Behavior of Listeria monocytogenes in commercial queso fresco and queso cotija and investigation of salt concentration as a control factor

ABSTRACT: Hispanic-style cheeses (HSC), particularly fresh soft varieties, have been associated with numerous outbreaks of Listeria monocytogenes, highlighting the need to identify product characteristics or interventions that could mitigate L. monocytogenes growth. This study characterized the behavior of L. monocytogenes in commercial HSC, specifically queso fresco (QF) and queso cotija (QC), with the goal of identifying compositional characteristics that could be used as a potential control measure to reduce foodborne illness risks associated with these products. Commercial QF and QC products from 2 production lots from 5 unique manufacturers were purchased at retail. Cheeses were divided into 20 g portions and were inoculated with a 5-strain L. monocytogenes cocktail (QF: 3 log cfu/g; QC: 5 log cfu/g). Populations of inoculated cheeses were monitored during storage at 7°C for 28 d by spread plating on Harlequin Listeria selective agar (37°C, 48 h). Samples from each production lot of commercial HSC were analyzed for aerobic plate count (tryptic soy agar with 0.6% yeast extract; 37°C, 48 h), lactic acid bacteria count (de Man, Rogosa, and Sharpe; 30°C, 48 h, anaerobic), and compositional properties (salt, moisture content, pH, water activity [aw]) at the beginning of each trial (n = 2). The QC products did not support growth of L. monocytogenes, likely due to the influence of reduced aw (0.92 ± 0.01) and pH (5.32 ± 0.25) values; however, populations remained stable during storage. The majority of commercial QF products (4/5; aw: 0.96 ± 0.00; pH: 6.32 ± 0.11) supported L. monocytogenes growth (>2 log cfu/g increase within 28 d); however, L. monocytogenes populations were consistently controlled in QF from a single manufacturer. Regression analysis suggested salt concentration as a likely predictor of L. monocytogenes control in QF (least squares regression). Inoculation studies on laboratory-manufactured QF, with 1% to 3% (wt/wt) salt, confirmed the inhibitory relationship between salt content and L. monocytogenes growth (least squares regression); however, the highest salt concentration tested did not inhibit L. monocytogenes growth. This study identifies product characteristics of commercial QF and QC products that influence L. monocytogenes behavior and informs strategies to improve the safety of HSC.