The invention is to provide a process for producing a liquid food, characterized by subjecting at least one of coffee, a milk composition, a food material (liquid) containing a component(s) having an antioxidation ability (radical-scavenging activity) and mixed liquids containing one(s) of them to an electrolysis treatment and/or a current passage treatment. Since the invention can prevent quality deterioration of coffee or milk composition over a long period of time without using additives and also improve safety, it is appropriate for producing canned coffee, milk-containing coffee or milk-containing tea drink to be sold in vending machines or can warmers. Further, since the invention can improve the antioxidation ability of foods, the resulting foods can suppress the in-vivo increase of active oxygen and free radicals, it greatly contributes to health.