A new model is validated to predict the growth of listeria in artisanal fresh cheeses

Listeriosis is one of the main zoonotic food-borne diseases and, although the number of cases is relatively low, its high mortality makes it a major public health problem. For the bacterium to cause disease, it is not enough for the food to be contaminated; it is also necessary for the pathogen to reach a certain concentration. As almost always happens in science, it is the dose that makes it poisonous. 

With the aim of studying precisely the proliferation of this bacterium, a research group with the Department of Bromatology and Food Technology at the University of Cordoba has developed a new mathematical model capable of predicting the growth of the pathogen within food, making it possible to estimate the shelf lives of food products under different foreseeable storage conditions. 

Specifically, the team has validated the model in artisanal fresh goat milk cheeses, a product that has been involved in several outbreaks of listeriosis in recent years and that could pose a greater risk than others, as it is considered a Ready-to-Eat Food (RTEF), a group including those products that do not require any heat treatment before being ingested, and that are prepared to be consumed as-is. 

According to researcher Olga Bonilla, the study's lead author, the work focused on developing a predictive growth model by analyzing different storage temperatures (4, 12, 18 and 25 degrees) in order to calculate, using predictive microbiology techniques, the time it takes for the food to reach the maximum concentration of the bacteria allowed by law. 

According to the results, the specific product the research group worked with would cease to comply with the regulatory limits after four days in refrigeration conditions at 4 degrees, when it is initially contaminated. These results point to the high growth potential of listeria in fresh goat milk cheeses. This does not mean, the researcher stressed, that these figures can be extrapolated to other types of cheese. "The results obtained in our study are specific for the particular conditions under which the mathematical model was developed, and must be validated in other types of cheeses," she said. It is crucial, therefore, to adjust the tool to each specific cheese and insert manufacturing, storage, composition and pH variables in order to offer reliable estimates.

According to Antonio Valero, another of the study's authors, "the valuable thing" about the model is that it allows the producer to demonstrate that its product "is safe against listeria" and, after introducing its own production parameters, to estimate the shelf life of the food "based on scientific evidence." This, the researcher concludes, is especially useful considering that last year the European Union modified its food safety regulations, applying more restrictive criteria with regard to the control of Listeria. 

The work, carried out by the HIBRO group at the University of Cordoba, also featured collaboration with research staff at the Polytechnic Institute of Bragança (Portugal), Ibn Zohr University (Morocco) and the University of Sohag (Egypt).