Sensitivity of Staphylococcus aureus and Lactobacillus helveticus in ovine milk subjected to high hydrostatic pressure

Ovine milk, standardized to 6% fat, was inoculated with Staphylococcus aureus CECT 534 and Lactobacillus helveticus CECT 414 at a concentration of 107 cfu/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50°C), and time (5, 10, and 15 min). Staphylococcus aureus was highly resistant to pressure; only pressurizations at 50°C of 500 MPa for 15 min achieved reductions of ≥7.3 log units. For L. helveticus, the number of surviving cells was reduced considerably at pressures of 400 MPa or more (up to 4.5 log units at 50°C for 15 min), and pressure was more effective at low (2 and 10°C) and moderately high (50°C) temperatures than at room temperature (25°C). Both species showed first-order kinetics of destruction in the range 0 to 60 min. The D values for S. aureus were 20 min (2°C at 450 MPa) and 16.7 min (25°C at 450 MPa), and D values for L. helveticus were 7.1 min (2°C at 450 MPa) and 9.1 min (25°C at 450 MPa). Lactobacillus helveticus showed higher rates of survival of pressure than those reported in previous studies for other Lactobacillus spp.