Wine barrels. Hundreds of wooden wine barrels stacked in rows in a vineyard warehouse. Red wine is matured in wooden barrels for 3-4 years before bottling to improve and soften its flavour. Air passes very slowly through the wood, encouraging the formation of complex esters produced when acid and alcohol react together in the wine. The aroma- tic esters gives the wine its distinctive flavour and smell. At the same time, the levels of tannin (the bitter principle from the grape skin in red wine) and acid fall, making the wine smoother and less tart. Subtle flavours may also be absorbed from the wood, sometimes giving a smoky quality. Photographed in Almaden Vineyard, California, USA.