“The best white wines—particularly Chardonnay—are barrel fermented in oak to give them added complexity; a toasty, yeasty flavor; and greater longevity. Some of these great Chardonnays will even be aged sur lie—on the “lees” or sediment of yeast that falls to the bottom of the barrels after fermentation, which gives the wine an added richness.” ( Anita L. LaRaia, 28 October 2005, http://www.informit.com/articles/article.aspx?p=421051&seqNum=3 )
Barrel fermentation is the process of letting grape juice sit in a large wooden barrel (usually oak), during which time yeast turns the sugar in the grapes into alcohol and the juice is converted into wine. In other words, it is alcoholic fermentation within an oak vat (as opposed to a steel one).
The action of barrel fermentation is especially beneficial to white wines. First, since white wines lack the tannins of reds, the wine can instead draw tannins from the wood barrels. Secondly, since the fermentation process occurs while the wine is soaking up the oak flavor, the wood flavor is weakened. This leaves the light flavors of butter, spice and oak in the wine, instead of an overwhelmingly woody taste. Finally, the wood imparts a rich, almost creamy character to the wine.
Barrel aging is completely different than barrel fermentation. In barrel aging, the already-fermented wine is stored in barrels (of wood, cement or steel) and left to age. ( logabottle.com, 2012, http://www.logabottle.com/home/wineguide.php?n=Barrel+Fermentation&t=1&id=10&gc=1 )
Please click the following link to watch a short video on the Carbon Dioxide gasses escaping through a fermentation-cap on a wooden barrel:
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