How does "reverse osmosis" work, and what's it for?

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Dear Dr. Vinny,

How does reverse osmosis assist in reducing high alcohol levels?

—Catherine, Cape Town, South Africa

Dear Catherine,

Some winemakers like the flavors obtained from extremely ripe grapes, but not the extra alcohol that results from the high sugar levels in the fruit. Reverse osmosis (or RO for short) is one technique they can use to reduce the alcohol without changing the fruit flavors and other elements in the wine.

In RO, wine passes through a filter—a really, really tight filter. Water and ethanol are the smallest molecules in wine, so they pass through the filter most easily. Some forms of acid also pass through, but most elements—including color, tannin and, basically, flavor—do not. (These elements are saved for later use.)

Then the colorless and tasteless water and alcohol mixture is distilled to separate the alcohol from the water. The water is then recombined with the color, flavor and tannins. Result: a small batch of wine with reduced alcohol.

This small batch is then blended back into the rest of the wine, thereby diluting the alcohol without losing any of the flavor elements (and only a tiny percentage of the volume).

RO is used in other applications, most commonly in household water purification systems. Some maple syrup producers also use RO to remove water from sap before boiling it down to syrup.

—Dr. Vinny

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