Abstract: The present invention concerns a kit of parts comprising a cooking bag for storing food to be cooked in a microwave oven, the cooking bag having a cooking bag opening, and a pressure release closure device, configured for restricting the cooking bag opening when attaching the pressure release closure device at the exterior of the cooking bag thereby contracting the cooking bag opening so as to establish a steam conduit whereby gas pressure in the cooking bag may be released from the cooking bag during cooking in a microwave oven. Further the present invention relates to a pressure release closure device and a method of preparing food using a pressure release closure device.