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  • 1 Horticultural Crops Quality Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705

The population of Listeria monocytogenes inoculated on lightly processed spinach held at 5C or 10C remained constant for 9 days, whereas the populations of mesophilic aerobic flora, pseudomonas, and enterobacteria increased sharply. We studied the cause of the bacteriostatic activity of spinach leaves against L. monocytogenes. Listeria monocytogenes were inoculated at the concentration of 2 × 104 cells/ml in raw, autoclaved and vacuum filtration sterilized spinach macerates as well as in tryptic soy broth used as control. The concentrations after 24 hat 28C were 2 × 104, 4 × 107, and 4 × 105 cells/ml in respective spinach macerates and 4 × 108 cells/ml in tryptic soy broth. The anti-listeria activity was still present in spinach macerates sterilized by vacuum filtration but not in autoclaved macerates. In conclusion, the bacteriostatic activity against L. monocytogenes might be due to antimicrobial compounds present in spinach leaves or produced by the other microorganisms. Studies are being continued to identify the compounds involved in the anti-listeria activity.

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