The influence of ultrasound and conventional heating under different processing conditions on the inactivation and potential subsequent growth of micro-organisms in orange juice was investigated. Although a limited level of microbial inactivation (<= 1.08 log CFU ml(-1)) was obtained by selected batch ultrasonic treatment: 500 kHz/240 W for 15 min, microbial growth was observed in the substrate following 14 days of storage at both refrigeration (5 degrees C) and mild abusive (12 degrees C) temperatures. The presence of pulp in the juice increased the resistance of micro-organisms to ultrasound. After continuous ultrasonic treatments at flow rates of 3000 L h(-1) negligible reductions of microbial counts were obtained. No ultrasound-related detrimental effects on the quality attributes of juice (limonin content, brown pigments and colour) were found. Therefore, to prevent the development of food-borne pathogens in orange juice it will be necessary to combine ultrasound with other processing methods with greater antimicrobial potency, as well as to achieve a very low initial concentration of bacteria, yeast and moulds in the juice. Such combinations will require further exploration of important synergistic effects that are relevant for industrial use. In this regard, the use of ultrasound in combination with mild heating for industrial use is discussed. (c) 2006 Elsevier Ltd. All rights reserved.