What is “reverse osmosis” in relation to wine? How and why is it used?

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Dear Dr. Vinny,

What is “reverse osmosis” in relation to wine? How and why is it used?

—Jianhang, Blenheim, New Zealand

Dear Jianhang,

Reverse osmosis (the cool kids call it “R.O.”) is a filtration method sometimes used in winemaking. The filter is so ridiculously fine that it can separate the water and alcohol (ethanol) in the wine from the tannins, pigments and the other components that are responsible for the wine’s flavors and aromas.

Why a winemaker might use reverse osmosis depends on their goal. Probably the most common reason that winemakers employ R.O. is to reduce a wine’s alcohol content. For instance, a winemaker might prefer the flavors of very ripe grapes, but not the higher alcohol levels that come with them. Water and alcohol are the smallest molecules in wine, so they pass through the filter most easily. The solution of water and alcohol can be distilled into whatever ABV the winemaker prefers, and then blended back together with the other components.

Reverse osmosis can also be used to concentrate flavors, by removing water. And R.O. can help diminish certain flaws, including brettanomyces, volatile acidity and even smoke taint (though the jury is still out on its effectiveness on those fronts). One of the concerns about using R.O. is that all that filtering might strip out a wine’s character—the natural order in which its flavors and aromas came together.

Just for context, some of the other ways in which you might encounter reverse osmosis in your daily life include household water purification systems and maple syrup (some makers use R.O. to remove water from sap before boiling it down to syrup).

—Dr. Vinny

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