Abstract: To alter rapidly the temperature of fluid 3 in a closed vessel 1, a nozzle 6 is disposed above the vessel 1, and an annular cross-section jet 17 of liquid at an appropriate temperature is directed at the vessel 1 so as to form a coherent film 18 covering its entire surface. Preferably the liquid, which may conveniently be water, is supplied at a high rate under a low pressure so as to preserve lamellar flow. Advantageously the annular cross-section jet 17 is conical, being directed downwardly and inwardly. The fluid 3 in the vessel 1 can be maintained at a desired temperature by continued application of the jet 17 of liquid.The invention is suitable for use with glass bottles and has application in food pasteurization.