Combined high pressure-sub-zero temperature processing of smoked salmon mince: phase transition phenomena and inactivation of Listeria innocua

Smoked salmon mince inoculated with Listeria innocua was subjected to various combined high pressure- low or sub-zero temperature treatments. Temperature in the sample and in the pressure-transmitting medium (PTM) were monitored against pressure in order to detail undercooling, ice nucleation and phase transition under pressure. Freezing at 0.1 MPa with subsequent storage at −28 °C or −40 °C for 5 days did not induce any microbial inactivation. Inactivation of L. innocua by pressurisation at 207 MPa for 60 min without ice crystal formation was higher (1.1 log cycle) at sub-zero temperatures (down to −21 °C) than at 4 °C (0.7 log cycle). Pressure-shift freezing from 207 MPa and −22 °C, followed by further freezing to −25 °C at 0.1 MPa, induced 1.4 or 1.9 log cycle reduction of L. innocua, depending on whether pressure was released rapidly (∼3 s) or slowly (18 min), respectively. Freezing samples at −28 °C and 0.1 MPa for 24 h followed by pressurisation at 207 MPa and −29 °C for 23 min, then fast pressure release resulted in a 2.5 log cycle reduction for L. innocua. When sample freezing was followed by pressure-assisted thawing at 207 MPa and in PTM at 10 °C for 23 min, a reduction of 1.2 log cycle was obtained. None of the combined high pressure-sub-zero temperature treatments was found to induce sub-lethal injury of L. innocua dispersed in smoked salmon mince.