A strain of <Emphasis Type="Italic">Meyerozyma guilliermondii</Emphasis> isolated from sugarcane juice is able to grow and ferment pentoses in synthetic and bagasse hydrolysate media

The search for new microbial strains that are able to withstand inhibitors released from hemicellulosic hydrolysis and are also still able to convert sugars in ethanol/xylitol is highly desirable. A yeast strain isolated from sugarcane juice and identified as Meyerozyma guilliermondii was evaluated for the ability to grow and ferment pentoses in synthetic media and in sugarcane bagasse hydrolysate. The yeast grew in xylose, arabinose and glucose at the same rate at an initial medium pH of 5.5. At pH 4.5, the yeast grew more slowly in arabinose. There was no sugar exhaustion within 60 h. At higher xylose concentrations with a higher initial cell concentration, sugar was exhausted within 96 h at pH 4.5. An increase of 350 % in biomass was obtained in detoxified hydrolysates, whereas supplementation with 3 g/L yeast extract increased biomass production by approximately 40 %. Ethanol and xylitol were produced more significantly in supplemented hydrolysates regardless of detoxification. Xylose consumption was enhanced in supplemented hydrolysates and arabinose was consumed only when xylose and glucose were no longer available. Supplementation had a greater impact on ethanol yield and productivity than detoxification; however, the product yields obtained in the present study are still much lower when compared to other yeast species in bagasse hydrolysate. By the other hand, the fermentation of both xylose and arabinose and capability of withstanding inhibitors are important characteristics of the strain assayed.     

Evaluation of hexose and pentose in pre-cultivation of <Emphasis Type="Italic">Candida guilliermondii</Emphasis> on the key enzymes for xylitol production in sugarcane hemicellulosic hydrolysate

The evaluation of hexose and pentose in pre-cultivation of Candida guilliermondii FTI 20037 yeast on xylose reductase (XR) and xylitol dehydrogenase (XDH) enzymes activities was performed during fermentation in sugarcane bagasse hemicellulosic hydrolysate. The xylitol production was evaluated by using cells previously growth in 30.0 gl^?1 xylose, 30.0 gl^?1 glucose and in both sugars mixture (30.0 gl^?1 xylose and 2.0 gl^?1 glucose). The vacuum evaporated hydrolysate (80 gl^?1) was detoxificated by ion exchange resin (A-860S; A500PS and C-150-Purolite^®). The total phenolic compounds and acetic acid were 93.0 and 64.9%, respectively, removed by the resin hydrolysate treatment. All experiments were carried out in Erlenmeyer flasks at 200 rpm, 30°C. The maximum XR (0.618 Umg _Prot ^?1 ) and XDH (0.783 Umg _Prot ^?1 ) enzymes activities was obtained using inoculum previously growth in both sugars mixture. The highest cell concentration (10.6 gl^?1) was obtained with inoculum pre-cultivated in the glucose. However, the xylitol yield and xylitol volumetric productivity were favored using the xylose as carbon source. In this case, it was observed maximum xylose (81%) and acetic acid (100%) consumption. It is very important to point out that maximum enzymatic activities were obtained when the mixture of sugars was used as carbon source of inoculum, while the highest fermentative parameters were obtained when xylose was used.     

Xylan-hydrolyzing thermotolerant <Emphasis Type="Italic">Candida tropicalis</Emphasis> HNMA-1 for bioethanol production from sugarcane bagasse hydrolysate

Sugarcane bagasse is one of the low-cost substrates used for bioethanol production. In order to solubilize sugars in hemicelluloses like xylan, a new thermotolerant isolate of Candida tropicalis HNMA-1 with xylan-hydrolyzing ability was identified and characterized. The strain showed relative tolerance to high temperature. Our results demonstrated 0.211 IU ml^?1 xylanase activity at 40 °C compared to 0.236 IU ml^?1 at 30 °C. The effect of high temperature on the growth and fermentation of xylose and sugarcane bagasse hydrolysate were also investigated. In both xylose or hydrolysate medium, increased growth was recorded at 40 °C. Meanwhile, the efficiency of ethanol fermentation was adversely affected by temperature since yields of 0.088 g g^?1 and 0.076 g g^?1 in the xylose medium, in addition to 0.090 g g^?1 and 0.078 g g^?1 in the hydrolysate medium were noticed at 30 °C and 40 °C, respectively. Inhibitory compounds in the hydrolysate medium demonstrated negative effects on fermentation and productivity, with maximum ethanol concentration attained after 48 h in the hydrolysate, as opposed to 24 h in the xylose medium. Our data show that the newly thermotolerant isolate, C. tropicalis HNMA-1, is able to efficiently ferment xylose and hydrolysate, and also has the capacity for application in ethanol production from hemicellulosic sources.     

Xylitol production by genetically modified industrial strain of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> using glycerol as co-substrate

Xylitol is commercially used in chewing gum and dental care products as a low calorie sweetener having medicinal properties. Industrial yeast strain of S. cerevisiae was genetically modified to overexpress an endogenous aldose reductase gene GRE3 and a xylose transporter gene SUT1 for the production of xylitol. The recombinant strain (XP-RTK) carried the expression cassettes of both the genes and the G418 resistance marker cassette KanMX integrated into the genome of S. cerevisiae. Short segments from the 5′ and 3′ delta regions of the Ty1 retrotransposons were used as homology regions for integration of the cassettes. Xylitol production by the industrial recombinant strain was evaluated using hemicellulosic hydrolysate of the corn cob with glucose as the cosubstrate. The recombinant strain XP-RTK showed significantly higher xylitol productivity (212 mg L^?1 h^?1) over the control strain XP (81 mg L^?1 h^?1). Glucose was successfully replaced by glycerol as a co-substrate for xylitol production by S. cerevisiae. Strain XP-RTK showed the highest xylitol productivity of 318.6 mg L^?1 h^?1 and titre of 47 g L^?1 of xylitol at 12 g L^?1 initial DCW using glycerol as cosubstrate. The amount of glycerol consumed per amount of xylitol produced (0.47 mol mol^?1) was significantly lower than glucose (23.7 mol mol^?1). Fermentation strategies such as cell recycle and use of the industrial nitrogen sources were demonstrated using hemicellulosic hydrolysate for xylitol production.     

Enhancing ethanol production from cellulosic sugars using <Emphasis Type="Italic">Scheffersomyces (Pichia) stipitis</Emphasis>

Studies were performed on the effect of CaCO₃ and CaCl₂ supplementation to fermentation medium for ethanol production from xylose, glucose, or their mixtures using Scheffersomyces (Pichia) stipitis. Both of these chemicals were found to improve maximum ethanol concentration and ethanol productivity. Use of xylose alone resulted in the production of 20.68 ± 0.44 g L^?1 ethanol with a productivity of 0.17 ± 0.00 g L^?1 h^?1, while xylose plus 3 g L^?1 CaCO₃ resulted in the production of 24.68 ± 0.75 g L^?1 ethanol with a productivity of 0.21 ± 0.01 g L^?1 h^?1. Use of xylose plus glucose in combination with 3 g L^?1 CaCO₃ resulted in the production of 47.37 ± 0.55 g L^?1 ethanol (aerobic culture), thus resulting in an ethanol productivity of 0.39 ± 0.00 g L^?1 h^?1. These values are 229 % of that achieved in xylose medium. Supplementation of xylose and glucose medium with 0.40 g L^?1 CaCl₂ resulted in the production of 44.84 ± 0.28 g L^?1 ethanol with a productivity of 0.37 ± 0.02 g L^?1 h^?1. Use of glucose plus 3 g L^?1 CaCO₃ resulted in the production of 57.39 ± 1.41 g L^?1 ethanol under micro-aerophilic conditions. These results indicate that supplementation of cellulosic sugars in the fermentation medium with CaCO₃ and CaCl₂ would improve economics of ethanol production from agricultural residues.     

Enhanced Xylitol Production by Precultivation of Candida guilliermondii Cells in Sugarcane Bagasse Hemicellulosic Hydrolysate

The present work evaluated the key enzymes involved in xylitol production (xylose reductase [XR] and xylitol dehydrogenase [XDH]) and their correlation with xylose, arabinose, and acetic acid assimilation during cultivation of Candida guilliermondii FTI 20037 cells in sugarcane bagasse hemicellulosic hydrolysate. For this purpose, inocula previously grown either in sugarcane bagasse hemicellulosic hydrolysate (SBHH) or in semidefined medium (xylose as a substrate) were used. The highest xylose/acetic acid consumption ratio (1.78) and the lowest arabinose consumption (13%) were attained in the fermentation using inoculum previously grown in semidefined medium (without acetic acid and arabinose). In this case, the highest values of XR (1.37 U mg prot⁻¹) and XDH (0.91 U mg prot⁻¹) activities were observed. The highest xylitol yield (∼0.55 g g⁻¹) and byproducts (ethanol and glycerol) formation were not influenced by inoculum procedure. However, the cell previously grown in the hydrolysate was effective in enhancing xylitol production by keeping the XR enzyme activity at high levels (around 0.99 U·mg_prot⁻¹), reducing the XDH activity (34.0%) and increasing xylitol volumetric productivity (26.5%) with respect to the inoculum cultivated in semidefined medium. Therefore, inoculum adaptation to SBHH was shown to be an important strategy to improve xylitol productivity.     

Kinetic behavior of Candida guilliermondii yeast during xylitol production from Brewer's spent grain hemicellulosic hydrolysate

Brewer's spent grain, the main byproduct of breweries, was hydrolyzed with dilute sulfuric acid to produce a hemicellulosic hydrolysate (containing xylose as the main sugar). The obtained hydrolysate was used as cultivation medium by Candidaguilliermondii yeast in the raw form (containing 20 g/L xylose) and after concentration (85 g/L xylose), and the kinetic behavior of the yeast during xylitol production was evaluated in both media. Assays in semisynthetic media were also performed to compare the yeast performance in media without toxic compounds. According to the results, the kinetic behavior of the yeast cultivated in raw hydrolysate was as effective as in semisynthetic medium containing 20 g/L xylose. However, in concentrated hydrolysate medium, the xylitol production efficiency was 30.6% and 42.6% lower than in raw hydrolysate and semisynthetic medium containing 85 g/L xylose, respectively. In other words, the xylose-to-xylitol bioconversion from hydrolysate medium was strongly affected when the initial xylose concentration was increased; however, similar behavior did not occur from semisynthetic media. The lowest efficiency of xylitol production from concentrated hydrolysate can be attributed to the high concentration of toxic compounds present in this medium, resulting from the hydrolysate concentration process.     

Xylitol production from a mutant strain of Candida tropicalis

Aims: To characterize the kinetics of growth, sugar uptake and xylitol production in batch and fed‐batch cultures for a xylitol assimilation‐deficient strain of Candida tropicalis isolated via chemical mutagenesis. Methods and Results: Chemical mutagenesis using nitrosoguanidine led to the isolation of the xylitol‐assimilation deficient strain C. tropicalis SS2. Shake‐flask fermentations with this mutant showed a sixfold higher xylitol yield than the parent strain in medium containing 25 g l^?1 glucose and 25 g l^?1 xylose. With 20 g l^?1 glycerol, replacing glucose for cell growth, and various concentrations of xylose, the studies indicated that the mutant strain resulted in xylitol yields from xylose close to theoretical. Under fully aerobic conditions, fed‐batch fermentation with repeated addition of glycerol and xylose resulted in 3·3 g l^?1 h^?1 xylitol volumetric productivity with the final concentration of 220 g l^?1 and overall yield of 0·93 g g^?1 xylitol. Conclusions: The xylitol assimilation‐deficient mutant isolated in this study showed the potential for high xylitol yield and volumetric productivity under aerobic conditions. In the evaluation of glycerol as an alternative low‐cost nonfermentable carbon source, high biomass and xylitol yields under aerobic conditions were achieved; however, the increase in initial xylose concentrations resulted in a reduction in biomass yield based on glycerol consumption. This may be a consequence of the role of an active transport system in the yeast requiring increasing energy for xylose uptake and possible xylitol secretion, with little or no energy available from xylose metabolism. Significance and Impact of the Study: The study confirms the advantage of using a xylitol assimilation‐deficient yeast under aerobic conditions for xylitol production with glycerol as a primary carbon source. It illustrates the potential of using the xylose stream in a biomass‐based bio‐refinery for the production of xylitol with further cost reductions resulting from using glycerol for yeast growth and energy production.     

Overcoming inhibitors in a hemicellulosic hydrolysate: improving fermentability by feedstock detoxification and adaptation of <Emphasis Type="Italic">Pichia stipitis</Emphasis>

In order to improve the fermentative efficiency of sugar maple hemicellulosic hydrolysates for fuel ethanol production, various methods to mitigate the effects of inhibitory compounds were employed. These methods included detoxification treatments utilizing activated charcoal, anion exchange resin, overliming, and ethyl acetate extraction. Results demonstrated the greatest fermentative improvement of 50% wood hydrolysate (v/v) by Pichia stipitis with activated charcoal treatment. Another method employed to reduce inhibition was an adaptation procedure to produce P. stipitis stains more tolerant of inhibitory compounds. This adaptation resulted in yeast variants capable of improved fermentation of 75% untreated wood hydrolysate (v/v), one of which produced 9.8 g/l ± 0.6 ethanol, whereas the parent strain produced 0.0 g/l ± 0.0 within the first 24 h. Adapted strains RS01, RS02, and RS03 were analyzed for glucose and xylose utilization and results demonstrated increased glucose and decreased xylose utilization rates in comparison to the wild type. These changes in carbohydrate utilization may be indicative of detoxification or tolerance activities related to proteins involved in glucose and xylose metabolism.     

Xylitol production from corn fiber and sugarcane bagasse hydrolysates by Candida tropicalis

A natural isolate, Candida tropicalis was tested for xylitol production from corn fiber and sugarcane bagasse hydrolysates. Fermentation of corn fiber and sugarcane bagasse hydrolysate showed xylose uptake and xylitol production, though these were very low, even after hydrolysate neutralization and treatments with activated charcoal and ion exchange resins. Initial xylitol production was found to be 0.43 g/g and 0.45 g/g of xylose utilised with corn fiber and sugarcane bagasse hydrolysate respectively. One of the critical factors for low xylitol production was the presence of inhibitors in these hydrolysates. To simulate influence of hemicellulosic sugar composition on xylitol yield, three different combinations of mixed sugar control experiments, without the presence of any inhibitors, have been performed and the strain produced 0.63 g/g, 0.68 g/g and 0.72 g/g of xylose respectively. To improve yeast growth and xylitol production with these hydrolysates, which contain inhibitors, the cells were adapted by sub culturing in the hydrolysate containing medium for 25 cycles. After adaptation the organism produced more xylitol 0.58 g/g and 0.65 g/g of xylose with corn fiber hydrolysate and sugarcane bagasse hydrolysate respectively.     

Kinetics of growth and ethanol formation from a mix of glucose/xylose substrate by Kluyveromyces marxianus UFV-3

The fermentation of both glucose and xylose is important to maximize ethanol yield from renewable biomass feedstocks. In this article, we analyze growth, sugar consumption, and ethanol formation by the yeast Kluyveromyces marxianus UFV-3 using various glucose and xylose concentrations and also under conditions of reduced respiratory activity. In almost all the conditions analyzed, glucose repressed xylose assimilation and xylose consumption began after glucose had been exhausted. A remarkable difference was observed when mixtures of 5 g L^?1 glucose/20 g L^?1 xylose and 20 g L^?1 glucose/20 g L^?1 xylose were used. In the former, the xylose consumption began immediately after the glucose depletion. Indeed, there was no striking diauxic phase, as observed in the latter condition, in which there was an interval of 30 h between glucose depletion and the beginning of xylose consumption. Ethanol production was always higher in a mixture of glucose and xylose than in glucose alone. The highest ethanol concentration (8.65 g L^?1) and cell mass concentration (4.42 g L^?1) were achieved after 8 and 74 h, respectively, in a mixture of 20 g L^?1 glucose/20 g L^?1 xylose. When inhibitors of respiration were added to the medium, glucose repression of xylose consumption was alleviated completely and K. marxianus was able to consume xylose and glucose simultaneously.     

Improvement of xylitol production by Candida guilliermondii FTI 20037 previously adapted to rice straw hemicellulosic hydrolysate

AIMS: To evaluate a simple and economical technique to improve xylitol production using concentrated xylose solutions prepared from rice straw hemicellulosic hydrolysate. METHODS AND RESULTS: Experiments were carried out with rice straw hemicellulosic hydrolysate containing 90 g l-1 xylose, with and without the addition of nutrients, using the yeast Candida guilliermondii previously grown on the hydrolysate (adapted cells) or on semi-defined medium (unadapted cells). By this method, the yield of xylitol increased from 17 g l-1 to 50 g l-1, and xylose consumption increased from 52% to 83%, after 120 h of fermentation. The xylitol production rates were very close to that (0.42 g l-1 h-1) attained in a medium simulating hydrolysate sugars. CONCLUSION: Yeast strain adaptation to the hydrolysate showed to be a suitable method to alleviate the inhibitory effects of the toxic compounds. Adapted cells of Candida guilliermondii can efficiently produce xylitol from hydrolysate with high xylose concentrations. SIGNIFICANCE AND IMPACT OF THE STUDY: Yeast adaptation helps the bioconversion process in hydrolysate made from lignocellulosic materials. This low-cost technique provides an alternative to the detoxification methods used for removal of inhibitory compounds. In addition, the use of adapted inocula makes it possible to schedule a series of batch cultures so that the whole plant can be operated almost continuously with a concomitant reduction in the overall operation time.     

Reduction of furan derivatives by overexpressing NADH-dependent Adh1 improves ethanol fermentation using xylose as sole carbon source with Saccharomyces cerevisiae harboring XR–XDH pathway

Several alcohol dehydrogenase (ADH)-related genes have been identified as enzymes for reducing levels of toxic compounds, such as, furfural and/or 5-hydroxymethylfurfural (5-HMF), in hydrolysates of pretreated lignocelluloses. To date, overexpression of these ADH genes in yeast cells have aided ethanol production from glucose or glucose/xylose mixture in the presence of furfural or 5-HMF. However, the effects of these ADH isozymes on ethanol production from xylose as a sole carbon source remain uncertain. We showed that overexpression of mutant NADH-dependent ADH1 derived from TMB3000 strain in the recombinant Saccharomyces cerevisiae, into which xylose reductase (XR) and xylitol dehydrogenase (XDH) pathway of Pichia stipitis has been introduced, improved ethanol production from xylose as a sole carbon source in the presence of 5-HMF. Enhanced furan-reducing activity is able to regenerate NAD⁺ to relieve redox imbalance, resulting in increased ethanol yield arising from decreased xylitol accumulation. In addition, we found that overexpression of wild-type ADH1 prevented the more severe inhibitory effects of furfural in xylose fermentation as well as overexpression of TMB3000-derived mutant. After 120 h of fermentation, the recombinant strains overexpressing wild-type and mutant ADH1 completely consumed 50 g/L xylose in the presence of 40 mM furfural and most efficiently produced ethanol (15.70 g/L and 15.24 g/L) when compared with any other test conditions. This is the first report describing the improvement of ethanol production from xylose as the sole carbon source in the presence of furan derivatives with xylose-utilizing recombinant yeast strains via the overexpression of ADH-related genes.     

Influence of the toxic compounds present in brewer's spent grain hemicellulosic hydrolysate on xylose-to-xylitol bioconversion by Candida guilliermondii

Fermentation media containing different concentrations of toxic compounds were prepared from brewer's spent grain (BSG) hemicellulosic hydrolysate, and used for xylose-to-xylitol bioconversion by Candida guilliermondii. Such fermentation media were composed of the hydrolysate in the following ways: raw (RH); concentrated four-fold (CH); concentrated and treated with activated charcoal (TCH); raw supplemented with sugars until a concentration four-fold higher (SRH); concentrated and subsequently diluted but supplemented with sugars until a concentration four-fold higher (SDCH). All media presented an initial xylose concentration of 85 g/l, except RH, which contained 23 g/l xylose. Fermentation results revealed that the sugars supplementation to raw hydrolysate favored the xylitol production. Nevertheless, xylitol production from CH was negatively affected due to the high concentration of toxic compounds present in the medium. The hydrolysate treatment with activated charcoal partially removed the toxic compounds, and the xylitol production was higher than in CH, but not so efficient as in SRH. It was thus concluded that to obtain an efficient xylose-to-xylitol bioconversion from BSG hydrolysate, the sugars concentration must be increased, but the toxic compounds concentration must be reduced to the same level present in the raw hydrolysate.     

Polyhydroxyalkanoate biosynthesis and simultaneous remotion of organic inhibitors from sugarcane bagasse hydrolysate by Burkholderia sp.

Burkholderia sp. F24, originally isolated from soil, was capable of growth on xylose and removed organic inhibitors present in a hemicellulosic hydrolysate and simultaneously produced poly-3-hydroxybutyrate (P3HB). Using non-detoxified hydrolysate, Burkholderia sp. F24 reached a cell dry weight (CDW) of 6.8 g L^?1, containing 48 % of P3HB and exhibited a volumetric productivity (P_P3HB) of 0.10 g L^?1 h^?1. Poly-3-hydroxybutyrate-co-3-hydroxyvalerate copolymers (P3HB-co-3HV) were produced using xylose and levulinic acid (LA) as carbon sources. In shake flask cultures, the 3HV content in the copolymer increased from 9 to 43 mol% by adding LA from 1.0 to 5.0 g L^?1. In high cell density cultivation using concentrated hemicellulosic hydrolysate F24 reached 25.04 g L^?1 of CDW containing 49 % of P3HB and P_P3HB of 0.28 g L^?1 h^?1. Based on these findings, second-generation ethanol and bioplastics from sugarcane bagasse is proposed.     

Kinetic behavior of Candida guilliermondii yeast during xylitol production from highly concentrated hydrolysate

Rice straw hemicellulosic hydrolysate containing a high xylose concentration was used as fermentation medium to evaluate the kinetic behavior of Candida guilliermondii yeast (FTI 20037) during the bioconversion of xylose into xylitol. Assays were conducted first with detoxified and non-detoxified (raw) hydrolysates and semi-synthetic medium in agitated flasks, and second with detoxified hydrolysate in a stirred-tank bioreactor at a given oxygen transfer rate. The results for the agitated flasks showed that in detoxified hydrolysate the xylose-to-xylitol bioconversion by the yeast was as effective as in synthetic medium and 47% higher than in raw hydrolysate. In the stirred-tank bioreactor, the kinetic behavior of the yeast in detoxified hydrolysate was slower, resulting in smaller values of fermentative parameters, probably due to unsuitability of the oxygen transfer rate employed (K_La=22 h⁻¹).     

Detoxification of hemicellulosic hydrolysates from extracted olive pomace by diananofiltration

Xylitol can be obtained from the pentose-rich hemicellulosic fraction of agricultural residues, such as extracted olive pomace, by fermentation. Dilute acid hydrolysis of lignocellulosic materials, produces the release of potential inhibitory compounds mainly furan derivatives, aliphatic acids, and phenolic compounds. In order to study the potential on the increase of the hydrolysate fermentability, detoxification experiments based on diananofiltration membrane separation processes were made. Two membranes, NF270 and NF90, were firstly evaluated using hydrolysate model solutions under total recirculation mode, to identify the best membrane for the detoxification. NF270 was chosen to be used in the diananofiltration experiment as it showed the lowest rejection for toxic compounds and highest permeate flux. Diananofiltration experiments, for hydrolysate model solutions and hydrolysate liquor, showed that nanofiltration is able to deplete inhibitory compounds and to obtain solutions with higher xylose content. Conversely to non-detoxified hydrolysates, nanofiltration detoxified hydrolysates enabled yeast growth and xylitol production by the yeast Debaryomyces hansenii, clearly pointing out that detoxification is an absolute requirement for extracted olive pomace dilute acid hydrolysate bioconversion.     

Evaluation of ethanol production by a new isolate of yeast during fermentation in synthetic medium and sugarcane bagasse hemicellulosic hydrolysate

A new xylose fermenting yeast was isolated from over-ripe banana by enrichment in xylose-containing medium. The phylogenetic analysis of ITS1-5.8S-ITS2 region sequences of ribosomal RNA of isolate BY2 revealed that it shows affiliation to genus Pichia and clades with Pichia caribbica. In batch fermentation, Pichia strain BY2 fermented xylose, producing 15 g l^?1 ethanol from 30 g l^?1 xylose under shaking conditions at 28°C, with ethanol yield of 0.5 g g^?1 and volumetric productivity of 0.31 g l^?1 h^?1. The optimum pH range for ethanol production from xylose by Pichia strain BY2 was 5–7. Pichia strain BY2 also produced 6.08 g l^?1 ethanol from 30 g l^?1 arabinose. Pichia strain BY2 can utilize sugarcane bagasse hemicellulose acid hydrolysate for alcohol production, efficiency of fermentation was improved by neutralization, and sequential use of activated charcoal adsorption method. Percent total sugar utilized and ethanol yield for the untreated hydrolysate was 17.14% w/v and 0.33 g g^?1, respectively, compared with 66.79% w/v and 0.45 g g^?1, respectively, for treated hemicellulose acid hydrolysate. This new yeast isolate showed ethanol yield of 0.45 g g^?1 and volumetric productivity of 0.33 g l^?1 h^?1 from sugarcane bagasse hemicellulose hydrolysate detoxified by neutralization and activated charcoal treatment, and has potential application in practical process of ethanol production from lignocellulosic hydrolysate.     

Fermentation performance of Candida guilliermondii for xylitol production on single and mixed substrate media

Semidefined media fermentation simulating the sugar composition of hemicellulosic hydrolysates (around 85 g l^-1 xylose, 17 g l^-1 glucose, and 9 g l^-1 arabinose) was investigated to evaluate the glucose and arabinose influence on xylose-to-xylitol bioconversion by Candida guilliermondii. The results revealed that glucose reduced the xylose consumption rate by 30%. Arabinose did not affect the xylose consumption but its utilization by the yeast was fully repressed by both glucose and xylose sugars. Arabinose was only consumed when it was used as a single carbon source. Xylitol production was best when glucose was not present in the fermentation medium. On the other hand, the arabinose favored the xylitol yield (which attained 0.74 g g^-1 xylose consumed) and it did not interfere with xylitol volumetric productivity (Q _P=0.85 g g^-1), the value of which was similar to that obtained with xylose alone.     

Effect of controlled oxygen limitation on Candida shehatae physiology for ethanol production from xylose and glucose

Carbon distribution and kinetics of Candida shehatae were studied in fed-batch fermentation with xylose or glucose (separately) as the carbon source in mineral medium. The fermentations were carried out in two phases, an aerobic phase dedicated to growth followed by an oxygen limitation phase dedicated to ethanol production. Oxygen limitation was quantified with an average specific oxygen uptake rate (OUR) varying between 0.30 and 2.48 mmolO₂ g dry cell weight (DCW)^?1 h^?1, the maximum value before the aerobic shift. The relations among respiration, growth, ethanol production and polyol production were investigated. It appeared that ethanol was produced to provide energy, and polyols (arabitol, ribitol, glycerol and xylitol) were produced to reoxidize NADH from assimilatory reactions and from the co-factor imbalance of the two-first enzymatic steps of xylose uptake. Hence, to manage carbon flux to ethanol production, oxygen limitation was a major controlled parameter; an oxygen limitation corresponding to an average specific OUR of 1.19 mmolO₂ g DCW^?1 h^?1 allowed maximization of the ethanol yield over xylose (0.327 g g^?1), the average productivity (2.2 g l^?1 h^?1) and the ethanol final titer (48.81 g l^?1). For glucose fermentation, the ethanol yield over glucose was the highest (0.411 g g^?1) when the specific OUR was low, corresponding to an average specific OUR of 0.30 mmolO₂ g DCW^?1 h^?1, whereas the average ethanol productivity and ethanol final titer reached the maximum values of 1.81 g l^?1 h^?1 and 54.19 g l^?1 when the specific OUR was the highest.