Harnessing heterosis: Enhancing industrial yeast functionality through interspecific hybridisation: Dissertation

Industrial fermentations of beer, cider and wine, can be very challenging for the yeast. Brewer's wort mainly comprises starch-derived sugars requiring specific transmembrane transporters. Apple and grape musts contain simpler sugars but with high fructose-to-glucose ratio and very high total sugar concentration, respectively. Additionally, environmental conditions, such as fermentation temperature, can hinder yeast performance. The temperature directly impacts the aromatic properties of a fermented beverage. Clean and fresh lager beer is a result of cold fermentation, only possible with the use of the hybrid yeast. Saccharomyces pastorianus combines the cold tolerant Saccharomyces eubayanus with good fermenter Saccharomyces cerevisiae. Genetically, lager yeast can be divided into two subgroups with different ratios of the parental DNA. Historically, these group differentiation matched the strains variable use of maltotriose.

In order to study maltotriose utilisation in lager yeast, the brewing performance of a diverse group of strains was linked to the uptake rates and presence of specific transporters. Contrary to the traditional characterisation, group I comprises strains with varying ability to consume maltotriose, enabled by the presence of SeAGT1 and MTT1 transporters. A58, a variant of the group I strain CBS1513, had a particularly high capacity to take up maltotriose, even at extreme temperatures (0°C). This property is a result of increased MTT1 copy number. The presence/absence of specific permeases accounts for much of the variation in lager yeast fermentation behaviour.

Besides variable sugar utilisation rates, lager yeast are phenotypically fairly homogeneous. The use of genetic modification in brewing yeast is not well accepted, limiting the possibilities for strain improvement. However, the discovery of S. eubayanus, raised the possibility to create new tailor-made lager yeast. By mating a domesticated ale strain with S. eubayanus hybrids were obtained with faster fermentation and superior alcohol content at low temperature (12°C). The main contribution of S. eubayanus to these new hybrid strains seems to be cold tolerance.

Yeast cryotolerance is also important in cider and wine making. Therefore, S. eubayanus was crossed with a S. cerevisiae wine-producing strain and the hybrids tested for their suitability for cider and wine making. In cider fermentation, hybrids maintained the superior fermentation rate of S. eubayanus without the unpleasant sulphur by-products generated by this parent. A consumer panel rated the ciders fermented with hybrid strains as identical to the wine strain cider. Both being significantly more pleasant than S. eubayanus' cider. The best performing hybrid was processed in active dry yeast form, and tested in winemaking conditions. The hybrid retained high viability and fermentation profile comparable to commercial strains but with increased production of pleasant aroma volatiles.

This project showed that interspecific hybridisation is an effective approach to improve low-temperature fermentation performance without compromising product quality.