Fate of Listeria monocytogenes in Commercial Ham, Formulated with or without Antimicrobials, under Conditions Simulating Contamination in the Processing or Retail Environment and during Home Storage

Abstract

Commercial cured ham formulated with or without potassium lactate and sodium diacetate was inoculated with Listeria monocytogenes and stored to simulate conditions of processing, retail, and home storage. The ham was sliced, inoculated with a 10-strain composite of L. monocytogenes (1 to 2 log CFU/cm2), vacuum packaged, and stored at 4°C to simulate contamination following lethality treatment at processing (first shelf life). After 10, 20, 35, and 60 days of storage, packages were opened, samples were tested, and bags with remaining slices were reclosed with rubber bands. At the same times, portions of original product (stored at 4°C in original processing bags) were sliced, inoculated, and packaged in delicatessen bags to simulate contamination during slicing at retail (second shelf life). Aerobic storage of both sets of packages at 7°C for 12 days was used to reflect domestic storage conditions (home storage). L. monocytogenes populations were lower (P < 0.05) during storage in ham formulated with lactate-diacetate than in product without antimicrobials under both contamination scenarios. Inoculation of ham without lactate-diacetate allowed prolific growth of L. monocytogenes in vacuum packages during the first shelf life and was the worst case contamination scenario with respect to pathogen numbers encountered during home storage. Under the second shelf life contamination scenario, mean growth rates of the organism during home storage ranged from 0.32 to 0.45 and from 0.18 to 0.25 log CFU/cm2/day for ham without and with lactate-diacetate, respectively, and significant increases in pathogen numbers (P < 0.05) were generally observed after 4 and 8 days of storage, respectively. Regardless of contamination scenario, 12-day home storage of product without lactate-diacetate resulted in similar pathogen populations (6.0 to 6.9 log CFU/cm2)(P ≥ 0.05). In ham containing lactate-diacetate, similar counts were found during the home storage experiment under both contamination scenarios, and only in 60-day-old product did samples from the first shelf life have higher (P < 0.05) pathogen numbers than those found in samples from the second shelf life. These results should be useful in risk assessments and for the establishment of “sell by” and “consume by” date labels for refrigerated ready-to-eat meat products.