Effect of storage at 4 and 10C on the growth of Listeria monocytogenes in and on queso fresco
Posted: April 20th, 2012 - 1:52pm
Source: Journal of Food Safety, Volume 32, Issue 2, pages 236–245, May 2012
A five-strain rifampicin-resistant Listeria monocytogenes cocktail (ca. 3.0 log10 cfu/g) was introduced as a postpasteurization contaminant in queso fresco (QF) that was manufactured using a commercial procedure. L. monocytogenes was either inoculated into (IN) the curds before forming the cheese block or onto (ON) slices (52–66 g), individually vacuum-packed and stored at 4 and 10C. Growth was monitored for up to 35 days. Gompertz analyses showed small differences in lag time because of temperature, but growth rate and generation time were faster at 10C than at 4C. After 20 days for both the IN and the ON treatments, the maximum population density was 7.80 ± 0.17, regardless of the 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 postprocessing interventions is necessary to prevent the presence and growth of L. monocytogenes.
Queso fresco (QF) is a popular, rennet-set, Hispanic-style fresh cheese made from pasteurized milk known for its crumbly texture and nonmelting properties. However, its high moisture content, near neutral pH and moderate salt content provide the ideal conditions for growth of various spoilage and pathogenic bacteria, notably Listeria monocytogenes. Past studies have examined the growth ofL. monocytogenes on QF but were typically conducted using retail cheese of unknown manufacturing, handling and storage procedures. In this study, the growth of L. monocytogenes was monitored in and on QF made according to a commercial procedure used in the U.S.A. These data and a primary model that was created from the data can be used as the basis for food safety assessments and process and formulation improvements, or to monitor the effectiveness of antimicrobials and process interventions in preventing the presence and growth of L. monocytogenes associated with QF.