The Role of Stems in Red Winemaking

How whole bunches influence structure, freshness, aroma and balance in red wines.

There are a tremendous number of choices a winemaker has to make when making red wine. Starting with the exact timing of harvest, many of these are process-related decisions concerning how the grapes are handled before, during and after fermentation. Part of that is deciding exactly what goes into the fermentation vat in the first place. Careful sorting, either by hand on sorting tables or with high-tech optical sorting devices, allows a winemaker to discard damaged, diseased or underripe berries and clusters, while also removing material that clearly should not make its way into the tank such as leaves, insects, vineyard debris and anything else that is not part of the crop. There is, however, one major exception to this instinct to exclude anything that is not a grape, and that is the deliberate inclusion of stems.

A grape berry is not attached directly to the vine itself. The berries hang from small stalks called pedicels, which branch from the rachis, the central framework of the bunch. The rachis is connected to the vine shoot by the peduncle, where the cluster is normally cut at harvest. When the bunches arrive at the winery, the winemaker must therefore decide whether to separate the berries from this framework, or whether some proportion of it should be allowed into the fermenting wine.

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Whole clusters in bins, ready to be processed into wine - Photo by Danielle Comer on Unsplash

This can take several forms. In whole-cluster, or whole-bunch, fermentation, intact clusters are placed in the vat with the berries still attached to the stems. These bunches may be left largely untouched, lightly foot-trodden, or gradually broken apart by the weight of the fruit and the movement of fermentation. Alternatively, the grapes may be destemmed first, with some or all the stems then deliberately added back to the fermenting must. This is called stem inclusion, but it is not quite the same thing as whole-cluster fermentation. Whole clusters affect the physical structure of the ferment, preserving intact berries and allowing some degree of intracellular, or semi-carbonic, fermentation, while also changing the pattern of extraction upon crushing. Added-back stems, by contrast, do not result in semi-carbonic fermentation or significant differences in the architecture of the cap and must on pressing, rather primarily influencing the wine through what they release into the must.

Each of these choices carries consequences. Depending on their maturity, lignification and the chemistry of the fruit around them, stems, whether whole bunch or otherwise included, can contribute tannin, potassium, water, aroma compounds and a distinctively savoury or herbal register. Used well, they can bring lift, structure and complexity to the final wine. However, when used carelessly, or without consideration for the realities of the vintage and chemistry of the must, they can raise pH, harden the tannins, dull the fruit or push a wine towards sappy bitterness and greenness.

As you might imagine, the various techniques that put stems into contact with the must can be a divisive topic among winemakers and there is no inherently right answer to whether stems should or should not be used. It is a stylistic decision which some liken to the use of spice in food, where a little can be beneficial, but too much can overpower the food. That said, there are proponents of 100% whole cluster out there that make superb wine. Similarly, there are those that insist that the only way to make quality wine is to always destem. I will not attempt to come to any particular conclusion here, as the answer inevitably is, it depends. What I do want to do however, is take a closer look at what stems actually contribute and some of the variables that result in different outcomes for the resulting wine. There are enough anecdotal accounts out there which cannot be reconciled, so taking some of the mystery out of what stems do would I think be helpful to gain a better understanding of the subject. One such collection of anecdotes can be found in this Guildsomm article by Kelli White.

What Stems Bring to the Vat

As mentioned in the introduction above, stems can be introduced to the must in a number of ways. This can be by so called true whole bunch fermentation, where the clusters are intact and berries are attached to the stems. Then there is partial whole bunch, where whole bunches are combined with a given percentage of destemmed and crushed fruit. Stems can be added back, much like one would immerse a tea-bag in water, in order to extract certain compounds from the stems but without any of the intracellular dynamics that whole clusters imply. The stems can be added fresh, or can be dried to lower their water content and change their aromatic and tannic properties. Lastly, and this is rather unusual, they can be introduced in the form of powders or extracts, presumably for better control.

Photo by The New York Public Library on Unsplash

Stems represent roughly 3-7% of the total bunch weight (Blackford et al. 2021) depending on variety and clone and the various bunch architectures they may have, as well as the number of grapes on the bunch. As per the Blackford et al. paper, the composition of stems can be broken down as follows:

Water: Represents roughly 55-80% of fresh stem weight. This varies by variety and factors like stem maturity and possibly growing conditions.

Cellulose and Hemicellulose: Ranges from between 12–38% of dry matter.

Lignin: 13–47% of dry matter. This is linked to lignification, perceived stem “ripeness”, woody texture and extractability. Varies by variety and maturity.

Proteins: Stem protein content ranges from 5 to 11% of dry matter (2–3% of fresh weight). According to Blackford et al. Protein figures are relatively consistent between studies with a mean of around 7%.

Ashes: Also quite consistent between studies referenced in Blackford et al., with a mean of 6.9% of dry matter, regardless of the grape variety or origin.

Acids: Represent around 1-2% of stem weight. A combination of organic acids such as malic and tartaric, (Wang et al., 2025) and phenolic acids (Souket et al., 2000).

Phenolic compounds: Highly variable, depending on variety, vintage, growing conditions and stem maturity. Broadly speaking, these can be divided into flavonoids and non-flavonoids. In stems, the most important flavonoids include flavan-3-ols and proanthocyanidins, which contribute to tannin, bitterness and astringency, while the non-flavonoid group includes compounds such as phenolic acids and stilbenes.

Main chemical components of stems (values expressed in % DM). Source: Blackford et al. 2021

How Extraction Works

Wine must is a relatively complex solvent system that is at once aqueous, acidic, hydroalcoholic, warm and mechanically agitated. Such a combination is very effective at extracting various compounds from solids present in the juice, which in addition to the stems we’re discussing here of course includes skins and seeds. At the start of the fermentation process, before any significant alcohol has been produced, any extraction is mainly water-driven. The acidic juice is able to extract the more water-soluble materials from the stems such as potassium and other minerals, as well as some organic acids, low molecular-weight phenolics and certain green smelling volatiles. This means that stems can influence the overall pH of the must very early on in the fermentation process, something that is driven mainly by the release of potassium.

Once the yeast get going, converting available sugars into alcohol, the capacity of the must as a solvent changes. It is no longer simply acidic grape juice, but an increasingly hydroalcoholic solution, capable of extracting compounds that water alone would extract less readily. Among these are stem-derived phenolics, including monomeric flavan-3-ols such as catechin and epicatechin, as well as larger proanthocyanidins, or condensed tannins. The former are often associated with bitterness, similar to that which can be drawn from seeds, while the latter contribute more directly to astringency. Proanthocyanidins bind to salivary proteins, reducing lubrication in the mouth and creating the dry, gripping sensation associated with tannic structure. Depending on how much is extracted, they can create significant textural complexity in the wine, giving rise to descriptors like silky, velvety, chalky, firm, angular or plush, as well as a host of others. How this comes across in the final wine depends on other variables as well, such as the alcohol level of the wine, its pH and the élevage and oxygen exposure over time. Their character, as with tannins extracted from seeds and skins, will shift as they polymerise and bind to other compounds like proteins. This also contributes to a certain degree of colour stabilisation. Generally speaking, water-ethanol solvents are more effective than either water or ethanol alone due to the different polarities of the various compounds present.

Another key byproduct of fermentation is heat. For the same reason that your dishwasher works best when the water it uses is piping hot, the rate at which compounds are extracted from stems increases as the temperature rises. That is because higher temperatures increase solubility and reduces the viscosity of the solvent. This, combined with time, the pH of the must, the surface area, or rather the physical contact of stems and juice, and movement, all contribute greatly to how much is extracted from the stems. This leads to some practical considerations for the winemaker.

Photo by Joao Grave on Unsplash

While the amount of stems included matters greatly, their physical location in the fermentation vessel is often just as important. A whole, intact rachis sitting in a relatively dry cap extracts differently from chopped or broken stem material that is fully submerged. Punchdowns, pumpovers, délestage, submerged caps, pressing, broken stems, and whether the cap is kept wet all affect how much solvent actually reaches the stem surface. This is why added-back stems can behave differently from true whole clusters with added-back stems often being more exposed, more broken, or more evenly wetted, whereas whole clusters also alter the physical architecture of the ferment. Where whole clusters are placed relative to the destemmed fruit (if it is not 100% whole cluster), also impacts how much contact the stems have with the liquid. If the whole clusters are placed at the bottom of the fermenter, the extraction from the stems will likely be higher than if they were placed at the comparatively drier environment at the top of the fermentation cap.

Cell-wall breakdown is also key. The stem tissue is what is known as lignicellulosic, consisting, as we’ve seen above, of cellulose, hemicellulose, lignin, pectic material and vascular tissue. These compounds are not just sitting on the surface but are partly embedded in the structure, both physical and cellular, of the stem. Lignification dramatically changes the permeability of the cells in the stem, and thus the kinds of compounds that are likely to be extracted.

Sensory Implications

Given all of this, it is easy to see why the sensory implications of stems can be so difficult to generalise, and why winemakers don’t seem able to come to any consensus regarding their use. The same practice that gives one wine aromatic lift, savoury complexity and structural poise can make another taste green, hard and overworked. Aromatically, stems are most often associated with herbal, spicy, floral and savoury notes: bay leaf, dried herbs, green tea, rose stem, black pepper, mint, bark, incense, forest floor and sometimes a faintly smoky or woody character. In varieties such as Pinot Noir, Gamay and Syrah, these notes can sit very naturally alongside the fruit, either lifting the perfume or pulling the wine away from simple primary fruit towards something more layered and architectural.

In Syrah, for instance, stems can amplify peppery, smoky and olive-like qualities that are already part of the variety’s register. In Pinot Noir and Gamay on the other hand, they can add perfume, spice and a kind of sappy freshness that many drinkers associate with whole-bunch winemaking. The risk, however, is that this herbal register tips too far. In varieties already prone to methoxypyrazine-derived aromas, such as Cabernet Sauvignon, or in fruit picked before flavour maturity has properly arrived, stem inclusion can push the wine towards bell pepper, cut grass, green bean, leafiness or a general impression of underripeness. A stem-included wine can be herbal without being underripe, just as a ripe wine can still be made to taste crudely vegetal if the stems are poorly lignified, the proportion is too high, or the extraction too forceful.

We’ve seen that stems are a source of phenolic compounds, particularly flavan-3-ols and proanthocyanidins, which can materially alter the tannic shape of a wine. The effect is not just the addition of further tannin, but also a potential change in the grain, dryness, bitterness and spatial impression of the wine on the palate. In successful examples, stem-derived phenolics can give the wine a more defined frame, lending firmness, length and a sense of direction to fruit that might otherwise feel soft, sweet or diffuse. This is one reason whole-bunch wines are often described as more vertical, savoury or architectural. They may feel less plush than fully destemmed equivalents, but more lifted, more tensile and more structured. In less successful examples, however, the same compounds can produce tannins that feel woody, angular, scratchy or abruptly drying, particularly on the finish, which are qualities you definitely want to avoid. Where stems are broken, bitterness is also a risk. This can also be a problem when the cap is aggressively worked, stems are insufficiently lignified or left in contact with the must for too long. This is why stem inclusion cannot be judged by percentage alone. Ten percent whole cluster at the hands of one winemaker may be far more dominant than a wine with thirty percent inclusion made by another.

In Syrah, whole cluster can accentuate savoury and peppery notes, like olive tapenade, black pepper and bay leaves.

Stems can also alter the perceived balance of a wine in ways that are not always obvious from tasting alone. Because stems can release potassium into the must, they may raise pH and reduce the sharpness of acidity, even though the resulting wine is often described as fresher. That apparent contradiction is one of the more interesting aspects of whole-cluster and stem-inclusion winemaking as the freshness may not come from acidity in the typical sense, but from the aroma and structural profile of the wine. These qualities are often a certain herbal lift and spicy complexity, lighter early extraction from intact berries, and a less saturated expression of fruit. Their high water content can also have a modest diluting effect, sometimes reducing alcohol or slightly changing the concentration of the must, while whole berries in a whole-cluster ferment may delay extraction and introduce a degree of intracellular fermentation, giving brighter fruit and aromatic lift.

There truly is no one size fits all when it comes to the subject of stems. For example, in a wine seeking plushness, density, deep colour and polished tannin, stems may be intrusive, while for a wine seeking perfume, savouriness, tension and ageworthy structure, they can be among the most effective levers available to the winemaker to shape the profile of the wine.

Risks

Whole cluster fermentations are often more demanding, and require careful management. This is not just so that the right qualities are extracted, but also because the structure of the cap is much looser, often leaving room for air pockets where unwanted spoilage organisms can get a foothold. This can be partially avoided by diligent cap management, but can easily lead to increased levels of volatile acidity (VA), which often comes across as a hint of nail varnish remover or sour vinegar. That is not to say this is a necessary outcome, but such air gaps, in addition to a slightly higher pH environment, brought on by the extraction of potassium from the stems can create a more hospitable environment for spoilage organisms. This means that hygiene and fruit health is even more important than it would normally be.

The increased pH environment can also impact the effectiveness of sulphite additions to the wine. As stems release potassium, it can bind with tartaric acid and encourage its precipitation, causing titratable acidity to fall, and increasing pH. This shift in pH affects how much of the SO₂ in the must exists as molecular SO₂, the form that is gives the wine important antioxidative and antimicrobial properties. So, for example, a wine at pH 3.35 needs far less free SO₂ to reach a given molecular SO₂ target than a wine at pH 3.75. That is why whole-cluster or stem-included wines can sometimes require more careful SO₂ management if the stems have pushed the pH up.

Final Thoughts

Given the complexity of this subject, this overview will inevitably be incomplete, but I hope it can shed some light on what whole cluster and stem inclusion actually means for a wine. For me, I quite like the idea of stems as spice, to be used sparingly to add further complexity and structural dimension to a wine, qualities I personally value more than pure fruit.

In an interview I listened to recently with Pax Mahle, an ardent proponent of 100% whole cluster usage when possible, I was struck by how the seemingly simple question of whether or not to include stems in the must impacts the entire winemaking approach. One example of this is in the crushing of the fruit, which in Pax’s wines is always done by foot and as gently as possible so as to minimise the amount of stems that are broken, which as discussed can lead to significantly greater extraction of qualities that might be unwanted. It is a good example of how a decision at one point in the process inevitably informs any future step, and whole cluster usage undoubtedly represents a significant fork in the road for any wine.

Curious to hear your experiences with whole cluster or stem included wines, and what your thoughts are on this stylistic decision.

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