Wine Science: Whole Cluster Fermentation and Carbonic Maceration

Many winemaking terms are thrown around by wine experts when offering descriptions of wines to consumers. Some are easily interpreted and understood. It’s a given that a barrel aged wine will typically have aromas of oak, for example. But other terms have begun popping up in casual wine descriptions that need a bit more explaining. In this article, we’ll discuss two of these terms that have become quite popular in the lexicon of wine speak, and often go hand in hand: whole cluster fermentation and carbonic maceration. Beaujolais has made these methods famous, but the process is practiced by winemakers all around the world. So let’s delve in a bit deeper and get some understanding of these wine terms!

What is whole cluster fermentation?

Let’s begin with a discussion of whole cluster fermentation. It is exactly what it sounds like: instead of fermenting grape berries that have been removed from their stems, entire grape bunches are picked, thrown into fermenting vessels, and the fermentation is allowed to start. This is an ancient method, used long before modern techniques of mechanical harvesting and sorting tables were common.

What does it do for a finished wine?

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Whole cluster fermentation means grapes and stems are all fermented together. We don’t normally think about wine being made with fermented grape stems. What characteristics are imparted to a finished wine by fermented stems, and why?

1.      Grape stems provide a structural component to the fermenting must. This results in physical properties that change the way the must ferments. Juice is circulated more homogeneously during the fermentation, and this ends up keeping the must cooler by a few degrees. This alone can significantly affect the finished product, as cooler fermentation temperatures tend to preserve aromatics and extract less color and tannin.

2.      Grape stems soak up color. When a winemaker desires a lighter wine, stem inclusion is a natural way to assist.

3.      Grape stems contribute their own flavors and tannins. Unripe stems result in a green bell pepper/herbaceous aroma due to extraction of 2-methoxy-3-isopropylpyrazine. Ripe stems can contribute increased tannin levels.

Whole cluster fermentation also means grapes are intact when they are put into the fermentation vessel. This changes the makeup of the fermentation via our next term up for discussion: carbonic maceration.

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What is carbonic maceration?

There are different ways to achieve alcoholic fermentation. The standard yeast method is familiar: grapes are crushed, releasing their sugary juice. Yeast eats the sugar, transforming it into energy and producing ethyl alcohol as a byproduct. This can go on until the juice becomes too alcoholic for yeast to survive, typically around 16%.

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Carbonic maceration produces alcohol in a different fashion, and releases different byproducts. When grapes are left whole, their own glycolytic enzymes will begin to break down sugars inside the grapes. During this fermentation process, the cell walls of the grape skins become permeable and various compounds leak out of the grapes. The fermentation can last up to about 10 days before the berries run out of sugar, at which point there is about 1.5% alcohol by volume inside the grapes.

During the process of carbonic maceration, malic acid in the grapes is broken down resulting in about half of it being converted to pyruvate, acetaldehyde, and then ethanol. Amino acids leaking out of the grapes give nutrients to the surrounding grape must. Additionally, the volatile compounds benzaldehyde, ethyl salicylate, vinylbenzene, and ethyl-9-decenoate are produced. Benzaldehyde is esterified by ethanol to produce ethyl cinnamate, which is known for its cinnamon, strawberry, and raspberry aromas. Remaining benzaldehyde offers aromas of kirsch. Ethyl salicylate’s minty aromas can sometimes be detected in finished wines, and vinylbenzene and ethyl-9-decenoate produce relatively innocuous sweet-ish aromas.

An interesting historical note…

In the 1934, a French scientist by the name of Michel Flanzy was presented with a problem to overcome. He was attempting to reduce grape deterioration during transportation from vineyards to wineries. He tried storing harvested bunches under carbon dioxide for long time periods. After 2 months, he discovered that some of the grapes had measurable amounts of alcohol. These grapes were crushed and vinified, and the resulting wine was tasted. The group found the wine to be “pleasantly unusual.” And thus the carbonic maceration method was born!

How does carbonic maceration occur in whole cluster fermentations?

Though no carbon dioxide is typically added to whole cluster fermentations to intentionally induce carbonic maceration, uncrushed grapes toward the bottom of the must are basically in an oxygen-free environment. These grapes will have some enzymatic fermentation occur, producing the byproducts of carbonic maceration. This process is referred to as semi-carbonic maceration.

So what is the overall result of whole cluster fermentation?

Wines produced by this method end up with these characteristics in general:

1.      Lighter coloration
2.      Lower acidity
3.      More complex tannin structure (though not necessarily higher overall tannins)
4.      Some herbaceous notes (from stems)
5.      Some kirsch, strawberry, raspberry, and cinnamon aromas (from semi-carbonic maceration)

So hopefully the next time you read a wine description and read the words whole cluster vinification, you’ll be armed with a little more knowledge to determine what’s in the bottle. Try a bottle of Tyler Zotovich Chardonnay for a good example of a whole cluster fermented white wine, or (for a really fun experience) a bottle of Oszkár Maurer Kadarka 1880 for an absolutely perfect example of a red!

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