NOW that is a contentious title to an article! Aromatic wine lovers would say oak is not required, and in the case of riesling, gewurztraminer and to a point semillon and sauvignon blanc, they are probably right. There are other wines, such as the majority of Italian white varieties that don’t need oak. But with red wines and chardonnay oak adds another dimension.
Oak has been used as a storage and transportation vessel for wine for two thousand years. It is fit for purpose, being a watertight wood and strong material for coopers to shape into barrels. The discovery that this storage container actually improved and added complexity to the flavour of wine was a windfall. The cocktail of compounds responsible for these flavours include ellagitannins, phenolic aldehydes and volatile phenols.
Quercus is the Latin name of the oak genus and it belongs to the beech (Fagaceae) family of trees. Sitting underneath the genus there are between 400 and 600 species of oak trees. Fortunately, we rely on only a handful of species. In Europe we have the Q. robur and Q. petraea (Q, is used as an abbreviation for Quercus) In America, barrels are made from the Q. alba or white oak. The tree is harvested when it is between 80 and 180 years old; like wine, the older the tree, the better the barrel. According to the cooperage Seguin Moreau, you get around four barrels from one trunk measuring 4m long and 80cm wide. The majority of European oak comes from France; but Hungarian, Russian and Slavonian sourced barrels are also used. French oak is named after the forest or region where the oak trees are grown, which includes Allier, Nièvre (or Nevers), Vosges, Tronçais and Limousin.
Q. robur is a pedunculate oak, whilst Q. petraea is a sessile oak. The difference in the two is down to the leaves and acorns. Pedunculate oak leaves have no stalks, but their acorns do and the opposite is true of the sessile.
Q. petraea likes cool climates and is able to grow on thin, poor soils. It is found in the cooler northern and eastern parts of France, especially in the Vosges Forest, and has a tight grain adding complexity and spice to the wine. Q. robur is more suited to richer soils and a slightly warmer climate. It is found in the central area of Limousin and gives more recognisable robust vanilla notes to the wine. Some say this is closer to American oak. In short, Q. robur tends to give more tannin whilst Q. petraea produces more aromatics and has a tighter grain. Some famous French cooperage or ‘tonnellerie’ include Seguin Moreau, Francois Frères and Louis Latour.
Slow growing trees develop tighter grains and this characteristic will slow down the pick-up of oak derived aromas and flavours. The opposite is true about coarser grained oak. This feature can be used in different ways by the winemaker in getting a wine ready for market. If a wine can only spend say six months in barrel, then coarser grain oak will have a bigger impact. On the other hand, if the winemaker chooses to mature a wine for 18 months or longer then fine grain oak would be advantageous. It also depends on the grape variety. Pinot noir is a delicate wine and can easily be drowned out by strong oak aromas and flavours, so fine grain oak is preferred. At the other end of the scale, shiraz is full bodied and intensely fruit driven so it can withstand a stronger flavoured oak.
Cost can come into the equation over which barrel to use and American oak is traditionally cheaper than French oak. One of the reasons for this is that American oak is denser and can be quarter sawn into logs, whilst French oak must be split along its medullary rays which grow at right angles to the growth rings. This is to ensure it is watertight. Sawing is cheaper than hand splitting. Another costly step in the process is the method and length of seasoning of the oak staves. Seasoning stabilises the oak so it can be made into stronger barrels. During this process moisture levels start at between 50% to 70% and drop to as little as 15%. The staves can either be naturally air-dried or kiln dried. Air dried is the preferred method in France, but takes a long time, approximately 2 to 4 years. Drying them in a kiln takes as little as six weeks and is used in American production. During seasoning bitter tannins and eugenol (clove oil) are also leached out of the staves. It also increases some flavour compounds including volatile phenols, phenolic aldehydes and furanic compounds and cis and trans ß oak lactones.
During their production barrels are browned over an open fire. This toasting process changes the structure of the oak and reduces the overall lactones present. The more a barrel is toasted the less oak tannins affect the wine, and more of a toasty character prevails. The process is akin to a barrel undergoing a Maillard reaction – what makes our morning slice of toast go brown. Barrels can be purchased with light, medium or heavy toast. Light toast allows the fruit to remain a strong component of the wine whilst medium or heavy toasting produces vanilla, coffee and toasty aromas.
There are many additional flavours and aromas coming from volatile compounds in oak.
American oak has more oak lactones, that gives wine vanillin and coconut aromas and flavours. European oak produces ellagitannins which is a hydrolysed extractable tannin. Ellagitannins do not add to the grape tannins but help protect the wine from oxidation in barrel and allows other aromas to develop. The volatile phenol eugenol, that is not lost in the seasoning process, gives the wine a clove, spice, cinnamon note. Whilst another group of compounds called the norisoprenoids can leave noticeable floral or balsamic aromas.
The role that oak plays in a wine cannot be under-estimated and whilst there has been a rise in unwooded wines you simply don’t get that all important aspect of complexity from simple fruit derived bottles.