A method for operating a food processing apparatus is provided. The food processing apparatus includes a reaction vessel that has a space that accumulates a liquid reactant used for a food product; a catalytic reactor that includes a reaction tube and a light source; and an introducing tube for introducing the reactant into the reaction vessel. The reaction tube has an outer surface where a photocatalyst is provided. The reaction tube transmits light. The light source generates heat at a time of light emission in which the light source emits light from an inner side of the reaction tube. The method for operating the food processing apparatus includes introducing the reactant into the reaction vessel from the introducing tube. In the introducing, the reactant is introduced up to a position at which a liquid surface of the reactant is positioned higher than an opening portion of the introducing tube.