There are hundreds of species in the Oak genus Quercus. Of these, four species intersect the world of wine. Three are used in barrel making and the other is cork, which traditionally is used to seal wine bottles.
What makes Oak suitable for making wine barrels? Well, for one it is strong wood. At the same time it remains fairly easy to work with. The microscopic structures of the wood make it possible to fashion watertight vessels.
Barrel Construction
The wood used to make barrels comes from the interior core of the trunk. This “stavewood” is dead and very tough. When a cooper uses French oak, the stavewood must be split rather than sawn. French oak becomes porous when sawn, not something you want in a wine barrel. American oak is sawn because micro-structures, called tyloses, interrupt the vertical fibers at regular intervals. The tyloses structures mean American oak doesn’t become porous when sawn.
The cooper tightly fits the staves around the inside of heated steel bands and barrel end pieces are fitted into place. When the bands cool and contract the stave are compressed even tighter making the barrel tight enough to hold liquid. Barrels use no glue or chemical adhesive which could affect the wine in undesirable ways.
The extra effort required in splitting means that French oak barrels are more expensive to produce than barrel make of American oak. The cooperage’s methhod of producing staves, the “house style” is possibly the most influential factor in determining the barrels affect on the wine.
Seasoning
Before the cooper can make a barrel, the wood to be used must be seasoned. The process humidifies the wood to the same level of the environment in which the barrel will be used. Seasoning, usually done outdoors, reduces undesirable acids and compounds that could impart bitterness to the wine while also raising the level of aromatic compounds.
Depending on the thickness of the staves, the seasoning process can take up to three years. It’s quicker and cheaper to age the staves in ovens, but taking this short-cut on seasoning means that the staves miss out important chemical changes and end up with fewer aromatic compounds and more bitter characteristics which will be imparted to the wine as it ages.
Toasting
The cooper heats the staves over a fire so they can more easily bend into the barrel shape. A side effect of this mild charring, known as “toasting,” combined with chemical properties of the wood to impart strong flavor to wine aged the barrels. The flavor from toasting is most strongest with new barrels, while barrels that have already held wine for aging have a less pronounced effect on the wine.
Effects of Barrels on Wine
A barrel’s flavor is imparted to the wine directly and, more subtly through a process of micro-oxygenation, a very controlled exposure of the wine to oxygen. Generally winemakers try to avoid contact with oxygen, but the slight change in the chemical profile of the wine from micro-oxygenation has a beneficial effect.
Oak barrels are great for wine, but they are expensive. Some producers of less distinguished wine have attempted to get the benefits of oak by adding oak chips, staves, or even liquid oak flavor to wine ages in stainless steel.
This is a just a brief overview of oak and it’s application in winemaking. Used with care, Oak can add unique and beneficial characteristics wine aged in barrels.
Check out some of our other articles on wine and winemaking.