Five-tube most-probable-number method using the Fung-Yu tube for enumeration of Listeria monocytogenes in restructured meat products during refrigerated storage.

The survival of Listeria monocytogenes in cooked, chopped hams stored at 5 degrees C for 5 weeks was studied. Slices of chopped ham (25 g) were inoculated with a three-strain mixture of L. monocytogenes (LM 101M, LM 103M, and Scott A) at levels of < or = 350 cfu/25 g and packaged in Stomacher bags. Prior to inoculation, L. monocytogenes cells were subjected to either heat-injury (56 degrees C, 30 min) or freeze-injury (-18 degrees C, 14 days). The organisms were detected by a five-tube most-probable-number (MPN) technique using the motility enrichment Fung-Yu tube, and the direct plating method. After storage at 5 degrees C for 1 week, there was a one log10 reduction in counts. Thereafter, Listeria recovered and heat- and freeze-injured cells grew to 10(7) and 10(8) within 5 weeks, respectively. Similar results were obtained from both methods. However, direct plating could not recover L. monocytogenes at low levels (< or = 100/25 g), whereas MPN counts were obtained at these low levels. The pH (6.22 +/- 0.10) of the chopped hams remained constant throughout the study. These results indicated that low numbers of L. monocytogenes surviving sublethal heat- or freeze-injury could initiate growth after recovery in chopped hams. The five-tube MPN method using the Fung-Yu tube was effective in enumerating both low and high levels of L. monocytogenes in food.