“Ah oak!” says the wine professional as she sips. Is she complimenting or criticising the wine? It could be either.

At its worst, oak can be used to add flavour to an otherwise insipid wine. Oak chippings in a sack are dipped into the maturing wine like a teabag. The vanilla-like flavour of the oak beefs up the wine which now tastes of something: but it’s oak not grapes.

At its best, an oak barrel interacts with the maturing wine, deepening its colour and bringing out more complex flavours with greater intensity. Quite how this happens is complex, but when successful the one thing you are not conscious of is the taste of oak.

Success in using oak barrels (the main alternative is stainless steel) depends on a number of things. The barrel should be made from oak that’s been seasoned in the open air for at least 12 months; the oak should have a tight grain; the barrel should be charred on the inside to add interesting toasty, smoky flavours; and, most would add, the oak should come from France, or possibly eastern Europe, rather than America. American oak is cheaper but much more likely to contribute its own vanilla or coconut taste. It also matters how old the barrel is; after being used three times it will no longer contribute any flavour to the wine. This is a shame considering that a barrel of French oak can easily cost £800. Old barrels still have a function. The winemaker may transfer the wine from newer to older barrels during the maturation process to ensure the oak flavours aren’t overdone.

What’s happening in the barrel to make such a difference? Two things: firstly, chemicals are coming out of the oak into the wine. I’ve mentioned vanilla but there are hundreds of others that gives flavours that make us think of coconut and dill, cinnamon and cloves, and mocha and toast from the charring process. Secondly, wood is porous, so water evaporates and air is sucked in. Up to 25 litres, known as “the angels’ share”, can escape from a 225 litre barrel in a year – it’s why cellars full of barrels smell so glorious. The oxygen that enters the barrel does it so slowly that it doesn’t turn the alcohol to vinegar but it does facilitate other interesting chemical reactions that improve flavour and soften those astringent tannins that make your tongue feel like sandpaper.

Some wines, usually whites, do not benefit from ageing in barrels: Sauvignon blanc, Reisling, Chablis, whites from the Loire. Other whites respond really well to oak provided it’s not overdone: Chardonnay’s the great example. Most reds benefit, except light fruity wines like Beaujolais.

How can you tell if a wine has been aged in oak? Read the small print on the back, though it usually won’t tell you. Only the Spanish are really helpful on this: ‘Crianza’ on the main label means it’s had 6 – 12 months in oak, ‘Reserva’ means at least 1 year, and ‘Gran Reserva’ means at least 2 years. Otherwise just try it. If it tastes uncomplicatedly fresh and fruity it’s probably been matured in a steel vat. If it’s more complex and intense than that it’s probably spent time in a barrel. Unless it’s wine from hot regions like Australia and the Languedoc, where it’s ‘easy’ to make full-bodied wines that have been matured in steel but emerge complex and intense. But that’s wine for you – nothing’s ever black and white.

Andrew Polmear