Study examines listeria growth from packaging to consumer

In the study, microbial behaviour and concentration of L. monocytogenes in ten lots of naturally contaminated loin chops were evaluated under four different storage scenarios throughout the product shelf life.

Microbial counts of L. monocytogenes in ten lots of contaminated loin chops, produced from the same company over a one-year period, were evaluated.

The researchers said that the results should be used by risk managers to predict the effect of cooking, or undercooking on pathogen inactivation in loin chops, and the risk of cross contamination when the cooking utensils are used on raw contaminated loin chops and sterile foods, either cooked or ready-to-eat.

The EU regulation (EC) No 2073/2005, amended by No 1441/2007, does not include microbiological limits for L. monocytogenes in fresh meat.

Four scenarios​

Pathogen monitoring was conducted under four different scenarios (S) over the product shelf life:

• within two hours after packaging at the processing facility (S1)
• after transport and storage at retail up to day four or five of the product shelf life established one week after packaging (S2);
• after transport and storage at retail, transport at car temperature and storage at 6 or 14 °C (S3 and S4 respectively) up to the end of the product shelf life

Overall, the percentage of samples showing counts above 10 cfu/g of L. monocytogenes in the samples collected immediately after packaging (S1) was 5% and increased to more than 15% during storage at retail (S2) and to 20% and 39% during storage at temperatures mimicking mean and abuse temperature conditions in the consumer's refrigerators.

Storage at mean refrigerator temperatures did not increase the populations of L. monocytogenes over 2 log colony forming unit per gr (cfu/g).

In contrast, final storage at abuse temperatures produced microbial counts greater than 3 log cfu/g for some lots, which would require a more severe heat inactivation treatment before consumption.

S1-S4 results​

S1 ranged from 20 to 350 cfu/g, with a mean value of 104.71 cfu/g (2.02 log cfu/g), after refrigeration in the retail cabinets (S2) maximum values of L. monocytogenes population increased to 3 log cfu/g.

A higher percentage of positives were found in samples from S3, although population levels did not increase significantly. However, refrigeration at abuse conditions until the end of the product shelf life (S4) produced a higher population of positive samples, with levels close to ∼3.8 log cfu/g.

“…​pork meat products which are cooked at 55–70 °C are quite likely free of live L. monocytogenes​ cells. However, products that are undercooked may not be L. monocytogenes​ free when high contamination levels are present in the raw materials,” ​said de Cesare et al.

“Moreover, cross contamination between contaminated meat and utensils that come into direct contact with foods intended to be consumed without further cooking might occur.”​

Source: Food Control, Volume 34, Issue 1 November 2013, pages 198-207

Online ahead of print, DOI: 10.1016/j.foodcont.2013.04.027​

“Modeling growth kinetics of ​Listeria monocytogenes​ in pork cuts from packaging to fork under different storage practices”​

Authors: A. De Cesare, A. Valero, A. Lucchi, F. Pasquali, G. Manfreda