Abstract: The package is closed by a lid and it is fitted with a valve fixed over an opening zone of section greater than 10 mm2, and preferably lying in the range 30 mm2 to 50 mm2, leading to the inside of the package and formed through said lid. This opening zone is either an orifice formed through a flat lid, or else a hollow chamber itself pierced by a vent orifice of small size. The valve comprises solely a flexible sheet extending over the opening zone together with a sealing compound disposed between the flexible sheet and the lid. The sealing compound is solid at ambient temperature and it passes to a non-sticky liquid state when its temperature is raised. During heat treatment, the valve opens and closes in a succession of venting sequences.