Few wine terms convey the romance of winemaking more than “barrel fermented.” It suggests wine’s connection to the natural world and the winemaker’s art. But what does “barrel fermented” mean?

Most wine barrels are 225-liter vessels made of oak, grown in the world’s great forests. Since “fermentation” is the process of turning grape juice into wine, “barrel fermentation” refers to wine that has been fermented in an oak barrel.

Winemakers didn’t always ferment their wine in barrels. In ancient times, they usually used earthen pots. It took the Europeans to discover that oak fermentation improves wine.

Why? Oak is porous; about 10% of the wine in a barrel evaporates (oldtimers call this “the angel’s share”). This concentrates the wine, making it more intense. Oak contains chemical compounds that leach into the wine, giving it butter, vanilla and spice aromas. Vintners also discovered that fire-toasting their barrels gave the wood caramel and smoky notes that make wine more rich and appealing. Finally, oak tannins add to wine’s silky texture. The winemaker’s challenge is to make sure that the oak is in balance, so it doesn’t overwhelm the wine itself.

Steve Heimoff is one of America’s most respected and well-known wine writers. The former West Coast Editor for Wine Enthusiast Magazine and a contributor to Wine Spectator, he has also authored two books on the subject of California wine, including “New Classic Winemakers of California: Conversations with Steve Heimoff,” published in the fall of 2007.