Rapid heating of the surface with steam at 100 °C can prevent listeria on fish, according to a Nofima researcher.
Listeria is eliminated using surface steam pasteurisation for a few seconds before it is packed and distributed
Listeria monocytogenes is a challenging pathogenic bacterium for the seafood industry, explained Torstein Skåra from the Norwegian research institute as part of his PhD work - which shows the effect of steam killing on the surface of the fish.
Surface pasteurisation experiments were performed using a special test rig and different contamination levels of bacteria.
Chilled products with extended shelf life, such as smoked fish, fried fish cakes, burgers and puddings, are vulnerable as the bacteria grow at refrigeration temperatures.
Skåra investigated different model organisms and compared the growth of Listeria monocytogenes strains isolated from fish processing environment.
“It is not used much for fish but it is used in animal slaughter houses on whole carcasses and has been used in the poultry industry a bit,” he told FoodQualityNews.com.
“We focussed to a great extent on food safety to kill the bacteria but fish is delicate and the ability to maintain quality is key, overheat and it becomes dry and the texture less appealing.”
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, he said.
Surface steam pasteurisation only affects the outermost part of the surface and has minimal effect on appearance or nutritional status, said Skåra.
To determine the thermal load on samples, tests used 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 was used to measure changes in the surface caused by heat, and mathematical models relate these changes to the thermal load the samples are exposed to.
The US has a number of contamination procedures available such as chlorine but in the EU legislation additives are not permitted to the processing step so steam, which is only water, used at high temperatures is a useful tool in killing bacteria, especially when the temperature can vary widely on surfaces only a few millimetres apart from each other.
When asked about factors affecting results, he said: “If the surface is not even, it can contain crevices or uneven areas and the way the bacteria is applied has an affect, whether it is dipped or sprayed onto the surface.”
Skåra added: “We want to go further on investigating possible efficiencies and the exact suspects relating to the inoculation procedure, location of the bacteria and processing steps used in raw fish, cooked or fried products prior to the final packaging step.”