April 12, 2024

How Does Listeria Get into Soft Cheeses?

The current Listeria monocytogenes outbreak linked to recalled Queseria Bendita soft cheeses (and sour cream) has sickened three people and killed one person in Washington state. Soft cheeses are not recommended as part of a pregnancy diet, along with deli meats and unpasteurized milk for this reason. But why are soft cheeses a Listeria risk?

CamembertFirst of all, any soft cheese made with unpasteurized milk is a risky food. Raw milk can, and does, contain many pathogenic bacteria from E. coli to Campylobacter to Listeria monocytogenes.

The FDA developed a draft assessment on the risk of consumers contracting listeriosis from soft cheeses in 2013. They found that soft cheese made from unpasteurized milk has a risk of containing Listeria bacteria 50 to 160 mites higher than soft cheeses made with pasteurized milk. Consumers should avoid eating raw milk cheeses, even those that have been aged, especially if they are in a high risk health group.

Second, the soft cheese has lots of moisture and a low acid content (higher pH). That makes it more friendly to bacterial growth even when made with pasteurized milk. Most aged cheeses have more acidity and much less moisture, along with a higher salt content. Those factors help reduce or eliminate bacterial growth except for starter cultures deliberately introduced during the cheese making process.

And third, many of these soft cheeses are produced by small, artisanal cheese makers who may not have the experience to understand the risks inherent in this product. Little or no food safety training is required for these vendors. Listeria can be present in the environment, in the water used to make the cheese, or in the milk itself. FDA inspectors visited more than 100 cheese making facilities in New York in April 2010, and found Listeria in 24 facilities; more than half of those were small artisanal operations.

Once Listeria is introduced into the environment, it persists, especially since it can grow at refrigerator and freezer temperatures. Post-pasteurization contamination of milk can also occur.

The symptoms of a Listeria infection include high fever, headache, muscle aches, confusion, and neck stiffness, along with nausea, abdominal cramps, and diarrhea. Pregnant women make up 30% of all Listeria monocytogenes food poisoning cases, even though their illness may be very mild. Any pregnant women who suffers flu-like symptoms should see their doctor immediately. Listeriosis can cause stillbirth, miscarriage, and infection in the newborn baby.

Pregnant women, young children, the elderly, and those with chronic diseases and compromised immune systems should avoid eating these soft cheeses: feta, Brie, Camembert, blue-veined cheeses, and Mexican style cheeses such as queso blanco, queso fresco, and panel. A Listeria infection in those populations can cause serious complications and death.

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