Distribution of Salmonella contamination of lymph node origin in ground beef and application of an antimicrobial intervention during the grinding process

Date

2016-05

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Abstract

Salmonella prevalence in ground beef has remained relatively steady over recent years despite the use of numerous pathogen decontamination interventions throughout the beef production system. Salmonella contamination of ground beef remains a public health concern. Peripheral lymph nodes of cattle have been shown to harbor Salmonella. These peripheral lymph nodes are often incorporated with trimmings used to produce ground beef and are therefore, a potential source of Salmonella contamination in ground beef. The objective of the first study was to develop a model to demonstrate how Salmonella, introduced via the inclusion of multiple peripheral lymph nodes of cattle, is distributed in ground beef during its production. In this study, six peripheral lymph nodes collected from a steer intradermally challenged with Salmonella were incorporated with 10 kg of beef trimmings per replicate (six replicates). This intradermal challenge method has been shown to repeatedly and consistently produce Salmonella-positive peripheral lymph nodes. The percentage of Salmonella-positive samples varied across replicate in that Salmonella was recovered from 88.1, 55.9, 50.5, 29.0, 68.0, and 48.5% of ground beef samples in replicates 1 through 6, respectively. Most samples contained Salmonella below the limit of quantification (1 CFU per g). For those with quantifiable concentrations, the mean concentration was 2.3 log10 CFU per 100 g ground beef sample. This model appears useful for introducing Salmonella into the grinding process via peripheral lymph nodes and might be useful to evaluate potential post-harvest intervention strategies to mitigate Salmonella contamination of this origin. The objective of the second study was to describe the distribution of Salmonella in both coarse and fine ground beef using a single peripheral lymph node as the vehicle of Salmonella contamination. This study consisted of two steps. In each step, a single peripheral lymph node harvested from a steer intradermally challenged with Salmonella was incorporated with 10 kg beef trimmings per replicate (six replicates). The purpose of Step 1 was to describe the distribution of Salmonella in coarse ground beef, whereas the purpose of Step 2 was to describe the distribution of Salmonella fine grinding the coarsely ground beef. In Step 1, the percentage of Salmonella-positive samples varied somewhat across replicate as Salmonella was recovered from 3.0, 14.0, 6.1, 14.0, 18.4, and 7.0% of coarse ground beef samples in replicates 1 through 6, respectively. No samples from Step 1 contained Salmonella above the limit of quantification (1 CFU/g). In Step 2, the percentage of Salmonella-positive samples in replicates 1 through 6 was much greater than Step 1 at 80.4, 88.9, 96.0, 97.9, 100.0, and 100.0%, respectively. In replicates 3 through 6, 1.0, 3.1, 1.1, and 61.6% of samples contained quantifiable concentrations of Salmonella, respectively. Of those with quantifiable concentrations, the mean concentration of Salmonella recovered from fine ground beef samples was 1.4 log10 CFU/g. Following a single pass through the grinder, Salmonella-positive samples were highly clustered; however, when the coarse ground product was mixed and ground a second time, Salmonella was more uniformly distributed. The objective of the final study was to test the decontamination efficacy of an aqueous solution of sulfuric acid and sodium sulfate as an intervention during re-grinding of beef in which Salmonella was inoculated onto the surface of initial beef trimmings. In replicates 1 through 3, beef trimmings were inoculated with Salmonella Anatum, and in replicates 4 through 6, trimmings were inoculated with Salmonella Montevideo. In replicate 1, approximately 15% of beef trimmings were inoculated on one side with Salmonella at a target concentration of 9 log10 CFU per ml, while in replicates 2 through 6, 15% of trimmings were inoculated with Salmonella at a concentration of 7 log10 CFU per ml. The trimmings were then ground once using a 0.95 cm grinder plate to produce coarsely ground beef and then divided into two 5-kg batches with one to serve as the control and one as the treated. The 25 ml of the undiluted intervention was spray-applied for two minutes to the coarsely ground beef while the product was being mixed (50 revolutions per minute). Following application, the coarsely ground beef was ground a second time using a 0.64 cm grinder plate. Following application of treatment, ground beef was loafed and divided into approximately 50 100-g samples per replicate. All beef samples were then cultured qualitatively and quantitatively for Salmonella. The application of an aqueous solution of sulfuric acid and sodium sulfate resulted in a 0.34 log10 CFU/g reduction (P = 0.013) in Salmonella concentration across all replicates. In replicates 1 through 3, Salmonella Anatum concentration was reduced (P < 0.001) by 0.54 log10 CFU/g. In replicates 4 through 6 Salmonella Montevideo concentration in treated samples was not significantly reduced (P = 0.061). The minimum inhibitory concentration of the aqueous solution for Salmonella Anatum and Salmonella Montevideo was determined to be 0.31% and 0.63% concentration of the stock aqueous solution, respectively.

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Keywords

Salmonella, Ground beef, Lymph nodes, Intervention

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