Reduction of Listeria monocytogenes on green peppers (Capsicum annuum L.) by gaseous and aqueous chlorine dioxide and water washing and its growth at7 degrees C

Reduction of Listeria monocytogenes Scott A on uninjured and injured surfac es of green peppers after 0.3- and 3-mg/ liter gaseous and aqueous ClO2 tre atment and water washing for 10 min at 20 degreesC was studied. Growth of t he L monocytogenes untreated or treated with 0.6 mg/liter ClO2 gas for 30 m in at 20 degreesC on green peppers also was investigated. A membrane-surfac e-plating method was used for resuscitation and enumeration of L. monocytog enes treated with ClO2. The bacterial viability on pepper surfaces was visu alized 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 mo nocytogenes on uninjured surfaces and about 3.5 log CFU/5 g on injured surf aces were inactivated by both 3-mg/liter and 0.6-mg/liter ClO2 gas treatmen ts. The 3-mg/liter aqueous ClO2 treatment achieved 3.7- and 0.4-log reducti ons 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 an d water washing. The significant difference (P < 0.05) between log reductio ns on uninjured and injured surfaces and the results from CLSM analysis sug gested that injured surfaces protected more bacteria from sanitation treatm ents than did uninjured surfaces. Not only could L. monocytogenes grow on g reen pepper surfaces at 7 degreesC, bacteria that survived the 0.6-mg/liter ClO2 gas treatment also could grow.