Ullage is a Middle English word which means “the space at the top of a barrel, bottle, or cask.” It is used in a wide variety of industries to refer either to the liquid missing from a container, or to the space left behind by that missing liquid. This term commonly pops up in assessments of bottles of wines, in which ullage can be a critical factor in the determination of quality and cost, especially in older wine bottles.
The roots of this word are rather interesting. Ullage is derived from the Latin oculus, “eye,” a term which was borrowed to refer to the bunghole in a cask. The French adopted this word as ouiller, “to fill a cask,” and the Normans brought it to England as “ullage,” using it to refer to the fill level of casks up to the bunghole. Eventually, ullage came to refer more generally to the empty space in any sort of container, from a bottle of juice to a rocket.
In wine, ullage is critical. Ullage happens over time as the wine in the bottle slowly evaporates, and the ullage level can change radically if the wine is handled badly or the cork is faulty. Ullage is usually described in terms of the shoulder and neck of the bottle. Many new wines are filled all the way to the neck, with the ullage slowly slipping past the shoulder over time. In looking at lists of old wines for sale, you may have noticed references to “ullage” which are used by buyers to get some clues into the condition of the wine and the cork.
If the ullage in a bottle of wine is significant, it can promote oxidation, which will cause the wine to go off. For this reason, wine barrels and bottles must be periodically topped off, ensuring that the level of wine stays high. Some wineries offer a recorking service on older vintages, allowing consumers to bring in bottles of wine so that their corks can be removed and the bottle can be topped off before being resealed.
When a new bottle of wine is purchased, if there is a lot of space in the bottle, it can be a sign that the cork is in bad condition, and the wine may have spoiled. For this reason, it's a good idea to check wine in the store before purchase to assess the ullage. When buying older vintages, ullage should always be considered, and if the ullage is high, the seller should be asked if the wine has been topped off. Topping off prevents oxidation, but it can also impact the flavor of the wine and its value.