For this article, I am taking the rather unusual step of announcing the source material first, simply because it would be difficult to write a better description of the process that Agrovin used to validate the introduction of their new ultrasound maceration equipment for wine or juice grape treatments. In warm climates, grapes often reach physiological maturity after sugar levels have already attained the desired ABV potential. In cooler climates, grapes may run out of time to achieve physiological maturity due to season ending climatologic conditions. In both cases, High Powered Ultrasound can provide extraction of flavor and texture compounds at lower ABV potential.
This article is the English translation of a Spanish paper titled: "Un nuevo enfoque para la reduccion del contenido de alcohol en los vinos tintos: el uso de ultrasonidos de alta potencia" from Foods 2020, Volume 9, page 726.
My input for this article has been to modify the text about the non-scientific aspect of their work into a format for presentation in WineBusiness Monthly. That said, large parts are quoted directly from Agrovin's reported work.
Overview of the project
To make lower-alcohol wines it might be best to choose the simplest approach; harvest the grapes earlier. However, this choice has implications for the composition and quality of the resulting wine as the grapes may lack phenolic ripeness. A new innovation in technology helps by including the use of ultrasound for wineries.
For this study, grapes were harvested at two different ripeness levels – 25.4° Brix and 29° Brix – and then transported to the winery and vinified. At the winery, a large-scale, high-power ultrasound system was used to treat the less ripe grapes just after crushing. There were three different vinifications that were macerated with the skins for seven days.
At bottling, the wine was analyzed for aromatic compounds as well as its physicochemical, chromatic and sensory characteristics. The results showed that the wine produced from the ultrasound-treated grapes had very similar characteristics to the wine produced from the less ripe grapes, especially in terms of total tannin and total phenolic content, but with an alcohol content that was 15% lower than the latter.
The results of this project indicate that this technology can be applied to grapes to promote the extraction of phenolic compounds, even before the phenolic maturity of the grapes has been completed, allowing the production of quality wines with a reduced alcohol content.
Review of grape constituent components
Color, one of the most important quality attributes of red wine, depends on the phenolic composition of that wine. Color components not only provide the color of the wine but also add to the body and texture. Color is, therefore, closely related to the phenolic composition of the grapes. Grape phenolic compounds, both anthocyanins and tannins, are mainly located in the skins. Primarily, tannin compounds are concentrated in the seeds as well, and all are extracted into the must during crushing and maceration.
Although some oenological techniques can help extract these compounds, this extraction can be severely limited by the grape cell walls where these compounds are located. The cell wall forms a barrier to extraction. If these cell walls are not easily broken, the extraction of phenolic compounds will be limited. From this understanding of the grapes' development, phenolic maturity is identified as the stage where the phenolic content of the skin is not only high but also easily extractable.
The difficulty when it comes to extracting the tannins from the seeds is also reduced due to the lignification of the seed. When grapes are phenologically-immature, phenolic compounds from the skin are not easily extracted, even when present at high concentrations, and yet high concentrations of astringent tannins from the seeds may be present. This situation changes upon reaching phenological maturity-when cell walls are easily degraded, and phenolic compounds are extracted.
Choosing the optimal time for harvest has generally been based on when a variety is well adapted to its terroir and technical maturity is reached (defined as an optimal level of sugar content in grapes for that grape variety). This also requires that the grapes' phenolic and aromatic maturity (i.e., when grapes have lost vegetal, herbaceous and fruity aromas) are reached at the same time.
Climate, however, exerts a major influence on vine phenology and grape composition. Among the most important effects related to climate change is vine phenology modification. If grapevine phenology is shifted to earlier dates, due to global warming, this indicates asynchronous development of grape composition. Sugar accumulation is faster than phenolic compound synthesis. Consequently, harvest is delayed, allowing the grapes to reach higher sugar levels in the berries than previously desired. The delay was thought to be necessary because optimal aromatic and phenolic ripeness was more important to wine quality than alcohol concentration. Alcohol removal is now the leading solution to this issue.
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