Inactivation of Listeria monocytogenes in raw and hot smoked trout fillets by high hydrostatic pressure processing combined with liquid smoke and freezing

High hydrostatic pressure (HHP; 200 MPa for 15 min), liquid smoke (0.50%, v/v) and freezing (−80 °C, overnight) was used to eliminate Listeria monocytogenes in BHI broth, raw and smoked trout. The bactericidal effect of liquid smoke solutions (L9 and G6), HHP and their combinations was evaluated against L.monocytogenes LO28, EGD-e and 10403S and further continued with the most resistant strain (10403S) to the combined treatment. For first time, a synergistic effect of liquid smoke and HHP was observed and was further enhanced by freezing prior to HHP. The effect of HHP and liquid smoke, prior to freezing was highest in BHI compared to raw and smoked trout. A major synergistic effect of HHP, liquid smoke and freezing was observed, reaching a 5.48 or 1.93 log CFU/g reduction when smoked or raw trout was used respectively. Furthermore, high injury levels occurred, among treatments reaching up to 55.98%. This paper illustrates for first time, the possibility of using a very low pressure in combination with liquid smoke and freezing to eliminate L.monocytogenes. It was demonstrated that treatment of trout samples with liquid smoke followed by freezing prior to pressurization at 200 MPa for 15 min reduced the number of L.monocytogenes by more than 5-log CFU/g. Such a remarkable bacterial inactivation at a very low pressure (compared to common industrial practices) is a significant achievement that could allow production of safer and novel products by HHP at an affordable price, as the cost of equipment manufacture as well as the maintenance and running costs could be reduced substantially at lower operation pressures. •L. monocytogenes 10403S was the most resistant strain compared with LO28 and EGD-e.•The synergistic effect of HHP and liquid smoke ranged from 0.50 to 3.00 log CFU/mL.•A 5-log reduction resulted from the synergistic effect of freezing, liquid smoke and HHP.•Smoking and freezing can allow reduction of HHP intensity, to levels as low as 200 MPa.