After fermentation and pressing, the wine remains cloudy with suspended solids, some of which winemakers may remove by “racking,” which allows the particles to settle, while the clearer juice above is transferred to aging vessels.
Chosen by winemakers, the aging vessel material can range from oak or steel to cement, depending on the desired wine style. Oak barrels add body and additional flavors, which are also influenced by barrel age, size, oak variety (French or American), the level of barrel “toasting” and duration of barrel aging. All oak barrels breathe, allowing limited oxygen, which helps the wine mature without the damaging effects of over-oxidization.
When winemakers want a strong oak influence, they usually age wine in relatively small, new oak barrels, such as the 225-liter barrique (developed in Bordeaux) or the 228-liter pièce (developed in Burgundy). Winemakers may also use large barrels, ranging up to the 900-liter tonneau or the 1,000-liter fuder, but these impart little additional oak flavor. Second-use oak may impart some subtle oak flavors, whereas older containers provide little flavor. The longer the wine remains in barrel, the greater the oak and oxygen influences.
American and European oak release flavors and tannins from the wood, including “vanillin,” which adds the characteristic vanilla flavor, and “lactones,” which can add coconut flavors. Grown in Wisconsin, Oregon and Minnesota, American white oak contains more lactones and releases more flavor. More traditional, subtle and expensive French oak is found in a number of French forests that each deliver unique attributes. Barrels made with other European oak are far less common and generally less interesting.
Cooperage—the process by which oak is transformed into barrels—also impacts the final wine. The barrel makers (coopers) cut and kiln-dry wood staves, assemble them into barrels and then use heat or fire to toast the inside walls. In particular, winemakers can select the degree of toasting from light to heavy. The higher the toasting, the spicier and more intense the flavor of the wine, and light toasting expresses more “oaky” wood flavor and tannins.
Regardless of the vessel, winemakers remain involved throughout the aging process.
As wines age in barrel, winemakers sometimes rack the wine as before to remove sediment and top off the barrels from other barrels as some wine is lost to evaporation. With oak barrels expensive, winemakers may use oak alternatives for less expensive wines, which can include oak chips, oak staves inserted into neutral vessels or even toasted oak powder. These alternatives may not present exactly the same flavors, but they can enhance a less expensive wine.
Stainless steel is used to age and simply store wine that the winemaker wants to be light-bodied and fruity—both red and white. It is particularly good when the winemaker seeks to enhance highly aromatic characteristics found in some wine varieties, such Sauvignon Blanc or Viognier. Other neutral containers include resin, glass-lined, cement or even fiberglass. However, because little air is exchanged in these containers, prolonged storage or aging can produce a reductive wine with unpleasant flavors.
When winemakers want to add flavor to white wines—with or without oak—they may age the wine with the yeast in the vessel—a process called “sur lie.” In that case, yeast contact adds rich, creamy and bready flavors, which are enhanced through periodic stirring or bâtonnage of the inactive yeast or “lees,” which releases additional flavors. The lees are then removed before bottling. The combination of lees stirring, malolactic fermentation and oak aging often produces the rich, creamy and full-bodied Chardonnay wines common in California and is sometimes used for other varietals, including Viognier and Sauvignon Blanc.