A freezing mould bag (10) comprises two sheets (12, 14) which are joined together in substantially overlapping relationship along the periphery of the sheets, defining a plurality of mould compartments (24) within the interior of the freezing mould bag. An inlet channel defined by joints (28, 30, 32) extends from the mould compartments (24) to an inlet opening (26). Two closure valve flaps (16, 18) are connected to the sheets (12, 14) at the inlet opening (26) and extend from the inlet opening within the inlet channel. The inlet channel comprises a first and a second segment, which first segment is of a configuration tapering from the inlet opening (26) towards the mould compartments, and which second segment has dimensions so as to to no substantial extent hinder liquid or water from flowing through the second segment. At the transition between the first and the second segment of the inlet channel, a constriction is defined, serving the purpose of generating a venturi effect for generating a relative pressure drop within the constriction for closing the inlet channel at the constriction as liquid or water is flowing from the second segment of the inlet channel through the constriction towards the first segment.