Putting microbial diversity to work in shaping wine style
The AWRI has a long history in microbial strain development and the successful deployment and commercialisation of newly-developed strains. This project aims to build on that history.
There is substantial scope to both refine existing strains (such as the ‘rose’ yeast and Saccharomyces bayanus hybrids) and optimise their use through varietal or wine style pairing. For example, building on knowledge gained through production of the ‘rose’ yeast, this project will explore the degree to which yeast aromatic profiles can be modulated by modification of carbon flux through amino acid biosynthetic pathways.
Molecular technology will help to determine what is possible, and will provide foundational knowledge on biosynthetic pathways and markers for strain selection. Identification of molecular markers for specific aromatic traits will permit targeted selection of strains isolated during bioprospecting work.
In addition, this project will tap into resources identified in other AWRI projects that have a bioprospecting focus. Novel microorganisms, after initial screening, will be put to work directly through use of appropriate winemaking techniques and/or be used as source material for introducing greater genetic diversity into existing wine strains. Together these breeding and selection strategies will deliver non-genetically modified germplasm that can be used by industry, and will provide new microorganisms for winemakers seeking a point of differentiation in their wines.
Driving enhancement of aromatics in sparkling wine
The suitability for production of sparkling wine of two third-generation phenylethanol (PE) (‘rose’ aroma) over-producing yeasts, with moderated tryptophol and tyrosol production and improved resilience in difficult ferments, was assessed. The trial design, using the traditional method of sparkling wine production, was constructed so as to reveal whether these new yeast strains were suitable for both primary and secondary fermentation phases and in which phase the new yeasts would have the greatest impact. In treatments where the yeasts were used for only one phase, they were paired with the yeast PDM (commonly used in sparkling wine production) in the complementary phase. The strains were all shown to be capable of completing either primary fermentation, secondary fermentation or both. The wines, made in the 2019 vintage, were disgorged in 2020 and an assessment of their chemical and sensory attributes was undertaken. This analysis revealed that it was during primary fermentation that these yeasts had the greatest impact in terms of driving concentrations of PE in the finished wines.
While the ageing time of these wines was relatively short (eight months) compared to the time traditional method sparkling wine might spend in bottle prior to commercial release, one obvious question is whether aroma compounds such as PE have sufficient longevity in bottle to contribute to sensory attributes after extended bottle ageing. The impact of bottle ageing on PE concentration was evaluated in an earlier trial of PE over-producing strains in still Chardonnay wine. These wines were analysed 3 and 15 months after bottling and showed that overall concentrations of PE remained stable over time (Figure 1). It is understood that bottle ageing of still and sparkling wines differs with regard to lees contact; however, this work shows that PE continues to influence sensory attributes well after wine is bottled.
The hybrid test – are two genomes better than one?
A previous project generated a Saccharomyces cerevisiae x Saccharomyces uvarum hybrid that exhibited low acetate production in high-sugar conditions. The concentration of acetate produced by this hybrid was not intermediate between the two parents, but significantly less than both, suggesting an unusual genetic interaction. To investigate this further, a large collection of progeny resulting from the sporulation of the hybrid was generated. This collection showed variance in the concentration of acetate produced, which could be used to map the determinants of the low-acetate trait. Quantitative trait loci analysis revealed chromosomal loss to be a driver of low acetate production in progeny from this hybrid, suggesting that genome instability can sometimes have advantages. While this spore population was produced with the aim of mapping genetic features that contribute to low acetate production, it will also be useful in identifying genetic determinants of other characteristics.
In previous work, a head-to-head comparison of seven hybrids of S. cerevisiae with all compatible non-cerevisiae Saccharomyces yeasts was undertaken in Riesling. The earlier trial revealed distinct features of individual hybrids from a flavour and aroma perspective as well as technical aspects relevant for production. But how do these hybrids perform in red winemaking? To answer that question, an equivalent trial in Shiraz fermentations was undertaken this year with the same hybrid set to assess their performance in red winemaking.
Contributions of yeast to non-volatile wine components
Previous work assessed the relative contribution of different hybrid yeasts to the extraction of phenolic compounds in Tempranillo. This work identified the major source of yeast-derived variation in wine phenolic concentrations to be the Saccharomyces cerevisiae parent of the hybrid. This observation is consistent with earlier work at the AWRI that highlighted differences between S. cerevisiae yeast strains in phenolic compound extraction. With these results in mind, an internal collaboration with AWRI researchers working on wine macromolecules was initiated, whereby 94 S. cerevisiae wine yeast strains were assessed for their capacity to influence phenolic extraction in red wines.
Simon Schmidt, Anthony Borneman