Towards fermentation of galacturonic acid-containing feedstocks with Saccharomyces cerevisiae

Abstract

The ambition to reduce our current dependence on fossil transportation fuels has driven renewed interest in bioethanol. Pectin-rich feedstocks like sugar beet pulp and citrus peel, which are currently sold as cattle feed, are promising raw materials for the production of bioethanol. This thesis explores the challenges related to the fermentation of pectin-rich hydrolysates with Saccharomyces cerevisiae. Galacturonic acid is a major constituent of pectin-rich hydrolysates. Achieving efficient conversion of this compound is desired. This requires introduction of a heterologous pathway for galacturonate metabolism. In this project, we functionally expressed uronate isomerase and tagaturonate dehydrogenase, the first two enzymes of a bacterial catabolic pathway. Introduction of the entire 5-enzyme catabolic route did not result in a S. cerevisiae strain capable of galacturonic acid fermentation. Galacturonic acid will therefore remain present in the fermentation medium. It was shown that the presence of galacturonic acid negatively affects fermentation characteristics in aerobic chemostat cultivations. In addition, it was demonstrated that, especially at low pH, galacturonic acid has a drastic impact on cellular viability and galactose, arabinose and xylose consumption. As long as galacturonic acid is not consumed by S. cerevisiae, these inhibitory effects of galacturonic acid will remain a key issue in the yeast-based production of bioethanol and other products from pectin-rich feedstocks.