Phenotypical and Physiological Characterization of a Flocculating Yeast Strain

Poster (konferens), 2010

Bioethanol from lignocellulosic materials is one of the desired alternatives to meet the increased demand of renewable fuels. However, there are challenges in several steps of lignocellulose processing, including pretreatment, hydrolysis and fermentation. Using flocculating strains in the fermentation process gives a number of advantages. For example, the cells can be accumulated in the bioreactors leading to high cell concentration and rapid fermentation. They are easily separated using sedimentation and can thus be recycled to the bioreactors. Some of these strains are also better than non-flocculating strains at tolerating the possible inhibitors in the cultivation media, such as furan aldehydes, organic acids and phenolic compounds. These inhibitors make it hard for the yeast to ferment the hydrolyzate and detoxification is often necessary. A flocculating yeast strain was isolated from a Swedish ethanol plant (Domsjö Fabriker AB) fermenting sulphite liquor, and registered at Culture Collection University of Gothenburg as CCUG 53310. It has been shown that this strain can successfully ferment lignocellulosic hydrolyzates, where freely suspended reference strains failed to assimilate any sugar. In order to understand and get the possibility to improve the fermentation, a phenotypic and physiological characterization of the yeast strain has been performed. The effect of different inhibitor classes present in hydrolyzate as well as of complete hydrolyzate, on the macromolecular composition of the yeast has been investigated. Different responses can be seen from the different inhibitor classes, providing evidence for differences in metabolism between yeast cells grown in the different media. The phenotypical studies have shown that the strain, that is constitutively flocculating, belongs to the Flo1 phenotype, meaning that its flocculation is inhibited only by mannose. It has also been shown that the flocculation is dependent on cell wall proteins, and Ca2+, suggesting calcium ion dependent proteins binding to carbohydrates in neighboring cells. The flocculating cells also exhibit a significantly higher hydrophobicity than the non-flocculating reference strain; this is a factor that might also contribute to the flocculation.