A vessel cap (100, 10) for exchanging gas in and pressuring a vessel headspace (30), the cap (100, 10) comprising a cap inlet (11), a seal (12) arranged to form a gas-tight seal on a vessel opening, a pressure reducing valve, a gas inlet port (25) arranged to allow incoming gas into the vessel headspace (30), a gas outlet port (31) arranged to allow outgoing gas to escape from the vessel headspace (30).