WINETECH Technical Yearbook 2020

ability (or lack thereof) can be exploited by the winemaker to specifically craft the desired wine style by selecting a yeast strain with specific properties and capabilities. IT’S ALL IN THE GENES Before thiol liberation, the yeast needs to transport the conjugated precursor from the juice into the yeast cell. For this transportation, the yeast needs to be in possession of general amino acid transporters located in the cell membrane. Once in the cell, the glutathionylated precursor is first transformed into the cysteinylated precursor before the aromatic thiol can be released. This transformation entails a complex mechanism involving multiple genes (Belda et al ., 2017A). For the cleavage of the cysteinylated precursors (and subsequent release of the aromatic thiols), a specific enzyme, namely the carbon-sulphur β-lyase enzyme, is required. Various genes have been identified as being responsible for the yeasts’ production capabilities of cleaving enzymes including BNA3, CYS3, STR3 and GLO1. However, the main gene responsible for the release of thiols is the IRC7 gene. Studies done in a synthetic wine medium with precursors present have shown that a full-length copy of IRC7 is required for the cleavage of precursors and the deletion of the gene resulted in most yeast strains being incapable of converting the

marked increase in 3MHA content (Anfang et al ., 2009), while sequential fermentationwith Torulaspora delbrueckii and Saccharomyces cerevisiae showed a significant increase in 4MMP production compared to fermentation with Saccharomyces cerevisiae alone (Belda et al ., 2017B). As mul t i-starter fermentat ions are increasingly being used for organoleptic and quality improvement, the possible advantages regarding thiol production should also be considered. CONCLUSIONS Commercial yeast manufacturers offer a wide range of yeast strains often with recommendations based on the ability to produce certain attributes. Knowledge of yeast genes required provides tools to distinguish potential yeast strains based on the ability to produce and convert thiols. This will help the winemaker make an informed decision when selecting a yeast strain based on specific target market segments. SUMMARY Studies have shown specific genes are in­ volved in the process of releasing aromatic thiols from the conjugated precursors and could serve as markers to identify yeast strains with high thiol releasing potential. A better understanding of the genes and mechanisms involved will allow winemakers to more reliably produce wines in their desired wine style.

categorised according to the amount of thiols produced during fermentation and results showed that the majority (70%) of the strains tested were considered to be low releasers of thiols, 20%weremoderate releasers and 10% were considered high releasers. In most cases, the low releasers possessed an inactive IRC7 gene, which limited the ability to release thiols, while the moderate and high releasers had an active or partially active IRC7 gene. Among the selection of commercial yeast strains tested, the strains recommended by suppliers for the production of white wine produced relatively higher concentrations of volatile thiols when compared to strains that were recommended for the production of red wine only or the production of both red and white wine. This shows some active selection from yeast manufacturers for strains with a higher thiol-releasing capacity for the production of fruit-driven white wines. NON-CONVENTIONAL YEASTS The use of non-conventional yeasts can be an important tool for thiol production during fermentation. Certain species, such as Torulaspora delbrueckii , Kluyveromyces marxianus and Metschnikowia pulcherrima , show significant potential with marked β-lyase activity and thiol production, but with high strain dependency (Belda et al ., 2016; Zott et al ., 2011). Mixed fermentation of Sauvignon blancmust with Pichia kluyveri and Saccharomyces cerevisiae also showed a

conjugated precursors into the aromatic thiols (Santiago & Gardner, 2015). This gene could potentially serve as a molecular marker to predict a yeast’s potential to release thiols. That said, in a juice medium (and not a synthetic medium with limited precursors available), the presence of the IRC7 gene was not a prerequisite for the production of varietal thiols (Roncoroni et al ., 2011), supporting the hypothesis that other unknown precursors also contribute significantly to the final concentration of varietal thiols in wine. The esterification of 3MH to form 3MHA also requires a gene encoding alcohol acetyltransferase (ATF1) (Swiegers et al ., 2005). YEAST STRAIN VARIABILITY Several studies have reported the limited capacity of most Saccharomyces cerevisiae yeast strains to release varietal thiols from the corresponding non-volatile precursors (usually less than 5% converted) (Ruiz et al. , 2019). A study (Cordente et al ., 2017) assessed the ability of 82 different yeast strains (not all commercially available) to produce thiols from a grape-like medium containing realistic concentrations of conjugated thiol precursors. Results showed a 20-fold difference between the yeasts’ ability to release 3MH and a 35 fold difference in the ability to produce 4MMP. The yeasts were

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