PhD defence, Gabriela Miguel Mellado

Thesis title:

“Pichia kluyveri yeast in brewing: Insights into phenotypic traits and genomic aspects”

Abstract:

Driven by the need of innovation, the application of non-Saccharomyces yeasts for brewing has seen a substantial growth in recent years. Pichia kluyveri, for instance, is a maltose-negative yeast applied for production of non- and low- alcoholic beer with an attractive fruity flavor, due to the enhanced production of aroma compounds that it releases. Despite of its brewing potential, intrinsic physiological and metabolic aspects of this yeast are unknown. In this context, the main aim of this project was to understand and to increase the knowledge on phenotypic and genomic aspects of brewing P. kluyveri strains, and, if possible, to establish a link between these aspects.

Three P. kluyveri strains showed similar fermentation parameters in synthetic wort in terms of growth, carbohydrate metabolism and metabolites, enhanced production of esters, a strong preference for methionine and a low preference for several amino acids including glutamate, glutamine and aspartate. Strain differences were observed regarding the development of volatile sulfur aroma compounds, especially in the release of hydrogen sulfide. Since no variations were detected in terms of absence or presence of related genes among the low and high producers of hydrogen sulfide strains, a distinctive genic regulation affecting sulfur-related enzymes might influence the sulfur compound pattern observed among strains.

Genomic analysis of P. kluyveri revealed the absence of some genes encoding key enzymes associated to the sulfur metabolism in S. cerevisiae. For instance, transsulfuration enzymes responsible of the cysteine formation from homocysteine were not found. However, homolog proteins exclusive from other non-Saccharomyces yeasts that allow the synthesis of cysteine through the o-acetyl serine pathway, and the consequent formation of homocysteine through the transsulfuration pathways, were found in P. kluyveri. 

Supervisor:

Associate Professor Nils Arneborg, Department of Food Science

Evaluation Committee:  

Head of the committee, Associate Professor Henrik Siegumfeldt, Department of Food Science

Professor Doris Rauhut, Hochschule Geisenheim University, Germany

Associate Professor Timothy J. Hobley, DTU National Food Institute