Effects of nutrients supplementation on fermentability of lignocellulosic hydrolysates under high gravity conditions

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Examensarbete för masterexamen
Master Thesis
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2012
Författare
Claesson, Kjersti
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Bioethanol produced from lignocellulosic materials is emerging as a promising alternative to fossil fuels. Fermentation of these substrates results in low yields due to the presence of inhibitory compounds and non-fermentable sugars. The main advantage of biomass bioconversion under HG (initial dry mater concentrations above 10% w/w) or very high gravity (VHG, above 30% w/w) conditions is the generation of high final ethanol concentrations (above 5% v/v), decreasing the cost of the distillation step, which is considered one of the main constraints in the bioethanol industry. However, many challenges are associated with the increase in initial dry mater. With this the generation of inhibitors during the pre-treatment step is increased, which in turn dramatically reduces the fermentability of the material. In this project, the effects of nutrients on the fermentative performance of Saccharomyces cerevisiae on toxic spruce hydrolysates were evaluated. Fermentations were performed with the addition of selected nutrients to evaluate their effects on inhibitor tolerance and fermentation rates. Yeast extract was found to have positive effects on the fermentation rate and biomass growth in comparison to other nutrients at fermentations in 78% of hydrolysate. The effect was evaluated with smaller inoculum sizes, 1 and 0.5 gL-1. For 1 gL-1 of initial cell dw, and less, the nutrient addition was crucial for biomass growth and ethanol formation. Cultivations performed with an initial cell density of 1 gL-1 and with yeast extract supplementation resulted in an ethanol concentration of 19.2 gL-1. In comparison, the unsupplemented cultivation produced an ethanol concentration of 1.5 gL-1. The conversion of inhibitory compounds was also affected by the supplementation. At 1 gL-1 of inoculums, HMF and furfural was not converted to less toxic compounds without the addition of yeast extract. The positive effects were found to be independent from the concentration starting at 0.5% w/v of yeast extract. The results were used for scaling-up experiments in bioreactors. In these controlled conditions the addition of yeast extract resulted in higher production rates and yields. Spruce hydrolysate was found to be fermentable at a concentration of 90%. However, the high concentration resulted in lower ethanol yield. A yield of 0.16 was reached after 36 hours for the fermentation at 90% of hydrolysate, which is considerably lower than the theoretical yield of 0.5. Fermentations at 80 % of hydrolysate reached yield of 0.35
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Energi, Industriell bioteknik, Energy, Industrial Biotechnology
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