Rising sugar content in grape must, and the concomitant increase in alcohol levels in wine, are some of the main challenges affecting the winemaking industry nowadays. Among the several alternative solutions currently under study, the use of non-conventional yeasts during fermentation holds good promise for contributing to relieve this problem. Non-Saccharomyces wine yeast species comprise a high number or species, so encompassing a wider physiological diversity than Saccharomyces cerevisiae. Indeed, the current oenological interest of these microorganisms was initially triggered by their potential positive contribution to the sensorial complexity of quality wines, through the production of aroma and other sensory-active compounds. This diversity also involves ethanol yield on sugar, one of the most invariant metabolic traits of S. cerevisiae. This review gathers recent research on non-Saccharomyces yeasts, aiming to produce wines with lower alcohol content than those from pure Saccharomyces starters. Critical aspects discussed include the selection of suitable yeast strains (considering there is a noticeable intra-species diversity for ethanol yield, as shown for other fermentation traits), identification of key environmental parameters influencing ethanol yields (including the use of controlled oxygenation conditions), and managing mixed fermentations, by either the sequential or simultaneous inoculation of S. cerevisiae and non-Saccharomyces starter cultures. The feasibility, at the industrial level, of using non-Saccharomyces yeasts for reducing alcohol levels in wine will require an improved understanding of the metabolism of these alternative yeast species, as well as of the interactions between different yeast starters during the fermentation of grape must.

a b s t r a c tWe have recently shown that ethanol yields in winemaking can be reduced by taking advantage of the respiratory metabolism of some non-Saccharomyces yeast species. Using an orthogonal design we have now addressed the impact of three environmental factors (temperature, nitrogen source, and oxygen supply level) on the aerobic metabolism in synthetic must of Saccharomyces cerevisiae, Metschnikowia pulcherrima, Kluyveromyces lactis, and Candida sake. An integrative parameter, Efficacy (efficacy for alcohol level reduction) was designed to simplify comparisons between strains or growth conditions. It integrates sugar consumption, ethanol yield, and acetic acid production data. We found a high relative impact of nitrogen source availability and temperature, as compared to aeration conditions, for several fermentation parameters, including ethanol yield. However, increasing oxygen supply showed a positive impact in terms of alcohol reduction and Efficacy for all the strains tested. The best results across assays were obtained for C. sake CBS 5093, with high sugar consumption rates, associated to low ethanol yields, and very low acetic acid production. Processes involving this yeast strain would benefit from high aeration levels and low nitrogen source availability; while fermentation temperatures would have little impact on its Efficacy for alcohol level reduction.