A batch gasification assembly comprises a pressure vessel having an open end and being configured to contain a beverage to be carbonated and a flexible seal that covers the open end of the pressure vessel. A closure mechanism for the pressure vessel comprises a retainer configured to receive and retain the open end of the pressure vessel, a top cap, and an over-center clamping mechanism that is configured to clamp the top cap onto the open end of the pressure vessel so that the flexible seal member is sandwiched between the top cap and the open end of the pressure vessel.