Survival of Escherichia coli O157:H7 and Listeria monocytogenes on Spinach and Blueberries Treated With Aqueous Chlorine Dioxide and Muscadine (Vitis rotundifolia) Extract

The antimicrobial effect of various concentrations of chlorine dioxide (ClO₂) and muscadine extract (ME) was investigated on Escherichia coli O157:H7 and Listeria monocytogenes under in vitro conditions and on treated spinach and blueberries. Tryptic soy broth (TSB) and produce were inoculated with a 3-strain cocktail of each pathogen. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using either 96-well plates incubated in a microplate reader for ClO₂ or test tubes incubated in a conventional incubator at 37 °C for 24 h for the ME. Pathogen reduction on inoculated produce (inoculation levels were 5.6-7.0 log CFU/g for spinach and 4.5-5.1 log CFU/g for blueberries) was evaluated after exposure to various antimicrobial treatments for 10 min + 30-45 min dry time + storage at 8 °C for up to six days. Under in vitro, L. monocytogenes was more susceptible than E. coli O157:H7 to ClO₂ and ME. On spinach, 300 mg/ml ME and 3 ppm ClO₂ + 300 mg/ml ME treatments resulted in the maximum reduction of E. coli O157:H7 (4.6 log CFU/g reduction at day 1) and L. monocytogenes (5.6 log CFU/g reduction at day 2), respectively. On blueberries, all the treatments significantly decreased E. coli O157:H7 counts (≤1.7 log CFU/g), and no differences (P > 0.05) were detected from day 1 to day 6. The treatment 3 ppm ClO₂ + 300 mg/ml ME led to nondetectable (P ≤ 0.05) L. monocytogenes levels on blueberries at days 1, 2, 4, and 6, leading to surface reductions >4.1 log CFU/g. Based on the study, ME combined with ClO₂ could be a potential candidate as postharvest treatment to reduce the presence of E. coli O157:H7 and L. monocytogenes on blueberries. Both pathogens were less susceptible to these combined treatments on spinach.