Most modern wines are fermented in stainless-steel tanks and sold in glass bottles, but winemakers still use traditional oak barrels. These can refine and improve most premium red wines and those whites, like Chardonnay, that are rich enough to benefit from oak treatment without being overwhelmed.

Why? Just as a chef might reduce a sauce over heat to concentrate its flavors, add butter to enrich it or add fragrant spices, vintners use oak barrels to adjust the intensity, texture and taste of their wine. Here are three ways how:

1) Oak Barrels Intensify Wine

Water and alcohol are absorbed through wood’s pores and evaporate when they reach the surface, which has an effect similar to simmering a stew.

The remaining flavor compounds, acids and tannins become more concentrated, increasing the quality and aging potential of the wine.

2) Oak Barrels Soften and Enrich Wine 

Air enters through wood’s pores, exposing the wine to very slow, continuous oxidation. This causes small-scale chemical reactions that soften harsh young wines and help them feel more plump and less harsh on the palate.

Oak aging enhances wine’s mouthfeel in much the way that a dab of butter can enrich and smooth a sauce.

3) New Oak Barrels Impart Flavors and Tannins

Oak contains soluble flavor compounds and tannins, which are imparted to wine over time. New barrels give wine a strong toasty flavor, like that of Cognac or Bourbon, which vintners use as a seasoning for wine.

Oak tannins are also essential preservatives for age-worthy fine wines.

However, just as tea bags make weaker tea with each repeated use, oak flavors and tannins fade as well. Older barrels reach what is called a “neutral” state by their fourth year, where most flavor and tannin has been depleted. Since 100% new oak can overpower wine’s fruit flavor, vintners typically rotate in 20%–50% new barrels when making each vintage.

Marnie Old is one of the country’s leading wine educators. Formerly the director of wine studies for Manhattan’s French Culinary Institute, she is best known for her visually engaging books published by DK – the award-winning infographic Wine: A Tasting Course for beginners and the tongue-in-cheek He Said Beer, She Said Wine.