Listeria monocytogenes has been detected in a number of frozen vegetables tested in Ireland.

The Food Safety Authority of Ireland (FSAI) said findings show a potential risk of illness for consumers who have non-ready-to-eat (RTE) frozen vegetables, fruits or herbs without cooking.

After a 2015 to 2018 listeriosis outbreak in Europe traced to uncooked non-RTE frozen sweetcorn and other vegetables, a national microbiological survey on frozen vegetables, fruits and herbs sold in Ireland was done between August and November 2019 by the FSAI. More than 50 people were sick from five countries and 10 died in the outbreak linked to vegetables made by Greenyard in Hungary.

Of 906 samples tested for Listeria monocytogenes in Ireland, 27 were positive, and 21 were non-RTE frozen vegetables. It was detected in four samples of RTE frozen fruits and two RTE frozen vegetables.

Low levels but a risk with frequent consumption
Listeria monocytogenes was below the limit of detection at under 10 colony forming units per gram (cfu/g) for most samples, apart from three which were non-RTE frozen petit pois, non-RTE frozen diced onion, and frozen chopped basil.

The work found such low levels posed a minimal risk to people who are in good health.

“However, the risk could be higher for consumers who are immunocompromised, or if consumer preparation and handling practices allow any of the Listeria monocytogenes contamination present in the non-RTE frozen vegetables to increase at the point of consumption to levels high enough to potentially cause listeriosis,” said the study.

Of 828 samples tested for Listeria spp., 37 were positive. No samples were contaminated with Salmonella.

Presence of E. coli as a hygiene indicator was assessed in 887 frozen vegetable, fruit and herb samples. Ten were contaminated with E. coli at levels of between 20 and 100 cfu/g and two were RTE frozen vegetables. There were two non-RTE samples of frozen spinach with E. coli levels of 1,100 cfu/g and 570 cfu/g.

Clear instructions to consumers to fully cook frozen vegetables intended by the manufacturer to be non-RTE was lacking on the packaging of 37 of 399 samples tested.

Pamela Byrne, FSAI chief executive, said caterers and food service businesses must ensure they follow the manufacturers’ instructions when preparing food for customers.

“It is vital that food manufacturers follow best practice guidelines and ensure frozen products that are not RTE are clearly labelled as such, with clear cooking instructions. They also need to ensure there are no serving suggestions presented on the packaging which could suggest that the products can be eaten thawed without prior cooking,” she said.

Public polling results
A survey was commissioned by safefood between January and March 2020 to understand the consumption habits of adults living on the island of Ireland in relation to uncooked RTE and non-RTE frozen vegetables, fruits and herbs. More than 800 participants were interviewed in the Republic of Ireland and Northern Ireland.

A third regularly ate more than one type of frozen vegetables, fruits and/or herbs uncooked. Frozen fruits such as strawberries, blueberries, raspberries, and mixed berries were most likely to be consumed without cooking, mainly in a dessert or smoothie.

Uncooked frozen vegetables such as sweetcorn, carrots, peas, peppers and spinach were also eaten by a few as part of a salad or as a garnish.

Just over half of adults had heard of Listeria. A wide variety of foods were associated with Listeria, with meat and chicken/poultry topping the list. One in five people had previously heard about the 2015 to 2018 outbreak of listeriosis linked to frozen vegetables.

The consumer survey showed that 203 adults in Ireland said they regularly consumed frozen sweetcorn but only 21 said they would eat it uncooked. A quarter said that sweetcorn does not need to be cooked before eating. The microbiological survey found that 9.8 percent of frozen sweetcorn samples were contaminated with low levels of Listeria monocytogenes.

Overall, 85 respondents said they either rarely or never follow cooking instructions on the packaging of vegetables, fruits and herbs when preparing frozen food for vulnerable groups.

Combining results from both surveys shows there were a small proportion of non-RTE frozen vegetables which some consumers said they regularly consume uncooked and that were contaminated with low levels of Listeria monocytogenes.

Gary A Kearney, interim safefood chief executive, said if a product says “cook before eating” people should always follow that advice.

“We know from social media that there’s a growing trend for people to eat frozen fruit and vegetables raw in things like smoothies and salads. While the risk of contracting a Listeria infection is low, it’s still a risk you can avoid by reading the manufacturer’s instructions and cooking these frozen foods before you eat them,” he said.

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