B12006: Risk assessment of Listeria monocytogenes in UK retail cheese

Study Duration: May 2005 to April 2006

Contractor: Food Safety Assurance

Although foodborne listeriosis is a rare disease in the UK, infections carry a high mortality rate. Vulnerable groups such as pregnant women are advised to avoid certain foods, which are known to carry a risk of Listeria monocytogenes contamination and growth of the organism. Listeriosis in a pregnant woman can lead to miscarriage, stillbirth or severe illness in the newborn baby.

The Food Standards Agency wishes to further develop the scientific basis for its advice on the consumption of cheese. The current Agency advice says �pregnant women are advised to avoid eating mould ripened soft cheeses of the Brie, Camembert and blue-veined types, whether pasteurised or unpasteurised�. However, the range of cheese types available to consumers in the UK is diverse and a review of information about these cheeses is required to further inform the Agency�s policy and advice in this area.

The work required will focus on the main categories of cheeses available in the UK and examine in detail key representative types within each category. Information on how the cheeses are made, together with the controlling factors such as pH, water activity (a_W) and salt content, will be assessed to determine the risk of the cheese being contaminated with L. monocytogenes and/or likely to support the growth and survival of the organism.

For the main categories of cheese consumed in the UK, the risk of Listeria monocytogenes contamination, survival or growth was found to be as follows;

• The majority of semi-hard and hard cheeses (e.g. Cheddar, Cheshire, Caerphilly and Red Leicester) appear to present little risk.
• For fresh, soft cheese (e.g. Mozzarella, Ricotta, Quarg and cream cheese) the risk is low, partly due to the processing and packaging used.
• For blue-veined cheese (e.g. Stilton, Gorgonzola, Roquefort and Danish Blue), surface film-ripened cheeses (e.g. Taleggio, Munster and Chaumes) and mould-ripened cheese (e.g. Brie, and Camembert), the project identified possible risks due to the production processes and controlling factors.

With the exception of hard cheeses, the use of raw milk introduces an additional risk factor for L. monocytogenes. However, most cheese consumed in the UK is made from pasteurised milk.

Predictive modelling based on controlling factors such as pH, water activity and salt content was found to over-estimate the growth or survival of L. monocytogenes. This may be because the lactic starter cultures are able to influence the growth and survival of L. monocytogenes in cheese.

The risk ranking of cheese categories provided in the final project report tends to support existing Agency advice on cheese consumption for pregnant women.

The final report is available from the Agency’s Information Centre.
To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk)

For any enquiries concerning this research project, please contact the relevant Programme contact or email: science@foodstandards.gsi.gov.uk