Eliminating Listeria with steam
In his recently completed PhD project, Torstein Skåra from Nofima has demonstrated that rapid heating of the surface with steam at 100 °C may be a future method for preventing unwanted bacteria on the surface of fish products.
Listeria monocytogenes is a challenging pathogenic bacterium for the seafood industry, as it can contaminate the surface of products with otherwise good microbiological status.
Chilled products with extended shelf life, such as smoked fish, fried fish cakes, burgers and puddings, are particularly vulnerable as the bacteria grow at refrigeration temperatures. These products are demanded by consumers because they are easy to prepare, have high nutritional value and long shelf life.
Consequently, the main objective has been to find a method that can eliminate Listeria from the products just before packing and distribution.
As part of his PhD work, Scientist Torstein Skåra has investigated the choice of different model organisms and compared the growth of Listeria monocytogenes strains isolated from fish processing environment.
Some model organisms (Listeria innocua) had a lower growth rate and longer lag phase than Listeria monocytogenes at 4 and 8 °C. These findings are important when different organisms and their growth and killing rate are used in mathematical models.
The Listeria bacteria is then eliminated using surface steam pasteurisation for a few seconds. The unique feature of surface steam pasteurisation is that it only affects the outermost part of the surface and, consequently, will have a minimal affect on appearance or nutritional status.
In order to determine the thermal load on samples, tests have been conducted using spectroscopic technologies and modelling of the heat transfer. Steam pasteurisation has been compared with the use of water bath treatment (between 70 and 95 °C).
Reflectance spectroscopy is used to measure changes in the surface caused by heat, and mathematical models have been developed to relate these changes to the thermal load the samples are exposed to.
In order to document the effects on fish products, Skåra performed a number of systematic experiments to quantify steam killing of Listeria on the surfaces of fish products.
Improving food safety
The surface pasteurisation experiments were performed using a special test rig and different contamination levels of bacteria. The specially designed test rig, purchased by Nofima, was developed during earlier EU collaboration and was built at the University of Bristol.
One of the main conclusions of the thesis is that a new method has now been developed to determine the effect of steam killing of Listeria on the surface of fish products. This PhD project has contributed to an improved understanding of the mechanisms involved in steam surface pasteurisation. Skåra concludes that given correct use of this technology it will be possible to improve food safety assurance for a range of seafood products with extended shelf lives.
This work has been implemented at Nofima, and has formed part of a larger project, ProSpect, in which 10 scientists from Stavanger and Tromsø have worked to improve food safety assurance in the production of fish.
The PhD project received financial support from the Research Council of Norway and was implemented in collaboration with KU Leuven University in Belgium.