Listeria found most often in cold-smoked and lightly salted fish

ERR reports that a doctoral study at the Estonian University of Life Sciences found Listeria bacteria in relatively few ready-to-eat foods examined in Estonia between 2012 and 2016, but contamination was concentrated in certain product groups, particularly cold-smoked and lightly salted fish.

Listeria monocytogenes differs from many other foodborne pathogens because it can grow even at refrigerator temperatures, although slowly. The main concern is therefore chilled ready-to-eat food consumed without further heating. If such food has a long shelf life, a small initial number of bacteria can increase over time and become dangerous.

Listeriosis is less common than many other foodborne infections, but it can be severe. Older people, pregnant women, newborns and those with weakened immune systems face the greatest risk. Because of the potentially serious course of the disease, ready-to-eat food producers must control their raw materials, production environments and hygiene, as well as set an appropriate shelf life.

Julia Koskari’s doctoral thesis provides one of the most extensive overviews so far of Listeria in ready-to-eat foods in Estonia. Over five years, she analysed more than 30,000 samples from meat, fish, dairy, prepared foods and bakery products, as well as fruit and vegetables, mixed salads and several other food categories.

Ready-to-eat food is defined as food intended by its producer or preparer for direct consumption without heat treatment or another process that would eliminate pathogenic microorganisms or reduce them to an acceptable level.

Between 2012 and 2016, 3.6 percent of the ready-to-eat food samples analysed using a detection method tested positive for Listeria monocytogenes. The differences between food groups were substantial. Fish products had the highest proportion of positive samples, at 11.6 percent, with particularly high levels found in lightly salted and cold-smoked fish.

The bacterium was found considerably less often in hot-smoked and other heat-treated fish products, and it was not detected in canned fish. The proportion of positive samples in salads, prepared foods, and fruit and vegetable products ranged from 2.1 to 3.9 percent.

In bakery and dairy products, the proportion was only 0.2 to 0.5 percent. Listeria was not detected in sauces, infant foods or special foods intended for medical purposes.

The presence of the bacterium does not always mean that a food contains an amount that poses a health risk. When bacterial levels were measured, 99.3 percent of samples contained fewer than 10 Listeria bacteria per gram or fell below the detection limit.