Purpose: This study assessed the impact of post-inoculation hold time on antimicrobial efficacy of ClO2, against the two bacterial pathogens most often responsible for fresh produce-related outbreaks.
Methods: Three batches of fresh-cut Romaine lettuce and cherry tomatoes were, respectively, dip-inoculated with 3- and 2-strain cocktails of Escherichia coli O157:H7 (K3995, K4492, K4830) and Salmonella spp. (S. Montevideo MDD22, S. Newport MD313) to contain ~7.90 log CFU/g and then held for 1 and 24 h before gassing. Both products were exposed to ClO2 gas generated from a MiniDox-M (ClorDiSys, Lebanon, NJ) at 3, 6 or 10 mg/l for up to 30 min. Thereafter, E. coli O157:H7 and Salmonella populations were, respectively, quantified in 25-g samples by direct plating appropriately diluted rinse solutions on trypticase soy agar (TSA) overlaid with sorbitol MacConkey agar supplemented with cefixime and potassium tellurite and TSA overlaid with xylose lysine deoxycholate agar.
Results: Holding inoculated lettuce for 1 and 24 h before treatment yielded similar E. coli O157:H7 reductions of 1.53 ± 0.08 and 1.48 ± 0.14 log CFU/g after 30 min of exposure, respectively. However, ClO2 gassing was significantly less efficacious (P < 0.05) for tomatoes at the longer post-inoculation hold time with Salmonella populations on 1- and 24 h- held tomatoes decreasing up to 7.19 ± 0.00 and 5.81 ± 0.13 log CFU/g, respectively. As expected, a significant interaction (P < 0.0001) was seen between ClO2 concentration and treatment time (concentration*time) for both lettuce and tomatoes.
Significance: When evaluating the efficacy of ClO2 for field-grown tomatoes, post-inoculation hold times of at least 24 h should be considered so as to not overestimate the effectiveness of ClO2 against foodborne pathogens.