Survival and growth of Listeria monocytogenes on queso fresco cheese stored under modified atmospheres

Abstract Text:

Cheese varieties characterized by high moisture and low acidity, such as Queso Fresco (QF), have been shown to support the growth of Listeria monocytogenes to very high levels during refrigerated storage. In addition to improving quality and extending shelf life, modified atmosphere packaging (MAP) has been used to control the growth of pathogenic microorganisms in various foods. The objective of this research was to determine the effect of five MAP conditions on the survival and growth of L. monocytogenes as post-processing contaminants on QF during refrigerated storage at 7°C. To test the hypothesis that MAP affects L. monocytogenes growth on QF during storage when compared to conventional methods of packaging (i.e., vacuum), 25g samples of QF were surface inoculated with an eight-strain cocktail of L. monocytogenes to achieve 4 log CFU/g.  Following microbial attachment, individual cheeses were placed in 75-micron high barrier pouches (nylon/ethylene vinyl alcohol/polyethylene), packaged under one of seven conditions (air; vacuum; 100% carbon dioxide (CO₂); 70% CO₂ /30% nitrogen (N₂); 50% CO₂/50% N₂; 30% CO₂/70% N₂; or 100% N₂), and stored at 7°C.  Samples were removed weekly through 28 days of storage for enumeration of L. monocytogenes.  Data were analyzed using one-way ANOVA.  Analyses identified overall effects of time and packaging treatment on the change in L. monocytogenes counts over 28 days (P <0.001). L. monocytogenes populations increased rapidly on cheese packaged under air, vacuum, and 100% N₂, with counts differing significantly (P<0.001) from the initial inoculum by day 7.  Changes in counts over time and counts on individual days did not differ between these treatments with means exceeding 7 log CFU/g on day 14 and stabilizing at >8 log CFU/g through day 28. Treatments that incorporated CO₂ at any percentage significantly limited pathogen growth over time compared to treatments without CO₂, including air and vacuum controls (P<0.001). Although pathogen growth was limited, the change in counts over 28 days in CO₂ treatments was significant (P<0.05), reaching a mean of 5.0 log CFU/g. Pathogen growth during storage did not differ significantly between treatments with varying percentages of CO₂.  These data demonstrate that vacuum packaging and conditions containing 100% N₂ do not impede the growth of L. monocytogenes on QF.  However, packaging under anaerobic modified atmospheres containing CO₂ may be a promising control for limiting L. monocytogenesgrowth on QF and other high moisture, low acid cheeses during cold storage. 

Keywords: packaging, Listeria monocytogenes, cheese