When new, oak used to make barrels contains aromatic molecules – vanilla, coconut, smoke, coffee, chocolate and toast for instance - that are passed on to the wine during the ageing process.
These aromas contribute to a wine’s complex character. New oak also imparts tannins – ellagitannins – which help create a wine’s structure. Although these components bolster complexity and structure in a wine, they can also jeopardise its balance of aroma and flavour. Attempting to conceal lack of substance in a wine by ageing it in oak will not do it any favours. Also, the intrinsic qualities of some grape varieties are undermined by use of oak.
Aromatic grape varieties in particular, which fully reveal themselves when fruit-forward, would be spoilt by oak ageing. A fine example of this is Sauvignon blanc. However, use of oak is not only designed to add aroma and tannins. Wines can also be aged in used barrels, referred to as ‘first, second, third use’ and so on, as long as strict hygiene rules are applied to avoid possible contamination.
A second use oak barrel imparts three times less aroma and tannins as new oak but is still extremely useful: oak is a porous substance that allows a tiny amount of airflow, primarily through the bung hole where the barrels are filled. This automatically leads to very slow oxidation which softens a wine’s tannins, stabilises its colour and allows aroma to mature. Also, oak ageing promotes clarification and stabilisation in wines. Barrels are nevertheless expensive and are often kept for making the finest wines.