| 1. |
- Almeida, Joao, et al.
(författare)
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Increased tolerance and conversion of inhibitors in lignocellulosic hydrolysates by Saccharomyces cerevisiae
- 2007
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Ingår i: Journal of Chemical Technology and Biotechnology. - : Wiley. - 0268-2575 .- 1097-4660. ; 82:4, s. 340-349
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Forskningsöversikt (refereegranskat)abstract
- During hydrolysis of lignocellulosic biomass, monomeric sugars and a broad range of inhibitory compounds are formed and released. These inhibitors, which can be organized around three main groups, furans, weak acids and phenolics, reduce ethanol yield and productivity by affecting the microorganism performance during the fermentation step. Among the microorganisms that have been evaluated for lignocellulosic hydrolysate ethanol fermentation, the yeast Saccharomyces cerevisiae appears to be the least sensitive. In order to overcome the effect of inhibitors, strategies that include improvement of natural tolerance of microorganism and use of fermentation control strategies have been developed. An overview of the origin, effects and mechanisms of action of known inhibitors on S. cerevisiae is given. Fermentation control strategies as well as metabolic, genetic and evolutionary engineering strategies to obtain S. cerevisiae strains with improved tolerance are discussed.
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| 2. |
- Petersson, Anneli, et al.
(författare)
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Fed-batch cultivation of Saccharomyces cerevisiae on lignocellulosic hydrolyzate
- 2007
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Ingår i: Biotechnology Letters. - : Springer Science and Business Media LLC. - 1573-6776 .- 0141-5492. ; 29:2, s. 219-225
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Tidskriftsartikel (refereegranskat)abstract
- Saccharomyces cerevisiae grows very poorly in dilute acid lignocellulosic hydrolyzate during the anaerobic fermentation for fuel ethanol production. However, yeast cells grown aerobically on the hydrolyzate have increased tolerance for the hydrolyzate. Cultivation of yeast on part of the hydrolyzate has therefore the potential of enabling increased ethanol productivity in the fermentation of the hydrolyzate. To evaluate the ability of the yeast to grow in the hydrolyzate, fed-batch cultivations were run using the ethanol concentration as input variable to control the feed-rate. The yeast then grew in an undetoxified hydrolyzate with a specific growth rate of 0.19 h(-1) by controlling the ethanol concentration at a low level during the cultivation. However, the biomass yield was lower for the cultivation on hydrolyzate compared to synthetic media: with an ethanol set-point of 0.25 g/l the yield was 0.46 g/g on the hydrolyzate, compared to 0.52 g/g for synthetic media. The main reason for the difference was not the ethanol production per se, but a significant production of glycerol at a high specific growth rate. The glycerol production may be attributed to an insufficient respiratory capacity.
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| 3. |
- Petersson, Anneli
(författare)
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Improving Ethanol Production from Lignocellulose Hydrolyzates: Fed-batch Fermentation, Yeast Cultivation and Strain Development for Increased Tolerance
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ethanol production by the yeast Saccharomyces cerevisiae from dilute acid lignocellulose hydrolyzate was studied with the aim to improve fuel ethanol production. Specifically, the problem of inhibition was addressed. Lignocellulose hydrolyzate contains not only sugars, but also several compounds, such as furfural and 5-hydroxymethyl furfural (HMF), which can inhibit yeast growth and ethanol production by the yeast. It was shown that severe inhibition could be avoided during both aerobic cultivation and anaerobic fermentation in properly controlled fed-batch processes. The reason for this is that the levels of inhibitors can be maintained at low levels since the yeast has the ability to convert these substances to less toxic compounds if severe inhibition is avoided. In aerobic fed-batch cultivation aiming at high biomass yield, it is essential not only to avoid inhibiting concentrations of e.g. furfural and HMF, but also to avoid ethanol formation due to over-flow metabolism. Closed-loop feed-rate control with constant ethanol set-point, allowed a specific biomass productivity of around 0.2 g g-1 h-1 with a yield of 0.46 g biomass/g fermentable sugar. Feeding strategies were also successfully developed for anaerobic fed-batch fermentation. It was possible to increase the specific ethanol productivity several-fold in controlled fed-batches compared to batch fermentation - from 0.06 g g-1 h-1 to 0.7 g g-1 h-1. In addition to process technology, work was also made concerning inhibitor tolerance of the yeast. An alcohol dehydrogenase enzyme able to reduce HMF was identified using genome-wide analysis. The enzyme ? encoded by the gene ADH6 ? was shown to catalyze NADPH-dependant reduction of HMF and over-expression of the gene gave an increased in vivo HMF conversion ability. Furthermore, the modified yeast showed increased fermentation rate in undetoxified lignocellulose hydrolyzate.
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