Industrial challenges in the use of Saccharomyces cerevisiae for ethanolic fermentation of lignocellulosic biomass
Sammanfattning: Popular Abstract in English The production and use of bioethanol can help reducing the dependency on oil, and also represents a renewable source of energy. Bioethanol can be obtained from biomass such as agricultural and forest residues, after appropriate pretreatment and fermentation. This thesis addresses some of the challenges that must be overcome for the production of ethanol from biomass to be economically feasible. Yeast is able to transform sugars into alcohol (ethanol) by fermentation, and this is the reason why it has been used throughout history to ferment the sugars present in cereals and grapes for the production of bread, beer and wine. In the same way, this microorganism can also be used to produce bioethanol from biomass. However, biomass consists of chains of complex sugars that must be broken down into simple sugars that can be fermented by yeast to produce ethanol. High temperature and pressure, and, sometimes, also chemicals, are usually needed to break down the complex mixture of sugars. However, severe treatment conditions cause the sugars to be degraded into aldehydes. Biomass also contains other compounds, such as acids and phenolics, which are also released during processing. These, together with aldehydes, inhibit the metabolism of the yeast, thus reducing its ability to ferment sugars. In this work, the effect of acetic acid on yeast was investigated. An adaptation procedure that enabled yeast to be less inhibited by the presence of acetic acid was developed, resulting in a reduction of the fermentation time. The more sugars the yeast is able to consume, the more ethanol can be produced, and the higher the process yield. However, yeast does not have the enzymes required for the consumption of xylose and arabinose, two of the simple five carbon sugars (or “pentose sugars”) that are present at significant amounts in various types of biomass. Therefore, five genes coding for the missing enzymes were introduced in an industrial yeast. The rate of consumption of these pentose sugars was later improved by gradually adapting the yeast so that it grew faster on these sugars. Another challenge in bioethanol plants is the lack of sterile conditions during operation. As a result, other microorganisms present in the environment can contaminate the plant and compete with yeast for the available sugars. For example, some bacteria produce acids instead of ethanol. In addition, the presence of acids inhibits yeast metabolism, further reducing ethanol production. However, the lack of sterility offers the possibility of isolating yeasts that have become naturally adapted over time to the severe fermentation conditions. In this work, a lactic acid bacterium that consumed simple sugars and produced lactic acid was isolated from an ethanol plant in Sweden. The effect of this bacterium on yeast fermentation was investigated. Possible changes in the process conditions that would reduce the level of this contaminant, and thus help maintain a high level of ethanol production by the yeast, were also studied. An indigenous yeast that dominated the fermentation process was also isolated at the same plant. This yeast systematically competed with the commercial yeast added, and eventually took over the fermentation, due to its better robustness. Such naturally adapted yeasts can provide useful information on yeast tolerance mechanisms. In addition, these yeasts could be used in other fermentation plants. Finally, competition experiments between to different yeasts were performed to investigate whether one yeast would dominate over another under conditions of limited sugar or oxygen supply.
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