The Norwegian Scientific Committee for Food and Environment (VKM) evaluated products within four groups: fish and seafood, meat, dairy products and fresh produce.
The latter category includes fruits, vegetables, berries, potatoes, nuts, seeds and cereal products.
“By certain conditions we mean packing methods, storage time and temperature from the food is produced until it is eaten," said Taran Skjerdal, lead expert in the project.
Mattilsynet (Norwegian Food Safety Authority) asked VKM to assess the Listeria risk to pregnant women and other vulnerable groups (such as children and the elderly).
It will review and update health advice from 2009 on Listeria monocytogenes to these consumers.
New knowledge on the growth and survival of Listeria in various products means advice should be updated, said Mattilsynet.
Products with increased risk
Cold-smoked and hot-smoked fish, cured fish, cooked shrimps, processed fish packed after cooling are products that may increase the probability of listeriosis.
Heat-treated meat products the manufacturer intends for further heat-treatment prior to consumption but are often not heat treated by the consumer may also increase the likelihood of infection. Such items are meat patties, meatballs and sausages.
Cured raw meat, smoked raw meat without curing or preservatives or heat treatment and chicken salad can also increase the chances.
Unpasteurized dairy products can increase likelihood as can pasteurised items like sliced or grated cheese, mozzarella used in salad, all spreadable and soft cheeses.
Some cut, fresh vegetables and fruit, especially melons, can increase the probability of listeriosis.
This applies also to sprouts and freshly cut ready-to-eat products with several ingredients as well as inappropriately preserved home-canned and fermented food.
Basis of risk assessment
Quantitative and qualitative methods were used to assess the likelihood that pregnant women and vulnerable groups could be exposed to doses that can cause disease.
Knowledge gaps concerned initial contamination rate; points of contamination, growth-inhibiting components of the food; natural background amounts of lactate and other preservatives; consumer behaviour; necessary infective dose and measurement uncertainty.
Probable values for pH, water activity, lactic acid bacteria, packaging atmosphere and time-temperature conditions were reported for the products modelled.
When these indicated there would be no growth or survival of L. monocytogenes during some of the steps, even if the product might be re-contaminated after any heat treatment, then further evaluation was considered unnecessary.
Products composed of several ingredients or that may have areas where the local pH, water activity, etc. may support growth of L. monocytogenes were evaluated further.
It was assumed the manufacturer, distributor and shops in the chain stored fish, meat and dairy products at 4˚C and the consumer put them in a refrigerator at 8˚C.
If data for the initial dose of L. monocytogenes were lacking, VKM assumed at the time of contamination it was 0.04 (1 per 25g), 1, and 10 cfu/g.
Should the assessment indicate a product is likely to support doses of the pathogen greater than 3 log cfu/g (1000 cfu/g), then it was described as having an increased likelihood of causing listeriosis in pregnant women and other vulnerable groups.
Measures to reduce risk
For such products, VKM evaluated whether measures such as decreasing shelf life or lowering storage temperature could reduce the dose by such an extent that illness could be avoided.
“There was wide variation in the probabilities that consumption of the different product groups described may expose pregnant women and other vulnerable groups to L. monocytogenes in doses that could cause disease. Similarly, the effects of risk mitigation measures varied widely,” said VKM.
L. monocytogenes growth is slower at 4˚C than 8˚C but is not entirely stopped. However, an improvement is achieved if low temperature is combined with a shorter storage time and/or other preservation techniques (modified atmosphere packaging/use of additives).
A shorter storage time means the pathogen has less time to grow, and for most products, the concentration early during the shelf life is lower than towards the end of storage time. If however, initial concentration is high, i.e., up to 10 cfu/g, then a short shelf life will have only a limited effect.
Other advice includes avoid leftovers that have been in the fridge for several days, only eat small quantities and food should be heated so the coldest point, usually the core, reaches 75°C.