Food containing water is improved. A fryer controls water which is an ingredient of food as an object placed between a pair of opposed plate antennas, to be more specific, the water activity of the object, by generating electromagnetic waves between the pair of opposed plate antennas. Then, the fryer applies a DC electric field of +100 V as an offset electric field on the water phase side relative to the oil phase in an AC electric field to irradiate food with electromagnetic waves at a frequency of 50 kHz. Thus, the interfacial polarization between the water phase and the oil phase can be increased to decrease the interfacial tension between the water phase and the oil phase by approximately 60 percent and bind water in a pearl-chain structure, thus decreasing the water activity.

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