Purpose: In response to this interest, ozone was evaluated during simulated iceberg lettuce processing for its effect on surface-adhered and planktonic bacteria, including naturally-occurring bacteria on head lettuce and inoculated human pathogens.
Methods: 22 heads of cored iceberg lettuce were submerged in 75 l of tap water and covered with 5400 g of ice, after which ozone was introduced with a recirculating injection system. After a 20 min rinse and hydration interval, 2 heads of lettuce were removed every 2 min, with inner and outer leaves immediately neutralized for microbial analysis. Dissolved ozone was measured every 10 min (Hach AccuVac #73), and lettuce (20, 26, 32, and 38 min of washing) and water samples (collected every 10 min) were quantified for total heterotrophs and coliforms. The same procedures were followed using lettuce inoculated with ~4.5 log CFU/g of E. coli O157:H7, L. innocua, and Salmonella Typhimurium. In both experiments, wash water parameters were measured every 10 min.
Results: Wash water containing ozone resulted in significantly lower (P < 0.05) populations of total heterotrophs and coliforms after 30 min (0.2 log CFU/ml and 0.6 log MPN/100 ml) when compared to the water control (2.5 log CFU/ml and 2.9 log MPN/100 ml), although pathogen populations did not differ (P > 0.05). Ozone doses applied did not have a significant effect on surface-adhered bacteria. Of the parameters studied, ORP was the most affected by continual ozone injection, with significantly higher (P < 0.05) values 10 through 40 min after ozone generation began.
Significance: The use of ozone can help alleviate bacterial build-up in the water and potentially aid in preventing cross-contamination, fulfilling goals for preventative controls in small-scale food preparation.