ULLAGE/ˈʌlɪdʒ/

The unfilled space in a barrel or wine bottle.

This is largely a technical word of the beverage technologist, the wine merchant or publican. The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was created, to fill a barrel up to the bung hole. (When wine ferments in the barrel, there’s a slow loss of liquid due to evaporation through the wood. It’s very important to keep the barrels full, as otherwise unwanted bacteria and yeasts can get in and cause nasty side fermentations.) In turn, a noun ouillage was created, which was the immediate source of our word, first recorded in Norman English about 1300, at first in the sense of the amount of liquid needed to fill a barrel up to the bung hole. By an obvious extension, ullage came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. And it is also applied to the unfilled air space at the top of a bottle of wine, which in this case is essential to allow for expansion of the contents as the temperature changes.

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