Modeling Contamination in Poultry Meat

Raw poultry meat is easily contaminated with harmful foodborne pathogens such as Salmonella and Listeria monocytogenes with contamination possible at multiple stages during the life of the meat. Seeking to better understand when contamination happens, and to what extent, Lerasle et al. developed a probabilistic model focusing on contamination levels of Salmonella and Listeria monocytogenes in poultry meat before and after a high pressure (HP) treatment.¹

For their investigation, the team developed a model  based on four crucial steps in the shelf-life of poultry:

1. Reception of raw meat materials, mincing and mixing meat.

2. Partitioning and packaging into 200g modified atmosphere packs.

3. High pressure treatment of the meat.

4. Storage in chilled conditions until the end of the shelf-life.

Since contamination can occur during the very early stages of meat processing, the team collected data from the primary meat processing plants in France to get initial contamination levels of Salmonella and Listeria monocytogenes. This data made up steps one and two. Step three was the main focus for this investigation. Step four came from existing literature and data models.

The meat used in this study came from turkey meat (79%), pork fat (11%), spices (less than 0.1%) and water (10%). While at the factory, workers minced and mixed meats to form a 100kg mixture and then stored it at 4 °C prior to partitioning and packaging meats. Researchers surveyed 25g meat samples for Salmonella and L. monocytogenes using cultures on Xylose-Lysine-Desoxycholate (XLD) agar and Oxoid Brilliance Listeria Agar (Thermo Scientific), respectively. They found that the level of contamination of the minced meat, before cooking, was estimated to be low. They determined levels were in the 99th percentile of the distribution and equal to −2.3 log cfu/g for Salmonella and 0.5 log cfu/g for L. monocytogenes.

To study the effects of HP treatment and potassium lactate on these pathogens, the team evaluated samples using standard factory pressure conditions, including pressure at 200, 350 and 500 MPa, holding time for 2, 8 and 14 minutes and potassium lactate concentrations of 0 or 1.8% w/w. They cultured samples using Oxoid TSYEA plates (Thermo Scientific). Salmonella was unable to grow in the presence of lactate.

The following table helps summarize the calculated probability of contamination in raw poultry.

	L. monocytogenes			Salmonella
Steps	Prevalence	P(X ≤ 1 cfu/g)	P(X ≤ 10 cfu/g)	Prevalence	P(X ≤ 1 cfu/g)	P(X ≤ 10 cfu/g)
Mixing into 100-kg meat	100%	100%	100%	100%	100%	100%
Partitioning into 200 g	7.60%	100%	100%	62%	100%	100%
High pressure treatment	7.10%	100%	100%	4.14%	100%	100%
Storage	7.10%	93%	100%	4.14%	100%	100%

After the HP pressure treatment, the prevalence  decreased further down to a low level (4.14% and 7.10%) for Listeria monocytogenes and Salmonella, respectively. The HP treatment did indeed reduce the microbial prevalence and contamination level; however, they found lactate only affected microbial growth during storage in chilled conditions until the end of the shelf-life.

Summarizing the findings, the team recommends using HP treatment and lactate formulation before cooking poultry meats to decrease contamination. They also suggest future work should focus on stability in spoilage microorganisms after HP treatment, potassium lactate addition.

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Reference

1. Lerasle M., et al. (2014) “Assessment of Salmonella and Listeria monocytogenes level in ready-to-cook poultry meat: effect of various high pressure treatments and potassium lactate concentrations.” International Journal of Food Microbiology, 2014 Sep 1;186:( pp. 74-83). doi: 10.1016/j.ijfoodmicro.2014.06.019. Epub 2014 Jun 27.