CUPA, -ae (f) -1. barrel, tub, cask, vat; especially for holding liquids
Lewis and Short (Latin Dictionary) suggest that the barrels mentioned in Caesar (Civil War 2.11) were filled with grain. Caesar’s description is also the earliest mention of barrels in Roman literature. Several other sources indicate that the Romans also barrels for wine (among other storage/transport vessels), but is also served as an appropriate vessel for the transport of beer in the ancient period (Nelson 2001, 155). Nelson relates, however, that the first securely attested association of wooden barrels and beer is found in the 7th c. CE (Nelson 2001, 157).
Nelson, M.C. 2001. “Beer in Greco-Roman Antiquity.” Ph.D. dissertation, the University of British Columbia.