Synergy of oxygen plasma bubbles with ultrasonic waves for Escherichia coli inactivation

This study evaluated the individual and combined effects of oxygen plasma bubbles and ultrasound on the inactivation of Escherichia coli ATCC 35218 suspended in clarified amber apple juice. The results revealed that ultrasound alone had a minimal bactericidal effect against 7-log E. coli contaminated juice over 20 min while the plasma treatment resulted in 5.2 log cycles of bacteria reduction in 8 min. The synergistic impact of ultrasound (US) was found to be pronounced when used in combination with plasma with 5-log reductions observed in 4 min. Reactive species scavenger tests as well as the bubble images revealed that O₂·^‐ played a pivotal role in this system while the bubble coalescence induced by US might also contribute to the synergistic bactericidal effect. The physiological changes of E. coli including cell membrane integrity, scanning electron microscopy (SEM), intracellular reactive oxygen species (ROS) level and cell content leakage ratio showed that the combination treatment significantly compromised the E. coli cell membrane, resulting in the disruption of intracellular redox homeostasis and leakage of cellular contents compared to the US treatment and the plasma treatment. The hypothesis was proposed that ultrasound leads to sonoporation of the cell membrane enabling more effective transport of ROS into the cell and the observed rapid cell death. Apart from reducing the treatment time to achieve the target 5-log reduction, the synergistic effect of US with CAP facilitated the retention of quality factors including color which is known to be affected under oxidizing environments.