Co-cultivation of non-conventional yeast with Saccharomyces cerevisiae to increase the aroma complexity of fermented beverages

Yeast are used as workhorses to convert hopped wort into beer. Conventionally, such yeasts belong to the genus Saccharomyces and most research on fermentation of wort for the production of beer has focussed on the species Saccharomyces cerevisiae and Saccharomyces pastorianus. Recently, there is an increasing interest in unravelling features of non-conventional yeast species for beer innovation. In this thesis, features of yeast isolates belonging to the species: Cyberlindnera fabianii, Pichia kudriavzevii and S. cerevisiae (all isolated from fermented masau (Ziziphus mauritiana) fruits in Zimbabwe), were studied with focus on aroma production. Additionally, a novel approach was used to apply these yeasts in co-cultivation with Brewers’ yeast (S. cerevisiae) for beer innovation. The characteristics and quality of the beer are mainly determined by aroma compounds in the final product such as esters, alcohols, aldehydes and acids. Yeast use various metabolic pathways such as glycolysis, the fermentative pathway, the tricarboxylic acid (TCA) cycle and the Ehrlich pathway to produce aroma compounds or the precursors for the synthesis thereof (Chapter 1). Among the aroma compounds, esters are of major importance, especially since they are perceived by the human olfactory system at very low concentrations. In general, esters are desirable compounds in beers due to their fruity flavour. Examples are isoamyl acetate (banana), isobutyl acetate (fruity, sweet), phenylethyl acetate (rose, apple, honey), ethyl acetate (sweet pear), ethyl hexanoate (apple, aniseed) and ethyl octanoate (sour apple). Together with an extensive range of other volatile organic compounds (VOCs) these compounds were previously profiled using headspace solid-phase-micro-extraction gas-chromatography mass-spectrometry (GCMS). Interestingly, comparative profiling of aromas showed that C. fabianii produces significantly higher amounts of isoamyl acetate and ethyl acetate compared to S. cerevisiae. It has