Destruction of Listeria monocytogenes during a ham cooking process

Two batches of uncooked whole hams were inoculated with Listeria monocytogenes phagovar 2389/2425/3274/2671/47/108/340 during brining. One batch of hams was contaminated with a high level (3.9 x 10(5) CFU/g), and the second batch was contaminated with a low level of L. monoytogenes (<10 CFU/g). These vacuum-packaged hams were then cooled according to the minimum standards allowed to obtain technologically and organoleptically acceptable hams, i.e., to a core temperature of 58.8 degrees C and an F-70-value of 32 min. They were then maintained undisturbed for 2 months at 9 degrees C, which is consistent with the worst conditions typically found in practice. Enumeration of the spoilage flora (30 degrees C mesophilic flora, lactic flora, and Enterobacteriaceae) during storage demonstrated that this treatment yielded a microbiologically satisfactory food product. In the case of L. manocytogenes, the heat treatment applied would in theory reduce the contamination level by 52 to 77 log units (assuming a D-value in the range of 1.82 to 3.38 min at 60 degrees C and a z-value in the range of 5.05 to 6.74 degrees C). No Listeria spp. was found during storage in the batch of hams contaminated with the low level. However, a very small number of L. monocytogenes was found in the highly contaminated batch of hams at the end of the storage period. These results should alert the commercial sector to the importance of cooking their products to a minimum core temperature of 65 degrees C, with an F-70-value of at least 40 min. If weakly contaminated products are cooked according to this protocol, the risk of L. monocytogenes surviving should be diminished.