Listeria was detected in almost a quarter of frozen vegetable samples in England, according to a study.

Between December 2018 and April 2019, 1,050 frozen fruit and vegetable samples were collected. Listeria monocytogenes or other Listeria species were detected in 167 samples of vegetables. Listeria monocytogenes was present in 10 percent of frozen vegetables.

The study of frozen fruit and vegetables from catering and retail premises in England assessed microbiological quality with respect to Listeria and E. coli. Findings were published in the International Journal of Food Microbiology.

Eleven samples contained more than 100 colony forming units per gram (cfu/g) of E. coli. Listeria monocytogenes or other Listeria species were detected in six samples of fruit and six fruit and vegetable mixes.

Obtaining baseline data
Work was prompted after the outbreak of listeriosis that affected 54 people in six countries with 10 deaths in 2015 to 2018 associated with frozen sweetcorn produced by Greenyard in Hungary. Researchers found the strain from this outbreak remained in the UK frozen vegetable food chain until April 2019 and caused a case of Listeria meningitis in England in February this past year.

Amongst all samples, 351 were fruit, 673 were vegetables, and 26 were a mixture of the two. A total of 885 were sampled from unopened packs. There were 25 different types of frozen fruit, with the largest single category being mixed fruits.

The most common types within samples containing a single fruit were blackberries, blueberries, raspberries and strawberries. There were 43 different vegetable types plus mixtures. The main types of single frozen vegetables were peas, sweetcorn, beans and carrots.

All the 26 fruit and vegetable mixtures were frozen smoothie mixes, 17 of which contained either spinach and/or kale plus various types of fruit.

Seventy-nine percent of samples containing fruit were listed as ready-to-eat (RTE) products on the packaging compared to only 30 of the vegetables.

Seventy-seven percent of the vegetables were not RTE and the intended use on packaging advised cooking or blanching. In 12 percent of the vegetables, whether the product was RTE or otherwise was not stated.

Examples of contaminated items
Eleven samples had E. coli above 100 cfu/g, of which six were higher than 500 cfu/g and four above 1,000 cfu/g. Examples include pre-packed jackfruit from India labelled as non-ready-to-eat; an open sample of runner beans from the UK; pre-packed banana from Vietnam; pre-packed lima beans from Bangladesh; pre-packed coconut from the Philippines; and pre-packed cabbage from Belgium.

Listeria monocytogenes was detected in 69 of 673 vegetables, and six of 26 fruit and vegetable mixes compared to three of 340 fruit samples. Three samples contained Listeria monocytogenes at levels of 10 cfu/g: pre-packed spinach from Poland or 20 cfu/g: stir-fry vegetable mix from Poland and sweetcorn from Belgium. The only fruit samples contaminated with Listeria monocytogenes were melon.

A range of vegetable samples were contaminated with Listeria monocytogenes, the most common types being: mushrooms; peppers; sweetcorn; and squash. It was also recovered from vegetable mixes, including those containing carrot, sweetcorn, peas or beans.

“These results are of concern, particularly in products that may be defrosted and held at refrigeration or ambient temperature before consumption,” said researchers.

“The contamination of 23 percent of the frozen fruit and vegetable smoothie mixes with Listeria monocytogenes is of particular concern since these products may not be consumed directly after defrosting, do not undergo any cooking process, and therefore provide opportunities for the growth of Listeria monocytogenes before consumption.”

Results by product origin
Overall, 673 of 1,050 samples originated from within Europe, with the majority of all categories from within the EU. Products from outside the EU originated from 22 different countries. However, this may be wher the product was packed, with the produce being grown in a country not indicated on the pack.

Six percent of 127 products from non-EU countries had E. coli levels above 100 cfu/g compared to 0.7 percent of 612 samples from EU nations.

Listeria monocytogenes and other Listeria species were detected in a larger proportion of items from EU countries with 47 of 612 samples for the former and 74 for the latter compared to non-EU countries with four of 127 samples for Listeria monocytogenes and five for other Listeria species.

Researchers said there must be education of consumers on the risks from non-ready-to-eat foods but this may be difficult without clear labelling.

There is also a need to communicate with food manufacturers to mitigate against cross-contamination within the food chain and prevent non-RTE ingredients being put into RTE foods such as sandwiches or smoothies without a heating step.