Alcohol degree management
The rise in average temperature induces earlier bud burst of the vines (milder winters) and a shortening of the vegetative period which leads to earlier veraison and maturity (direct effect of temperature on the growth and physiology of the vine). ). Since 2010, in Europe, the harvest has started two to three weeks earlier than before the 1980s. The ripening period is therefore approaching the heart of summer, with higher temperature conditions which amplify the effects of climate change
The increase in the sugar content of grape must and the concomitant increase in the alcohol level in wine are some of the main challenges facing the wine industry today. This sheet briefly presents adaptation solutions.
IN THE VINEYARD
The choice of plant material is certainly the most important lever to play on the date of maturity (extract from Van Leeuwen,.
the clonal selection of the main varieties by looking for late clones and producing grapes less rich in sugar. Faced with the constantly changing needs of winegrowers, it is essential to conserve the necessary resources to always be able to select new clones adapted to the needs of the moment (van Leeuwen and Roby, 2013).
Then, it is possible to choose rootstocks which lengthen the cycle of the vine, such as 420A, to the detriment of early rootstocks, such as Riparia Gloire de Montpellier.
Finally, the most powerful lever is to play on the differences in precocity between grape varieties. When a wide variety of grape varieties are grown in one location, the difference in maturity date between the earliest and the latest grape is more than two months. (Van Leeuwen, 2018)
Several viticultural techniques make it possible to delay the reproductive cycle of the vine.
- Late pruning (in February or March) delays bud break by a few days.
- In a context of global warming, increasing the height of the trunk would allow the grapes to ripen in a slightly cooler microclimate, which should delay maturity.
- The degradation of the leaf-fruit ratio makes it possible to delay veraison (Parker et al., 2014). However, it has many other consequences, some of which can be interesting such as a decrease in the sugar content of the grape without increasing the acidity (Parker et al., 2015). On the other hand, other effects that can potentially jeopardize the quality of the wine, such as a decrease in the content of phenolic compounds, or a possible increase in the herbaceous flavor of the grape, must be studied in more depth.
An easy modification to implement is the advancing of the harvest date. This does not allow the grapes to ripen in cooler conditions, but it can prevent a major imbalance in the sugar / acidity balance and an excessive increase in the pH of musts and wines (Van Leeuwen, 2018)
IN THE CELLAR
Choice of yeasts
Among the many alternative solutions currently under study, the use of unconventional yeasts during fermentation shows promise to help alleviate this problem. Non-Saccharomyces wine yeast species include a large number of species. This diversity also implies the yield of ethanol on sugar, one of the most invariant metabolic traits of S. cerevisiae. Critical aspects discussed include selection of appropriate yeast strains, identification of key environmental parameters influencing ethanol yields (including the use of controlled oxygenation conditions), and management of mixed fermentations, through inoculation sequential or simultaneous starter cultures of S. cerevisiae and non-Saccharomyces. The feasibility of using non-Saccharomyces yeasts to reduce alcohol levels in wine will require a better understanding of the metabolism of these alternative yeast species, as well as the interactions between different yeast starters during must fermentation. grape. (Ciani et al, 2016)
Management of the grape sugar level
The reduction in the sugar content of the musts by pre-fermentation treatment:
- either by extracting the excess sugar by the Ultra-Filtration / NanoFiltration (UF-NF) membrane coupling process;
The two successive filtration steps make it possible to subtract from the must, approximately 34g.L-1 of sugars (corresponding to approximately 2% of ethanol) in the form of a sugar concentrate at approximately 400 g.L-1
- or by dilution of the must by adding completely dealcoholized wine (Wine without Alcohol: VSA).
The dilution with VSA is easy to implement but it nevertheless requires the prior development of a quality VSA by distillation at controlled temperature under vacuum. The major drawback of this process lies in regulatory aspects (blending a wine with must) which do not currently allow it. There is no technical limit to this reduction mode. The reduction limit has not been established, it will only depend on the final sensory quality of the wine. (IFV, BES)
The extraction of alcohol during fermentation:
- either by total dealcoholization by distillation of part of the batch and reassembly with the untreated batch;
- or by partial dealcoholization by stripping (CO2 entrainment) of the entire batch.
The principle of stripping is to entrain alcoholic water vapor under the effect of strong CO2 bubbling. This alcoholic water vapor is then condensed and collected, it constitutes the stripping condensate. The alcoholic degree of this condensate depends on the alcoholic degree of the treated product. This technique is not alcohol-selective, so it is necessary to dealcoholize these condensates and then reintroduce them (reintroduce the water fraction removed during treatment). As for distillation, it is possible to extract alcohol in concentrated form at around 92%. (IFV, BES)
The extraction of alcohol from finished wine by post fermentation treatments using different technologies: Vacuum distillation, stripping, mixed membrane-distillation coupling (Reverse Osmosis distillation OI-D, Nanofiltration distillation NF-D), any membrane coupling (Reverse Osmosis -OI-CM Membrane Contactor, Nanofiltration- NF-CM Membrane Contactor).
Several routes are possible to produce quality wines with reduced alcohol content, each with their advantages, disadvantages and limitations. As the sensory criteria for controlled dealcoholization of the order of 2% are satisfactory, the choice of the technology to be implemented will rather depend on economic and environmental criteria.
Authors: Maddy Tintinger (INRAE), Jean-Marie Sablayrolles (UMR SPO)
IFV, BES M. Les différentes stratégies microbiologiques et technologiques pour la production de vin à teneur réduite en alcool, in « Programme de recherche Vins De Qualité à Teneur Réduite en Alcool (VDQA) », 28. https://www.vignevin.com/wp-content/uploads/2019/03/Partie_3_PDF.pdf
Ciani, Maurizio, Pilar Morales, Francesca Comitini, Jordi Tronchoni, Laura Canonico, José A. Curiel, Lucia Oro, Alda J. Rodrigues, et Ramon Gonzalez. « Non-conventional Yeast Species for Lowering Ethanol Content of Wines ». Frontiers in Microbiology 7 (4 mai 2016). https://doi.org/10.3389/fmicb.2016.00642.
Parker A., Hofmann R, Van Leeuwen C., McLachlan A. and Trought M., 2014. Leaf area to fruit mass ratio determines the time of veraison in Sauvignon blanc and Pinot noir grapevines. Aust. J. Grape Wine Res., 20, 422-431.
Parker A., Hofmann R, Van Leeuwen C., McLachlan A. and Trought M.., 2015. Manipulating the leaf area to fruit mass ratio alters the synchrony of soluble solids accumulation and titratable acidity of grapevines: implications for modelling fruit development. Aust. J. Grape Wine Res., 21, 266-276. doi:10.1111/ajgw.12132.
Van Leeuwen C. et Roby J.-P., 2013. Préservation des ressources génétiques de la vigne. Cohabitation entre sélection clonale institutionnelle, sélection massale et sélection clonale privée. Revue des œnologues, 148, octobre 2013, 13-16.
Van Leeuwen C., et Darriet P.. « Le changement climatique en viticulture : les leviers d’adaptation au vignoble », november 2018.
UMR SPO - Sciences Pour l'Oenologie
Les recherches menées au sein de l'UMR SPO visent à mieux comprendre les mécanismes impliqués lors de l'élaboration du vin.