Gaseous ozone treatment of baby spinach within the existing production chain for inactivation of Escherichia coli O157:H7

• Escherichiacoli O157:H7 count reduction during vacuum cooling drops under high ozone demand.
• Sanitizer applied after vacuum cooling is less effective due to vacuum internalization.
• Vacuum cooling removes liquid from produce surfaces and increases efficiency of gas.
• Application of ozone during vacuum cooling reduces internalized bacterial counts.
• Combination of short-term and long-term ozone treatment is synergistic.

Sanitization of fresh fruits and vegetables, particularly leafy greens, is limited by penetration of sanitizers to the location of internalized pathogens. It is necessary, therefore, to adapt sanitization operations to practices in the existing produce chain. In this work, we investigated experimentally in a pilot scale, various potential sanitization options using gaseous ozone during and post vacuum cooling on the inactivation of Escherichia coli O157:H7 ATCC 43889. It was found that vacuum cooling causes bacterial internalization, making them harder to reach by sanitizer. However, the application of ozone during the vacuum cooling step significantly reduced (p < 0.05) population of viable internalized bacteria which otherwise remain unaffected by sodium hypochlorite and UV light surface treatments. The presence of free water inside the vessel available for evaporation during vacuum cooling was found to impact the microbial reduction during combined vacuum cooling and the gaseous ozone treatment. The efficacy of application of high ozone concentration (1.5 g/m3) short term during the vacuum cooling step in combination with low ozone concentration (0.032–0.528 g/m3) long term sanitization treatments (days) was evaluated. This combination of gaseous treatment was found more effective in microbial reduction compared to a single treatment with a strongly expressed synergistic effect. The post-treatment spinach quality evaluation, however showed an increasing degree of damage as the time of treatment increases even at low ozone concentrations.