France is just one of many European countries producing a significant amount of oak; the others include some Baltic States and Eastern European countries, as well as Portugal. France, however, remains the most significant, as not only does it produce oak in great quantities (close to a quarter of the country is covered by forest), but also the quality of French oak has become the standard by which all other oaks are judged. When selecting wood for a barrel, the cooper would be most concerned with the grain, which is the term used to describe how tightly the fibrous channels in the wood are packed. A tight grain means the wood is less porous, and therefore is more likely to make a watertight barrel, but the grain also affects how the much oak flavor is imparted to the wine. French and European Oak will taste like high quality vanilla ice-cream and butter on the back of the tongue in wines.
The most predominant oak in France and the rest of Europe is Quercus robur, generally a wide grained species, which is tolerant of a variety of growing conditions. Otherwise, the oak forests are filled with the less common Quercus sessiliflora, a tight grained species, and a large variety of hybrids of the two. The most significant forests for the production of oak suitable for coopering are in Vosges, Limousin, Sarthe and the central regions of Nièvre and Alliers. Certain forests, such as Tronçais in the central region, are particularly well known. As you would expect, smaller forests in Alsace, Burgundy, the Loire Valley and Champagne provide some wood for local use. The forest of origin is of some importance, as the climate in which the tree has grown will affect the density of the grain, and thus the quality and suitability of the wood. Vosges, Nièvre, Alliers and Sarthe oaks all have a tighter grain, and are regarded as excellent sources of wood, whereas Limousin oak has a wide grain and is less highly regarded.
Other European oaks, as alluded to above, come from the Baltic States and Eastern Europe. Indeed, Baltic oak was once highly prized, but communist rule and concordant poor forest management has severely reduced its use by European winemakers. In recent years this situation has changed, and trade of oak for coopering between these countries and Europe, as well as the USA, has begun again. Portuguese oak, however, has always been of huge importance. Not because of its use for the production of barrels, which does occur in a limited fashion, but because the bark of the cork oak is harvested for the manufacture of corks, used to seal bottles of wine.
American oak has a history of use in Spain and Australia, as well as the obvious markets of North and South America. As well as being richer in tannins, the bold, spicy vanillin flavors imparted by maturation in American oak were deemed suitable for big, full-bodied red wines such as Rioja and ripe Australian Shiraz. American Oak will taste like basic vanilla ice cream or dill pickles on the back of the tongue.
The predominant species in the USA is Quercus alba, but there are a large number of other species, not all of them suitable for cooperage. In general North American oaks are wide grained, because the reliably warm climate and long summer season results in a greater amount of growth each year. This wide grain would suggest that the wood is more porous than in European species, but in fact the opposite is true. The American oaks are much richer in compounds called tyloses than the Europeans, and thus possible sources of leakage are less. Hence American oak may be sawn into staves, rather than split longitudinally as in Europe. This practice of sawing rather than splitting is one factor that alters the flavors that the oak imparts to the wine.
The market for American oak has increased in recent years, in part because it is less expensive than French oak. It is also being applied to a much wider range of styles, not just robust red wines.
Once the staves have been formed, either by splitting or sawing, there is still a long wait before the barrel can be made, and many decisions to make along the way. Firstly, the wood is dried, which may be rapidly achieved using a kiln, but many prefer the practice of air-drying. A large cooperage would have a huge amount of oak air-drying, or "weathering" at any one time, as shown here, because the process is much slower than kiln-drying. In most cases the wood would be left outside for well over a year. High quality wine producers will often buy in wood and weather it themselves, in order to ensure the process is carried out just to their liking. During the weathering process the color will darken to a silvery-grey hue, and the ground beneath the wood will stain darkly. Many winemakers say that this is due to the leeching of tannins from the wood, but this is a contentious issue.
The cooper may then assemble the staves to form a barrel. This is frequently performed around a small fire, usually burning the wood shavings the cooper has generated when dressing the staves in preparation. This fire heats the staves whilst they are bent, but it also has the effect of toasting the inner surface of the barrel. The amount of toast will greatly affect the flavors the barrel will impart to the wine. Alternatively, the barrel may be formed around a source of steam, or other heat source, and again different flavors will result.
The size of the container also varies the influence on the wine. A small barrel will provide a greater surface area to volume of wine ratio, and the flavors will be more pronounced than in wine matured in a large barrel.
The marriage of wine and oak
Having requested (or supplied) the correct oak, specifying the barrel size and the degree of toast, there are still many choices facing the winemaker. New oak barrels have a more profound influence on the wine than old barrels, so the winemaker may wish to mix the barrels. In Burgundy, where using 100% new oak would swamp the wine, the good producers are rightly cautious, only using a few new barrels each year. In Bordeaux, however, many châteaux will use 100% new oak each year, or even 200% in the case of some of the 'vin-de-garagistes', racking from one set of new oak barrels to another set after a year. A greater oak influence may also be achieved by fermentation in the barrel.
The winemaker must also decide how long to leave the wine in the barrel before bottling. This will vary greatly according to the style of wine, and the local appellation regulations. The longer the time in barrel, the greater the influence of oak on the final flavor profile of the wine. This period is referred to as barrel maturation, and it is not just the contact between oak and wine that is of importance. It is believed that the contact between the wine and the small amount of air present in the barrel has a beneficial effect on the way the wine matures.
The Economical Alternatives
Oak barrels are very expensive. Consequently, some winemakers will add oak flavors to the wine by a variety of other methods, not all necessarily legal.
One obvious method for reducing the cost incurred by oak aging is to purchase second-hand barrels. As many top châteaux use only new oak, there is a good supply of one-year-old barrels, which will be in prime condition. Barrels in less than perfect condition, whether just purchased or in the winemakers cellar, can be repaired by the insertion of the necessary number of replacement staves. This not only breathes new life into the barrel, but also adds a touch of new oak influence to the wine. Old barrels may also be revitalized by disassembly, shaving down the staves, and retoasting during reassembly. This has been practiced by many top winemakers purely due to the prohibitive cost of new oak.
Oak flavors can be added simply by lowering a few staves of oak into the tank containing the wine, which avoids the business of cooperage altogether. Even cheaper is the option of adding oak off cuts, chips or shavings, all dangled into the wine in a porous sack, rather like a giant teabag. Whether or not this is legal depends on your location Australian winemakers may do this within the law, whereas it is very much illegal for the vignerons of Bordeaux. Another option is the use of powdered oak extract, although this is generally illegal regardless of location.