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- Albers, Eva, 1966, et al.
(författare)
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Selective suppression of bacterial contaminants by process conditions during lignocellulose based yeast fermentations
- 2011
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Ingår i: Biotechnology for Biofuels. - : Springer Science and Business Media LLC. - 1754-6834 .- 1754-6834. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- BackgroundContamination of bacteria in large-scale yeast fermentations is a serious problem and a threat to the development of successful biofuel production plants. Huge research efforts have been spent in order to solve this problem, but additional ways must still be found to keep bacterial contaminants from thriving in these environments. The aim of this project was to develop process conditions that would inhibit bacterial growth while giving yeast a competitive advantage.ResultsLactic acid bacteria are usually considered to be the most common contaminants in industrial yeast fermentations. Our observations support this view but also suggest that acetic acid bacteria, although not so numerous, could be a much more problematic obstacle to overcome. Acetic acid bacteria showed a capacity to drastically reduce the viability of yeast. In addition, they consumed the previously formed ethanol. Lactic acid bacteria did not show this detrimental effect on yeast viability. It was possible to combat both types of bacteria by a combined addition of NaCl and ethanol to the wood hydrolysate medium used. As a result of NaCl + ethanol additions the amount of viable bacteria decreased and yeast viability was enhanced concomitantly with an increase in ethanol concentration. The successful result obtained via addition of NaCl and ethanol was also confirmed in a real industrial ethanol production plant with its natural inherent yeast/bacterial community.ConclusionsIt is possible to reduce the number of bacteria and offer a selective advantage to yeast by a combined addition of NaCl and ethanol when cultivated in lignocellulosic medium such as wood hydrolysate. However, for optimal results, the concentrations of NaCl + ethanol must be adjusted to suit the challenges offered by each hydrolysate.
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| 2. |
- Koppram, Rakesh, 1986, et al.
(författare)
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A novel process configuration of Simultaneous Saccharification and Fermentation for bioethanol production at high solid loadings
- 2012
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Ingår i: Advanced Biofuels in a Biorefinery Approach, February 28 - March 1, 2012, Copenhagen, Denmark.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Simultaneous saccharification and fermentation (SSF) is a process option for lignocellulosic bioethanol production that has proven to have several advantages compared to separate hydrolysis and fermentation. The economical viability and commercialization of cellulose-to-ethanol demands the process to work under high-solid loadings to result in high sugar yield and final ethanol titer in S. cerevisiae based SSF process. In a conventional batch SSF process practical limitations to high-solid loadings include, poor mixing and accessibility of enzymes to substrates and high inhibitors concentration that reduces the yeast viability and metabolism. In order to overcome these limitations, we propose a novel SSF process configuration involving feeding of substrate, enzyme and yeast. It is possible to overcome mixing issues associated with a batch SSF at high-solid loadings by a feed of substrate, enzyme and yeast. The feed of freshly cultivated yeast throughout the fermentation process ensures active metabolic state of yeast. In addition, the substrate feed ensures low inhibitors concentration at any given time point increasing the survival ability of yeast compared to a batch SSF. The enzyme feed ensures slow release of glucose providing an opportunity for xylose consuming yeast strain to co-consume xylose together with glucose. The aim of the current work is to understand how different combinations of feeding strategies influence the outcome of the SSF process. In the longer perspective, we aim at deducing an optimized SSF process that can handle very high-solid loadings with efficient hydrolysis and fermentation process at low enzyme and yeast loadings, respectively.
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| 5. |
- Westman, Johan, 1983, et al.
(författare)
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Effects of encapsulation of microorganisms on product formation during microbial fermentations
- 2012
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Ingår i: Applied Microbiology and Biotechnology. - : Springer Science and Business Media LLC. - 1432-0614 .- 0175-7598. ; 96:6, s. 1441-1454
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Forskningsöversikt (refereegranskat)abstract
- This paper reviews the latest developments in microbial products by encapsulated microorganisms in a liquid core surrounded by natural or synthetic membranes. Cells can be encapsulated in one or several steps using liquid droplet formation, pregel dissolving, coacervation, and interfacial polymerization. The use of encapsulated yeast and bacteria for fermentative production of ethanol, lactic acid, biogas, l-phenylacetylcarbinol, 1,3-propanediol, and riboflavin has been investigated. Encapsulated cells have furthermore been used for the biocatalytic conversion of chemicals. Fermentation, using encapsulated cells, offers various advantages compared to traditional cultivations, e.g., higher cell density, faster fermentation, improved tolerance of the cells to toxic media and high temperatures, and selective exclusion of toxic hydrophobic substances. However, mass transfer through the capsule membrane as well as the robustness of the capsules still challenge the utilization of encapsulated cells. The history and the current state of applying microbial encapsulation for production processes, along with the benefits and drawbacks concerning productivity and general physiology of the encapsulated cells, are discussed.
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| 6. |
- Westman, Johan, 1983, et al.
(författare)
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Encapsulation-Induced Stress Helps Saccharomyces cerevisiae Resist Convertible Lignocellulose Derived Inhibitors
- 2012
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Ingår i: International Journal of Molecular Sciences. - : MDPI AG. - 1661-6596 .- 1422-0067. ; 13:9, s. 11881-11894
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Tidskriftsartikel (refereegranskat)abstract
- The ability of macroencapsulated Saccharomyces cerevisiae CBS8066 to withstand readily and not readily in situ convertible lignocellulose-derived inhibitors was investigated in anaerobic batch cultivations. It was shown that encapsulation increased the tolerance against readily convertible furan aldehyde inhibitors and to dilute acid spruce hydrolysate, but not to organic acid inhibitors that cannot be metabolized anaerobically. Gene expression analysis showed that the protective effect arising from the encapsulation is evident also on the transcriptome level, as the expression of the stress-related genes YAP1, ATR1 and FLR1 was induced upon encapsulation. The transcript levels were increased due to encapsulation already in the medium without added inhibitors, indicating that the cells sensed low stress level arising from the encapsulation itself. We present a model, where the stress response is induced by nutrient limitation, that this helps the cells to cope with the increased stress added by a toxic medium, and that superficial cells in the capsules degrade convertible inhibitors, alleviating the inhibition for the cells deeper in the capsule.
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| 7. |
- Westman, Johan, 1983, et al.
(författare)
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Flocculation Causes Inhibitor Tolerance in Saccharomyces cerevisiae for Second-Generation Bioethanol Production
- 2014
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Ingår i: Applied and Environmental Microbiology. - : American Society for Microbiology. - 1098-5336 .- 0099-2240. ; 80:22, s. 6908-6918
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Tidskriftsartikel (refereegranskat)abstract
- Yeast has long been considered the microorganism of choice for second-generation bioethanol production due to its fermentative capacity and ethanol tolerance. However, tolerance toward inhibitors derived from lignocellulosic materials is still an issue. Flocculating yeast strains often perform relatively well in inhibitory media, but inhibitor tolerance has never been clearly linked to the actual flocculation ability per se. In this study, variants of the flocculation gene FLO1 were transformed into the genome of the nonflocculating laboratory yeast strain Saccharomyces cerevisiae CEN.PK 113-7D. Three mutants with distinct differences in flocculation properties were isolated and characterized. The degree of flocculation and hydrophobicity of the cells were correlated to the length of the gene variant. The effect of different strength of flocculation on the fermentation performance of the strains was studied in defined medium with or without fermentation inhibitors, as well as in media based on dilute acid spruce hydrolysate. Strong flocculation aided against the readily convertible inhibitor furfural but not against less convertible inhibitors such as carboxylic acids. During fermentation of dilute acid spruce hydrolysate, the most strongly flocculating mutant with dense cell flocs showed significantly faster sugar consumption. The modified strain with the weakest flocculation showed a hexose consumption profile similar to the untransformed strain. These findings may explain why flocculation has evolved as a stress response and can find application in fermentation-based biorefinery processes on lignocellulosic raw materials.
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| 10. |
- Westman, Johan, 1983, et al.
(författare)
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Improved inhibitor tolerance and simultaneous utilisation of hexoses and pentoses during fermentation of inhibitory lignocellulose hydrolysates by yeast at high local cell density
- 2014
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Ingår i: Lignobiotech III, Concepción, Chile, 26-29 October 2014.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Issues still creating a barrier for successful commercialization of second generation bioethanol are the inhibitory compounds present in the lignocellulose derived media and the inability of Saccharomyces cerevisiae to efficiently utilise pentoses. Both of the issues have been addressed by construction of recombinant yeast strains, often in combination with evolutionary engineering. However, hexoses and pentoses are mainly fermented sequentially by these yeasts, prolonging the total fermentation time. In our research, we have shown that encapsulation of S. cerevisiae cells in semi-permeable alginate-chitosan liquid core gel capsules increased the tolerance to lignocellulose hydrolysates and specifically furan aldehydes. The potential formation of concentration gradients of these convertible inhibitors through the cell pellet inside the capsule has been given as explanation to the increased tolerance [1]. Gradients of carbohydrates in the capsules were further hypothesised to lead to an improvement in the simultaneous utilisation of hexose and pentose sugars by the cells. To verify this hypothesis we constructed and encapsulated the xylose fermenting S. cerevisiae strain CEN.PK XXX. We found that encapsulation of the strain not only increased the inhibitor tolerance of the yeast, but also promoted simultaneous utilisation of glucose and xylose. Furthermore, during the 96 hour fermentations of a medium with glucose and xylose, the encapsulated yeast consumed at least 50% more xylose compared to the suspended cells. This led to approximately 15% higher final ethanol titres in batch fermentations. As proof of concept, an inhibitory spruce hydrolysate was fermented by suspended and encapsulated cells. The suspended cells fermented the hexoses and pentoses mainly sequentially, after a long lag phase. The encapsulated yeast, on the other hand, did not display a lag phase, and consumed glucose, mannose, galactose and xylose simultaneously from the start of the batch. However, encapsulation of yeast cells in an alginate membrane would likely not be economically permissible in an industrial setting. We therefore investigated whether keeping the cells tight together would be sufficient, even without a membrane. To this end we constructed a set of flocculating yeast strains with different flocculation strengths by expression of different variants of a flocculation gene. We found that the strongest flocculating strain, forming large dense cell flocs in the batch reactor, increased the tolerance towards furfural and increased the fermentation rate in an inhibitory spruce hydrolysate, compared to the non-flocculating strain. Overall, yeast at high local cell density comes out as a promising option for production of second generation bioethanol.References.[1] J.O. Westman et al., Int. J. Mol. Sci., 13, 11881-11894, 2012.
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