ARS | Publication request: Effect of storage at 4 and 10 C on growth of listeria monocytogenes in Queso Fresco

Submitted to: Journal of Food Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 13, 2012
Publication Date: April 1, 2012
Citation: Leggett, L.N., Tomasula, P.M., Van Hekken, D.L., Porto Fett, A.C., Shoyer, B.A., Luchansky, J.B., Renye Jr, J.A., Farkye, N. 2012. Effect of storage at 4 and 10 C on growth of listeria monocytogenes in Queso Fresco. Journal of Food Safety. 32:236-245.

Interpretive Summary: Queso Fresco (QF) is a popular, rennet-set, Hispanic-style fresh cheese made from pasteurized milk known for its crumbly texture and non melting properties. However, its high moisture content, near neutral pH and moderate salt content provide the ideal conditions for growth of various bacterial species and pathogens such as L. monocytogenes. Past studies have examined the growth of L. monocytogenes on QF but were typically conducted using retail cheeses of unknown make and handling procedures. In this study, the growth of L. monocytogenes on QF made using a popular make procedure in the U.S. was examined. Spoilage bacteria and other potential pathogens were also identified. This data and a primary model that was created from the data can be used as the basis for food safety assessments, process and formulation improvements, or to monitor the effectiveness of antimicrobials and process interventions in preventing growth of L. monocytogenes on QF.

Technical Abstract: A five-strain rifampicin - resistant Listeria monocytogenes cocktail (ca. 3.0 loglOCFU/g) was introduced as a post-pasteurization contaminant in Queso Fresco (QF) that was manufactured using a commercial make procedure. L. monocytogenes was either inoculated into (IN) the curds before slicing or on (ON) the slices (52 to 66 g), individually vacuum - packaged and stored at 4C and IOC. Growth was monitored for up to 35 days. Gompertz analyses showed no differences in lag time (LT) because of temperature but growth rate (GR) and generation time (GT) were faster at IOC than 4C. After 20 days for both the IN and ON treatments, the maximum population density was 7.80 + 0.17 regardless of storage temperature. These results indicate that QF manufacture must be conducted using Good Manufacturing Practices and under hygienic conditions and that the use of antimicrobials and/or or post-processing interventions are necessary to prevent L. monocytogenes growth.