This text was developed from several articles originally published in OENO One and IVES Technical Reviews. The original studies, cited throughout the article, examine the dynamics of sugar accumulation in grape berries and their implications for ripening monitoring and the selection of the most appropriate harvest date.
Joint monitoring of sugar content per berry and berry weight makes it possible to distinguish active sugar accumulation from simple concentration resulting from water loss. Recent studies on ripening dynamics show that identifying the sugar accumulation plateau can provide an additional reference for guiding harvest decisions towards the desired aromatic and structural profile.
Sugar concentration remains one of the main parameters used to monitor ripening and determine harvest timing. However, an increase in °Brix does not necessarily mean that the berry is continuing to accumulate sugars.
Once net sugar influx into the berry has ceased, water loss can continue to increase sugar concentration. At the same time, the evolution of aroma compounds, phenolic compounds, and their extractability continues. Consequently, two grape samples with similar sugar concentrations may be in different physiological and enological conditions.
The concept of berry sugar loading, developed and subsequently applied by Deloire and colleagues, proposes integrating °Brix measurements with the amount of sugar accumulated per berry and changes in berry mass. Studies conducted on Shiraz and Cabernet Sauvignon show how this dynamic can be used to identify a physiological reference point—the sugar accumulation plateau—from which aromatic and phenolic development tends to become progressively uncoupled from so-called technological maturity.
The Limitations of °Brix in Assessing Grape Ripening
During the first stage of ripening, beginning at veraison, the berry actively receives sugars through the phloem, while its water content and berry mass increase simultaneously.
When sugar is expressed as the amount per berry (mg/berry) and plotted over time, the curve rises until it reaches a stage at which sugar accumulation stabilizes: the plateau.
From this point onward, an apparently contradictory situation may arise:
sugar per berry ≈ stable
berry weight ↓
°Brix ↑
The increase in °Brix is therefore mainly due to a concentration effect caused by water loss and does not necessarily reflect further sugar accumulation.
In the studies by Antalick et al. conducted on populations of Shiraz and Cabernet Sauvignon berries, the plateau was identified at approximately 20 ± 1 °Brix. However, the authors emphasize that this value should not be regarded as a universal threshold: what is truly important is to monitor the evolution of the curve for each individual vineyard block.
This interpretation has been further supported by more recent studies. A study published in 2025 on 58 Merlot genotypes showed that a significant proportion of the final differences in sugar concentration among the different plant materials was associated with water loss occurring after phloem unloading had ceased, rather than with greater sugar accumulation.
For the winery, the practical implication is direct: a late increase in potential alcohol may indicate overripening and dehydration rather than a true progression in physiological maturity.
Why Identifying the Sugar Accumulation Plateau Matters
The plateau can be used as a kind of physiological “day zero.”
Rather than comparing vineyard blocks exclusively on the basis of °Brix, it is possible to assess how much time has elapsed since the berry essentially completed its main phase of sugar accumulation.
This approach is particularly interesting because the studies by Antalick et al. highlighted a relatively reproducible sequence in the profiles of wines obtained from successive harvests.
After the plateau, the wines shifted from profiles dominated by:
red/fresh fruit
towards profiles characterized by:
black fruit → plum → riper fruit.
This evolution was observed in both Shiraz and Cabernet Sauvignon, as well as in vineyards located in different mesoclimates, although the rate of the process depended on the variety and the specific vineyard conditions.
The value of the method, therefore, does not lie in establishing that all Cabernet Sauvignon vineyards should be harvested, for example, a fixed number of days after the plateau, but rather in calibrating, for each variety and winegrowing area, the relationship between physiological dynamics and wine style.
How Tannin Extraction Changes After the Plateau
The change does not concern the aromatic profile alone.
In sequential harvest trials, wines obtained at more advanced stages of ripening showed higher concentrations of phenolic compounds and, in particular, tannins.
Between harvests associated with a fresh-fruit profile and those associated with ripe-fruit characteristics, tannin concentration in the wine increased by approximately 25 to 50%, depending on the vineyard and variety.
A particularly relevant finding was that this difference did not appear to be explained simply by a marked increase in the accumulation of phenolic compounds in the grapes. During this stage, their extractability also increases.
Ripening alters the cellular structures of berry skins and seeds, facilitating extraction during fermentation. In addition, grapes harvested later generally produce wines with a higher alcohol content, further increasing the extraction capacity of the medium.
This leads to an important practical implication:
harvest timing and extraction strategy should be planned together.
A vineyard block intended for a fresh wine with moderate tannin levels may require a relatively early harvest during the post-plateau phase, combined with restrained extraction. Conversely, if the aim is to achieve black-fruit notes, greater volume, and more structure, a later harvest may provide higher extraction potential, which should be considered when managing pump-overs, délestage, temperature, and maceration time.
DMS and Grape Aromatic Ripening
Among the volatile compounds analyzed, one of the most interesting findings concerns the evolution of dimethyl sulfide (DMS).
Its concentration increased consistently from wines corresponding to fresh-fruit stages to those associated with ripe-fruit profiles, in both Shiraz and Cabernet Sauvignon. At appropriate concentrations, DMS can act synergistically, enhancing certain sensory perceptions associated with black fruit.
Increases in γ-nonalactone were also observed, particularly in Cabernet Sauvignon. This compound is sensorially associated with plum and cooked-fruit notes. In this case, the authors emphasize that the increase may be related both to ripening and to berry dehydration.
This observation introduces an important distinction for the winemaker:
advanced maturity and dehydration are not necessarily equivalent, even though both can lead to an increase in °Brix.
Monitoring berry weight makes it possible to begin distinguishing between these two phenomena.
How Harvest Date Influences Fermentation Aromas
The aromatic composition of wine does not depend exclusively on the metabolites initially present in the grapes.
The trials showed that harvest date also affects the production of fermentation-derived esters by Saccharomyces cerevisiae, although the response varies depending on the grape variety.
In Shiraz, for example, grapes harvested at more advanced ripening stages produced higher concentrations of certain acetates, whereas Cabernet Sauvignon showed a different response.
This confirms that harvest timing alters the must matrix and, consequently, the subsequent metabolism of the yeast. The aromatic evolution observed during ripening therefore cannot be explained solely by the concentration of aromas or precursors present in the berry.
Effect of Temperature and Vine Water Status on Sugar Accumulation
Subsequent studies have provided a better understanding of why the plateau is not always reached at the same time.
A study published in OENO One in 2024 analyzed 18 Merlot vineyard blocks from Saint-Émilion, Pomerol, and neighboring appellations over seven vintages between 2012 and 2019, excluding 2017 because of spring frost damage. Temperature, vine water status, nitrogen nutrition, berry weight, and sugar accumulation dynamics were assessed jointly.
The results, subsequently presented in IVES Technical Reviews in 2025, show that the mean temperature between veraison and the plateau is one of the main determining factors.
Higher temperatures:
bring forward the onset of the plateau;
shorten the duration of the sugar accumulation phase.
Moreover, faster sugar accumulation was associated with a lower sugar concentration at the time the plateau was reached. Berry size also proved to be important: larger berries showed lower sugar concentrations when reaching this stage.
Vine water status showed a more limited effect, although water deficit tended to bring forward the plateau and, as expected, reduce berry weight.
Interestingly, under the conditions examined, assimilable nitrogen was not found to be a significant variable in explaining these dynamics.
Climate Change, Dehydration, and Sugar Concentration
This distinction becomes particularly relevant in warm vintages.
High temperatures accelerate sugar accumulation and shorten the interval between veraison and the plateau, while subsequent water loss can continue to increase sugar concentration.
Under conditions of high temperature and water deficit, an increasing divergence may therefore occur between:
potential alcohol ↗ and aromatic and phenolic maturity.
The 2025 study on intravarietal diversity in Merlot further supports this concept: once phloem unloading had ceased, a substantial proportion of the differences in sugar concentration observed among genotypes was attributable to differences in their response to late-season water loss.
From an enological perspective, this means that in warm vintages it becomes even more important not to interpret every increase in °Brix automatically as an equivalent advance in ripening.
How to Apply Sugar-per-Berry Monitoring in the Vineyard
One of the advantages of this approach is that the basic data required are relatively simple:
– sugar concentration;
– average berry weight;
– sampling date.
However, the protocol requires representative and repeatable sampling, because heterogeneity among berries can introduce considerable variability.
From an operational standpoint, monitoring can be set up as follows.
- Start sufficiently early
It is advisable to begin around veraison, rather than only when the vineyard block is approaching the desired potential alcohol level for harvest.
This makes it possible to observe the rising phase of the curve and subsequently identify when it begins to stabilize. - Maintain a consistent sampling protocol
Samples should represent different positions within the cluster, both sides of the row, and different areas of the vineyard block.
The objective is not to characterize the behavior of individual berries, but to obtain a reproducible estimate of the berry population within the vineyard block. - Measure °Brix and average berry weight
These parameters can be used to estimate the amount of sugar accumulated per berry.
In simplified terms:
sugar per berry ≈ sugar concentration × berry mass or volume
with the appropriate conversions between the units used.
More important than the absolute value is the evolution of the curve over successive sampling dates. The original methodology also considers the sugar accumulation rate, expressed as mg/berry/day. - Identify the stabilization phase
When the amount of sugar per berry stops increasing significantly, the plateau phase is reached. From this point onward, berry weight becomes particularly important.
If the following pattern is subsequently observed:
°Brix ↑ + stable sugar/berry + berry weight ↓
the increase in potential alcohol is interpreted mainly as a concentration effect resulting from water loss. - Intensify enological monitoring
Once the plateau has been identified, conventional ripening monitoring becomes more meaningful when integrated with:
– berry tasting;
– pH and acidity;
– malic acid;
– tannin status and extractability;
– anthocyanins and color potential;
– berry weight;
– sanitary status;
– any signs of dehydration;
– the desired sensory profile of the wine.
The plateau does not, by itself, determine the harvest date. Rather, it represents a physiological reference point from which the other parameters can be interpreted more effectively.
Differences Between Vineyard Blocks and Ripening Dynamics
One of the most useful aspects of the study conducted in Bordeaux is precisely the high variability observed among vineyard blocks and vintages.
The dynamics depended on local temperature, the timing of veraison, berry size, and, to a lesser extent, vine water status. This opens up an interesting perspective for wineries working with a large number of vineyard blocks.
Rather than classifying incoming grapes solely according to °Brix or the expected harvest date, it may be possible to group vineyard blocks according to their ripening dynamics.
The same authors emphasize that this approach could be used in cooperatives or large wineries to identify vineyard blocks with similar ripening patterns and decide whether to vinify them together or use them in specific blends.
This is a particularly interesting application: monitoring is no longer used solely to decide when to harvest, but can also help determine which vineyard blocks are best vinified together.
Beyond °Brix: Interpreting the True Ripening Status of Grapes
°Brix remains essential for estimating potential alcohol and planning harvest operations, but it provides only part of the information.
Simultaneous monitoring of sugar per berry and berry weight makes it possible to answer a different question: whether the observed increase in concentration still reflects active sugar accumulation or instead results from progressive water loss.
Identifying the plateau therefore provides a reference point from which the subsequent evolution of the grapes can be interpreted.
Available studies indicate that, after this point:
- °Brix may continue to increase without a corresponding increase in sugar accumulation;
- the aromatic profile continues to evolve;
phenolic extractability increases; - the amount of tannins extracted during winemaking may increase significantly;
- dehydration plays an increasingly important role;
- thermal and water conditions determine how quickly this stage is reached.
However, there is no universal number of post-plateau days that can be directly transferred across varieties, regions, and production systems. The method requires local calibration, particularly when it is intended to be related to specific sensory profiles.
Its practical value lies precisely in adding a physiological dimension to traditional ripening monitoring.
For the winemaker, therefore, the question is no longer simply:
“How many degrees has the grape reached?”
but rather:
“Is it still accumulating sugar, or is it simply losing water?”
and, above all:
“At what stage of its aromatic and phenolic development do we want to harvest it?”
Antalick, G., Šuklje, K., Blackman, J.W., Schmidtke, L.M. & Deloire, A. (2021). Performing sequential harvests based on berry sugar accumulation (mg/berry) to obtain specific wine sensory profiles. OENO One, 55(2), 131–146.
Antalick, G., Šuklje, K., Blackman, J.W., Schmidtke, L.M. & Deloire, A. (2023). Plasticity of red wine sensory profiles: improved understanding through grape berry sugar loading profiles. IVES Technical Reviews.
de Rességuier, L., Inchboard, L., Parker, A.K., Petitjean, T. & van Leeuwen, C. (2024). Drivers of grape berry sugar accumulation in field conditions at local scale. OENO One, 58(4).
de Rességuier, L., Inchboard, L., Parker, A.K., Petitjean, T. & van Leeuwen, C. (2025). Characterization of sugar accumulation dynamics in grapes and the factors influencing this process at local scale. IVES Technical Reviews.
Lesbats-Sichel, V., This, P., Lacombe, T., Le Cunff, L. & Romieu, C. (2025). The intravarietal diversity of sugar concentration in Merlot depends more on late evaporation than sugar loading. IVES Technical Reviews.
