Insights Into Oxygen-Plasma-Bubble Inactivation of Escherichia coli in Apple Juice

An ‘in-bottle’ antimicrobial processing approach for apple juice is demonstrated using oxygen plasma bubbles. The effect of oxygen plasma chemistry, interfaced via gas bubbles and induced electric field, is reported for Escherichia coli ATCC 35218 inactivation. The operating parameters of discharge frequency, gas flow rate and voltage were investigated in situ for filled juice bottles. Apple juice, with various pH, were inoculated with E. coli and exposed to plasma for different times to access the impact of pH on inactivation efficacy. The results showed that as the pH of the juice decreased, the inactivation efficacy increased, achieving a reduction of over 5 logs. Longer treatment times corresponded to enhanced inactivation efficacy under consistent pH conditions. Use of reactive specie scavengers underscored the significant role of the superoxide anion (O₂⁻) radical for the observed E. coli inactivation. Additionally, intracellular reactive oxygen species (ROS) accumulation levels and scanning electron microscopy (SEM) analysis demonstrated that the plasma effects disrupted the intracellular redox homeostasis and significantly damaged the cell membrane integrity of E. coli, with low pH acting synergistically with plasma treatment. The electrical simulation results supported the important role of increased conductivity, due to juice acidity, which is found to enhance the local current density at the edge of the bacterial cell which has an important contribution to cell death in addition to the pivotal role of ROS.