Carboy

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A 5-gallon glass carboy acting as a fermentation vessel for beer. It is fitted with a fermentation lock.

A carboy is a glass or plastic vessel that is often used in fermenting beverages such as wine, mead, and beer. Usually it is fitted with a rubber stopper and a fermentation lock to prevent bacteria from entering during the fermentation process.

During the homebrewing process, a primary carboy is used for fermentation. Once primary fermentation is complete, the beer is either transferred to a secondary carboy for conditioning or it can be transferred directly to bottles for conditioning. (This process of transferring is usually called racking.)

Polypropylene carboys are also commonly used in laboratories to transfer purified water. They are typically filled at the top and have a spigot at the bottom for dispensing.

The word carboy is from the Persian qarabah, from Arabic qarraba, big jug.

Carboys come in various volumes ranging from 1 gallon to 6.5 gallons (4 to 25 litres). A 1 gallon carboy is usually called a jug. A 15 gallon carboy is often called a demijohn. "Demijohn" is an old word that formerly referred to any glass vessel with a large body and small neck, enclosed in wickerwork. The word is said to derive from the name of a Persian town, Damaghan, but this is not supported by any historical evidence. According to OED the word comes from French dame-jeanne, literarily "Lady Jane", as a popular appellation. This is in accordance with the historical evidence at present known, since the word occurred initially in French in the 17th c., and no earlier trace of it has been found elsewhere.

In Britain, demijohn refers to a 1-gallon glass brewing vessel.

In Southeast U.S. slang, a demijohn jug, of any size, is referered to as a jimmyjohn.