Oak barrels have been used since Roman times to store wine. It is only in the last hundred years or so that the use of oak in winemaking has taken on a more significant and considered role. It affects the colour, flavour, tannin profile and texture of the wine making it more complex and interesting.
Maturing wine in small oak barrels does many things, but two main effects are:
- allows the oak to impart its own character to the wine
- allows oxidation to occur in minute amounts.
Wine stored in oak becomes more aromatically complex as compounds are extracted from the wood. Some flavours and aromas descriptions can include cedary, toasty, spicy, sweet, nutty, vanilla and chocolate. The uptake of these aromas must be carefully monitored to ensure that that it becomes a harmonious part of the wine’s overall structure and does not dominate it. This is managed by the amount of new oak that the wine is stored in and the length of time the wine spends in barrel. For lighter bodied wines like Pinot Noir the wine may only stay in oak for 6 months. For more fuller bodied wines such as Cabernet Sauvignon at least 12 months is the norm. Over time less and less oak character is extracted and the barrel takes on a new role usually as a flower pot.
It seems counter intuitive to expose wine to oxidation when it is maturing. When we drink a bottle of wine it is exposed to a large amount of air and oxidises very quickly hence the need to consume it within a day or so before it starts tasting like vinegar. A little bit of oxidation during maturation is good as it helps to intensify the colour due to reactions between the pigments and tannins in the wine. The wine has better colour than if it was just store in a vat and is more stable over time. This reaction also helps to soften the tannins making the flavour of the wine more attractive.
Over time water evaporates out of the barrel and as the level of wine goes down the surface area of the wine exposed to air is increased and thus the risk of oxidation. The oxidation is controlled by keeping the barrels full. Topping up of barrels is done every couple of weeks.
Racking the wine every 3 months also exposes the wine to oxidation, but again in a controlled manner. Racking involves removing the wine from barrel, cleaning the barrel, returning the wine and topping it up. (Losses are referred to as the angels share). This promotes the controlled oxidation of tannins, thus softening them. It also allows any sediment to be removed thus helping to clarify the wine.
We may not fully understand all of the interactions between oak and wine but there is a synergy there that enhances the richness, interest and complexity of the wine.