| 1. |
- Sárvári Horváth, Ilona, 1960, et al.
(författare)
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Effects of furfural on the respiratory metabolism of Saccharomyces cerevisiae in glucose-limited chemostats
- 2003
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Ingår i: Applied and Environmental Microbiology. - 0099-2240 .- 1098-5336. ; 69:7, s. 4076-4086
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Tidskriftsartikel (refereegranskat)abstract
- Effects of furfural on the aerobic metabolism of the yeast Saccharomyces cerevisiae were studied by performing chemostat experiments, and the kinetics of furfural conversion was analyzed by performing dynamic experiments. Furfural, an important inhibitor present in lignocellulosic hydrolysates, was shown to have an inhibitory effect on yeast cells growing respiratively which was much greater than the inhibitory effect previously observed for anaerobically growing yeast cells. The residual furfural concentration in the bioreactor was close to zero at all steady states obtained, and it was found that furfural was exclusively converted to furoic acid during respiratory growth. A metabolic flux analysis showed that furfural affected fluxes involved in energy metabolism. There was a 50% increase in the specific respiratory activity at the highest steady-state furfural conversion rate. Higher furfural conversion rates, obtained during pulse additions of furfural, resulted in respirofermentative metabolism, a decrease in the biomass yield, and formation of furfuryl alcohol in addition to furoic acid. Under anaerobic conditions, reduction of furfural partially replaced glycerol formation as a way to regenerate NAD+. At concentrations above the inlet concentration of furfural, which resulted in complete replacement of glycerol formation by furfuryl alcohol production, washout occurred. Similarly, when the maximum rate of oxidative conversion of furfural to furoic acid was exceeded aerobically, washout occurred. Thus, during both aerobic growth and anaerobic growth, the ability to tolerate furfural appears to be directly coupled to the ability to convert furfural to less inhibitory compounds.
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| 2. |
- Taherzadeh Esfahani, Mohammad, 1965, et al.
(författare)
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Continuous Cultivation of Dilute-Acid Hydrolysates to Ethanol by Immobilized Saccharomyces cerevisiae
- 2001
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Ingår i: Applied Biochemistry and Biotechnology. - 0273-2289 .- 1559-0291. ; 95:1, s. 45-57
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Tidskriftsartikel (refereegranskat)abstract
- The continuous cultivation of immobilized Saccharomyces cerevisiae CBS 8066 on dilute-acid hydrolysates of forest residuals was investigated. The yeast cells were immobilized in 2–4% Ca-alginate beads. The 2% beads were not stable. However, the 3 and 4% beads were stable for at least 3 wk when an extra resource of calcium ions was available in the medium. The continuous cultivation of a dilute-acid hydrolysate by the immobilized cells at dilution rates of 0.3, 0.5, and 0.6 h−1 resulted in 86, 83, and 79% sugar consumption, respectively, and an ethanol yield between 0.45 and 0.48 g/g. The hydrolysate was fermentable at a dilution rate of 0.1 h−1 in a free-cell system but washed out at a dilution rate of 0.2 h−1. The continuous cultivation of a more inhibiting hydrolysate was not successful by either free- or immobilized-cell systems even at a low dilution rate of 0.07 h−1. However, when the hydrolysate was overlimed, it was fermentable by the immobilized cells at a dilution rate of 0.2 h−1.
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| 3. |
- Horváth, I. S., et al.
(författare)
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Effects of furfural on anaerobic continuous cultivation of Saccharomyces cerevisiae
- 2001
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Ingår i: Biotechnology and Bioengineering. - : Wiley. - 0006-3592 .- 1097-0290. ; 75:5, s. 540-549
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Tidskriftsartikel (refereegranskat)abstract
- Furfural is an important inhibitor of yeast metabolism in lignocellulose-derived substrates. The effect of furfural on the physiology of Saccharomyces cerevisiae CBS 8066 was investigated using anaerobic continuous cultivations. Experiments were performed with furfural in the feed medium (up to 8.3 g/L) using three dierent dilution rates (0.095, 0.190, and 0.315 h-1). The measured concentration of furfural was low (<0.1 g/L) at all steady states obtained. However, it was not possible to achieve a steady state at a specific conversion rate of furfural, qf, higher than approximately 0.15 g/g·h. An increased furfural concentration in the feed caused a decrease in the steady-state glycerol yield. This agreed well with the decreased need for glycerol production as a way to regenerate NAD+, i.e., to function as a redox sink because furfural was reduced to furfuryl alcohol. Transient experiments were also performed by pulse addition of furfural directly into the fermentor. In contrast to the situation at steady-state conditions, both glycerol and furfuryl alcohol yields increased after pulse addition of furfural to the culture. Furthermore, the maximum specific conversion rate of furfural (0.6 g/g·h) in dynamic experiments was significantly higher than what was attainable in the chemostat experiments. The dynamic furfural conversion could be described by the use of a simple Michaelis-Menten-type kinetic model. Also furfural conversion under steady-state conditions could be explained by a Michaelis-Menten-type kinetic model, but with a higher anity and a lower maximum conversion rate. This indicated the presence of an additional component with a higher anity, but lower maximum capacity, either in the transport system or in the conversion system of furfural.
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| 4. |
- Millati, R., et al.
(författare)
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Effect of pH, time and temperature of overliming on detoxification of dilute-acid hydrolyzates for fermentation by Saccharomyces cerevisiae
- 2002
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Ingår i: Process Biochemistry. - 1359-5113 .- 1873-3298. ; 38:4, s. 515-522
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Tidskriftsartikel (refereegranskat)abstract
- The effects of different variables in detoxification of a severely inhibiting dilute-acid hydrolyzate by overliming were investigated. Overliming was carried out by increasing the pH to 10, 11 or 12 at two different temperatures, 25 and 60 °C, holding the pH and temperature at constant values for different periods of time, 0, 1, 20 and 170 h, and then adjusting the pH to 5.5. All hydrolyzates were then fermented in batch cultivation by Saccharomyces cerevisiae in shake flasks, whereupon one was then selected for continuous cultivation in a bioreactor. The most significant effect of overliming was a sharp decrease in the concentration of furfural and hydroxymethylfurfural, whereas the concentration of acetic acid remained unchanged and the decrease in the total phenolic compounds was less than 30%. Detoxification at pH 12 for more than 1 h was effective, whereas no effect was obtained at pH 10 and the hydrolyzates had to remain at pH 11 for more than 20 h to become fermentable. On the other hand, decrease in sugar concentration during overliming was a serious problem at pH 12, especially at the higher temperature, where up to 70% sugars were degraded. The fermentability of a detoxified hydrolyzate was also tested in a continuous cultivation by immobilized S. cerevisiae in Ca-alginate. The hydrolyzate was fully fermentable at different dilution rates between 0.2 and 1.0 h-1.
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| 5. |
- Modig, Tobias, et al.
(författare)
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Inhibition effects of furfural on alcohol dehydrogenase, aldehyde dehydrogenase and pyruvate dehydrogenase.
- 2002
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Ingår i: Biochemical Journal. - 0264-6021 .- 1470-8728. ; 363:Pt 3, s. 769-776
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics of furfural inhibition of the enzymes alcohol dehydrogenase (ADH; EC 1.1.1.1), aldehyde dehydrogenase (AlDH; EC 1.2.1.5) and the pyruvate dehydrogenase (PDH) complex were studied in vitro. At a concentration of less than 2 mM furfural was found to decrease the activity of both PDH and AlDH by more than 90%, whereas the ADH activity decreased by less than 20% at the same concentration. Furfural inhibition of ADH and AlDH activities could be described well by a competitive inhibition model, whereas the inhibition of PDH was best described as non-competitive. The estimated K(m) value of AlDH for furfural was found to be about 5 microM, which was lower than that for acetaldehyde (10 microM). For ADH, however, the estimated K(m) value for furfural (1.2 mM) was higher than that for acetaldehyde (0.4 mM). The inhibition of the three enzymes by 5-hydroxymethylfurfural (HMF) was also measured. The inhibition caused by HMF of ADH was very similar to that caused by furfural. However, HMF did not inhibit either AlDH or PDH as severely as furfural. The inhibition effects on the three enzymes could well explain previously reported in vivo effects caused by furfural and HMF on the overall metabolism of Saccharomyces cerevisiae, suggesting a critical role of these enzymes in the observed inhibition.
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| 6. |
- Nilsson, Anneli, et al.
(författare)
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On-line estimation of sugar concentration for control of fed-batch fermentation of lignocellulosic hydrolyzates by Saccharomyces cerevisiae
- 2002
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Ingår i: Bioprocess and Biosystems Engineering. - : Springer Science and Business Media LLC. - 1615-7605 .- 1615-7591. ; 25:3, s. 183-191
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Tidskriftsartikel (refereegranskat)abstract
- A feed control strategy, based on estimated sugar concentrations, was developed with the purpose of avoiding severe inhibition of the yeast Saccharomyces cerevisiae during fermentation of spruce hydrolyzate. The sum of the fermentable hexose sugars, glucose and mannose, was estimated from on-line measurements of carbon dioxide evolution rate and biomass concentration by use of a simple stoichiometric model. The feed rate of the hydrolyzate was controlled to maintain constant sugar concentration during fed-batch fermentation, and the effect of different set-point concentrations was investigated using both untreated and detoxified hydrolyzates. The fed-batch cultivations were evaluated with respect to cellular physiology in terms of the specific ethanol productivities, ethanol yields, and viability of the yeast. The simple stoichiometric model used resulted in a good agreement between estimated sugar concentrations and off-line determinations of sugar concentrations. Furthermore, the control strategy used made it possible to maintain a constant sugar concentration without major oscillations in the feed rate or the sugar concentration. For untreated hydrolyzates the average ethanol productivity could be increased by more than 130% compared to batch fermentation. The average ethanol productivity was increased from 0.12 to 0.28 g/g h. The productivity also increased for detoxified hydrolyzates, where an increase of 16% was found (from 0.50 to 0.58 g/g h).
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| 7. |
- Nilsson, Anneli, et al.
(författare)
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Use of dynamic step response for control of fed-batch conversion of lignocellulosic hydrolyzates to ethanol
- 2001
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Ingår i: Journal of Biotechnology. - 1873-4863 .- 0168-1656. ; 89:1, s. 41-53
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Tidskriftsartikel (refereegranskat)abstract
- Optimization of fed-batch conversion of lignocellulosic hydrolyzates by the yeast Saccharomyces cerevisiae was studied. The feed rate was controlled using a step response strategy, in which the carbon dioxide evolution rate was used as input variable. The performance of the control strategy was examined using both an untreated and a detoxified dilute acid hydrolyzate, and the performance was compared to that obtained with a synthetic medium. In batch cultivation of the untreated hydrolyzate, only 23% of the hexose sugars were assimilated. However, by using the feed-back controlled fed-batch technique, it was possible to obtain complete conversion of the hexose sugars. Furthermore, the maximal specific ethanol productivity (q(t.max)) increased more than 10-fold, from 0.06 to 0.70 g g(-1) h(-1). In addition, the viability of the yeast cells decreased by more than 99% in batch cultivation, whereas a viability of more than 40% could be maintained during fed-batch cultivation. In contrast to untreated hydrolyzate, it was possible to convert the sugars in the detoxified hydrolyzate also in batch cultivation. However, a 50% higher specific ethanol productivity was obtained using fed-batch cultivation. During batch cultivation of both untreated and detoxified hydrolyzate a gradual decrease in specific ethanol productivity was observed. This decrease could largely be avoided in fed-batch cultivations. (C) 2001 Elsevier Science B.V. All rights reserved.
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| 8. |
- Purwadi, R., et al.
(författare)
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Kinetic study of detoxification of dilute-acid hydrolyzates by Ca(OH) 2
- 2004
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Ingår i: Journal of Biotechnology. - : Elsevier BV. - 0168-1656 .- 1873-4863. ; 114:1-2, s. 187-198
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Tidskriftsartikel (refereegranskat)abstract
- Detoxification of dilute-acid hydrolyzates by addition of Ca(OH) 2 (overliming) and cultivation of the detoxified hydrolyzates by Saccharomyces cerevisiae were examined. The examined overliming involves increasing the pH of the hydrolyzates to 9, 10, 11 or 12, keeping up to 90 min at different temperatures of 30, 45 and 60°C, followed by readjustment of the pH to 5. Increasing the pH, time and/or temperature resulted in more effective degradation of furans and resulted in better fermentability for both of the tested hydrolyzates, but higher loss of the sugars was observed as well. Overliming of glucose and furfural solution at pH 12 showed a rapid decrease in concentration of these chemicals followed by a slow degradation process. Therefore, a kinetic model was proposed for the detoxification, where the sugars or furans make transient complexes with calcium ions and this complex will then be converted to the degradation product. The ANOVA analysis of the model resulted in an average R 2 of 0.99 for the model fitted to all the experimental data points.
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| 9. |
- Taherzadeh, Mohammad J, 1965-, et al.
(författare)
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Inhibition effects of furfural on aerobic batch cultivation of Saccharomyces cerevisiae growing on ethanol and/or acetic acid
- 2000
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Ingår i: Journal of Bioscience and Bioengineering. - : Elsevier Science Publishers B.V.. - 1389-1723 .- 1347-4421. ; 90:4, s. 374-380
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Tidskriftsartikel (refereegranskat)abstract
- Physiological effects of furfural on Saccharomyces cerevisiae growing on ethanol (15 g · l-1) or acetate (20 g · l-1) as the carbon and energy source were investigated. Furfural (4 g · l-1), which was added during the exponential growth phase in batch cultures, was found to strongly inhibit cell growth on both carbon sources. No biomass formation occurred in the presence of furfural. However, furfural was in both cases converted to furfuryl alcohol and furoic acid, and growth resumed after complete conversion of furfural. During growth on ethanol, a rapid initial conversion of furfural to furfuryl alcohol was observed during the first few minutes after the addition of furfural, after which the conversion rate decreased to approximately 0.15 g · g-1 · h-1 for the remaining conversion time. Acetaldehyde accumulated in the medium during the first few hours of conversion. Interestingly, addition of acetate after furfural addition resulted in an increased conversion rate of furfural and a higher carbon dioxide evolution rate, but no growth was observed until after complete conversion of furfural. Furfural addition to cells growing on acetate as the sole carbon source induced no formation of acetaldehyde, and the furfural conversion rate was lower than that on ethanol. The relationship between inhibition effects of furfural and NADH consumption is discussed.Physiological effects of furfural on Saccharomyces cerevisiae growing on ethanol (15 g.l-1) or acetate (20 g. l-1) as the carbon and energy source were investigated. Furfural (4g.l-1), which was added during the exponential growth phase in batch cultures, was found to strongly inhibit cell growth on both carbon sources. No biomass formation occurred in the presence of furfural. However, furfural was in both cases converted to furfuryl alcohol and furoic acid, and growth resumed after complete conversion of furfural. During growth on ethanol, a rapid initial conversion of furfural to furfuryl alcohol was observed during the first few minutes after the addition of furfural, after which the conversion rate decreased to approximately 0.15 g.g-1.h-1 for the remaining conversion time. Acetaldehyde accumulated in the medium during the first few hours of conversion. Interestingly, addition of acetate after furfural addition resulted in an increased conversion rate of furfural and a higher carbon dioxide evolution rate, but no growth was observed until after complete conversion of furgural. Furfural addition to cells growing on acetate as the sole carbon source induced no formation of acet aldehyde, and the furfural conversion rate was lower than that on ethanol. The relationship between inhibition effects of furfural and NADH consumption is discussed.
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| 10. |
- Taherzadeh, Mohammad J, 1965-, et al.
(författare)
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On-line control of fed-batch fermentation of dilute-acid hydrolyzates
- 2000
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Ingår i: Biotechnology and Bioengineering. - New York, NY, United States : John Wiley & Sons Inc. - 0006-3592 .- 1097-0290. ; 69:3, s. 330-338
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Tidskriftsartikel (refereegranskat)abstract
- Dilute-acid hydrolyzates from lignocellulose are, to a varying degree, inhibitory to yeast. In the present work, dilute-acid hydrolyzates from spruce, birch, and forest residue, as well as synthetic model media, were fermented by Saccharomyces cerevisiae in fed-batch cultures. A control strategy based on on-line measurement of carbon dioxide evolution (CER) was used to control the substrate feed rate in a lab scale bioreactor. The control strategy was based solely on the ratio between the relative increase in CER and the relative increase in feed rate. Severely inhibiting hydrolyzates could be fermented without detoxification and the time required for fermentation of moderately inhibiting hydrolyzates was also reduced. The feed rate approached a limiting value for inhibiting media, with a corresponding pseudo steady-state value for CER. However, a slow decrease of CER with time was found for media containing high amounts of 5-hydroxymethyl furfural (HMF). The success of the control strategy is explained by the conversion of furfural and HMF by the yeast during fed-batch operation. The hydrolyzates contained between 1.4 and 5 g/l of furfural and between 2.4 and 6.5 g/l of HMF. A high conversion of furfural was obtained (between 65-95%) at the end of the feeding phase, but the conversion of HMF was considerably lower (between 12-40%). (C) 2000 John Wiley and Sons, Inc.Dilute-acid hydrolyzates from lignocellulose are, to a varying degree, inhibitory to yeast. In the present work, dilute-acid hydrolyzates from spruce, birch, and forest residue, as well as synthetic model media, were fermented by Saccharomyces cerevisiae in fed-batch cultures. A control strategy based on on-line measurement of carbon dioxide evolution (CER) was used to control the substrate feed rate in a lab scale bioreactor. The control strategy was based solely on the ratio between the relative increase in CER and the relative increase in feed rate. Severely inhibiting hydrolyzates could be fermented without detoxification and the time required for fermentation of moderately inhibiting hydrolyzates was also reduced. The feed rate approached a limiting value for inhibiting media, with a corresponding pseudo steady-state value for CER. However, a slow decrease of CER with time was found for media containing high amounts of 5-hydroxymethyl furfural (HMF). The success of the control strategy is explained by the conversion of furfural and HMF by the yeast during fed-batch operation. The hydrolyzates contained between 1.4 and 5 g/l of furfural and between 2.4 and 6.5 g/l of HMF. A high conversion of furfural was obtained (between 65-95%) at the end of the feeding phase, but the conversion of HMF was considerably lower (between 12-40%).
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| 11. |
- Taherzadeh, Mohammad J, 1965-, et al.
(författare)
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Physiological effects of 5-hydroxymethylfurfural on Saccharomyces cerevisiae
- 2000
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Ingår i: Applied Microbiology and Biotechnology. - : Springer Verlag. - 0175-7598 .- 1432-0614. ; 53:6, s. 701-708
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Tidskriftsartikel (refereegranskat)abstract
- The physiological effects of 5-hydroxymethylfurfural (HMF) on Saccharomyces cerevisiae CBS 8066 in the presence and absence of furfural were studied. Experiments were carried out by pulse addition of HMF (2-4 g/l) as well as HMF (2 g/l) together with furfural (2 g/l) to batch cultivations of S. cerevisiae. Synthetic medium with glucose (50 g/l) as carbon and energy source was used. Addition of 4 g/l of HMF caused a decrease (approx. 32%) in the carbon dioxide evolution rate. Furthermore, the HMF was found to be taken up and converted by the yeast with a specific uptake rate of 0.14 (±0.03) g/g h during both aerobic and anaerobic conditions, and the main conversion product was found to be 5-hydroxymethylfurfuryl alcohol. A previously unreported compound was found and characterized by mass spectrometry. It is suggested that the compound is formed from pyruvate and HMF in a reaction possibly catalysed by pyruvate decarboxylase. When HMF was added together with furfural, very little conversion of HMF took place until all of the furfural had been converted. Furthermore, the conversion rates of both furfural and HMF were lower than when added separately and growth was completely inhibited as long as both furfural and HMF were present in the medium.
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| 12. |
- Taherzadeh, Mohammad J, 1965-, et al.
(författare)
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Production of mycelium biomass and ethanol from paper pulp sulfite liquor by Rhizopus oryzae
- 2003
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Ingår i: Bioresource Technology. - : Elsevier Ltd. - 0960-8524 .- 1873-2976. ; 88:3, s. 167-177
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Tidskriftsartikel (refereegranskat)abstract
- The cultivation conditions for Rhizopus oryzae grown in synthetic medium and paper pulp spent sulfite liquor (SSL) were investigated to achieve high biomass and ethanol yields using shake flasks and bioreactors. The fungus assimilated the hexoses glucose, mannose and galactose, and the pentoses xylose and arabinose as well as acetic acid which are present in SSL. The assimilation of hexoses was faster than pentoses during cultivation in a synthetic medium. However, all sugars were assimilated concomitantly during growth in SSL supplemented with ammonium, magnesium, calcium, phosphate, sulfate and trace amounts of some other metal ions (SSL-S). The medium composition had an important influence on biomass yield. The highest biomass yields, viz. 0.18 and 0.43 g biomass/g sugar were obtained, when the cells were cultivated in shake flasks with a synthetic medium containing glucose as carbon and energy source and SSL-S, respectively. The corresponding yields in a bioreactor with more efficient aeration were 0.22 and 0.55 g/g. In addition to the biomass, ethanol, lactic acid, and glycerol were important extracellular metabolites of the cultivation with maximum yields of 0.37, 0.30 and 0.09 g/g, respectively. When the source of sugars in the medium was exhausted, the fungus consumed the metabolites produced, such that the liquid medium was depleted of potential oxidizable nutrients. In general, there was a direct competition between lactic acid and ethanol among the metabolites. Poor medium compositions and cultivation conditions resulted in higher yields of lactic acid, whereas the ethanol and biomass yields were higher in rich media. SSL-S supported good growth of mycelium and a high ethanol yield.
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| 13. |
- Taherzadeh, Mohammad J, 1965-, et al.
(författare)
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Strategies for enhancing fermentative production of glycerol - A review
- 2002
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Ingår i: Enzyme and Microbial Technology. - 0141-0229 .- 1879-0909. ; 31:1-2, s. 53-66
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Forskningsöversikt (refereegranskat)abstract
- The present paper reviews the metabolic basis of different methods for fermentative glycerol production. The most important microbial production organism is the yeast Saccharomyces cerevisiae but other yeast species, as well as molds, algae, and bacteria are of potential interest for glycerol production. A large variety of methods have been applied to increase the fermentative glycerol yield. The first methods were based on physiological control, e.g. chemically induced overproduction of glycerol through NADH entrapment by the addition of chemical steering agents (such as bisulfite). More recently, genetic engineering of the glycolytic pathway has been used to improve production, involving modulated function of e.g. triose phosphate isomerase, phosphoglycerate mutase, PDC or alcohol dehydrogenase. Direct intervention in the glycerol pathway, such as overexpression of G3P dehydrogenase, has also been tried. The applied strategies can be divided into three principal groups; (a) deactivation or down-regulation of NADH oxidation sites alternative to G3P dehydrogenase, (b) increase of NADH generation or, (c) direct changes in the carbon flux to glycerol. (C) 2002 Published by Elsevier Science Inc.
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