Oak ageing

Oak does three separate things, and conflating them causes a lot of confusion. It adds flavour compounds (vanillin, lactones, guaiacol). It allows very slow oxygen ingress, which polymerises and softens tannin. And it concentrates the wine slightly through evaporation.

Only the first of these depends on the oak being new. Old, neutral barrels still provide the oxygen exposure and evaporation while contributing almost no flavour, which is why traditional Barolo aged in large old casks tastes of Nebbiolo rather than wood.

Origin matters: French oak is generally tighter-grained and gives subtler spice; American oak gives more overt vanilla and coconut, from higher lactone content.

Size matters more than most people realise. A 225-litre barrique has far more surface area per litre of wine than a 5,000-litre cask.

Toasting is the variable nobody sees on a label. Barrel staves are heated over a fire to bend them, and the degree of that toasting changes what it produces: light toast gives more vanilla and coconut, heavy toast gives smoke, coffee and clove. Two barrels of the same wood and the same age can taste substantially different for this reason alone.

The alternatives are worth knowing about, because they are widespread and rarely disclosed. Oak chips, staves and powder are legal in most jurisdictions and used in a great deal of inexpensive wine. They contribute flavour and nothing else — no oxygen exchange, no texture from time — which is why an oaky cheap wine often tastes of vanilla without the roundness barrel ageing gives.

The trend since the 2000s has been away from obvious oak almost everywhere. Larger barrels, older barrels, longer ageing in neutral wood, and more concrete and amphora, are all responses to a period when new French oak had become a default rather than a decision.