A method for heating and pasteurizing a material, comprising placing the material inside of a semi-sealed chamber at atmospheric pressure that contains an atmosphere produced by replacing air in the chamber with a gas component produced by spraying or injecting hot water droplets and steam at a temperature of at least 100° C. into the chamber which is heated to a temperature of at least 100° C. which is equal to or higher than the temperature of the sprayed hot water droplets and steam, which gas component has a humidity of at least 95% and contains no more than 1% oxygen; contacting said material to the gas component for exposing it to continual temperature variation during heating ranging from 10-50° C. inside the chamber at temperature(s) ranging from 90-180° C. for a time sufficient to heat or pasteurize it. An apparatus for performing this method.