A warm refrigerator allows the listeria to grow

September 17/2019

Listeriosis caused by the Listeria bacterium (Listeria monocytogenes) is more common in Finland than in the rest of the world. Overall, the incidences of listeriosis have been on the rise in Finland. The rise is suspected to be due, among other things, to the aging of the population, leading to a decline in resistance. Changing eating habits can also affect the incidence of diseases, including listeriosis.

Listeriosis is a zoonotic disease, that is, one that can be transmitted from animals to humans. People are most often infected with food, such as undercooked meat or fish, or poorly washed greens or root vegetables. Unpasteurized milk and products made from it should be avoided. In rare cases, people may get infected with a direct contact with a diseased animal.

“The ability of the listeria bacterium to cause disease depends on the amount of bacteria obtained from the food. In healthy adults, listeria rarely causes symptomatic disease. Instead, the disease can be dangerous for various risk groups. The risk groups for listeriosis include the elderly as well as pregnant women and people with low levels of resistance”, says Petra Pasonen, a researcher at the Finnish Food Authority.

The growth of listeria at refrigerator temperature was predicted by a mathematical model

In the risk assessment, the growth of listeria was predicted at different refrigerator temperatures by a mathematical model of growth. The mathematical model used in the study combined bacterial concentration data, food consumption data, growth pattern, and case numbers into an overall risk assessment. These data were used to predict the impact of consumer behaviour, that is, how foods are stored in the refrigerator, on the risk.

Storage at 10 degrees increased the incidence of listeriosis in the elderly by nearly 20 times compared to the recommended storage in the maximum temperature of 3 degrees. According to the model, cases of listeriosis also tripled in the healthy adult population.

“Improper food storage alone did not increase the risk of developing illness in the elderly. 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”, says Pasonen.

In the risk assessment, the listeria levels of cold-smoked and salt-cured salmon products were generally so low that they were unlikely to be sufficient to cause the disease. In addition, the concentrations had fallen in recent measurements as conditions for the manufacture and storage of fishery products in establishments and shops had been improved.

Listeria cannot be detected by senses

Listeria does not affect the organoleptic quality. A product perceived as completely edible may have a high concentration of listeria bacteria.

Storing food at a refrigerator temperature does not completely prevent the growth of listeria. Unlike many other bacteria, listeria can also grow at a refrigerator temperature. The risk assessment supports the Finnish Food Authority's recommendation for cold-smoked and salt-cured fish products to be stored at a temperature of 0 to 3 degrees.

“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”, says Pasonen.

The completed risk assessment utilized the laboratory results from cold-smoked and salt-cured fish products obtained from year 2004 to 2010 and the food consumption data from the Finravinto 2007 study by the Finnish Institute for Health and Welfare.

The results of the risk assessment have been published in a peer-reviewed scientific publication:
Pasonen, P., Ranta, J., Tapanainen, H., Valsta, L., Tuominen, P.
Listeria monocytogenes Risk Assessment on cold-smoked and salt cured fishery products in Finland - a Repeated Exposure Model. International Journal of Food Microbiology 2019: Vol. 304, pp. 97 - 105. 

Further information:
Petra Pasonen, Researcher, Risk Assessment Research Unit
+358 50 439 7754