Continuous-flow ultrasonic washing system for fresh produce surface decontamination

A pilot-scale continuous-flow washing system with three pairs of ultrasonic transducers operating at 25, 40, and 75 kHz was designed and fabricated, and used to investigate the efficacy of ultrasound treatment for produce sanitization. A uniform ultrasound distribution in the channel was achieved, as shown by pitting on aluminum foil and log reduction of Escherichia coli O157:H7 population on spinach held at different locations in the channel. The inactivation normalized by acoustic power density for one-minute treatments at 25, 40, and 75 kHz was 0.056, 0.061, and 0.057 log CFU/(W/L), respectively. Blockage reduces the exposure of screened leaves to ultrasound, and results in significantly lower microbial count reduction. Compared to treatment with chlorine alone, combined treatment with chlorine and ultrasound in the continuous-flow system achieved additional log reductions of 1.0 and 0.5 CFU/g for E. coli cells inoculated on spinach, for washing in single-leaf and batch-leaf modes, respectively. Industrial relevance: Continuous-flow ultrasonic washing of fresh produce has the potential to provide the produce industry with a means to significantly enhance microbial safety. However, care must be taken to minimize the screening/blockage of ultrasound by produce leaves, minimize the variance in the residence-time distribution, and assure a near-uniform acoustic field distribution in the washing facility.