Inactivation kinetics of a piezotolerant Staphylococcus aureus isolated from high-pressure-treated sliced ham by high pressure in buffer and in a ham model system: Evaluation in selective and non-selective medium

The kinetics of inactivation by high pressure of a pressure-resistant strain of Staphylococcus aureus isolated from pressure-treated packaged sliced ham, in buffer and in a ham model system was studied. Selective (BP agar) and enrichment media (BHI agar) were used for enumeration in order to count healthy and sublethally injured cells of the pathogen. A first-order kinetic inactivation was observed in both suspension media, and a very significant increase in D values was apparent when the microorganism was suspended and pressurized in the model food system compared to buffer. In the case of phosphate buffer as suspension medium, the z_p values obtained were 107.5 and 113.6 MPa for the two recovery media, i.e. BP and BHI agars, respectively. In contrast, in the case of the food model system, a two-phase linear relation was apparent and the PDT (Pressure Death Time) curve can be divided into two linear sections, so that two z_p values could be defined, one for each section. Z_p values of 100 and 79.4 MPa correspond to pressures < 500 MPa for the BP and BHI counts, respectively, while z_p values of 416.7 and 333.3 MPa correspond to higher pressures > 500 MPa for the selective and non-selective medium, respectively. When S. aureus had been pressurized in phosphate buffer, the BHI agar was slightly better in cell recovery, while in the case of the ham model system, the BP agar proved superior and gave significantly higher colony counts.

Industrial relevance

The paper provides significant information for the food processing industry as it deals with the effect of high-pressure technology on a piezotolerant pathogen that may survive in sliced ham. This technology is already applied in ham products and this paper supports the need for the use of real food in pressure studies in order to avoid underestimation of the effect and hence the processing times. It is also shown that different recovery media, i.e. selective and non-selective, should be used to avoid underestimation of the surviving cells.