Design of a batch ultrasonic reactor for rapid pasteurization of juices

Juices are usually thermally pasteurized, but high temperatures negatively affect their nutritional and sensory qualities. For this reason, nonthermal pasteurization techniques with little or no heat applied have been developed to minimize the quality losses. Among them, high-intensity ultrasound has been considered as a promising technique for juice pasteurization. Despite the extensive studies, a long pasteurization time is needed to achieve the required 5-log microbial reduction when a liquid is ultrasonically processed in an existing typical ultrasonic reactor, while inactivating some bacteria with a high tolerance to ultrasound is still a challenge. This is because there exist passive zones in the reactor as revealed by numerical simulations using COMSOL Multiphysics. This problem was successfully resolved by developing a new batch ultrasonic reactor which delivered almost uniform high intensity ultrasound throughout the reactor during processing, and therefore, substantially shortened the pasteurization time. Using this reactor to inactivate Escherichia colt at 60 degrees C and Staphylococcus aureus at 62 degrees C in peptone water, a 5-log microbial reduction was achieved in 0.38 min and 0.55 min, respectively. This was the first reported study that reached a 5-log reduction of S. aureus using ultrasonic processing. SEM images illustrated that high intensity ultrasound imposed physical lethal damages to the cell structure, resulting in microbial inactivation. The substantially shortened pasteurization time can reduce processing costs and negative effects of ultrasound treatment on organoleptic and nutritional properties of juices.