An apparatus is provided for the uniform and rapid heating of foodstuffs, usually for pasteurization and sterilization purposes, by means of microwave energy. The cross-sectional shape of the outlet of the wave guides through which the microwave energy is supplied is selected so as to be geometrically similar to that of the product to be treated. A spacing is maintained between the outlet aperture and the surface of the product, which spacing is not greater than the wavelength of the microwave energy be used. These features provide rapid and appropriate processing of a food product while, at the same time, preventing the harmful overheating of the edge regions of the product.