Topping up the barrels at Château Fougas
Ageing in oak casks automatically entails a loss of wine which either evaporates or is absorbed by the wood, particularly during the first months after the casks have been filled.
Oxygen goes through the cask, mainly through the tiny cracks between the staves then via the bung-hole and, to a lesser extent, through the wood itself. This regular intake of oxygen in small quantities leads to controlled oxidation allowing the wine to become imbued with the tannins and aromas of the oak. Ullage thus forms inside the cask due to evaporation.
Topping up involves regularly refilling barrels with a wine of the same quality to compensate for the shortfall and avoid too great a risk of oxidation which would subsequently turn the wine into vinegar!
During the first year, every seven to twelve days, the barrels have to be topped up by refilling the headspace with wine up to the bung-hole.
The shortfall of wine in a Bordeaux-style, 225-litre barrel amounts to approximately 10 to 15% of its total capacity during the maturation phase.