Measures for the inhibition of Listeria monocytogenes in a traditional Norwegian fermented fish product (rakfisk) and their effect on sensory attributes

Listeria monocytogenes is a food-borne pathogen of concern in the traditional Norwegian fermented fish product known as rakfisk. This product is made from freshwater salmonid fish through mild-salting and maturation in brine at 3-7 °C for 3-12 months. L. monocytogenes can potentially grow during this manufacturing process if initial contamination occurs, and since the product is consumed without any heat treatment, it may cause outbreaks of listeriosis. One of the most severe outbreaks of listeriosis in Norway during recent years was caused by contaminated rakfisk. Consequently, rakfisk producers are seeking measures to decrease the risk of growth of L. monocytogenes and ideally inactivate the bacteria in the process. We investigated different mitigation strategies, used alone or in combinations, in the production of rakfisk. These included the addition of glucose, a standard starter culture, an anti-Listeria protective culture, dried buffered vinegar, nisin and an anti-Listeria bacteriophage, and their effect on the growth and survival of L. monocytogenes. Our findings showed that some of these measures, particularly the protective culture and dried buffered vinegar, provided complete growth inhibition of L. monocytogenes during the rakfisk process. If protective culture was combined with glucose or buffered vinegar was combined with nisin or bacteriophages, a listericidal effect was observed. Sensory properties of both pilot and industrially produced rakfisk made with and without (controls) these measures showed minor differences. These measures could thus be considered for practical use by rakfisk producers to combat L. monocytogenes.