Assessing the Mechanism of Microbial Inactivation during Ozone Pocessing

Ozone has numerous applications in food industry because of its advantages over traditional preservation techniques. Damage to cell membranes and cytoplasmic contents was proposed as involved in ozone inactivation but there is no available information concerning oxidative stress effect of ozone on regulated knockout genes and the protection or sensitivity of microbial mutants (lacking these genes) against ozone or ozone generated radicals. The aim of this work was to investigate the mechanism of action of ozone on microbial populations during the treatment of liquid food systems. E. coli BW25113 and its isogenic mutants in soxR, soxS, oxyR, rpoS, dnaK genes were treated with ozone at a previously optimized concentration of 6µg/mL for a period up to 4 min in a 100 mL bubble column. Ozone gas was generated using a corona discharge ozone generator. Oxygen was supplied via air cylinder and the flow rate (0.06 L/min) was controlled using a flow regulator. A significant effect of ozone exposure on microbial inactivation was observed while the cell membrane integrity and permeability were affected by ozonation. Scanning Electron Microscopy (SEM) analysis showed slightly altered cell surface structure. The results of this study suggest that cell lysis was not the primary mechanism of microbial inactivation. The absence of oxidative stress-related genes resulted in increased susceptibility of E. coli cells to ozone treatment suggesting that they play an important role for protection against the radicals produced by ozone. However, DnaK which has previously been shown to protect against oxidative stress did not protect against ozone treatment. Furthermore, RpoS was important for survival against ozone through an unidentified mechanism. This preliminary study provides important information about the mechanism through which ozonation acts against microorganisms for the production of safe liquid food products.