Listeria also reduces the shelf life of foods that have been chilled to prolong their usability. Nofima is researching into steam pasteurisation of fish products, to control the Listeria bacteria without destroying product quality.
The actual muscle tissue of fish is normally sterile, so that any contamination occurs on the surface, through contact with equipment, water or working surfaces. For chilled, prepared fish products with an extended shelf life, such as smoked fish, fried fish cakes, fish pudding etc., such contamination before packing can have serious consequences if the bacteria grow to high concentrations.
The recently harmonised regulations of the Norwegian authorities are for a maximum level of Listeria monocytogenes in foods from zero up to 100 Listeria bacteria per gram by the use-by date. The aim of the project is therefore to use technology to reduce the amount of Listeria before packing and ensure that the tolerance laid down in legislation is not exceeded.
Killing bacteria, keeping quality
The amount of heat required to kill 100 bacteria per cm2 is 15 million times lower than the heat needed to cook a surface layer with the same thickness as a bacterium.
In the light of this knowledge, it is natural to look at surface pasteurisation as an effective way of eliminating Listeria. Using surface pasteurisation, we could achieve a safe and long-lasting product without any great loss of taste or texture, and this can be done with the aid of steam or hot air through a tunnel on the packing line. The product can them be packed using aseptic packaging technology. Aseptic packaging is common for liquid products and entails sterilising the packaging, before packing and sealing in the product under sterile conditions. The innovation is to use this technology on non-sterile products.
The surface tolerates more
Surface pasteurisation with steam has already been tried for whole animal carcasses and chicken products, but hardly at all for fish products. Nofima has procured equipment that will allow it to perform systematic tests involving steaming the surface of an experimental fish product. Tests that have been performed on liquids show that Listeria is rapidly killed at temperatures as low as 58-60°C. The lethal effect on Listeria in a real fish product is very different from when testing on liquids. Researchers are now investigating the effects of fish structure, water activity and salt content. Two strains of Listeria innocua are being used as model organisms in the project and trials are going on to compare the speed of growth of these with Listeria monocytogenes from the fisheries industry.