Effect of inoculum preparation procedure, and storage time and temperature on the fate of Listeria monocytogenes on inoculated salami.

Abstract

Although dry/semidry fermented sausages are characterized as being of low-to-moderate risk for human listeriosis on a per-serving and per-annum basis, data are lacking relative to the fate of postprocessing Listeria monocytogenes contamination during storage of such products. This study evaluated the effect of inoculum preparation and storage conditions on the fate of L. monocytogenes on vacuum-packaged salami. Commercially produced salami was sliced and inoculated (4 ± 1.3 log CFU/cm2) with one of four types of inocula. All inocula consisted of the same 10-strain L. monocytogenes composite, cultivated as individual strains prior to mixing for inoculation. Active cultures of individual strains were prepared (30°C, 24 h) in either tryptic soy broth (containing 0.25% glucose) plus 0.6% yeast extract (TSBYE), tryptic soy broth without glucose plus 0.6% yeast extract (TSBYE-G), TSBYE-G plus 1% glucose (TSBYE+G), or in TSBYE, and then habituated (7°C, 72 h) in sterile salami homogenate (10% [wt/wt] with distilled water). Inoculated salami slices were vacuum packaged, stored at 4, 12, or 25°C, and analyzed (three samples per treatment in each of two replicates) periodically for surviving bacterial counts. In general, pathogen levels decreased during storage and reached levels below the detection limit (-0.4 log CFU/cm2) between 27 and 90 days of storage, depending on temperature of storage and inoculum type. Death rates (log CFU/cm2/day) were found to increase as storage temperature increased, with the exception of the acid-adapted (TSBYE+G) cells, which decreased more rapidly at 4°C than at 12 or 25°C. The habituated inoculum was inactivated at a faster rate than other inocula at 12 and 25°C, but performed similarly to nonadapted (TSBYE-G) and partially acid-adapted (TSBYE) inocula at 4°C. These data may be used to supplement existing information for use in future risk assessments.