Efeito da disponibilidade de oxigénio na produção de bioetanol por Scheffersomyces stipitis
authors Tiago Manuel dos Santos Henriques
supervisors Ana Maria Rebelo Barreto Xavier and Luísa Alexandra Seuanes Serafim Leal
thesis type Master Thesis
author keywords Bioethanol, Hardwood Spent Sulphite Liquor (HSSL), Scheffersomyces stipitis, Oxygen, Dissolved oxygen tension, Aeration
abstract Bioethanol is a renewable alternative to fossil fuels. However, the first generation bioethanol, produced from food feedstocks, triggered serious economic and social problems, therefore it is essential to find strategies to enable the commercial viability of second generation bioethanol, produced from lignocellulosic feedstocks. Hardwood spent sulfite liquor (HSSL) is a by-product from paper industry that can be used as substrate for second generation bioethanol production, owing to its high sugars content. However, pentoses are the largest fraction of HSSL sugars. Therefore, the fermentation of HSSL is performed by Scherffersomyces stipitis, since this yeast is able to ferment both hexoses and pentoses. Nevertheless, S. stipitis only produces ethanol under microaerophilic conditions, so the establishment of optimal aeration conditions is the main challenge for ethanol production by S. stipitis. Thus, this work aimed to establish an aeration strategy that allows for the efficient bioethanol production from HSSL by S. stipitis C4, which is an adapted strain to this substrate. In this way, experiments were carried out in Erlenmeyer, in order to characterize S. stipitis C4 growth, and in bioreactor, in order to study the ethanol production by S. stipitis C4 in two different aeration strategies. In the first strategy was used just one aeration stage, with dissolved oxygen tension (DOT, %) control, and in the second strategy were used two aeration stages, with DOT control in the first stage and with oxygen restriction in the second stage. In the Erlenmeyer experiments, the S. stipitis C4 growth was completely inhibited by HSSL. Regarding to the bioreactor experiments, the DOT control did not allow for an ethanol production in one stage aeration strategy. However, efficient ethanol production was possible in two stage aeration strategy with synthetic medium. In this strategy, the maximum specific growth rate and the biomass yield increased with the use of a higher DOT value in the first aeration stage. In addition, the ethanol productivity during second aeration stage also increased with the use of a higher DOT value in the first aeration stage. In turn, the oxygen restriction in second aeration stage avoided the ethanol reassimilation by S. stipitis C4. Thus, the best results for ethanol production were obtained with DOT control to 50% during the first aeration stage and with 0 mLAir.min-1 e 250 rpm during the second aeration stage. The application of this aeration strategy to 60% HSSL/40% synthetic medium allowed to reach a maximum specific growth rate of 0.17 h-1 in the first aeration stage, which demonstrates the increases of S. stipitis C4 tolerance to inhibitors by the high oxygen availability. Furthermore, the whole fermentation ethanol volumetric production rate and ethanol yield was 0.03 g.L-1.h-1 e 0.38 g.g-1, respectively. Thus, the high conversion efficiency of sugar in ethanol (74.4%) demonstrates the two stage aeration fermentation as a promising strategy for second generation bioethanol production from HSSL.
year published 2015
link http://hdl.handle.net/10773/15673
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