What is Whole-Bunch Fermentation and Why Does It Matter?

Whole bunch of red grapes in vineyard
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“Whole bunch,” “whole cluster,” “stems” and “stemmy” are all terms that pop up in wine conversation. But what do they actually mean?

Grapes come into the winery as whole bunches. Winemakers have a choice: de-stem them or leave the grape bunch intact. What they decide impacts how the wine will eventually taste.

To de-stem means to take the berries off the bunch with a machine that separates the fruit from their stalks, or stems. Modern destemming machines do this very gently, so that whole, uncrushed berries come out at the other end. Other machines crush and de-stem simultaneously. But some winemakers forgo this process and ferment with the entire grape bunch intact, stems and all.

In red wines

Whole-bunch winemaking is the original way of making wine. Before crusher-destemmers were invented in the 20th century, almost all red wines were fermented with their stalks. This created wines that were often rustic and tannic, especially when the stems were not lignified, or still green.

Today, most red wines are made from destemmed grapes. However, fermenting whole bunches can be used as a stylistic tool in winemaking. The practice is often associated with Pinot Noir or, less frequently, with Syrah. It’s rarely or only experimentally with other red grape varieties.

Whole-bunch fermentation affects the aroma, texture and tannic structure of a wine. Some of Burgundy’s most famous estates use the technique to create their signature style. It’s achieved by either not destemming all the bunches, or by destemming and adding some of the stems back in during fermentation.

Whole-bunch ferments result in an aromatic headiness that often comes across as floral, herbal, spicy and scented. This can be polarizing. Some people find these qualities attractive, others are put off by it.

If whole bunches are used, there can be an element of intracellular fermentation. That’s an anaerobic fermentation that happens inside an intact, uncrushed grape that causes it to break down on its own. This produces different flavors and changes the aromatic composition of the wine.

However, whole-bunch fermentation is not the same as carbonic maceration, even if some intracellular fermentation takes place in both processes.

What is Carbonic Maceration?

The presence of stems also affects the length and temperature of the ferment, which changes the flavor composition. The stems release phenolic compounds that add to the tannic structure of the wine. With a trend for lighter, more delicate wines, the use of stem tannins can be more subtle and corrals the fruit in a gentler frame than oak. The grape stems need to be ripe, or they can add harsh, green tannins that can be detrimental to the wine.

Stems in the ferment also absorb color, which is why whole-bunch fermented Pinot Noir tends to be paler and more translucent. Most winemakers report that some plots seem to be more suited to the style than others, and that the percentage of whole bunches used can change from vintage to vintage.

Whole bunch of white wine grapes on vine
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In white wines

In white winemaking, the difference between using destemmed or crushed grapes and whole bunches lies in extraction.

In whole bunches, the grapes remain intact until pressure is applied. Then they burst, which sends their juices through the press. The spaces created in bunches by the stems act as drainage channels within the press.

The advantage of this method is that the grape juice picks up few of the phenolic compounds and potassium from the grape skins. Unless hard pressure is applied, the juice remains relatively clear and very light.

Since potassium acts to buffer acidity, this is a preferred method for making crisp, fresh wines.

This is also the way Champagne grapes are pressed. The red grapes pick up almost no color and retain wonderful acidity.

However, for Riesling grapes, where acidity can be sky-high, destemming, crushing and then macerating the juicy pulp can soften acidity by leeching as much potassium from the grape skins as possible. Destemming and crushing extracts the aromatic compounds, especially terpenes, which reside mainly in grape skins.

While there are pros and cons to each technique, based on a winemaker’s desired outcome, the decision to destem or press whole bunches of grapes can notably impact a wine style, acidity and aromatics.

Published on March 18, 2020
Topics: Wine Basics