How does whole cluster fermentation reduce alcohol levels?

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Dear Dr. Vinny,

In a recent article about France's Rhône Valley, Wine Spectator senior editor Kristen Bieler notes that “whole cluster fermentation increases freshness and decreases alcohol levels.” Can you explain how and why this happens?

—Tarek, Monterey, Calif.

Dear Tarek,

First, let's catch everyone up to speed on whole cluster fermentation. After grapes are harvested, winemakers have to decide if they want to crush and ferment the entire grape clusters all together or crush just the grapes, following a process called destemming. With whole cluster fermentations, you're using not just the grapes but also the stems. 

Bieler says there are a few things at play in terms of how whole cluster fermentation imapcts a wine's character. “The stems impart herbaceous white pepper character, which creates a fresher taste profile,” she says. “Stems also absorb some of the alcohol from the fermentation—some producers have observed up to 1 percent decrease in alcohol with whole cluster fermentations. Others have suggested that the slight alcohol reduction is a product of the stems containing water, which causes a slight dilution.” She adds that winemakers working with stems are often also using open-top fermentors, which can also lead to alcohol loss via evaporation.

—Dr. Vinny

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