What is "ullage"

Ullage \Ul"lage\ (?; 48), n. [OF. eullage, ovillage, the filling up of a cask, fr. ouillier, oillier, euillier, to fill a wine cask; properly, to add oil to prevent evaporation, as to a flask that is nearly full, fr. OF. oile oil. See Oil.] (Com.) The amount which a vessel, as a cask, of liquor lacks of being full; wantage; deficiency.

"amount by which a cask or bottle falls short of being full," late 15c., from Anglo-French ulliage (early 14c.), Anglo-Latin oliagium (late 13c.), Old French ouillage, from ouiller "to fill up (a barrel) to the bung," literally "to fill to the eye," from ueil "eye" (perhaps used colloquially for "bung"), from Latin oculus (see eye (n.)).

n. 1 In a wine bottle, the empty space between the cork and the top of the wine. 2 In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel 3 The topping-up of such a barrel with fresh wine 4 In an industrial setting, the empty space in a tank, such as for fuel

n. the amount that a container (as a wine bottle or tank) lacks of being full

Ullage or headspace is the unfilled space in a container, particularly with a liquid.

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.