The microbiological effect of ozone and chlorine treatments on minimally processed lettuce

Chlorine is an effective sanitizer against many foodborne microorganisms. However, it may cause the formation of carcinogenic trihalomethane compounds. Ozone is an effective disinfectant having greater oxidation potential than chlorine. Limited studies have been done to determine the optimum concentration and contact time for ozone and if there is a synergistic interaction with chlorine when treating minimally processed produce. Our objective was to determine the sanitizing efficacy of ozone and chlorine, alone or in combination on microbial reduction on fresh-cut lettuce and to develop data that was of used for the ready-to-eat salad industry based on the sensory characteristics and shelf-life of these products. Iceberg lettuce was cut into 2 by 5 cm strips and inoculated with log 8 CFU/g of a mixture of natural microflora strains isolated from cut lettuce stored at 10°C. 100 g samples were treated with 1 L distilled water solutions containing combinations of 0, 100, 150 or 200 ppm chlorine and 0, 2.5, 5.0 or 7.5 ppm ozone for a total of 16 treatments. Lettuce-water solutions were stirred constantly for 10 min and then lettuce was sampled for Aerobic Plate Counts (APC) and Psychrotrophic Plate Counts (PPC), four repetitions were used in this study. Commercially processed salads treated with chlorine, ozone or an ozone-chlorine mixture were evaluated for shelf-life using visual inspection by an untrained panel (n=30). Water samples were also analyzed for UV-Vis and total solids to determine the effect of the treatments in the processing water. Lettuce treated with only chlorine had reductions in APC up to 1.38 Logs and 1.71 Logs for PYS. Samples treated with ozone decreased in APC up to 1.12 Logs and 2.00 Logs for PPC. Lettuce treated with the combinations of chlorine and ozone had the greatest reduction in APC up to 2.5 Logs and 1.91 Logs for PPC. Sensory evaluation showed that the commercially processed samples treated with chlorine were least desirable having the shortest shelf-life with product decay after 16 days of treatment. Samples treated with ozone alone had a shelf-life of at least 20 days. Lettuce treated with the combination had the longest shelf-life retaining good visual sensory characteristics until at least 25 days after treatment. Results suggest that washing fresh-cut salads with an ozone-chlorine sanitation treatment can improve and extend the shelf-life of these products compared to either ozone or chlorine solutions individually