Kinetics of destruction of Escherichia coil and Pseudomonas fluorescens inoculated in ewe's milk by high hydrostatic pressure

Ewe's milk standardized to 6% fat was inoculated with Escherichia coli and Pseudomonas fluorescens at a concentration of 107 and 108 cfu ml-1 respectively, and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (50-300 MPa), temperature (2, 10, 25 and 50°C), and time (5, 10 and 15 min). Violet red bile agar and crystal-violet-tetrazolium count were used to determine E. coli and P. fluorescens respectively. Pressurization at low and moderately high temperatures produced higher P. fluorescens inactivation than treatments at room temperature, while pressurization at only moderately high temperature produced high E. coli inactivation; low and room temperatures produced similar reductions. On E. coli, reductions of 6.5 log units were produced with 300MPa for 15 min at 50°C, while on P. fluorescens, reductions of 5.9 log units were produced with 250 MPa for 15 min at 50°C. Both micro-organisms showed a first-order kinetics of destruction in the range O-30 min, with D-values (at different temperatures and pressures from 150 to 300 MPa) between 2.5 and 18.8 min for E. coli, and 2.8-23.3 min for P. fluorescens. A baroprotective effect of ewe's milk (6% fat) on both micro-organisms was observed, in comparison with other studies using different means and similar pressurization conditions.