A centrifugation/quadrant plate technique for the simplified differential-bacteriological examination of adequately heat-processed foods

Reduction of listeria monocytogenes on green peppers (Capsicum annuum L.) by gaseous and aqueous chlorine dioxide and water washing and its growth at 7 degrees C

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 degrees C was studied. Growth of the L. monocytogenes untreated or treated with 0.6 mg/liter ClO2 gas for 30 min at 20 degrees 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 degrees C, bacteria that survived the 0.6-mg/liter ClO2 gas treatment also could grow.

Response surface modeling for the inactivation of Escherichia coli O157:H7 on green peppers (Capsicum annuum L.) by chlorine dioxide gas treatments

The effects of chlorine dioxide (ClO2) gas concentration (0.1 to 0.5 mg/liter), relative humidity (RH) (55 to 95%), treatment time (7 to 135 min), and temperature (5 to 25 degrees C) on inactivation of Escherichia coli O157:H7 on green peppers were studied using response surface methods. A four-factor, central, composite, rotatable design was used. The microbial log reduction was measured as a response. A direct membrane-surface-plating method with tryptic soy agar and sorbitol MacConkey agar was used to resuscitate and enumerate ClO2-treated E. coli O157:H7 cells. The statistical analysis and the predictive model developed in this study suggest that ClO2 gas concentration, treatment time, RH, and temperature all significantly (P < 0.01) increased the inactivation of E. coli O157:H7. ClO2 gas concentration was the most important factor, whereas temperature was the least significant. The interaction between ClO2 gas concentration and RH indicated a synergistic effect. The predictive model was validated, and it could be used to determine effective ClO2 gas treatments to achieve a 5-log reduction of E. coli O157:H7 on green peppers.