PURPOSE: To carry out a treatment such as thermal sterilization, disinfection, inactivation of enzyme, etc., of a fluid containing proteins and starches such as liquid egg, raw material of a fish paste product, etc., without causing coagulation of the fluid, by heating the fluid with a microwave heater for a short time, keeping at the temperature over a specific interval and quickly cooling the product.

CONSTITUTION: A thermally coagulating fluid is quickly heated at a temperature above a level to initiate the coagulation of the fluid by an apparatus composed of a preheating apparatus 1, a microwave heater 2, a retaining apparatus 3 to retain the fluid at a coagulation initiation temperature in a state before or immediately before the start of coagulation, a vacuum vessel apparatus 10 to rapidly decrease the fluid temperature by the heat of evaporation and a cooling apparatus 7. The above apparatuses are connected with each other via pipes 11.