Abstract
Three brands of commercial roast beef were purchased and artificially inoculated with a 5-strain Listeria monocytogenes cocktail at 2 inoculation levels (approximately 3 and 6 Log CFU/g). Although all 3 brands contained sodium diacetate and sodium lactate, inoculated Listeria cocktail survived for 16 d in all 3 brands; significant increases in L. monocytogenes numbers were seen on inoculated Brand B roast beef on days 12 and 16. Numbers of L. monocytogenes increased to 4.14 Log CFU/g for the 3 Log CFU/g inoculation level and increased to 7.99 Log CFU/g for the 6 Log CFU/g inoculation level by day 16, with the pH values being 5.4 and 5.8 respectively. To measure the cell viability in potential biofilms formed, an Alamar blue assay was conducted. Brand B meat homogenate had the highest metabolic activities (P < 0.05). By comparing its metabolic activities to Brands A and C and the inoculated autoclaved meat homogenates, results indicated that the microflora present in Brand B may be the reason for high metabolic activities. Based on the denaturing gradient gel electrophoresis and the Shannon–Wiener diversity index analysis, the “Brand” factor significantly impacted the diversity index (P = 0.012) and Brand B had the highest microflora diversity (Shannon index 1.636 ± 0.011). Based on this study, results showed that antimicrobials cannot completely inhibit the growth of L. monocytogenes in ready-to-eat roast beef. Native microflora (both diversity and abundance), together with product formula, pH, antimicrobial concentrations, and storage conditions may all impact the survival and growth of L. monocytogenes.
Practical Application
This study evaluated the survival and growth of Listeria monocytogenes artificially inoculated on ready-to-eat roast beef, investigated cell viability and metabolic activity in biofilm, and identified factors that may impact L. monocytogenes survival. In addition to previous literature, this study found that the diversity of native microflora in roast beef is another important factor that may impact the persistence of Listeria populations. The diversity and the abundance of the native microflora is a direct reflection of the manufacturing environment. This indicates that the control strategies taken by the ready-to-eat meat manufacturers have a direct impact on meat safety for the following storage and retail steps.
