Non-Saccharomyces yeasts for reduction of ethanol content in wine

Over recent years, the average ethanol concentration of wine has increased as consequence of increased grape maturity. In fact, climate change has deeply influenced the vine phenology and grape composition, resulting in rising of grapes sugar concentration and, consequently, high alcohol content in wine. This increased ethanol content can have negative consequences on the wine characteristics, affecting the sensory properties of the wines, as high alcohol content reduces the perception of flavour and aroma complexity. Furthermore, wine containing high levels of alcohol gives rise problems to the human health and also to the economic aspects as it determines an increase of taxes. Among the several solutions currently under study, biotechnological approach based on the use of non-Saccharomyces yeasts during alcoholic fermentation holds good promise for contributing to reduction ethanol and contemporary to improve wine characteristics. Non-Saccharomyces wine yeast species comprise a high number of species, characterized by high physiological diversity. The oenological interest of these microorganisms was initially triggered by their potential positive contribution to the sensorial complexity of wines, through the production of aroma and other sensory-active compounds which are not produced by single fermentation with Saccharomyces cerevisiae. The current interest toward these yeasts is addressed also to the ethanol yield on sugar, one of the most invariant metabolic traits of S. cerevisiae. In fact, non-Saccharomyces yeast can divert carbon away from ethanol production affecting ethanol yield, fermentation efficiency, production of biomass and final by-products. One of these alternative pathways would be sugar respiration under suitable fermentation conditions, especially for Crabtree-negative yeast species, through partial and controlled aeration of the grape juice. However, the oxygenation levels required for yeast respiration could promote, as a side effect, the oxidation of key components for the sensory quality of wines, such as phenolics and aromatic compounds. As a consequence, a strict control of fermentative conditions during oxygen addition is necessary in order to avoid undesirable effects on aromatic quality of wine, such as too high levels of volatile acidity. The main aim of this research was testing different non-Saccharomyces strains in mixed fermentation with S. cerevisiae to select the most promising strain combination to be used as a tool for reducing the ethanol content in wines. In the first step, 33 non-Saccharomyces wild strains, belonging to Debaryomyces polymorphus, Hanseniaspora uvarum, Starmerella bacillaris and Zygosaccharomyces bailii species, were tested for parameters of technological interest, such as production level of hydrogen sulphide (H2S), resistance to ethanol, SO2 and copper, screening for killer-sensitive pattern, evaluation of enzymatic activities (ester-hydrolase, β‐glucosidase and β‐xylosidase activities). Some non-Saccharomyces strains, selected on the basis of results obtained by technological screening, were tested during mixed fermentations at laboratory scale, testing different inoculation protocols, such as simultaneous and sequential inoculum. After this step, one mixed starter culture, characterized by good oenological aptitude and highest ability to reduce ethanol content in wine, was tested in different process conditions, such as fermentation with oxygen addition and use of immobilized cells, in order to evaluate the influence of this parameters on starter behaviour. In the last step, the performance of selected mixed starter culture was validated at pilot scale in order to individuate the mixed starter culture and fermentation conditions to be proposed to winemakers for production of wine with reduced alcohol content and increased aromatic complexity.