On wine: How oak barrels affect wine

The ancient Celts were a people of the forest. They used a lot of wood in their daily lives, for housing, cookery and art. Sometime around the 5th century B.C., they came up with an ingenious new way with wood. They formed it into barrels for storing liquids such as water, ale and, when this craft was bequeathed to the West after the Dark Ages, wine.

Wood buckets for food preparation and storage long predated the closed-end barrel, but nothing beat the latter for moving liquids from one place to another - just across the winery cellar, for example, or down river to an export dock. Wood barrels were large, hollow wooden wheels, strong and reusable.

The best came from forests of oak plentiful especially in France, but also scattered throughout Europe. Sometimes coopers made barrels from chestnut, sometimes acacia, but they always preferred oak and still do to this day.

But while, for millennia, oak functioned to store and carry wine, it’s only in the past 100 years or so that have we cared about what it does to the wine it houses, especially how it flavors and matures it.

“The flavor of the barrel prior to the 20th century was considered a negative,” says Larry Brooks, winemaker at Tolosa Winery in San Luis Obispo. “The taste of oak is a relatively modern phenomenon.”

The very structure of oak (white oak, not red oak; and just three types of white oak, one from the United States and two from Europe) gives it its great strength and contains those elements that winemakers now seek.

An oak tree trunk is a bundle of tubes, vessels and fibers running parallel to and overlapping each other, hence the wood’s strength, with groups of other fibers that run radially from the outside to the center. When cut into staves, these rays help prevent liquid from leaking around the longitudinal fibers.

An oak tree grows twice during each year, commonly called “spring growth” and “summer growth.” Its rate of growth depends on where it grows, how many neighbor oaks compete with it for water and light, how cool its climate is and other horticultural factors.

The slower a tree grows, the tighter its grain. By and large, in cooler climates such as those in Europe, the spring and summer growth alternate in tightly packed rings. In warmer climates, such as in our country, the grain is wider or looser.

The contact of the wine with these growth rings determines what kinds of flavors and aromas get delivered to the wine, plus how much wood tannin.

Returns diminish, however. As a barrel ages, it gives less to its wine. Half of what it has is delivered to the wine in its first use, then 25 percent the second year, and so on, less and less each year of barrel use. (Hence, winemakers may use the neutral character of very old barrels as a substitute to fermenting in, say, stainless steel.)

Wine takes what the oak apportions it. In general, wider grained oak (most American and Limousin from France, for example) lends in-your-face aromas, typically vanilla or dill.

You’d think that tighter grained oak would squeeze out less; it doesn’t. Because there are more growth rings, it gives more: more tannin, but of plusher sorts; quieter aromas, still the American’s vanilla but embedded in cream or caramel.

Winemakers, then, choose their oak for it properties. For example, Silver Oak Napa Cabernet Sauvignon has long been aged in 100 percent American oak.

“At first, (using American oak) was convenient,” says David Duncan, president and CEO of Silver Oak Cellars in Oakville. “But we’re after its flavor profile and how marrying it with the wine lengthens the tannin structure.” The winery now owns its own Midwestern cooperage and ships about 30 barrels a day to California.

But fewer winemakers appear to interest themselves in the provenance of their oak wood. What they’re after these days are two things: tight grain and how the wood is seasoned (typically outdoors for two to three years rather than being kiln-dried.

“I think the tightness of the grain has more of an impact on flavor and tannin than anything else,” says Joel Aiken, winemaker at Amici Cellars in Calistoga and Aiken Wines in St. Helena. “Also, I think what’s more important than where (the trees) are grown is how they’re managed.”

And seasoning the oak outdoors for a long time is crucial, even for American oak destined for wine (not bourbon) production. That is a new advance for American oak.

“I do see from my experience the need for it to be well-seasoned,” says Mel Knox, a San Francisco-based barrel maker and broker.

“Studies in France indicate that molds and enzymes formed on and in the wood during seasoning can neutralize bitter phenolics in the wood. At the same time glucoses and polysaccharides are released from the wood. This process takes at least 18 months.”

That’s something a kiln cannot do.

Bill St John has been writing and teaching about wine for more than 40 years. He is based in Chicago.