Reduction of Listeria monocytogenes on Green Peppers (Capsicum annuum L.) by Gaseous and Aqueous Chlorine Dioxide and Water Washing and Its Growth at 7°C

Abstract

Reduction of Listeria monocytogenes Scott A on uninjured and injured surfaces of green peppers after 0.3- and 3-mg/liter gaseous and aqueous ClO2 treatment and water washing for 10 min at 20°C was studied. Growth of the L. monocytogenes untreated or treated with 0.6 mg/liter ClO2 gas for 30 min at 20°C on green peppers also was investigated. A membrane-surface-plating method was used for resuscitation and enumeration of L. monocytogenes treated with ClO2. The bacterial viability on pepper surfaces was visualized using confocal laser scanning microscopy (CLSM). Live and dead cells of L. monocytogenes were labeled with a fluorescein isothiocyanate-labeled antibody and propidium iodide, respectively. More than 6 log CFU/5 g L. monocytogenes on uninjured surfaces and about 3.5 log CFU/5 g on injured surfaces were inactivated by both 3-mg/liter and 0.6-mg/liter ClO2 gas treatments. The 3-mg/liter aqueous ClO2 treatment achieved 3.7- and 0.4-log reductions on uninjured and injured surfaces, respectively; whereas, water washing alone showed 1.4- and 0.4-log reductions, respectively. ClO2 gas treatment was the most effective in reducing L. monocytogenes on both uninjured and injured green pepper surfaces, when compared with aqueous ClO2 treatment and water washing. The significant difference (P < 0.05) between log reductions on uninjured and injured surfaces and the results from CLSM analysis suggested that injured surfaces protected more bacteria from sanitation treatments than did uninjured surfaces. Not only could L. monocytogenes grow on green pepper surfaces at 7°C, bacteria that survived the 0.6-mg/liter ClO2 gas treatment also could grow.