Explainer Science

What is Whole Cluster Fermentation?

Understanding the increasingly popular technique and its impact on wines

Meg Maker By September 14, 2021
Illustration by Richard A. Chance.

Centuries ago, before mechanization, nearly all red wines were made “whole cluster.” Winemakers simply tossed entire bunches of ripe grapes into a vat, stomped them, maybe, then let the yeasts and bacteria go wild. Later, they pressed off the wine into aging vessels, composting the sticky, stemmy remains.

The approach fell out of favor in the 20th century, considered rustic and unsophisticated, although it lingered in Burgundy and the northern Rhône for reasons that have as much to do with tradition as chemistry. 

Recently, whole cluster wine making has come roaring back, and the term is even touted on the front of wine labels. So what does whole cluster fermentation do to red wine, and why is it so desirable? 

The answer depends on three key factors:

  • the ripeness of the stems.
  • the percentage of whole clusters used. 
  • whether or not the berries get crushed.

First, the stems 

Like grapes, stems also ripen over a growing season. The stalky framework of a cluster the rachis, and the tiny branches attached to each berry, the pedicels turn from green and vegetal to brown and lignified. 

When green, the stalks have chemicals called methoxypyrazines that contribute herbaceousness and astringency to a wine. That vegetal note can highlight Syrah’s desirable green olive character, but it can be too much for grapes like Merlot, Cabernet Sauvignon, and Carménère, which already have a leafy quality that derives from the same compounds. 

Once mature, the stalks contribute nutty wood tannins, which is why they’re often used with less tannic grapes like Pinot Noir and Gamay. Some winemakers actually prefer the more silky texture of stem tannins to the more robust tannins and oak flavors a wine acquires from aging in oak barriques. 

Counterintuitively, a slower, cooler growing season can allow the stalks to reach full maturity at the same moment as the berries. But vintages with heat spikes can force a winemaker to harvest fruit before the stems have fully ripened.

Next, the percentage of whole clusters 

The proportion of stems in a tank affects the vinification process in a myriad of ways.

Stems sop up the dark pigments in the wine. This can be desirable for inky varieties like Syrah, but too much can over-lighten Pinot Noir; use of whole cluster fell out of favor in Burgundy in part because of a modern fashion for darker wines.

  • Stems also keep a fermentation cooler and more aerated.
  • They form a cap on top that lets gases and heat escape, although the mass can be tricky to manage.
  • In general, though, winemakers like the effect of a slower, cooler fermentation.

In short: Generally, a winemaker uses 100% whole clusters only when the stems are nutty and lignified, and dials back the proportion when most or all of them are green.

Finally, whether the berries stay intact 

A winemaker might fill a tank with 100% uncrushed bunches, or might crush some and leave others whole. 

  • Yeasts go to work on the juice from crushed berries. 
  • Whole berries start to ferment from the inside, a process known as intracellular fermentation or partial carbonic maceration. This technique, also used in Beaujolais winemaking, yields attractive candied red fruit flavors.
  • Eventually the whole berries start to burst open, and the fresh pop of juice gives the yeasts a trickle of new sugar to chomp on. 
  • This keeps the fermentation bubbling along low and slow.

The upshot: Whole cluster is a key winemaking technique that allows a winemaker to modulate the flavor, texture, and structure of a wine. 

Critics of the technique claim it mutes a wine’s terroir expressiveness. Others counter that stems, just as much as berries, are legitimate gifts from the land and their use also has important historic precedent.

What’s clear is that given global climate change, winemakers must remain flexible in their methods, adapting their strategy to ruthlessly fickle vintages, not merely to fashion and taste.

Whole cluster wines to try:

  • Port wine is made using whole grape clusters. The bunches are worked aggressively to extract maximum color and flavor before the addition of grape spirit and subsequent aging.
  • Beaujolais wines are made whole cluster in order to encourage carbonic maceration. The resulting red berry fruit flavor reaches its apogee in Beaujolais Nouveau.
  • For 100% whole cluster American wines, try Dirty & Rowdy’s reds and red blends, Enfield Wine Company’s Haynes Syrah, or Pinot Noirs from Kutch in Sonoma or Domaine de la Côte in the Sta. Rita Hills.