New research from Oregon State University provides insights into formulations and treatments that can help slow Listeria monocytogenes growth in soft, fresh cheeses like queso fresco, addressing a pathogen-commodity pair of concern with a history of causing foodborne illness outbreaks.

Queso Cotija Poses Less Risk of Listeria Growth Than Queso Fresco 

In a study published in Dairy Science, researchers with Oregon State University’s College of Agricultural Science and the Oregon Agricultural Experiment Station sought to characterize L. monocytogenes behavior in queso fresco and queso cotija. The researchers purchased samples of the two cheese types from two production lots and five unique manufacturers. The cheeses were inoculated with a five-strain L. monocytogenes cocktail and stored for 28 days at 7 °C, during which time the bacterial populations on the cheeses were monitored.

The researchers found that the queso cotija samples did not support the growth of L. monocytogenes, likely due to decreased water activity and lower pH, but populations remained stable during storage, indicating that L. monocytogenes contamination still poses a concern.

On the other hand, the majority of queso fresco products supported L. monocytogenes growth at levels greater than 2 colony-forming units per gram (CFU/g) across 28 days; however, L. monocytogenes populations were consistently controlled in samples of one queso fresco product from a single manufacturer, suggesting that formulation or production processes can influence risk.

Salt Slows Listeria Growth in Queso Fresco

To better understand the characteristics of the cheeses that influence L. monocytogenes growth, the researchers conducted a regression analysis and discovered that salt concentration is a likely predictor of growth in queso fresco. Inoculation studies on queso fresco, with 1–3 percent salt, showed that salt inhibits L. monocytogenes growth but does not stop it completely. Interestingly, the highest salt concentration tested did not inhibit L. monocytogenes growth.

Organic Acid Treatments Show Promise

In a related study published in the Journal of Food Protection, the researchers evaluated whether treating contaminated queso fresco cheeses with food-grade organic acids could inhibit L. monocytogenes growth while preserving the cheeses’ organoleptic properties. The researchers applied acetic, lactic, and propionic acids to the surface of L. monocytogenes-inoculated cheeses. They found that dipping the cheeses in stronger acetic and peracetic acid concentrations prevented L. monocytogenes growth during 28 days of storage at 7 °C. Lactic acid only worked when the pH of the queso fresco was adjusted to be more acidic, which may affect taste.