A flexible heat exchange jacket is provided which has channels for flow of a heat exchange fluid along one side, with inlets and outlets attached to a source of heat exchange fluid. The jacket can be attached in a watertight manner around the circumference of a cylindrical process container containing a liquid for heat treatment. Preferred embodiments include devices for heating and/or cooling the heat exchange fluid prior to entering the jacket, mixers for the liquid under treatment within the container, and heaters for the liquid within the container and/or the bottom of the container itself. A dairy pasteurizer version combines a cylindrical process container with a heat exchange jacket installed around its exterior with heating and refrigeration units for the heat exchange fluid, heat sensing and mixing devices, and a control system programmed to execute a pasteurization cycle.