The present invention relates to a method for the production of a self-venting packaging for use in heating a food item in a microwave oven, said packaging comprising a rupturable valve seal (3), said rupturable valve seal comprising an inside-inside seal, creating a weak point on said packaging, suitable to break in case of overpressure inside the packaging, said method comprising the following subsequent steps: providing an open plastic packaging; filling said packaging with a food item; —sealing the plastic packaging to obtain a closed pack via a closing seal with the food item therein ready to be sterilised; heat treating said food item; providing a rupturable valve seal (3) on the pack by ultrasound seal technology.