2026-09-09
New viticulture research is pushing winemakers to look beyond °Brix, the standard measure of grape sugar concentration, when deciding when to pick fruit, arguing that the key question late in ripening is not only how much sugar is in the grape juice, but whether the berry is still actively gaining sugar or simply losing water.
The argument comes from a group of studies published in recent years in OENO One and IVES Technical Reviews and highlighted this week by the industry publication Infowine. Together, the papers focus on how sugar accumulates in individual berries and how that process changes near harvest, with direct implications for alcohol levels, aroma, tannin extraction and the style of the finished wine.
For decades, growers and wineries have relied heavily on °Brix to track ripening and schedule harvest. The method remains central because it helps estimate potential alcohol and gives a quick, practical signal in the vineyard. But researchers say that late in the season, rising °Brix can be misleading. Once net sugar import into the berry has largely stopped, berries may continue to dehydrate. That water loss can push sugar concentration higher even if the total amount of sugar per berry is no longer increasing.
That distinction matters because grapes with similar sugar concentration can be in different physiological states and can produce different wines.
The research centers on what scientists call berry sugar loading, an approach that combines standard sugar concentration readings with average berry weight to estimate how much sugar each berry has actually accumulated over time. When the amount of sugar per berry is plotted through the ripening period, the curve rises and then stabilizes at what researchers describe as a plateau. After that point, berry weight can fall while °Brix still rises.
In practical terms, that means a grower can see higher sugar readings without a corresponding increase in actual sugar imported into the fruit.
Studies led by researchers including Gilles Antalick and Alain Deloire on Shiraz and Cabernet Sauvignon found that this plateau offered a useful physiological reference point for harvest decisions. In those trials, the plateau was identified around 20±1°Brix, though the researchers cautioned that it should not be treated as a universal threshold across all vineyards, varieties or regions. The more important point, they said, is to track the shape of the curve within each block or parcel.
The work suggests that once grapes reach that plateau, their aromatic and phenolic development can begin to separate from what growers often call technological ripeness, the traditional balance of sugar, acidity and pH used to guide harvest. That separation is especially important in warm years, when heat can speed sugar accumulation early and water loss later, creating the impression of further ripening through rising °Brix even when the underlying physiology is changing in a different way.
In sequential harvest trials on Shiraz and Cabernet Sauvignon, researchers found that wines made from fruit picked after the sugar-loading plateau moved through fairly consistent sensory stages. Wines made earlier after the plateau showed fresher red-fruit characters, while later picks shifted toward black fruit, plum and riper profiles. The pace of that change varied by variety and site, but the broad pattern appeared across different mesoclimates.
The findings do not mean all vineyards should be harvested a set number of days after the plateau. Researchers say the value of the method is local calibration. A winery can learn how a given variety in a given site behaves after sugar loading slows, and then match harvest timing to the style it wants to produce.
The studies also point to structural changes in the wine, not just aromatic ones. In the sequential harvest trials, wines from later harvest dates showed higher phenolic content and, in particular, higher tannin concentrations. Between wines associated with fresher fruit profiles and those tied to riper fruit profiles, tannin concentration increased by about 25% to 50%, depending on the vineyard and variety.
Researchers said the difference did not appear to come simply from much larger pools of phenolic material in the grapes. Extractability also changed. As berries continue to mature, the cellular structures of skins and seeds are modified, which can make tannins easier to extract during fermentation. Later-picked fruit also tends to produce wines with higher alcohol, increasing the solvent power of the fermenting must and further aiding extraction.
That has direct implications in the winery. If a parcel is intended for a fresher style with moderate tannin, growers may want to pick relatively early in the post-plateau period and use a gentler extraction regime. If the goal is darker fruit, more body and more structure, later harvest can offer greater extraction potential, which in turn affects decisions on pump-overs, temperature, maceration length and other cellar practices.
The research also identified changes in specific aroma-related compounds. One of the clearest findings involved dimethyl sulfide, or DMS, whose concentration rose consistently from wines tied to fresher stages to those tied to riper stages in both Shiraz and Cabernet Sauvignon. At suitable levels, DMS can reinforce sensory impressions linked to black fruit. Researchers also observed increases in gamma-nonalactone, especially in Cabernet Sauvignon, a compound associated with plum and cooked-fruit notes. In that case, the increase may reflect both grape maturation and berry dehydration.
That distinction is important for winemakers trying to separate desired ripeness from simple concentration caused by water loss. Both processes can lift °Brix, but they are not the same.
The work also suggests that harvest timing influences not only compounds present in the grapes, but the aromatic outcome of fermentation itself. Trials showed that picking date changed how Saccharomyces cerevisiae produced fermentative esters, although the response differed by variety. In Shiraz, riper harvests produced higher concentrations of some acetates, while Cabernet Sauvignon behaved differently. The result supports the idea that harvest timing changes the composition of the must and therefore the later metabolism of yeast.
Another part of the research looked at what drives the sugar-loading plateau and why it arrives earlier in some vineyards than others. A study published in 2024 examined 18 Merlot parcels in Saint-Émilion, Pomerol and nearby Bordeaux appellations across seven vintages from 2012 to 2019, excluding 2017 because of spring frost. Researchers evaluated temperature, vine water status, nitrogen nutrition, berry weight and sugar accumulation dynamics.
They found that average temperature between veraison and the sugar-loading plateau was one of the main drivers. Higher temperatures tended to bring the plateau earlier and shorten the period of active sugar accumulation. Faster accumulation was also linked to lower sugar concentration at the time the plateau was reached. Berry size mattered as well, with larger berries generally showing lower sugar concentrations at that stage. Water deficit had a more limited effect, though it tended to move the plateau earlier and, as expected, reduce berry weight. In those field conditions, assimilable nitrogen was not a significant factor in explaining the dynamics.
The climate message is clear. In hot vintages, grapes may hit their physiological sugar-loading limit sooner, while later dehydration keeps driving up concentration. That can widen the gap between potential alcohol on one side and aromatic or phenolic maturity on the other. A 2025 study on 58 Merlot genotypes reinforced that point by finding that a significant share of final differences in sugar concentration came from late water loss after phloem unloading had stopped, rather than from greater sugar loading itself.
For producers facing repeated warm seasons, that finding may help explain why grapes can appear to keep “ripening” on paper while the fruit in the vineyard behaves more like fruit moving into over-ripeness or dehydration.
The method itself is not technically complex, though it requires disciplined sampling. Growers need sugar concentration, average berry weight and repeated sampling dates beginning around veraison rather than only near harvest. The goal is to track the rise in sugar per berry, identify when it stabilizes and then watch what happens next. If °Brix continues to rise while sugar per berry remains flat and berry weight declines, the likely explanation is concentration from water loss rather than fresh sugar import.
Researchers say that once the plateau has been identified, standard ripeness monitoring becomes more informative when combined with tasting berries, checking pH, acidity and malic acid, measuring color and tannin potential, watching berry weight and health status, and looking for signs of dehydration. For wineries handling many vineyard blocks, the approach could also help sort parcels by ripening behavior rather than by °Brix alone, making it easier to decide which lots should be fermented together and which are better suited for different blends.
The practical shift is subtle but important. Instead of asking only what alcohol level the fruit might yield, growers can begin asking whether the grapes are still building sugar or simply concentrating it, and whether the vineyard has reached the sensory and structural point the winery wants before the pickers enter the rows.