Abstract: The present disclosure concerns apparatus for pasteurizing and/or sterilizing foodstuffs. In one representative embodiment, an apparatus for pasteurizing or sterilizing a packaged foodstuff includes at least one cavity in which the foodstuff to be pasteurized or sterilized is positioned. The cavity is configured such that, as microwave energy radiates into the cavity, the cavity operates as a single-mode cavity for sterilizing or pasteurizing the foodstuff. The present disclosure also concerns methods for optimizing a microwave system for sterilizing or pasteurizing a foodstuff.