The purpose of this research was to determine the effect of meat species, ingredients, and level of high hydrostatic pressure on growth of Listeria monocytogenes in ready-to-eat meats. The overall findings of the research show the benefits of high hydrostatic pressure (HHP) in controlling L. monocytogenes by serving as a post-packaging treatment capable of decreasing the number of L. monocytogenes by over 3 log CFU/g at 600 MPa.

The first research study presented here showed that HHP is a technology capable of decreasing the number of L. monocytogenes organisms by 3-4 log CFU/g in conventionally-cured processed meats. The study suggested that turkey and ham (pork) had very similar levels of growth of L. monocytogenes without HHP. The inclusion of nitrite was the key variable in the study that caused a decrease in growth of L. monocytogenes without HHP. However, when formulations were processed via 600 MPa HHP after inoculation with 3 log CFU/g of L. monocytogenes, the level of L. monocytogenes was decreased to below the detection limit in all treatments and remained below the detection limit in all treatments for at least 119 days after pressure processing.

A second study showed that use of a reduced level of pressure at 400 MPa was not adequate in decreasing the number of L. monocytogenes by >1 log CFU/g. The addition of nitrite from either a natural source (pre-converted vegetable juice powder), at 50 or 100 ppm, or from sodium nitrite, at 100 or 200 ppm, were equal in their inhibition of L. monocytogenes. For some unexplained reason the greater concentrations of natural nitrite (150 and 200 ppm) that used HHP had, towards the end of storage, greater growth of L. monocytogenes than all other treatments. Some hypotheses were presented, including a greater pH level in the treatments with a greater concentration of natural nitrite, but further research is needed to understand the root cause of the greater level of growth in these treatments.

The third study evaluated the use of the reduced pressure level of 400 MPa HHP in combination with antimicrobial compounds to provide added reduction of L. monocytogenes above what each of the interventions could achieve on their own in cured meats, using either natural or conventional formulations. The addition of antimicrobial ingredients to the formulations or use of post-lethality antimicrobial sprays in conventional items gave a 1-2 log CFU/g reduction of L. monocytogenes numbers, while combining each of these with 400 MPa HHP gave about a 3 log CFU/g reduction in L. monocytogenes numbers. However the combination of ingredient, spray, and 400 MPa HHP gave a >4 log CFU/g reduction in L. monocytogenes in conventional products formulated with sodium nitrite. Similar effects were witnessed in natural products (formulated using a natural nitrite source) but at reduced levels of efficacy, as the combination of ingredient, spray, and 400 MPa HHP gave an initial 2.1-2.4 log CFU/g reduction in L. monocytogenes. In conventional and natural products, when an antimicrobial ingredient was added (Danisco LM220 for conventional or MOStatin for natural) in combination with HHP, the level of L. monocytogenes continued to fall over the extended shelf life (182 days) of the sliced ham.