Transfer of Listeria monocytogenes during Slicing of Cooked Ham

Introduction: Slicing of processed Ready-to-Eat meat products at retail level can be a critical step for cross-contamination with pathogenic bacteria, especially Listeria monocytogenes. Predictive models can be helpful in establishing the best practices to minimize the transfer during this procedure.

Purpose: The study aimed at generating data on transfer of Listeria monocytogenes during slicing of cooked ham at retail, to be used in the development of a cross-contamination predictive model. 

Methods: Experiments were carried out with pieces of cooked ham purchased in supermarkets and checked for absence of Listeria monocytogenes using ISO 11290-2:1998 method. Initially, a meat matrix was created in a manual meat slicer by slicing a L. monocytogenes-negative piece of ham. Another piece of ham was experimentally contaminated by immersion for 30 min in a suspension containing L. monocytogenes (8 log CFU/ml) and sliced, causing the experimental contamination of the slicer. Subsequently, new pieces of non-contaminated ham were sliced, until 40 (first trial) and 200 (second trial) slices were obtained. The extent of pathogen transfer (cross-contamination) was determined counting L. monocytogenes in all slices in the first trial and in the first ten sequential slices and then in every 5th/10th slice in the second trial. 

Results: Two transfer scenarios were observed: counts of Listeria monocytogenes in the first slices were 5 log CFU/g and after the 25th slice the counts decreased gradually but formed a long tail. Even the last slice was L. monocytogenes positive. 

Significance: These results confirm that slicing at retail level is an important source of cross-contamination of Ready-to-Eat meat products. These data, combined with other experiments with lower levels of experimental contamination, will be useful for the development of cross-contamination predictive models.