A method of heating, for instance sterilization or pasteurization of food packed in microwave transparent packaging material and where the heat treatment is a continuous one by transporting the product through a processing channel. Microwaves of different strength are applied for heating the product. Portions of the product are heated quickly by the microwaves following curve (10)(10'''), other portions are heated not so quickly following curve (11)(11'''). The product is allowed to pass through a strong microwave field until the portion of the product which is heated fastest reaches a certain temperature somewhat higher than the temperature desired for the heat treatment. Thereafter the product is heated further in a microwave field of such a lower power that said fast heated portion of the product maintains the said predetermined temperature, and with such a duration that the product portion which is heated slowly reaches the temperature desired for the heat treatment.
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