Ullage (wine)

Ullage (from the French ouillage) is a winemaking term that has several meanings but most commonly refers to the headspace of air between wine and the top of the container holding the wine. It can also refer to the process of evaporation that creates the headspace itself or it can be used as a past tense verb to describe a wine barrel or bottle that has gone through the evaporation process (to be ullaged, etc.). The headspace of air is a mixture mostly of alcohol and water vapors with carbon dioxide that is a by-product of the fermentation process. In containers that are not completely air-tight (such as an oak wine barrel or a cork-stoppered wine bottle), oxygen can also seep into this space. While some oxygen is beneficial to the aging process of wine, excessive amounts can lead to oxidation and other various wine faults. This is why wine in the barrels is regularly "topped up" and refilled to the top with wine in order to minimize the head space. In the bottle, the ullage or "fill level" of the wine can be an important indicator of the kind of care and storage conditions that the wine was kept in. After-market resellers and wine auction houses will often inspect the ullage levels of older vintages to determine the potential quality and value of wine.

At the winery, the natural process of evaporation creates ullage in the barrel by causing some of the alcohol and water particles to escape as vapors, a loss sometimes referred to as the "angel's share". If the wine is in a container that is not completely air-tight, these vapor molecules (along with carbon dioxide) will diffuse out of the container through openings in the wood and around the bung and be replaced with oxygen molecules. While some oxygen is beneficial in the maturation and break-down of some phenolic compounds such as tannin, excessive amounts of oxygen can interact with Acetobacter present in the wine and start the process of turning the wine into vinegar. To prevent this possible oxidation and spoilage wineries will regularly "top up" the barrels by replacing the lost liquid with new wine.

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