Brewing [edit]. In brewing, a carboy or demijohn is a glass or plastic vessel used in fermenting beverages such as wine, mead, cider, perry, and beer. Usually it is fitted with a rubber stopper and a fermentation lock to prevent bacteria and oxygen from entering during the fermentation process. 4.5 L (1.2 US gal) carboy is usually called a jug. A 57 L (15 US gal) carboy is usually called a demijohn (in the Philippines, dama juana. ). Demijohn is an old word that formerly referred to any glass vessel with a large body and small neck, enclosed in wickerwork.