Nikola Gyurchev

• Non-domesticated Saccharomyces yeasts have promising brewing characteristics for beer diversification particularly when used in the generation of de novo interspecific hybrids. Here, non-cerevisiae Saccharomyces interspecific hybrids are explored in wort fermentations for exotic lagers and non-alcoholic beers with complex aroma profiles. Maltose and maltotriose utilization, typically associated with domesticated ale/lager brewing strains, were found in multiple Saccharomyces isolates using high-throughput screening. Saccharomyces mikatae successfully grew on maltotriose as the sole carbon source, a trait until recently unidentified for this species and only recently demonstrated in Saccharomyces jurei, another “wild” species of the clade. Remarkably, de novo hybrids between maltotriose-utilizing S. mikatae/S. jurei and the maltotriose-negative Saccharomyces eubayanus displayed heterosis on maltotriose outperforming both parents. The hybrids were only able to ferment maltose in lager brewing conditions; nevertheless, favorable fruity esters were produced. This study shows that novel hybrids can add to the diversity of lager brewing. Strains unable to grow on either sugar were identified, making them suitable candidates for brewing low-alcohol beers. Wort fermentations hinted the potential for low-alcohol brewing using novel maltose/maltotriose negative Saccharomyces hybrids producing low levels of off-flavors with one S. arboricola isolate highlighted for future studies. Interspecific Saccharomyces hybrids are sterile, hindering genetic analysis of traits of interest. One desired trait in lager brewing is cold-tolerance, with alternative hybrid combinations utilizing cold-tolerant yeasts possible. The cold-tolerance of Saccharomyces arboricola was investigated by successfully restoring fertility in Saccharomyces cerevisiae x S. arboricola hybrids and mapping quantitative trait loci (QTL) responsible in a multigeneration population with a focus on mitochondrial-nuclear interactions. QTL were successfully mapped in the S. cerevisiae genomes of the hybrids with the majority exclusive to specific mitotype. QTL in S. arboricola were not mapped due to lower diversity in the strains used. The method described here provides valuable lessons toward unraveling genetic determinants behind brewing traits in Saccharomyces hybrids.