Dr. Carien Coetzee
2 February 2021
Part 3: Fermentation and post-fermentation
Part 2 of this blog series addressed options for the reduction of sugar content through the adjustment of vineyard practices and grape/juice processing. In Part 3 of this series, strategies for the reduction of alcohol content during fermentation and post-fermentation are summarised. As with the viticultural and processing strategies, studies done using different winemaking strategies also showed varying degrees of success. Other than that, individual strategies might lead to relatively small reductions in alcohol content. The focus should be on the accumulative effect from the vineyard to the bottle resulting in a product with significantly lower alcohol content. It is advised to trial the processes using your unique grape composition, winery equipment and winemaking style to ascertain which of these strategies are of value to reach your specific goal.
Fermentation: Saccharomyces cerevisiae wine yeasts
Studies by academics as well as commercial yeast companies, comparing commercial Saccharomyces cerevisiae wine yeasts under the same fermentation conditions, have demonstrated that there are very little differences in sugar to alcohol conversion rates1,2. Therefore, shopping for low alcohol-producing yeasts among the usual favourites might not be a rewarding task. There are, however, some Saccharomyces cerevisiae yeasts that constantly produce slightly less alcohol compared to competitor yeasts. These yeasts often have alternative metabolic pathways during fermentation resulting in greater yeast biomass formation or the formation of higher amounts of other metabolic end-products, such as glycerol, acetaldehyde, pyruvate, acetic acid, malic acid and succinic acid instead of ethanol1. Some of these “slightly lower alcohol” commercial yeast strains have been generated through non-genetic modification techniques.
Gene modification (GM) approaches to reduce ethanol yield in winemaking are designed to partially divert carbon metabolism away from ethanol formation. Numerous studies have demonstrated various levels of success3. One research project, funded by Winetech at the Department of Viticulture and Oenology, Stellenbosch University, was successful in creating a wine yeast that produced less alcohol and no unwanted by-products. In this study, genetic modification techniques (using no foreign DNA) were used to obtain a yeast that shifted some of its sugar metabolism towards trehalose, a storage sugar that has no organoleptic qualities1,4.
While gene modification approaches can be effective at reducing ethanol concentration, the public perception towards genetically modified organisms in food and beverage production is generally not positive. It is unlikely that this yeast will be used in the industry in the near future, however, the project paved the way for researchers to try and apply non-GM techniques to possibly obtain a similar result.
Fermentation: Non-Saccharomyces cerevisiae wine yeasts
Certain non-Saccharomyces cerevisiae wine yeasts show potential to significantly reduce the ethanol content in wines2. Studies conducted either with/without sequential inoculation of non-Saccharomyces cerevisiae with the ethanol tolerant Saccharomyces cerevisiae yeasts showed a potential decrease of up to 1.6% v/v ethanol when compared to a control treatment fermented with Saccharomyces cerevisiae alone.
Unlike Saccharomyces cerevisiae, many non-Saccharomyces cerevisiae yeasts are able to use oxygen for growth and therefore divert carbon to other metabolic pathways (instead of ethanol production). Therefore, oxygenation (in combination with certain non-Saccharomyces cerevisiae yeasts) could possibly be used as a technique to stimulate respiration instead of fermentation, thereby reducing the ethanol content of the final wine. Strong aeration during the first 48 hours of fermentation with a non-Saccharomyces cerevisiae yeast reduced the ethanol content with 2.2% v/v5. The risk of oxidation when using these techniques should be considered.
Anecdotal evidence from around the world has shown that open top fermenters tend to give lower final alcohol concentrations when compared to closed systems. The same goes for aerated pump-overs compared to closed system pump-overs1. This is likely due to the evaporation of ethanol during fermentation. Aeration during fermentation and a higher fermentation temperature also allows for faster evaporation of alcohol, which makes it one of the fermentation strategies for lowering final alcohol levels.
Fermentation: Stop fermentation early
By stopping the fermentation before all of the sugar has been fermented, the alcohol content can be reduced. This will leave some natural residual sugar which can add body and balance to wine. The perception of dryness can be created by maintaining a higher natural acidity and high dissolved carbon dioxide levels.
The blending of high alcohol wines with wines containing less alcohol is an effective strategy to reduce excessive ethanol concentrations. The potential undesirable properties of an early harvested wine (with lower alcohol content) can be removed by treating the wine with a fining agent such as activated carbon. The “stripped” wine can then be used as a blending component to create a product with reduced alcohol content. Of course, this technique could have an effect on the sensory quality of the wine. Therefore, it is advised to do bench trials with fining agent dosages and blending volumes and ratios to obtain the desired alcohol content while ensuring the desirable sensory properties are preserved.
Post Fermentation: Alcohol removal
The physical removal of alcohol from wine includes technological systems, such as reverse osmosis, osmotic distillation and pervaporation, spinning cone column and supercritical CO2 extraction6. These techniques allow for effective and precise control of alcohol reduction. Often, these strategies are accompanied by a loss of other volatile aroma compounds and the extent of this loss will depend on the amount of alcohol removed and the technique employed. Possibilities to recover some of the lost volatile compounds and the partial reconstitution of the wine also exist.
Most of these physical removal techniques appear to have minimal effect on wine flavour composition and wine sensory profile when the ethanol is reduced by no more than 2% v/v7. It might become challenging to maintain the desired sensory composition when lowering the alcohol with more than 2 % v/v. The ideal is to find the balance and compromise between ethanol removal, energy demand and the potential impact on the wine composition and sensory attributes.
More strategies to improve the sensory perception of wines with reduced alcohol content
The following strategies6 are options for increasing the perception of fullness, balance and body in a wine that might otherwise be perceived as flat and lifeless due to the reduced alcohol content:
- Skin and lees contact
- The addition of certain preparations such as mannoproteins, gum arabic and tannins
- Ensuring sufficient carbon dioxide concentrations
- Select a yeast strain that can metabolise malic acid, thereby reducing excess acidity
The following strategies are options for masking unripe flavours
- The use of products such as untoasted oak dust shavings at the destemmer is said to help combat green flavours
- Select a yeast strain that is going to add complexity to the wine by providing flavour and intensity.
Many strategies to produce wine with reduced alcohol content exists. Strategies can be used in combination to achieve the desired wine composition. It is important to not only proceed blindly with applying different strategies but to make sure you are familiar with your target market’s preferences and to identify key sensory drivers to guide the wine style.
(1) O’Kennedy, K. Lowering Alcohol in Wine: Yeast Selection. Winetech Technical 2020, December.
(2) Rossouw, D.; Bauer, F. Microbial Strategies for Wine Ethanol Reduction. Winetech Technical 2017, 1 April.
(3) Varela, C.; Dry, P. R.; Kutyna, D. R.; Francis, I. L.; Henschke, P. A.; Curtin, C. D.; Chambers, P. J. Strategies for Reducing Alcohol Concentration in Wine. Australian Journal of Grape and Wine Research 2015, 21, 670–679. https://doi.org/10.1111/ajgw.12187.
(4) Rossouw, D.; Heyns, E. H.; Setati, M. E.; Bosch, S.; Bauer, F. F. Adjustment of Trehalose Metabolism in Wine Saccharomyces Cerevisiae Strains To Modify Ethanol Yields. Applied and Environmental Microbiology 2013, 79 (17), 5197–5207. https://doi.org/10.1128/AEM.00964-13.
(5) Morales, P.; Rojas, V.; Quirós, M.; Gonzalez, R. The Impact of Oxygen on the Final Alcohol Content of Wine Fermented by a Mixed Starter Culture. Applied Microbiology and Biotechnology 2015, 99 (9), 3993–4003. https://doi.org/10.1007/s00253-014-6321-3.
(6) Varela, C.; Dry, P. R.; Kutyna, D. R.; Francis, I. L.; Henschke, P. A.; Curtin, C. D.; Chambers, P. J. Strategies for Reducing Alcohol Concentration in Wine. Australian Journal of Grape and Wine Research 2015, 21, 670–679. https://doi.org/10.1111/ajgw.12187.
(7) Gil, M.; Estévez, S.; Kontoudakis, N.; Fort, F.; Canals, J. M.; Zamora, F. Influence of Partial Dealcoholization by Reverse Osmosis on Red Wine Composition and Sensory Characteristics. European Food Research and Technology 2013, 237 (4), 481–488. https://doi.org/10.1007/s00217-013-2018-6.