In the context of winemaking, you may have encountered terms such as “destemming” and “whole-cluster fermentation.”
Both refer to how grape bunches are handled at the point of vatting.
A grape cluster consists of many berries attached to a green, stem-like structure. This stem portion is called the “rachis” or more generally the “stem.”
In the winery, the practice of removing these stems before fermentation and using only the berries is called “destemming.” In contrast, using intact bunches, including the stems, is referred to as “whole cluster,” and placing these intact bunches in the fermenter and fermenting the juice around them is called “whole-cluster fermentation.”
Whole-cluster fermentation is used primarily in red winemaking. In white winemaking, the standard practice is to press the grapes immediately after harvest. During this process, skins, seeds, and stems remain in the press, and only the juice is transferred to tanks for fermentation. As a result, at the fermentation stage the material is almost always juice only, leaving very little room for whole-cluster fermentation in white wines.
Even in red winemaking, whole-cluster fermentation is not necessarily the default.
Historically, the situation was different. Before mechanization, destemming could only be carried out by hand, so many wineries ended up fermenting with whole clusters simply as a matter of practicality. The subsequent spread of mechanical destemmers and other equipment made destemming far easier, and “destemming as standard” became the norm in red winemaking.
In recent years, however, this trend has shifted again. More and more wineries around the world are deliberately incorporating whole-cluster fermentation into their red wine programs. The popularity of so-called “natural wine,” which emphasizes minimal intervention during vinification, is one factor reinforcing this movement.
Whole-cluster fermentation occupies a position close to the origin of red winemaking if we look back historically. At the same time, it remains a technique whose specific effects are surprisingly poorly quantified.
This article organizes what is currently known about the impact of whole-cluster fermentation on wine, addressing several key questions in turn.
Are Some Grape Varieties More Suitable for Whole-Cluster Fermentation?
In short, yes. Some grape varieties are more suited to whole-cluster fermentation than others.
Very simply put, whole-cluster fermentation is a vinification method in which not only skins and seeds but also stems are macerated in the fermenting must. The primary goal is to extract compounds contained in the stems into the wine.
A useful analogy is stock-making in cooking. Consider, for example, a rich pork-bone broth. You do not eat the bones themselves, but you simmer them for a long time to extract their soluble components. In red winemaking, stems play a similar role: they are not eaten, but maceration allows their internal components to be extracted into the wine.
It is important to emphasize that extraction does not selectively pull out only desirable components. Just as scum must be skimmed in stock-making, in wine there are always compounds that ideally would not be extracted.
Among the compounds we would like to extract in winemaking, the most prominent are “phenolic compounds” (phenolics), which are involved in color, aroma, bitterness, and astringency. On the other hand, a representative group of compounds we would prefer not to extract are the methoxypyrazines. Methoxypyrazines are known for contributing “green” aromatic notes such as grass, herbs, green bell pepper, or capsicum.
Methoxypyrazine concentrations vary substantially by grape variety. Among red varieties, Cabernet Sauvignon and Cabernet Franc are particularly rich in these compounds. In these varieties, even without whole-cluster fermentation, methoxypyrazine-derived green or herbal notes are readily apparent in the wine. Therefore, introducing whole clusters in such varieties carries a high risk of excessively emphasizing green, herbaceous character.
Overt methoxypyrazine character is, in most contexts, evaluated negatively. For this reason, high-methoxypyrazine varieties such as Cabernet Sauvignon and Cabernet Franc are generally considered less suitable for whole-cluster fermentation.
By contrast, Pinot Noir, which has a relatively low total phenolic content, and Syrah, in which methoxypyrazines and rotundone (a sesquiterpene responsible for black-pepper notes) function as part of the varietal aroma, are often cited as examples of varieties that respond well to whole-cluster fermentation. In these grapes, stem-derived phenolics and aroma compounds are more likely to contribute positively to depth and complexity in the finished wine.
Is Whole-Cluster Fermentation Always an “All-or-Nothing” Choice?
No. Even when whole-cluster fermentation is used, it is not necessary to vinify 100% of the fruit as whole clusters.
Because of the word “whole,” a statement such as “this wine was made with whole-cluster fermentation” is often interpreted as meaning “no destemming at all.” In practice, however, many wineries use a mixture of whole clusters and destemmed fruit.
A common pattern is to ferment about 15–20% of the fruit as whole clusters, with the remaining 80% or so destemmed. The proportion of whole clusters is finely adjusted according to grape variety, site conditions, vintage, and the target style of the wine.
As noted above, whole-cluster fermentation does not exclusively extract desirable components; it also extracts compounds with potentially undesirable sensory impacts. Moreover, even compounds that are desirable at moderate levels may become organoleptically negative if present in excess.
In addition, both the qualitative profile and the quantity of extractable stem components vary from year to year, and the taste profile of the grapes themselves changes with each vintage. The fact that a relatively high stem proportion gave a well-balanced wine in one year does not guarantee that the same ratio will be optimal in the next.
In other words, the proportion of whole clusters is determined by integrating multiple factors, rather than as a simple binary choice between “all whole cluster” and “no whole cluster.”
Does Whole-Cluster Fermentation Always Yield Fresher Wines with Softer Acidity?
The fair answer would be: “Largely yes in sensory terms, but with important caveats.”
When stems are macerated in must, potassium ions (K⁺) contained in the stems are extracted into the liquid phase. The extracted potassium reacts with tartaric acid in the juice to form potassium hydrogen tartrate (commonly referred to as “tartrates” or “wine diamonds”), which crystallizes and precipitates to the bottom of the tank or barrel.
Tartaric acid is one of the principal acids in wine. When a portion of it is removed as a tartrate precipitate, the amount of acid remaining in solution decreases accordingly. As a result, perceived acidity in the wine often appears lower.
At the same time, several important caveats must be considered. First, virtually all analytical work on whole-cluster lots reports an increase in pH. This increase is consistent with an effective deacidification effect due to the binding of tartaric acid by potassium and calcium from stems.
However, some trials have reported little to no change in titratable acidity (TA). In other words, despite strong indications from pH changes that deacidification has occurred, the total amount of titratable acid remains statistically unchanged in some cases, and a clear difference in perceived sourness is not always observed.
Furthermore, stems contribute compounds such as potassium that can reduce acidity by forming tartrate salts, but stems themselves contain very little acid. They do not supply additional acidity to the system. Consequently, the use of stems will rarely, if ever, increase perceived acidity. Their effect is essentially restricted to “acidity decreases or remains unchanged.” In fact, there are very few reported cases in which measurable acidity increased as a result of stem use.
Despite this, wines made with whole-cluster fermentation are frequently described in sensory evaluations as having enhanced “freshness.” In general, wine freshness is thought to depend largely on acidity. Many readers will associate the vivid freshness of, for example, high-acid Riesling from cool-climate regions with relatively elevated acid levels.
In whole-cluster fermentation, however, acidity tends to decrease and certainly does not increase in most trials. Yet tasters still frequently perceive an increase in freshness. At present, there is no definitive mechanistic explanation for this discrepancy. It is plausible that green and herbal notes extracted from stems, in combination with various fruit-associated aroma compounds, interact to produce a sensory impression interpreted as “freshness.”
Thus, whole-cluster fermentation offers a representative example in which relatively modest differences in chemical composition can still lead to clearly noticeable differences in sensory evaluation.
Can Stem Use Serve as a Practical “Last Resort” to Reduce Alcohol Level?
It cannot be ruled out entirely, but it is difficult to regard this as a generally effective strategy.
Stems typically account for approximately 3–7% of cluster weight and about 30% of cluster volume. Moreover, 55–80% of stem weight consists of water. From this, one often encounters the claim that vinifying with whole clusters will introduce stem water into the must, thereby diluting sugars and producing a lower-alcohol wine.
When we collate actual trial results, however, clear reductions in alcohol content attributable solely to whole clusters are rare. In those cases where whole-cluster lots do show slightly lower alcohol, they frequently also exhibit higher residual sugar, suggesting that incomplete fermentation—rather than dilution—is the more plausible explanation.
It is worth recalling that most of the must surrounding the stems is also predominantly water. Before fermentation begins, when sugar concentrations are high, osmotic pressure differences can indeed draw water from stems into the juice. However, as fermentation proceeds and ethanol accumulates, stems become immersed in a liquid with lower density due to its alcohol content. Under these conditions, it is equally plausible that stems absorb liquid rather than release water.
At present, detailed kinetic data on water exchange between stems and must are limited. Nevertheless, based on existing measurements of alcohol levels in finished wines, it is hard to support the scenario in which stem-sourced water is extracted in sufficient quantity to produce a practically meaningful reduction in alcohol. The experimental data available so far tend to support this more skeptical view.
Does Skipping Destemming Always Increase Phenolic Content?
The answer is: “In most cases, yes—but there are notable exceptions.”
The main reason for using whole clusters is to extract a higher amount of phenolic compounds, particularly tannins, from stems. If whole-cluster fermentation failed to increase phenolic content, its value would be significantly diminished.
In reality, however, there are documented cases in which whole-cluster fermentation did not lead to any measurable increase in phenolic content. This pattern is especially pronounced in trials where the proportion of whole clusters is low and the majority of fruit is destemmed. That said, exceptions are not limited to trivial levels such as a few percent; there are trials with around 20% whole clusters where no significant increase in phenolics was observed.
By contrast, there are very few reports of unchanged phenolic levels when the whole-cluster proportion exceeds roughly 50%. Even when a wine is described as having undergone “whole-cluster fermentation,” the exact proportion of whole clusters used is rarely disclosed on the label. Consequently, one must keep in mind that “wines made with some whole-cluster component but with little phenolic impact from stems” do exist.
Where Do “Silky Tannins” Come From?
One of the descriptors most frequently associated with whole-cluster fermentation is “silky tannins.” It is widely asserted that whole-cluster fermentation softens tannins, producing a smoother, silk-like mouthfeel.
The mechanism behind this phenomenon is rarely discussed in the literature. One relatively convincing hypothesis centers on increased levels of polymeric anthocyanins.
Analyses of whole-cluster lots consistently show elevated concentrations of polymeric anthocyanins. Anthocyanins are phenolic compounds responsible for the red color of red wines, and they readily react with tannins to form higher-molecular-weight species.
Stems contain substantial quantities of phenolic compounds that differ in composition from those found in skins and pulp. When these additional phenolics enter the must in large amounts, they open up reaction pathways that are less prominent in conventional red winemaking. One consequence is enhanced polymerization of anthocyanins.
When tannins are incorporated into larger polymeric structures with anthocyanins, their bitterness, astringency, and drying sensation tend to become less aggressive than in their monomeric or oligomeric forms. It is therefore reasonable to infer that the “silky mouthfeel” often observed in whole-cluster wines is linked to higher levels of polymeric anthocyanins.
Interestingly, sensory trials frequently report stronger bitterness and astringency in whole-cluster lots, yet these are often interpreted positively as contributing to “structure” or “backbone.” This is particularly relevant for Pinot Noir, which is naturally low in phenolics and prone to producing relatively light wines. In that context, the reinforcement of tannin structure via whole-cluster fermentation is often regarded as stylistically desirable.
Does Whole-Cluster Fermentation Impart a Distinctive Aroma Profile?
Broadly speaking, yes—both in positive and negative senses. Incorporating stems into the fermentation process generally produces a clearly identifiable change in aroma and flavor.
When tasting red wines made with whole-cluster fermentation, one can often recognize their stem component even without prior explanation. The characteristic aromatic signature arises from two main sources.
The first is direct extraction from stems. Stems contain aroma compounds and precursors that are virtually absent from skins or pulp. When such notes are evident in a wine, they frequently serve as a strong indicator that stems were used, and the intensity of these aromas often correlates with the proportion of whole clusters.
The second source is enzyme-mediated aroma formation.
In “carbonic maceration,” a method widely associated with Beaujolais Nouveau, intact, undestemmed clusters are placed in a sealed tank, which is then saturated with carbon dioxide (CO₂) before yeast fermentation begins. Under this anaerobic environment, enzyme-driven reactions inside the intact berries generate distinctive fruity and spicy aromas.
In whole-cluster fermentation, the fermenter contains a mixture of crushed berries with juice already exposed and intact berries still attached to stems. Once yeast fermentation begins in the free juice, CO₂ accumulates in the tank and creates a low-oxygen environment. Intact berries within this environment undergo biochemical changes similar to those in carbonic maceration. This state is often described as “semi-carbonic maceration,” but from a process standpoint it is essentially identical to carbonic maceration, albeit affecting only part of the fruit.
In full carbonic maceration, enzyme-mediated reactions occur throughout the entire volume of fruit, whereas in whole-cluster fermentation they are limited to the subset of intact berries. Even so, the fruity and spicy notes produced in this fraction can make a substantial contribution to overall aromatic complexity.
However, the distinctive aromatic imprint of carbonic maceration is so strong that it can easily mask the underlying varietal character of the grapes. This is one of the major risks of the technique.
Whole-cluster fermentation carries a similar risk. For that reason, some winemakers attempt to minimize semi-carbonic reactions—by adjusting cap management, temperature, or tank filling—while still benefiting from direct stem extraction.
Are “Unripe Phenolics” Really Responsible for Green Aromas?
Among the various risks associated with whole-cluster fermentation, the development of green, vegetal characters is one of the most critical.
This is often attributed to “unripe phenolics,” with explanations along the lines of “whole-cluster fermentation extracts phenolics, and when these are unripe they cause green aromas.” From a chemical standpoint, this explanation is not particularly accurate.
The main compounds responsible for grassy, vegetal, or herbal wine aromas include higher alcohols such as 1-hexanol, methoxypyrazines, and unsaturated alcohols such as 1-octen-3-ol, which imparts mushroom-like notes. None of these belongs to the phenolic class.
Among these, methoxypyrazines are especially important because their sensory thresholds are extremely low; even trace amounts are easily perceived. Stems are known to contain high levels of methoxypyrazines. In many trials, extraction of methoxypyrazines has been confirmed even when no increase in other green- or mushroom-like alcohols was observed.
Furthermore, compounds such as ethyl cinnamate, which contributes clove-like spicy notes, and benzaldehyde, which can evoke almond or certain vegetable nuances, have also been reported to increase in concentration in whole-cluster fermentations. These compounds likewise contribute to plant-like and spicy aromatic impressions.
One occasionally hears the claim that “better stem ripeness now allows higher whole-cluster proportions.” In practice, this likely reflects not so much a change in “phenolic ripeness” as an increase in lignification of stem tissues, which reduces extractability, combined with improved grape ripeness that lowers fruit-derived methoxypyrazine levels so that vegetal notes are less prominent relative to fruit.
Does Whole-Cluster Fermentation Always Lighten Wine Color?
Many trials do show this tendency, but vintage and varietal effects are at least as significant.
Whole-cluster wines are often said to exhibit paler color. The main proposed mechanism is adsorption of anthocyanins—the principal red pigments in wine—onto stem surfaces that are in contact with the juice and wine.
In Pinot Noir, for example, numerous studies have reported paler color in whole-cluster lots. However, there are also reports in other varieties where whole-cluster fermentation has yielded darker wines.
Even within Pinot Noir, in some vintages color differences between whole-cluster and destemmed lots are negligible, while in others the whole-cluster lot may even be slightly deeper in color. Overall, current evidence suggests that vintage-to-vintage variation and varietal differences often exert a greater influence on color than the simple presence or absence of whole clusters.
Is a High Whole-Cluster Proportion a Prerequisite for High-Quality Wine?
No. The use of whole clusters does not in itself guarantee “better” or “higher-quality” wine.
As already discussed, the impact of stem-derived extraction on wine is not uniformly positive. The same applies to aromas arising from semi-carbonic reactions in uncrushed berries.
High whole-cluster proportions certainly produce highly distinctive wines. However, distinctiveness does not automatically translate into superior quality or greater drinkability. From a winemaking risk-management perspective, high stem usage can even be disadvantageous.
Whole-cluster fermentation tends to increase wine pH. As pH rises, the antimicrobial and antioxidant efficacy of sulfur dioxide (SO₂) declines sharply. At the same time, the higher pH environment favors microbial activity, increasing the risk of spoilage yeasts such as Brettanomyces and other unwanted microorganisms.
In addition, there is a well-documented tendency for volatile acidity (VA) to increase in proportion to the whole-cluster ratio. The likely cause is the presence of stems within the cap, which creates additional void spaces where acetic acid bacteria and other bacteria can proliferate. While very low levels of VA are sometimes tolerated as part of a wine’s character, it is generally considered, like Brett-derived off-odors, to be a classic off-flavor when elevated.
In other words, a high whole-cluster proportion cannot be separated from an increased risk of microbial spoilage and fault development.
This does not mean that whole-cluster fermentation is meaningless. Stem-derived components and semi-carbonic processes can introduce complex and layered aromatics and structure—key attributes of many fine wines. The distinct aromatic and textural signatures of whole-cluster fermentation can be powerful tools in building depth and complexity.
Nevertheless, comparable complexity and structural interest can also be achieved using exclusively destemmed fruit. Ultimately, the decision about which processes to employ—and which risks to accept in pursuit of a desired style—is a matter of each producer’s philosophy and preferences.
