According to two UK food agencies, the likelihood of vulnerable people contracting listeriosis from blue cheese is very low.
A Food Standards Agency (FSA) and Food Standards Scotland (FSS) risk assessment found the severity of Listeria monocytogenes infection in vulnerable people is high. However, the frequency of listeriosis in these groups from consumption of blue cheese is considered very low.
Vulnerable people include pregnant women, people over 65 years of age, infants, and those with weakened immune systems.
Vulnerable people do not frequently consume blue cheese. When eaten, it is usually in low amounts. UK data suggests the prevalence of Listeria in blue cheese is around 1 to 3 percent of samples at retail. Blue cheeses include Stilton, Roquefort, Gorgonzola, and Danish Blue. The majority of blue cheeses for sale at retail in the UK are made from pasteurized milk.
The findings mean there will be no change to FSA and FSS advice. However, NHS websites listed Stilton as an example as a safe food for pregnant women, which has now been changed.
Listeria risk from blue cheese
Blue cheese can become contaminated at different stages of the cheesemaking process. Listeria monocytogenes could be present in the raw milk and in the brine used in cheesemaking, or it may contaminate cheese during processing, handling, cutting, and packaging.
A search found two listeriosis outbreaks and one case that may have been caused by blue cheese, but none were in the UK. The first, in 2011, was in the United States, with 15 cases and one death. The second was in 1989 and 1990 in Denmark, with 26 cases and six deaths. In 2003, a 63-year-old man was sickened in Italy.
Most semi-soft cheeses do not let Listeria monocytogenes grow, but blue cheeses may be an exception. This is because the mold in them makes the interior less acidic, which helps Listeria growth. Blue cheese is needled to allow air into the center for mold growth. Such cheese can have different acidity, moisture content, and salt levels.
Data from the Scottish Food Sampling Database for 2014 to 2021 showed that of 1,085 tests for Listeria monocytogenes on cheese products, 13 were deemed unsatisfactory. Non-compliant products included two pasteurized blue cheese wheels and two small pasteurized blue cheeses.
In the same period, 1,192 tests for generic Listeria species were carried out on cheese products, and ten were unsatisfactory. Three of these were Gorgonzola samples.
A search of the FSA incidents and alerts system from 2000 resulted in 17 notifications of Listeria spp. in blue cheese products for the UK market, three of which were Stilton. A search of EU signals from 2019 to 2023 identified 39 recalls of blue cheese, mostly Gorgonzola, due to Listeria spp. The number of samples positive for Listeria spp. in Stilton in 2023 was six of 106, according to Dairy UK.
A survey of unpasteurized milk cheeses at retail in England between 2019 and 2020 sampled 77 blue cheeses, of which two were positive for Listeria monocytogenes. Researchers found a significant correlation between storage at high temperatures and poor microbiological results.
(To sign up for a free subscription to Food Safety News, click here.)
