The invention relates to a method and a device for cooking and vacuum packing mussels with their shells and other foodstuffs by microwave heating so that they can be stored as chilled or frozen goods for an extended period. When mussels are vacuum packed, many of the shells often break, and moreover the subsequent cooking in water is too slow for the mussel meat to coagulate satisfactorily. The invention solves this problem by packing the product to be preserved, in the first place mussels, in a somewhat flexible plastic container which is sealed. On the top side of the container there is a venting hole which permits the escape of steam during boiling but which is sealed immediately as microwave heating ceases, whereupon a vacuum arises inside the container as the steam condenses. A vacuum packed and pasteurised product is thus obtained in one and the same operation.