Fresh Hispanic-style cheeses are a growing part of the U.S. dairy market, but they have also been tied to repeated Listeria monocytogenes outbreaks and recalls. The risk is straightforward. Listeria can survive in the cold and, in some foods, can still multiply in the refrigerator. When that happens, a small problem can become a serious one before a product is eaten.
Because queso fresco is a ready-to-eat food, any detectable level of Listeria monocytogenes is considered adulteration under federal food safety rules, which can trigger recalls and enforcement actions.
In response, researchers in Oregon State University’s College of Agricultural Sciences and the Oregon Agricultural Experiment Station studied two common cheeses, queso fresco and queso cotija, to better understand risk and to test practical ways to reduce it. In one study, published in the Journal of Dairy Science, the team bought queso fresco and queso cotija from five manufacturers.
They added Listeria to small portions and tracked what happened during 28 days of refrigerated storage. They also measured basic traits that influence whether bacteria can grow, such as acidity, moisture and salt.
The results showed a clear difference between the two cheese types. Queso cotija generally did not let Listeria multiply during refrigerated storage, likely because it is drier and more acidic. But the bacteria often did not go away. It held steady, which means contamination can still matter even when growth is limited.
Most queso fresco products, by contrast, did let Listeria multiply in the refrigerator. One brand was an exception. In that product, Listeria did not grow, suggesting that recipe or production differences can change risk from one company to another.
The team also tested whether increasing salt could reliably protect queso fresco. When they compared the store-bought cheeses, higher salt was linked with less growth. But when the researchers made queso fresco in the lab with 1% to 3% salt, Listeria still grew every time. More salt slowed growth, but it did not stop it. The takeaway is that salt may help, but it cannot be the only fix.
In a related study, published in the Journal of Food Protection, the researchers tested a second approach aimed at a common weak spot in fresh-cheese safety: contamination that can happen after pasteurization during handling and packaging. They asked a practical question: Could a short surface treatment keep Listeria from multiplying without changing the whole cheese?
They tested brief dips using food-grade organic acids that are already used in some foods, including acetic acid (the main acid in vinegar) and propionic acid (a common preservative). They found that stronger surface treatments with these acids kept Listeria from increasing during 28 days of refrigerated storage. A third acid, lactic acid, worked well only when conditions were more acidic than queso fresco typically is.
Together, the studies point to a clear message for industry and regulators. Queso cotija is generally less likely to support Listeria growth, but contamination can persist. Queso fresco is more likely to support growth during refrigeration. Salt can slow growth but is not enough on its own. Stronger safety gains are more likely when prevention is layered, including clean production practices and steps that can slow or stop growth if contamination occurs.
The study team included:
- Lead investigator Jovana Kovacevic, associate professor and Extension food safety specialist in the Department of Food Science and Technology, who leads food safety research and outreach at the OSU Food Innovation Center
- Joy Waite-Cusic, associate professor of food safety systems in the Department of Food Science and Technology
- Zoe Andersen, master’s student in the Department of Food Science and Technology and Food Innovation Center
- Samantha Kilgore, faculty research assistant in the Department of Food Science and Technology and Food Innovation Center
The OSU Food Innovation Center in Portland, which opened in 1999 as one of the nation’s first urban agricultural experiment stations, offers guidance on food safety and shelf-life testing to help businesses maintain quality and meet regulatory requirements.
Funding for this research was provided by BUILD Dairy and the Western Dairy Center at Utah State University, with additional support from Dairy Management Inc. in Rosemont, Illinois.
This research helps protect public health while supporting a stable dairy economy. It gives regulators and dairy processors clearer evidence about which cheeses are more likely to allow Listeria to multiply during refrigerated storage and why.
Better control can mean fewer illnesses, fewer costly recalls, less disruption for businesses and employees, and stronger consumer confidence in a growing category of dairy products.