, deletion of either COX9 or QCR9 resulted in higher ethanol production rates than the parental strain by 37% and 27%, respectively, with slight growth disadvantages. Also, deletion of QCR6 led to improved ethanol production rate by 24% with no growth disadvantage. The beneficial effects of these gene deletions were consistently demonstrated in different strain backgrounds and with four common hexoses. The combination of stoichiometric modeling and genetic screening using a systematic knockout collection was useful for narrowing a large set of gene targets and identifying targets of interest.

Metabolic engineering for improved production of ethanol by Corynebacterium glutamicum.

Recombinant Corynebacterium glutamicum harboring genes for pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adhB) can produce ethanol under oxygen deprivation. We investigated the effects of elevating the expression levels of glycolytic genes, as well as pdc and adhB, on ethanol production. Overexpression of four glycolytic genes (pgi, pfkA, gapA, and pyk) in C. glutamicum significantly increased the rate of ethanol production. Overexpression of tpi, encoding triosephosphate isomerase, further enhanced productivity. Elevated expression of pdc and adhB increased ethanol yield, but not the rate of production. Fed-batch fermentation using an optimized strain resulted in ethanol production of 119 g/L from 245 g/L glucose with a yield of 95% of the theoretical maximum. Further metabolic engineering, including integration of the genes for xylose and arabinose metabolism, enabled consumption of glucose, xylose, and arabinose, and ethanol production (83 g/L) at a yield of 90 %. This study demonstrated that C. glutamicum has significant potential for the production of cellulosic ethanol.

Producing Ethanol from Water Hyacinth through Simultaneous Saccharification and Fermentation with Acclimatized Yeasts

The production rate and yield of ethanol was markedly increased when water hyacinth, pretreated with microwave-assisted dilute acid, was fermented with acclimatized yeasts through simultaneous saccharification and fermentation. Water hyacinth hydrolyzate microwaved with 1% (v/v) dilute sulfuric acid was fermented with the acclimatized mixed yeasts Pichia stipitis and Pachysolen tannophilus at a volume ratio of 1:1. The highest ethanol yield was 0.22 g/g (raw biomass of water hyacinth), which was 76.3% of the theoretical ethanol yield. A maximum ethanol production rate of 0.19 g/(L•h) was obtained after 24 h.

Insights into acetate toxicity in Zymomonas mobilis 8b using different substrates.

BackgroundLignocellulosic biomass is a promising renewable feedstock for biofuel production. Acetate is one of the major inhibitors liberated from hemicelluloses during hydrolysis. An understanding of the toxic effects of acetate on the fermentation microorganism and the efficient utilization of mixed sugars of glucose and xylose in the presence of hydrolysate inhibitors is crucial for economic biofuel production.ResultsA new microarray was designed including both coding sequences and intergenic regions to investigate the acetate stress responses of Zymomonas mobilis 8b when using single carbon sources of glucose or xylose, or mixed sugars of both glucose and xylose. With the supplementation of exogenous acetate, 8b can utilize all the glucose with a similar ethanol yield, although the growth, final biomass, and ethanol production rate were reduced. However, xylose utilization was inhibited in both media containing xylose or a mixed sugar of glucose and xylose, although the performance of 8b was better in mixed sugar than xylose-only media. The presence of acetate caused genes related to biosynthesis, the flagellar system, and glycolysis to be downregulated, and genes related to stress responses and energy metabolism to be upregulated. Unexpectedly, xylose seems to pose more stress on 8b, recruiting more genes for xylose utilization, than does acetate. Several gene candidates based on transcriptome results were selected for genetic manipulation, and a TonB-dependent receptor knockout mutant was confirmed to have a slight advantage regarding acetate tolerance.ConclusionsOur results indicate Z. mobilis utilized a different mechanism for xylose utilization, with an even more severe impact on Z. mobilis than that caused by acetate treatment. Our study also suggests redox imbalance caused by stressful conditions may trigger a metabolic reaction leading to the accumulation of toxic intermediates such as xylitol, but Z. mobilis manages its carbon and energy metabolism through the control of individual reactions to mitigate the stressful conditions. We have thus provided extensive transcriptomic datasets and gained insights into the molecular responses of Z. mobilis to the inhibitor acetate when grown in different sugar sources, which will facilitate future metabolic modeling studies and strain improvement efforts for better xylose utilization and acetate tolerance.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0140-8) contains supplementary material, which is available to authorized users.

English

Algeria is one of the important date-producing countries with a yearly production of about 850,000 tons. The number of date palms is more than 18 million palm trees with more than one thousand varieties, however only 30% of this product is of good quality, and the rest is consumed locally or directly fed to the cattle. This study aims at transforming low quality types of dates using technical biotechnology (fermentation) into bioethanol. Our research shows that the average rate of ethanol production is 350 ml per kilogram of dates, thereby achieving a profit margin up to 2.9 € per kilogram, not to mention the byproducts of fermentation: The nuclei of dates, fibers etc. The application of this study allows the exploitation and marketing of poor quality date and a thus taking large profit that helps promote date palm trees cultivation and the production of all its types. Key words: Fermentation, bioethanol, anaerobic, biomass, palm tree, dates, El-ouel.

Systematic and evolutionary engineering of a xylose isomerase-based pathway in Saccharomyces cerevisiae for efficient conversion yields.

BackgroundEfficient xylose fermentation by yeast would improve the economical and sustainable nature of biofuels production from lignocellulosic biomass. However, the efficiency of xylose fermentation by the yeast Saccharomyces cerevisiae is suboptimal, especially in conversion yield, despite decades of research. Here, we present an improved performance of S. cerevisiae in xylose fermentation through systematic and evolutionary engineering approaches.ResultsThe engineering of S. cerevisiae harboring xylose isomerase-based pathway significantly improved the xylose fermentation performance without the need for intensive downstream pathway engineering. This strain contained two integrated copies of a mutant xylose isomerase, gre3 and pho13 deletion and XKS1 and S. stipitis tal1 overexpression. This strain was subjected to rapid adaptive evolution to yield the final, evolved strain (SXA-R2P-E) which could efficiently convert xylose to ethanol with a yield of 0.45 g ethanol/g xylose, the highest yield reported to date. The xylose consumption and ethanol production rates, 0.98 g xylose g cell−1 h−1 and 0.44 g ethanol g cell−1 h−1, respectively, were also among the highest reported. During this process, the positive effect of a pho13 deletion was identified for a xylose isomerase-containing strain and resulted in up to an 8.2-fold increase in aerobic growth rate on xylose. Moreover, these results demonstrated that low inoculum size and the cell transfer at exponential phase was found to be the most effective adaptation strategy during a batch culture adaptation process.ConclusionsThese results suggest that the xylose isomerase pathway should be the pathway of choice for efficient xylose fermentation in S. cerevisiae as it can outperform strains with the oxidoreductase pathway in terms of yield and ethanol production and xylose consumption rates. Consequently, the strain developed in this study could significantly improve the prospect of biofuels production from lignocellulosic biomass.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0122-x) contains supplementary material, which is available to authorized users.

Maltose and glucose utilization during fermentation of barley and sorghum lager beers as affected by β-amylase or amyloglucosidase addition

The effect of supplementation of amyloglucosidase or β-amylase during mashing of sorghum and barley beers was studied. The research focused in the increments in maltose and glucose concentrations, and their consumption during 144 h fermentation of lager beers. Barley, red sorghum and white sorghum malt beers, produced with regular or waxy sorghum adjuncts, and supplemented with these exogenous enzymes, were produced. The addition of exogenous enzymes increased up to 20% the total sugar content. Regression lines to describe sugar consumption were obtained (R2 > 0.951). Amyloglucosidase treatments had higher glucose contents and up to 4 times higher glucose consumption rates compared with unsupplemented or with β-amylase treatments. The latter treatments had maltose as predominant sugar. Amyloglucosidase treatments had higher ethanol production rates, however the maximum production rate was delayed 24 h, compared with maltose-rich treatments. These worts observed incomplete fermentations and contained higher residual sugars. β-amylase treatments were not significantly different compared with their regular counterparts in terms of sugar and ethanol production. The selection of adequate sorghum cultivars for malt and adjuncts as well as the use of an osmotolerant yeast should be considered when exogenous enzymes are supplemented in brewing.

Aromatic inhibitors derived from ammonia-pretreated lignocellulose hinder bacterial ethanologenesis by activating regulatory circuits controlling inhibitor efflux and detoxification.

Efficient microbial conversion of lignocellulosic hydrolysates to biofuels is a key barrier to the economically viable deployment of lignocellulosic biofuels. A chief contributor to this barrier is the impact on microbial processes and energy metabolism of lignocellulose-derived inhibitors, including phenolic carboxylates, phenolic amides (for ammonia-pretreated biomass), phenolic aldehydes, and furfurals. To understand the bacterial pathways induced by inhibitors present in ammonia-pretreated biomass hydrolysates, which are less well studied than acid-pretreated biomass hydrolysates, we developed and exploited synthetic mimics of ammonia-pretreated corn stover hydrolysate (ACSH). To determine regulatory responses to the inhibitors normally present in ACSH, we measured transcript and protein levels in an Escherichia coli ethanologen using RNA-seq and quantitative proteomics during fermentation to ethanol of synthetic hydrolysates containing or lacking the inhibitors. Our study identified four major regulators mediating these responses, the MarA/SoxS/Rob network, AaeR, FrmR, and YqhC. Induction of these regulons was correlated with a reduced rate of ethanol production, buildup of pyruvate, depletion of ATP and NAD(P)H, and an inhibition of xylose conversion. The aromatic aldehyde inhibitor 5-hydroxymethylfurfural appeared to be reduced to its alcohol form by the ethanologen during fermentation, whereas phenolic acid and amide inhibitors were not metabolized. Together, our findings establish that the major regulatory responses to lignocellulose-derived inhibitors are mediated by transcriptional rather than translational regulators, suggest that energy consumed for inhibitor efflux and detoxification may limit biofuel production, and identify a network of regulators for future synthetic biology efforts.

A thermo- and toxin-tolerant kefir yeast for biorefinery and biofuel production

Although biorefinery has become a common concept to convert biomass into biofuels and value-added chemicals for better cost-performance, good microbial hosts that can be used to implement the concept are still wanting. In this study, a Kluyveromyces marxianus yeast, named KY3, was isolated from a Taiwanese kefir microbial consortium. We showed that KY3 could grow on a broad spectrum of substrates, including hexose and pentose sugars. It is heat and toxin tolerant, can grow under a wide range of pH values (pH 2.5–9), and shows a high ethanol production rate at elevated temperatures. It also can produce value-added aromatic chemicals, such as 2-phenylethylethanol and 2-phenylethyl acetate, during the fermentative process. A genetic transformation was achieved in KY3 to express a rumen fungal β-glucosidase gene, and the transgenic host (KY3–NpaBGS) could efficiently convert cellobiose to ethanol. Furthermore, it was shown that a novel dual-microbe co-culture system of Bacillus subtilis and KY3–NpaBGS can be employed for bioethanol production from cellulosic material. Thus, KY3 has a high potential to be a good host for biorefinery.

Comparative study of the kinetic approach on the alcoholic fermentation procedure conducted in laboratory and scale-up systems by inverse gas chromatography

Summary The major objective of the present work was to compare the kinetic study of alcoholic fermentations conducted in the presence of wheat supported biocatalysts in laboratory scale and in a scale-up system of 80 L and to compare these results with those reported in literature. The kinetic study of fermentation processes was accomplished with the technique of reversed flow gas chromatography (RFGC), which is a version of inverse gas chromatography. The wine yeast species used was Saccharomyces cerevisiae AXAZ-1, and fermentations were conducted between 20 and 2°C. At low temperatures, maximal ethanol productivity and fermentation rate were reduced. The rate constants, determined through a mathematical model obtained from RFGC, were higher in the laboratory scale comparing to the scale-up system at the temperatures of 20 and 15°C. However, with the reduction of temperature, both systems presented almost similar values proving the great fermentative ability of immobilized cells even at extremely low tem...

Xylose and xylose/glucose co-fermentation by recombinant Saccharomyces cerevisiae strains expressing individual hexose transporters

Since the uptake of xylose is believed to be one of the rate-limiting steps for xylose ethanol fermentation by recombinant Saccharomyces cerevisiae strains, we transformed a hxt-null strain lacking the major hexose transporters (hxt1Δ-hxt7Δ and gal2Δ) with an integrative plasmid to overexpress the genes for xylose reductase (XYL1), xylitol dehydrogenase (XYL2) and xylulokinase (XKS1), and analyzed the impact that overexpression of the HXT1, HXT2, HXT5 or HXT7 permeases have in anaerobic batch fermentations using xylose, glucose, or xylose plus glucose as carbon sources. Our results revealed that the low-affinity HXT1 permease allowed the maximal consumption of sugars and ethanol production rates during xylose/glucose co-fermentations, but was incapable to allow xylose uptake when this sugar was the only carbon source. The moderately high-affinity HXT5 permease was a poor glucose transporter, and it also did not allow significant xylose uptake by the cells. The moderately high-affinity HXT2 permease allowed xylose uptake with the same rates as those observed during glucose consumption, even under co-fermentation conditions, but had the drawback of producing incomplete fermentations. Finally, the high-affinity HXT7 permease allowed efficient xylose fermentation, but during xylose/glucose co-fermentations this permease showed a clear preference for glucose. Thus, our results indicate that approaches to engineer S. cerevisiae HXT transporters to improve second generation bioethanol production need to consider the composition of the biomass sugar syrup, whereby the HXT1 transporter seems more suitable for hydrolysates containing xylose/glucose blends, whereas the HXT7 permease would be a better choice for xylose-enriched sugar streams.

Single-step fermentation of agricultural hemp residues for hydrogen and ethanol production

We have investigated a one-step fermentation process using purified hemp cellulose (PHC), hemp fibres (HF), and hemp hurds (HH), compared to reagent-grade α-cellullose (AC) for ethanol and hydrogen production by Clostridium thermocellum. Exponential phase growth and production rates on PHC were comparable to those observed with AC. Net production of ethanol for AC (8.47 mM) was only slightly higher than PHC (6.56 mM), but significantly higher than HF (5.48 mM) and HH (3.52 mM), while the final hydrogen yield was comparable for AC (12.70 mM), PHC (11.01 mM), HF (10.91 mM), and HH (4.72 mM). End-product yields were dependent on the intrinsic cellulosic content as well as the presence of other polymers present in substrate biomass. Rates of ethanol and hydrogen production were similar in early log phase but varied in mid-log, and accessibility to cellulose was shown to determine yield and metabolic flux distribution. Our data suggest that improving the accessibility of the cellulolytic bacterium, C. thermocellum, to cellulose fibres has a greater effect on increasing the yield of fermentation products than simply increasing the concentration of cellulose in the substrate.

High temperature stimulates acetic acid accumulation and enhances the growth inhibition and ethanol production by Saccharomyces cerevisiae under fermenting conditions

Cellular responses of Saccharomyces cerevisiae to high temperatures of up to 42°C during ethanol fermentation at a high glucose concentration (i.e., 100g/L) were investigated. Increased temperature correlated with stimulated glucose uptake to produce not only the thermal protectant glycerol but also ethanol and acetic acid. Carbon flux into the tricarboxylic acid (TCA) cycle correlated positively with cultivation temperature. These results indicate that the increased demand for energy (in the form of ATP), most likely caused by multiple stressors, including heat, acetic acid, and ethanol, was matched by both the fermentation and respiration pathways. Notably, acetic acid production was substantially stimulated compared to that of other metabolites during growth at increased temperature. The acetic acid produced in addition to ethanol seemed to subsequently result in adverse effects, leading to increased production of reactive oxygen species. This, in turn, appeared to cause the specific growth rate, and glucose uptake rate reduced leading to a decrease of the specific ethanol production rate far before glucose depletion. These results suggest that adverse effects from heat, acetic acid, ethanol, and oxidative stressors are synergistic, resulting in a decrease of the specific growth rate and ethanol production rate and, hence, are major determinants of cell stability and ethanol fermentation performance of S. cerevisiae at high temperatures. The results are discussed in the context of possible applications.

Evaluation of the tolerance of acetic acid and 2-furaldehyde on the growth of Pichia stipitis and its respiratory deficient

The use of lignocellulosic residues for ethanol production is limited by toxic compounds in fermenting yeasts present in diluted acid hydrolysates like acetic acid and 2-furaldehyde. The respiratory deficient phenotype gives the cell the ability to resist several toxic compounds. So the aim of this work was to evaluate the tolerance to toxic compounds present in lignocellulosic hydrolysates like acetic acid and 2-furaldehyde in Pichia stipitis and its respiratory deficient strains. The respiratory deficient phenotype was induced by exposure to chemical agents such as acriflavine, acrylamide and rhodamine; 23 strains were obtained. The selection criterion was based on increasing specific ethanol yield (gethanolg(-1)biomass) with acetic acid and furaldehyde tolerance. The screening showed that P. stipitis NRRL Y-7124 ACL 2-1RD (lacking cytochrome c), obtained using acrylamide, presented the highest specific ethanol production rate (1.82gg(-1)h(-1)). Meanwhile, the ACF8-3RD strain showed the highest acetic acid tolerance (7.80gL(-1)) and the RHO2-3RD strain was able to tolerate up to 1.5gL(-1) 2-furaldehyde with a growth and ethanol production inhibition of 23 and 22%, respectively. The use of respiratory deficient yeast phenotype is a strategy for ethanol production improvement in a medium with toxic compounds such as hydrolysed sugarcane bagasse amongst others.

Kinetic modeling of simultaneous saccharification and fermentation of corn starch for ethanol production.

Fuel ethanol production, using a simultaneous saccharification and fermentation process (SSF) of native starch from corn flour, has been performed using Saccharomyces cerevisiae and a granular starch hydrolyzing enzyme. The quantitative effects of mash concentration, enzyme dose and pH were investigated with the use of a Box-Wilson central composite design protocol. Proceeding from results obtained in optimal fermentation conditions, a kinetics model relating the utilization rates of starch and glucose as well as the production rates of ethanol and biomass was tested. Moreover, scanning electron microscopy (SEM) was applied to investigate corn starch granule surface after the SFF process. A maximum ethanol concentration of 110.36 g/l was obtained for native corn starch using a mash concentration of 25%, which resulted in ethanol yield of 85.71%. The optimal conditions for the above yield were found with an enzyme dose of 2.05 ml/kg and pH of 5.0. These results indicate that by using a central composite design, it is possible to determine optimal values of the fermentation parameters for maximum ethanol production. The investigated kinetics model can be used to describe SSF process conducted with granular starch hydrolyzing enzymes. The SEM micrographs reveal randomly distributed holes on the surface of granules.