Exploiting yeast diversity in whisky fermentations for biocatalysis of desirable flavour compounds

Abstract

Yeast is an important component in producing flavourful fermented products. The Scotch Whisky industry currently only uses four different yeast strains all belonging to Saccharomyces cerevisiae. In recent years, the brewing and wine industries have proven that a variety of yeasts can impart flavourful notes, and this has prompted research into alternative yeasts for whisky production. In this study, a variety of non-Saccharomyces yeasts were investigated for their potential of becoming new Scotch Whisky yeasts. While most of the yeast strains tested had a decreased fermentation performance compared to the tested commercial distilling yeast strains, yeast strains of Dekkera bruxellensis and Schizosacchaormyces pombe showed high sugar to ethanol conversion. Flavourwise, the yeast strains tested showed a variety of different flavours resulting in different ratios of detected congeners opening a field of wider flavour research. The flavour differences could not be linked to differences between species, Saccharomyces spp. tested showed a large variety in produced congeners. A correlation between degree of fermentation and flavour could be found with completed fermentations being more sweet, fruity, and floral and less complete fermentations having increased feinty, cereal and sulphur notes. D. bruxellensis CBS 11269 was shown to be especially fruity. The congener production across four different yeast strains and how they were influenced by different fermentation conditions (time, temperature, original gravity, pitching rate) was investigated. The results showed that the yeast strains had the largest impact, but the fermentation conditions also significantly influenced the congener ratios. Of the assessed factors, temperature and original gravity were discovered to have the largest impact.

To examine the flavour differences between yeast strains, Napping was implemented as a new quick sensory technique for Scotch Whisky and produced similar product separation as Quantitative Descriptive Analysis and gas chromatography – mass spectrometry. Furthermore, the flavour and congener changes from wash to low wines and new make spirit were investigated. The result was that assessing the flavour and congener profile on wash and low wine level is sufficient for screening purposes and drastically reduces the workload. Additionally, the alcohol by volume before distillation does not impact the flavour and congeners detected in the spirit if the same distilling regime is used. Pre-treatment (boiling, autoclaving, filtration) of wort before fermentation was also assessed to investigate different ways of reducing the impact of the microflora in wort but all three techniques had too large an impact on the fermentation performance and congener development and would not be recommended for industrial applications.

Date of Award	24 Feb 2022
Original language	English
Awarding Institution	Abertay University
Sponsors	IBioIC, Biotechnology and Biological Sciences Research Council & Scotch Whisky Research Institute
Supervisor	Graeme Walker (Supervisor), John Grigor (Supervisor) & Frances Jack (Supervisor)