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Dear Dr. Vinny,

What effect does reverse osmosis have on wine?

—Pravin, Prune, Maharashtra, India

Dear Pravin,

Reverse Osmosis (or "RO", as the cool kids say) is a special type of filtration. Wine passes through a very fine filter that water and ethanol pass through, as do some of the forms of acid in wine. But most of the elements of wine like tannins and the components responsible for pigment, flavor and aroma, are left behind.

What happens next depends on the intended purpose of the reverse osmosis procedure. Usually the solution of water and alcohol is distilled to separate the two and then some of the alcohol is typically removed before combining all of the elements (including the pigment and flavor components that were filtered out) back together again. Another application is to reduce the amount of water to concentrate the flavors. The process has also been employed in attempts to reduce brett, volatile acidity and smoke taint notes in a wine.

The understanding is that all of the stuff that makes up the flavor and aromatics of a wine never go through the filter, but certainly the whole purpose of RO is to alter a wine’s profile. It might sound like some mad scientist stuff, but RO is used for other industries, like water purification and desalination, and even to concentrate sap before making maple syrup.

—Dr. Vinny

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