IONIZING RADIATION SENSITIVITY OF LISTERIA MONOCYTOGENES AND L. INNOCUA INOCULATED ON ENDIVE (CICHORIUM ENDIVA)

Authors: Niemira, Brendan; Fan, Xuetong; Sokorai, Kimberly; Sommers, Christopher

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2003
Publication Date: 1/6/2003
Citation: NIEMIRA, B.A., FAN, X., SOKORAI, K.J., SOMMERS, C.H. IONIZING RADIATION SENSITIVITY OF LISTERIA MONOCYTOGENES AND L. INNOCUA INOCULATED ON ENDIVE (CICHORIUM ENDIVA). JOURNAL OF FOOD PROTECTION. 2003. V. 66 (6). P. 993-998.

Interpretive Summary: Leafy green salad vegetables, such as endive, can be contaminated with bacteria that cause serious illness in humans. Ionizing radiation can inactivate these pathogenic bacteria, making the food product safer to eat. To determine how well this process works with endive, leaf pieces and leaf homogenates of endive were inoculated with one of two bacteria, either the pathogenic bacterium Listeria monocytogenes or Listeria innocua, a non-pathogenic surrogate bacterium. The radiation sensitivity of the two strains were found to be similar, although L. innocua was more sensitive to the type of suspending leaf preparation. During refrigerated storage following irradiation, the population of L. monocytogenes on inoculated endive was briefly suppressed by a very low radiation dose calibrated to achieve a 99 percent reduction. However, the pathogen regrew after 5 days until it exceeded the bacterial levels on the untreated control after 19 days in storage. Treatment with a higher radiation dose, equivalent to 99.99 percent reduction, suppressed L. monocytogenes throughout the course of refrigerated storage. Doses up to the 99.99 percent kill level had no significant effect on the color or texture of endive leaves. These results show that endive leaves may be treated with doses sufficient to achieve at least a 99.99 percent reduction of L. monocytogenes with little or no impact on product quality. This information will assist food processors in eliminating harmful bacteria from fresh cut leafy green vegetables while preserving desirable sensory properties, resulting in a safer, healthier food supply for consumers.

Technical Abstract: Ionizing radiation inactivates the pathogenic bacteria that can contaminate leafy green vegetables. Leaf pieces and leaf homogenate of endive (Cichorium endiva) were inoculated with the pathogen Listeria monocytogenes or Listeria innocua, a non-pathogenic surrogate bacterium. The radiation sensitivity of the two strains were found to be similar, although L. innocua was more sensitive to the type of suspending leaf preparation. During refrigerated storage following irradiation, the population of L. monocytogenes on inoculated endive was briefly suppressed by 0.42 kGy, a dose calibrated to achieve a 99% reduction. However, the pathogen regrew after 5 days until it exceeded the bacterial levels on the control after 19 days in storage. Treatment with 0.84 kGy, equivalent to 99.99% reduction, suppressed L. monocytogenes throughout the course of refrigerated storage. Doses up to 1.0 kGy had no significant effect on color of endive leaf material, wether taken from the leaf edge or the leaf midrib. The texture of leaf edge material was unaffected by doses up to 1.0 kGy, while the maximum dose tolerated by leaf midrib material was 0.8 kGy. These results show that endive leaves may be treated with doses sufficient to achieve at least a 99.99% reduction of L. monocytogenes with little or no impact on the product's texture or color.