One of the questions that came up during our wine tastings at the Haro Bodegas was: why is wine matured in oak barrels rather than any other wood.
Historically wine used to be stored and transported in clay amphora, which are heavy. When the Roman Empire did their global expansion they struggled to export sufficient wine to keep the troops happy due to the weight involved. It wasn’t until they conquered Northern Europe did they learn from the Gauls the method of transporting wine in wooden barrels. Apparently we used it to transport beer.
Oak was chosen as the wood is soft and easy to bend into the traditional barrel shape. The Phoenicians had been using palm wood but it was harder and difficult to shape. It had to be toasted for a long period to make it pliable. Oak needs very little toasting so barrels could be created much faster. Oak was also in abundant supply in the forests of continental Europe at the time. Finally, oak, with its tight grain, offered a waterproof storage medium.
It was quickly realised that the oak barrels also imparted new scents to the wine of cloves, cinnamon, allspice or vanilla, and when drunk they had additional flavors present, such as caramel, vanilla or even butter. Indeed the longer the wine remained inside the barrels, the more qualities from the oak would be imparted into the wine, and thus began the practice of aging wine in oak.
In more recent times some brave wine makers have experimented with other woods but oak remains the best due as it seems to be just sufficiently porous to allow the right amount of interaction with the wine. Besides which we’ve all become used to the oak flavours and tend to expect them.
To get the oak effect you need to have new barrels. Once the barrels have been used several times they cease to impart any flavour, though can still be used for storage and to allow the wine to age without any oak effects.
Apparently first use barrels are kept for Crianza or Reserva wines, because the porous oak absorbs some of the wine. You clearly do not want to lose your most expensive wines. Second or third use barrels are kept for the Gran Reservas as these do not absorb as much wine but still impart good oak effects.