Inactivation kinetics of Listeria innocua in steam surface pasteurisation of fish products

Master's thesis in Biological chemistryThis master project is a part of the Strategic Institute programme, ProSpect, granted by the Norwegian Research Council (NFR). In order to develop safe minimally processed seafood and to increase quality and shelf-life, the project ProSpect combined principles from aseptic packaging with surface pasteurisation with steam. This master project had special focus on thermal steam pasteurisation of fish products. A special designed “BugDeath” test rig has been purchased for the experiments. Listeria monocytogenes was selected as target organism for the pasteurisation. To avoid the extra precautions associated with working with pathogenic bacteria, specific strains of Listeria innocua were selected as surrogate organisms for L. monocytogenes. L. innocua is a non-pathogenic organism, but more heat tolerant than L. monocytogenes. The objective of the current study was to determine inactivation kinetics of different L. innocua strains (ATCC 33090 from American Type Culture Collection and CCUG 35613 from the Culture Collection, University of Gothenburg) by using two different experimental designs: 1) using classical heat treatment in capillary tubes, 2) using steam on fish product surfaces. Another objective of this study was to investigate and to compare differences in inactivation kinetic in capillary tubes and on fish surface. In the fist experimental design with heat treatment in capillary tubes two strains of L. innocua (ATCC 33090 and CCUG 35613) were used. Both strains were cultivated and heat treated in the tryptone soya broth with yeast extract (TSBYE). In the second experimental design with steam pasteurisation on fish product surfaces L. innocua ATCC 33090 was used. This strain was cultivated in the TSBYE and heat treated on surimi “model-product”. In both designs L. innocua strains after heat treatment were regenerated on the tryptone soya agar with yeast extract (TSAYE). The D-values were used to determine inactivation kinetics and describe the heat resistance of the microorganisms. The calculations of heat resistance for treatment in capillary tubes were based on the linear first-order kinetics. The D-values were calculated for temperatures 59, 59.5, 60 ° C. D-values for L. innocua ATCC 33090 were D59 =2.62 minutes, D59.5 =2.1 minutes, D60 =1.58 minutes and for L. innocua CCUG 35613 were D59 =2.54 minutes, D59.5 =1.91 minutes, D60 =1.50 minutes. The inactivation of L. innocua in steam surface pasteurisation of fish products did not follow log-linear kinetics. Bacterial numbers of L. innocua ATCC 33090 declined rapidly during the first 15 s of steam treatment in “BugDeath” rig. This initial rate of decline slowed during the next 45 s. However, after 60 s of steam treatment bacterial numbers declined very slowly, so that bacterial numbers were still present after steam treatment for 4 min. In this experiment steam treatment of samples for 60 s gave total 3-4 log CFU reduction, but after 60 s had not considerable reduction of bacterial numbers and most likely had an undesirable effect for quality of the product