Dr. Carien Coetzee
Basic Wine
30 January 2021
Part 2: Viticultural practices and processing
Wines with reduced alcohol content can be achieved using different strategies (see Part 1). In Part 2 of this blog series, strategies for the adjustment of viticultural practices and grape processing will be briefly addressed. It is important to remember that each strategy may result in varying degrees of success. This is due to the inevitable difference in application, varying environmental factors and complex interaction between grape variety, rootstock and growing conditions which differs from one producer to the next.
1) Viticultural practices for obtaining lower sugar content in the grapes
Viticultural practices: Managing the harvest date
- Harvest earlier
Harvesting earlier will result in lower sugar content in the must and lower alcohol content in the resulting wine. Ripening should be carefully monitored to avoid excess unripe aromas, astringency and acidity due to harvesting earlier than usual. Experienced low alcohol winemakers will advise to taste the berries constantly and to not be afraid of the seemingly unripe grape berries.
Harvest date is critical for determining wine style, particularly for grape varieties such as Sauvignon blanc which develop significantly during ripening. Green nuances (which is not always undesirable) are usually more prominent earlier in the ripening season when the methoxypyrazines are present at higher concentrations. Thiol precursors usually increase with ripening1,2. Therefore, grapes harvested later often have reduced green characters and more “ripe” tropical fruit characters, but with higher sugar content. Depending on the intended wine style and the target consumer group, the harvest date can be intentionally selected to obtain the lowest sugar content in the specific time frame when the desired aroma compounds are present.
- Harvest in segments
Harvesting in segments and vinifying the fractions separately can be useful to obtain wines with different alcohol concentrations and varying aroma profiles. These fractions can then be blended to obtain a wine with the desired sensory properties while achieving reduced alcohol content.
Viticultural practices: Site selection
When selecting a site for the production of grapes intended for low alcohol wines, there are a few factors to consider. It is advised to choose a site that produces naturally lower acids and have hot nights with lower diurnal fluctuations (which can help reduce sugar accumulation). Other factors such as grape variety, rootstock and trellising can also have an impact on the sugar accumulation during ripening. This is especially important when planning a new vineyard intended for this purpose.
Viticultural practices: Slow down sugar accumulation
Other than choosing a specific site and planning the harvest date for low sugar musts, certain viticultural practices can be adjusted to reduce the sugar accumulation during ripening while still maintaining the development and ripening of flavours and phenolics.
- Reduce the leaf area
The rate of sugar accumulation in the berries is mainly determined by the leaf area:fruit mass ratio3,4. Flavour and phenolic ripeness, however, may be independent of this ratio5. The reduction of leaf area after fruitset may slow down sugar accumulation relative to other compositional changes during ripening thereby obtaining lower sugar content while minimising undesirable effects on flavour and mouthfeel5. The leaf area can be reduced by leaf removal, removing laterals and/or by topping of shoots to reduce leaf number per shoot5.
Studies have shown that the timing and the technique (for example the location in the canopy) of the removal can have different effects5 and should be explored. Some also increase the crop load to adjust the leaf area:fruit mass ratio, however, this should be done with caution to avoid possible negative effects.
- Irrigation
Water stress can be used to slow down sugar accumulation, however, the response of the vineyard depends on the grape variety, the timing, and the intensity of the water stress. Water supply only from veraison to harvest proved to reduce sugar accumulation without modifying phenol composition and wine quality in Cabernet Sauvignon6. Generally speaking, late water supply could be a strategy useful to resume shoot growth in the central phase of berry sugar accumulation in order to drive more photosynthates to the new vegetation and, thus, to reduce photosynthates available for the bunches4.
It is widely believed that late-season irrigation increases berry size and dilutes berry sugars (together with other quality-related components of the berries). However, this is not supported by experimental studies which have shown only small reductions in wine alcohol content in certain growing seasons (therefore not a consistent observation)7. Thus, it seems that pre-harvest irrigation is not an effective way to “dilute” the sugar content in the berry at harvest.
- Growth regulators
Studies would suggest that the exogenous application of growth regulators to either the bunch zone or the entire canopy may be useful for delaying sugar accumulation8,9
2) Processes for reducing the sugar content in the juice after harvest
Processing: Dilution and blending
- Water addition
Changes to regulations by Food Standards Australia New Zealand have permitted the adjustment of must sugar levels with the addition of water in order to ensure a sound fermentation progress as well as mitigating excessive alcohol levels. To date, the dilution of grape must with water, thereby reducing sugar concentration, is not permitted in South Africa as specified in the Liquor Products Act 60 of 1989. However, water added as part of certain wine additions, are permitted with limitations and can have a slight dilution effect. It is important to note that other wine constituents will also be subjected to the dilution factor and should be considered.
- Blending with low sugar grapes/musts
Blending early-harvested low sugar grapes/musts with more mature grapes/musts can adjust the sugar content and the final alcohol concentration of the resulting wine. Care should be taken to not increase unripe characteristics which could have a negative sensory impact on the final wine.
Processing: Decreasing sugar concentration by membrane technology
Although not allowed under the current regulations, sugar removal by membrane technology such as nanofiltration can be used to partially remove sugar from grape must. Together with the decrease in sugar content (resulting in lower alcohol in the final wine), a study also reported a significant decrease in colour and flavour concentration when compared to the untreated control wines10. The technical challenges related to the use of the equipment and the loss in colour and flavour would suggest that this approach is unlikely to find significant commercial traction5 even if permitted.
Processing: Decreasing sugar concentration through the addition of enzymes
An enzyme, glucose oxidase, can convert glucose into gluconic acid and hydrogen peroxide. The addition of the enzyme converts the sugar molecule which will subsequently not be available for the yeast to ferment thereby reducing the final ethanol concentration11. The production of gluconic acid can potentially increase the acid content and reduce the pH, and also potentially bind to sulphur dioxide, leaving less SO2 available for antioxidant and antimicrobial functions12. A study has shown that the addition of the enzyme can lead to the browning (possibly due to the production of hydrogen peroxide) and a decreased fruit aroma intensity13. Even though the potential is there, it is unlikely that this treatment will be a viable option for the production of quality wines with lower alcohol content.
Conclusion
Viticultural strategies to obtain lower sugar musts are mostly based on growing techniques having a direct or indirect impact on the berry sugar accumulation. This is not always easy to reach since grapevines have high physiological plasticity that provides several compensatory responses acting as a buffer in respect to the imposed treatments. More sure techniques for obtaining lower sugar musts lies with identifying a suitable harvest date and experimenting with different harvest fractions with varying sugar content.
References
(1) Roland, A.; Vialaret, J.; Razungles, A.; Rigou, P.; Schneider, R. Evolution of S -Cysteinylated and s -Glutathionylated Thiol Precursors during Oxidation of Melon B. and Sauvignon Blanc Musts. Journal of Agricultural and Food Chemistry 2010, 58 (7), 4406–4413. https://doi.org/10.1021/jf904164t.
(2) Capone, D. L.; Jeffery, D. W. Effects of Transporting and Processing Sauvignon Blanc Grapes on 3-Mercaptohexan-1-Ol Precursor Concentrations . Journal of Agricultural and Food Chemistry 2011, 59 (9), 4659–4667.
(3) Stoll, M.; Scheidweiler, M.; Lafontaine, M.; Schultz, H. . Possibilities to Reduce the Velocity of Berry Maturation through Various Leaf Area to Fruit Ratio Modifications in Vitis Vinifera L. Riesling. Progres Agricole et Viticole 2010, 7 (3), 68–71
(4) Novello, V.; De Palma, L. Viticultural Strategy to Reduce Alcohol Levels in Wine. OENOVITI INTERNATIONAL Network 2013, Alcohol le.
(5) 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.
(6) Fernández, O.; Sánchez, S.; L, R.; Lissarrague, J. R. Effects of Different Irrigation Strategies on Berry and Wine Composition on Cabernet Sauvignon Grapevines Grown in Madrid (Spain). Ciência e Técnica Vitivinícola, Proceedings 18th International Symposium GiESCO, Porto, 7-11 July 2013, 112-117. 2013, 28.
(7) McDonnel, C. The Effect of Crop Load and Extended Ripening on Wine Quality and Vine Balance in Vitis Vinifera Cv Cabernet Sauvignon. Univeristy of Adelaide, PhD Thesis 2011.
(8) BÖTTCHER, C.; HARVEY, K.; FORDE, C. G.; BOSS, P. K.; DAVIES, C. Auxin Treatment of Pre-Veraison Grape (Vitis Vinifera L.) Berries Both Delays Ripening and Increases the Synchronicity of Sugar Accumulation. Australian Journal of Grape and Wine Research 2011, 17 (1), 1–8. https://doi.org/10.1111/j.1755-0238.2010.00110.x.
(9) Symons, G. M.; Davies, C.; Shavrukov, Y.; Dry, I. B.; Reid, J. B.; Thomas, M. R. Grapes on Steroids. Brassinosteroids Are Involved in Grape Berry Ripening. Plant Physiology 2006, 140 (1), 150–158. https://doi.org/10.1104/pp.105.070706.
(10) García-Martín, N.; Perez-Magariño, S.; Ortega-Heras, M.; González-Huerta, C.; Mihnea, M.; González-Sanjosé, M. L.; Palacio, L.; Prádanos, P.; Hernández, A. Sugar Reduction in Musts with Nanofiltration Membranes to Obtain Low Alcohol-Content Wines. Separation and Purification Technology 2010, 76 (2), 158–170. https://doi.org/10.1016/j.seppur.2010.10.002.
(11) Biyela, B. N. E.; du Toit, W. J.; Divol, B.; Malherbe, D. F.; van Rensburg, P. The Production of Reduced-Alcohol Wines Using Gluzyme Mono® 10.000 BG-Treated Grape Juice. South African Journal of Enology and Viticulture 2009, 30 (2), 124–132. https://doi.org/10.21548/30-2-1432.
(12) Pickering, G. J.; Heatherbell, D. A.; Barnes, M. F. The Production of Reduced-Alcohol Wine Using Glucose Oxidase-Treated Juice. Part III. Sensory. American Journal of Enology and Viticulture 1999, 50 (3), 307 LP – 316.
(13) Pickering, G. J.; Heatherbell, D. A.; Barnes, M. F. The Production of Reduced-Alcohol Wine Using Glucose Oxidase Treated Juice. Part I. Composition. American Journal of Enology and Viticulture 1999, 50 (3), 291 LP – 298.
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