Effect of ozone on Listeria innocua in bell peppers (Capsicum annuum L.) and in contaminated water

The development of innovative technologies, promoting fruits and vegetables safety to reduce the risk of related food-bourne diseases, is an actual concern. Ozone (O3) is recognized as a strong oxidant and can be applied in food surface disinfection, sanitation of food plant equipment and reuse of wastewater. Several studies demonstrated that ozone reduces the microbial load of fresh fruits and vegetables, thus improving products’ safety. The objective was to study the effect of ozone on the microbial load of red bell peppers and on contaminated water. Listeria innocua was selected as target microorganism (apart from being non-pathogenic, it is physiologically very close to L. monocytogenes, and both can be isolated in the same products). Experiments were carried out using an ozone generator interconnected to a container (30L) filled with tap water. Ozone was continuously incorporated in water and its content indirectly measured by potential difference. Artificially contaminated bell peppers, with Listeria (initial load ~ 107 CFU/mg), were washed in water (control) and in ozonated water (O3 concentration ~ 0.25 p.p.m.), for different times till a maximum of 60 minutes. Artificially contaminated water, with Listeria (initial load ~ 107 CFU/mg), was treated with ozone (O3 concentration ~ 0.25 p.p.m., continuously incorporated in the water), for different times till a maximum of 15 minutes. Results showed that, in average, ozone-washings reduced 1.6 ± 0.4 log-cycles (± standard deviation) of Listeria in bell peppers, being this process equivalent to a simple water-washing. The time of treatment did not affect the results. The effect of ozone on contaminated water was different. Listeria in water suffered an inactivation dependent on the time of treatment, similar to a thermal inactivation kinetic behaviour (initial lag phase ~ 5 minutes, followed by a maximum death rate of ~ 0.5 min-1). After 15 minutes, Listeria load decreased 5 log-cycles.