Purpose: In this study, we investigated the efficacy of gaseous ozone during vacuum cooling and subsequent application of bacteriophage for inactivation of E. coli O157:H7 on fresh produce.
Methods: Spinach leaves were spot-inoculated with low or high concentrations of E. coli O157:H7 (5.3 or 6.6 log CFU/g, respectively). Inoculated leaves were subjected to gaseous ozone during vacuum cooling as follows: initially 28.5 in. Hg vacuum was applied, followed by gaseous ozone treatment at 1.5 g ozone/kg ozone-oxygen gas mix, at 10 psig vessel pressure, and 30 min holding time. Ozone-treated leaves were sprayed with E. coli phage at 8.3 log PFU/g, then held for five minutes.
Results: At 5.3 log CFU/g inoculum level, gaseous ozone alone significantly (P < 0.05) reduced E. coli population by 3.1 log CFU/g when compared to inoculated untreated control. Sequential treatment of ozone and bacteriophage significantly (P < 0.05) reduced E. coli populations below the detection limit of the enumeration method (i.e., < 1 log CFU/g). At 6.6 log CFU/g, gaseous ozone and sequential treatments significantly (P < 0.05) reduced E. coli counts by 1.8 and 5.2 log CFU/g, respectively.
Significance: Regardless of inoculum size, sequential treatments considerably decreased the risk of contamination of fresh produce with E. coli O157:H7. Considering the realistic low contamination levels of fresh produce and low infective dose of E. coli, sequential application of gaseous ozone and bacteriophage seems a promising post-harvest disinfection method to secure the safety of produce.