Metabolic, Organoleptic and Transcriptomic Impact of Saccharomyces cerevisiae Genes Involved in the Biosynthesis of Linear and Substituted Esters

Addition of Saccharomyces eubayanus to SCOBY fermentations modulates the chemical and volatile compound profiles in kombucha

Kombucha is a fermented beverage derived from a sweetened tea fermentation inoculated with a bacteria-yeast consortium referred to as Symbiotic Culture of Bacteria and Yeast (SCOBY). Different SCOBY cultures can impact the beverage's quality and make the whole process highly variable. Adding Saccharomyces yeast cultures to the fermentation process can avoid stalled fermentations, providing a reproducible beverage. Here, we explored using different Saccharomyces eubayanus strains together with SCOBY in the context of kombucha fermentation. Our results show that yeast x SCOBY co-cultures exhibited a robust fermentation profile, providing ethanol and acetic acid levels ranging from 0,18-1,81 %v/v and 0,35-1,15 g/L, respectively. The kombucha volatile compound profile of co-cultures was unique, where compounds such as Isopentyl acetate where only found in yeast x SCOBY fermentations. Metabarcoding revealed that the SCOBY composition was also dependent on the S. eubayanus genotype, where besides Saccharomyces, amplicon sequence variants belonging to Brettanomyces and Starmerella were detected. These differences concomitated global changes in transcript levels in S. eubayanus related to the metabolism of organic molecules used in kombucha fermentation. This study highlights the potential for exploring different S. eubayanus strains for kombucha fermentation, and the significant yeast genotype effect in the profile differentiation in this process.

Aroma characteristics of volatile compounds brought by variations in microbes in winemaking

Wine is a highly complex mixture of components with different chemical natures. These components largely define wine's appearance, aroma, taste, and mouthfeel properties. Among them, aroma is among the most important indicators of wine's sensory characteristics. The essence of winemaking ecosystem is the process of metabolic activities of diverse microbes including yeasts, lactic acid bacteria, and molds, which result in wines with complicated and diversified aromas. A better understanding of how these microbes affect wine's aroma is a crucial step to producing premium quality wine. This study illustrates existing knowledge on the diversity and classification of wine aroma compounds and their microbial origin. Their contributions to wine characteristics are discussed, as well. Furthermore, we review the relationship between these microbes and wine aroma characteristics. This review broadens the discussion of wine aroma compounds to include more modern microbiological concepts, and it provides relevant background and suggests new directions for future research.

The growth and metabolome of Saccharomyces uvarum in wine fermentations are strongly influenced by the route of nitrogen assimilation

Nitrogen is a critical nutrient in beverage fermentations, influencing fermentation performance and formation of compounds that affect organoleptic properties of the product. Traditionally, most commercial wine fermentations rely on Saccharomyces cerevisiae but the potential of alternative yeasts is increasingly recognised because of the possibility to deliver innovative products and process improvements. In this regard, Saccharomyces uvarum is an attractive non-traditional yeast that, while quite closely related to S. cerevisiae, displays a different fermentative and aromatic profile. Although S. uvarum is used in cider-making and in some winemaking, better knowledge of its physiology and metabolism is required if its full potential is to be realised. To address this gap, we performed a comparative analysis of the response of S. uvarum and S. cerevisiae to 13 different sources of nitrogen, assessing key parameters such as growth, fermentation performance, the production of central carbon metabolites and aroma volatile compounds. We observed that the two species differ in the production of acetate, succinate, medium-chain fatty acids, phenylethanol, phenylethyl acetate, and fusel/branched acids in ways that reflect different distribution of fluxes in the metabolic network. The integrated analysis revealed different patterns of yeast performance and activity linked to whether growth was on amino acids metabolised via the Ehrlich pathway or on amino acids and compounds assimilated through the central nitrogen core. This study highlights differences between the two yeasts and the importance that nitrogen metabolism can play in modulating the sensory profile of wine when using S. uvarum as the fermentative yeast.

The Impact of the Fermentation Strategy on the Flavour Formation of Ilzer Rose (Malus domestica Borkh.) Apple Wine

The flavour and the volatilome of apple wines made from the Austrian heritage variety Ilzer Rose was in the scope of this study. The apple wines were produced by adopting oenological practises that are not commonly used in fruit wine production. Different fermentation strategies including the addition of enzymes with β-glucosidase activity, addition of a fining agent, maceration of the mash along with mash fermentation were applied. The volatile compounds of the juices as intermediates and the resulting apple wines were analysed using headspace-SPME GC-MS. CATA technique with a well-trained panel was applied for sensory evaluation. The results show that the flavour of single-variety apple wine can be significantly altered by taking oenological measures. High correlations were found between the results of the analytical investigation and the sensory evaluation. Maceration of the mash leads to an increase in the fruity character of the products, also reflected by significantly higher fruit ester quantities in the wine. During mash fermentation, spontaneous malolactic fermentation was induced leading to a product with new, but thoroughly interesting sensory properties of the apple wine. The results of this study demonstrate that the integration of oenological measures may open a wide field to the development of a high diversity in apple wine flavour.