A direct contact steam injection heater with a coaxial steam nozzle has a converging/diverging combining tube. The converging/diverging combining tube eliminates the need for an adjustable combining tube sleeve to accommodate various flow rates of a liquid or slurry entering into the heater. The heater is especially well suited for sanitary applications because, without the combining tube sleeve, the heater can be made so that surfaces in contact with the liquid or slurry do not have pockets or folds in which liquid or particulates can accumulate. Contact surfaces in the heater are substantially self-cleaning because of turbulent mixing action, and the heater can be quickly disassembled and reassembled for manual cleaning.