Oak imparts a spicy, vanillin taste to wine stored in barrels, and this "oak" character is considered desirable in most red and some white wines. Unfortunately, after barrels are four or five years old, they no longer produce the desirable flavors, so wineries must replace their barrels from time to time. New, French barrels cost about $800, so on a capacity basis, barrels are very expensive wine storage containers. Barrels are also more expensive to store, maintain and handle then large tanks. Even so, most high quality red wines are aged in barrels for some time before they are released.

Besides the spicy, vanillin flavors, other changes occur to wine stored in oak barrels. From three to four gallons of wine are quickly absorbed into the wood when new barrels are first filled. Depending on storage conditions, one to three liters of wine disappears from tight barrels each month. Wines stored in barrels for significant times show the effects of oxidation aging. The color of wines stored in barrels becomes darker. The added flavors, absorbed wine, evaporation losses, oxidation aging and darker colors influence wine quality.

When used barrels are stored empty, the wine soaked into the wood can turn to vinegar in just a few days. Then the barrels become contaminated with vinegar bacteria, and the barrels must be discarded because sterilizing oak barrels is practically impossible. Empty barrels can be safely stored if they are gassed with sulfur dioxide at frequent intervals, but barrels stored this way must be carefully watched. Many commercial winemakers avoid this problem by not emptying their barrels until new wine is available. When the barrels are emptied, they are washed with clean water and immediately refilled with new wine.

Wine disappears from barrels during storage, and a partial vacuum develops as the liquid level drops. After an initial soak, old barrels and new barrels show the same amount of liquid loss each month, so the missing liquid is not just soaking into the wood. Water and alcohol diffuse through the barrel staves and evaporate into the air. Measurements show more water leaves when barrels are stored in low humidity cellars, and more alcohol leaves when barrels are stored in high humidity cellars. A negative pressure develops as the water and alcohol leave barrels, and the vacuum proves that air does not diffuse through staves into barrels. Contrary to the popular belief, barrels do not "breath." After prolonged aging, the slow lose liquid from barrels produce more concentrated wine.

The effects of oxidation are always evident in wines stored in barrels. Only small quantities of oxygen are needed to bulk age wine properly, and sufficient oxygen comes from the air that enters the barrels each time the bungs are removed (for tasting, testing, racking, etc.). The size of the storage containers is the important for bulk aging quality red wines, and 50-gallon to 200-gallon containers seem to be the optimum size. However, red wines can be effectively aged in large tanks by using a judicious racking schedule.

Color changes are observed in wine stored in barrels. White wines become more golden colored. Red wines become darker and show more brownish shades. Part of the color change is due to oxygen uptake, but some changes come from phenolic materials extracted directly from the oak. The darker color makes red wines more rich and robust looking. Unfortunately, the gold shades in white wines are sometimes mistaken for excess oxidation.

The effects of barrel aging can be summarized as follows. Wines stored in newer barrels develop desirable flavors and undergo slow oxidative aging. These wines also become darker in color, and wines develop more concentrated flavors because of the loss of liquids. Consequently, barrel aging contributes more than simple vanillin flavor changes. Home winemakers often add oak chips to wine stored in glass carboys. The chips add the spicy, vanillin taste and some color changes, but glass carboys do not concentrate flavors or provide the oxidative aging. An inexpensive way for home winemakers to simulate barrel aging is to add oak chips to wine stored in older, inert barrels. Wineries replace their barrels periodically, and clean, older barrels can often be purchased for less than $50. Here, the added oak chips provide the spicy flavors and added color, and the older barrels provide the slow oxidative aging and the important flavor concentration.