Results of a risk assessment in Finland show those preparing and handling food may play a critical role in the incidence of listeriosis.
The stage after retail, including food handling and storage by the consumer or professional kitchens, is essential to protect against Listeria infections, according to the study published in the International Journal of Food Microbiology.
To investigate listeriosis risk, an exposure assessment was developed, and laboratory results for cold smoked and salt-cured salmon products were used. Listeria monocytogenes exposure was modeled for elderly consumers aged 65 to 74 as a risk group and working population aged 25 to 64 as a reference. Incidence was assessed by estimating bacterial growth in the food products at three temperatures.
Storage temperature impact
Results showed elderly people are at a greater risk of acquiring listeriosis than healthy adults. Risk for the elderly is not eliminated even if products have been stored at the recommended temperatures.
Storage at 10 degrees C (50 degrees F) increased the incidence of listeriosis in the elderly by nearly 20 times compared to recommended storage at the maximum temperature of 3 degrees C (37 degrees F). According to the model, cases of listeriosis also tripled in the healthy adult population.
The Finnish Food Authority (Ruokavirasto) recommends storing cold-smoked and salt-cured fish products at 0 degrees to 3 degrees C (32 to 37 F). In this scenario, on average only five elderly people became ill with listeriosis. If storage temperature was at 10 degrees C, about 80 elderly people could be affected. In the reference group, the respective number of cases was nine and 28.
Growth of Listeria was predicted at different refrigerator temperatures by a mathematical model. This model combined bacterial concentration and food consumption data, growth pattern, and case numbers. The data were used to predict the impact of consumer behavior, on how foods are stored in the refrigerator, on the risk.
Petra Pasonen, a researcher at the Finnish Food Authority, said improper food storage alone did not increase the risk of the elderly developing illness.
“Older people also consumed cold-smoked and salt-cured fish products more frequently and in larger portions compared to the rest of the population. This, in turn, contributes to the risk of developing illness among the elderly.”
Finland’s Listeria burden
A Listeria monocytogenes concentration of up to 100 colony forming units per gram is allowed in products put on the market during their shelf life in food capable of supporting its growth.
Finland is among the countries with the most listeriosis cases in the European Union. Incidence has been on the rise due to factors such as aging of the population and changing eating habits.
Although most appear to be sporadic and the maximum bacterial concentration is not usually exceeded at retail, cold smoked and salt-cured fish products have been identified as risky items especially for the elderly.
Cold smoked and salt-cured salmon are common in Finland and thought to account for most listeriosis cases there. Previous research showed the elderly eat them more often and portions are bigger compared with younger people.
In the risk assessment, Listeria levels in cold-smoked and salt-cured salmon were generally so low that they were unlikely to be sufficient to cause disease. Also, concentrations had fallen in recent measurements as conditions for the manufacture and storage of fish products in factories and shops had been improved.
The predicted initial Listeria monocytogenes concentration was moderately low with a mean of 97 cfu/g and median of 3 cfu/g. There are no reliable estimates of the dose-response relationship for Listeria in humans.
Elderly people were predicted to eat cold-smoked and salt-cured salmon on 12 days per year compared to 10 for the reference individual. Probability of consumption the next day is relatively high: about 55 percent in the elderly population and 27 percent in the reference population. Consumed amounts were on average 85 grams in the elderly population and 71 grams in the reference population.
Elderly people more than 75 years old were excluded from the risk assessment because of a lack of data, as were some food groups such as deli meats, dairy products and frozen vegetables. The model also did not include ways of storing or handling such as freezing and heating or eating from two different packages.
Storing food at refrigerator temperature does not completely prevent growth of Listeria. The bacterium also survives freezing temperatures.
“Listeria stays in frozen and dried foods for long periods, even years. Risk groups should avoid eating cold-smoked and salt-cured fish products altogether, since even a small amount of Listeria can cause a serious disease for people at risk. In products stored for a long time in a refrigerator that is too warm, the levels of Listeria may increase and the products should no longer be eaten after the expiry date,” said Pasonen.
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