Why Ouillage Matters
Wine in barrel loses roughly 2-5% of its volume annually through evaporation — what the French poetically call the part des anges (angel's share). As wine evaporates, an air space (ullage) forms at the top of the barrel. This oxygen exposure accelerates oxidation, turning wine vinegary if left unchecked. Regular topping up with wine of the same quality keeps the barrel full and the wine protected.
Frequency and Practice
- Weekly to biweekly — the standard schedule during the first year of barrel aging, when evaporation is fastest through new oak
- Monthly — typical for second- and third-year barrels with less evaporative loss
- Same wine — top-up wine should be from the same lot or a compatible batch
- Hygiene — the bung hole and tools must be sterile to prevent contamination
Ouillage vs. Sous Voile (Jura Exception)
In most wine regions, preventing oxidation through ouillage is fundamental. The Jura, however, deliberately avoids ouillage for its vin jaune, allowing a film of yeast (voile) to form on the wine's surface in partially filled barrels. This controlled oxidative aging, lasting six years and three months, produces vin jaune's distinctive nutty, curry-spiced character — the polar opposite of the reductive, fruit-forward styles that ouillage protects.