Temperature-assisted high hydrostatic pressure inactivation of Staphylococcus aureus in a ham model system: evaluation in selective and nonselective medium

Abstract

The purpose of this study was to investigate the inactivation kinetics of Staphylococcus aureus in a ham model system by high hydrostatic pressure at ambient (25 degrees C) and selected temperatures (45, 55 degrees C). Selective [Baird Parker (BP) agar] and nonselective [brain heart infusion (BHI) agar] growth media were used for enumeration in order to count viable and sublethally injured cells. The micro-organism was exposed to a range of pressures (450, 500, 550, 600 MPa) at ambient temperature (25 degrees C) for up to 45 min. Additionally, the behaviour of the micro-organism was evaluated at mild temperatures in combination with high pressure treatment, namely: (i) 350, 400 and 450 MPa at 45 degrees C; and (ii) 350 and 400 MPa at 55 degrees C, for up to 12 min. Inactivation kinetics were calculated in terms of D(p) and z(p) values. Survival curves of S. aureus at ambient temperature were mostly linear, whereas when temperature was applied, tailing was observed in most survival curves. The estimated D(p) values and therefore the number of surviving cells, were substantially higher on the selective BP agar in the whole range of pressures applied, indicating that S. aureus showed greater recovery in the selective BP agar than the nonselective BHI agar. Samples pressurized at ambient temperature needed higher pressures (over 500 MPa) to achieve a reduction of the population of the pathogen more than 5 log CFU ml(-1). The same level of inactivation was achieved at lower pressure levels when mild heating was simultaneously applied. Indeed, more than 6 log CFU ml(-1) reductions were obtained at 400 MPa and 55 degrees C within the first 7 min of the process in BHI medium. Elevated temperatures allowed lower pressure levels and shorter processing times of pathogen inactivation than at room temperature. Greater recovery of the pathogen was observed in the selective (BP agar) medium, regardless of pressure and temperature applied. The obtained kinetics could be employed by the industry in selecting optimum pressure/temperature processing conditions. Attention must be given to the selection of the enumeration medium, as the use of an inappropriate medium would lead to underestimation of the surviving cells, thus imposing a risk in the microbiological safety of the product.