Some recommended cooking times only ‘marginally safe’

By Joseph James Whitworth contact

- Last updated on GMT

Examples of products used in the study
Examples of products used in the study

Related tags: Microbiology

Salmonella was still detected in a kebab and a quail roulade when they were cooked following label instructions supposed to be enough to eradicate the pathogen, according to research.

When products in the study were cooked following the guidelines, Salmonella Typhimurium (absence per 25g) was eliminated from meats with lower initial levels (10 and 100 cfu/g) but not 1000 cfu/g.

Salmonella was detected in the two 25g samples so the results show that recommended cooking times for these types of products are only marginally safe, said the authors.

Improper cooking did not fully eliminate Salmonella even at 10 cfu/g.

The study reproduced and assessed the effect of several types of cooking treatments (from label instructions and not following label instructions) on Salmonella Typhimurium DT 104 artificially inoculated in five types of poultry-based meat preparations (burgers, sausages, ready-to-cook-kebabs, quail roulades and extruded roulades) likely to be contaminated.

Three contamination levels (10 cfu/g; 100 cfu/g and 1000 cfu/g) and three cooking techniques (grilling, frying and baking) were applied to enable quantitative monitoring of Salmonella inactivation and to have relevant inoculum levels.

Temperature to eliminate

The temperature of 70 °C, considered appropriate to inactivate most pathogenic bacteria, was only sometimes reached at the end of the applied cooking treatments, said the researchers.

Even when label instructions were followed, temperature reached was not always 70 °C and it cannot always be considered safe, since Salmonella survived cooking to this heat, the instructions given on labels were not entirely satisfactory, found the study.

“Clearly, safe cooking instructions need to be provided to consumers with an appropriate safety margin to account for the wide range of conditions in which meat products will actually be prepared and heat treated.”​ 

Kebabs were the most common Salmonella-positive meat product after cooking, followed by sausages, burgers and extruded roulades.

In terms of cooking treatment applied, Salmonella Typhimurium was detected mostly after frying.

Next to cross-contamination, inadequate cooking is one of the most important factors contributing to foodborne illness, said the researchers.

Label instructions difference

After improper cooking, 26 out of 78 samples were positive, and 23 out of 26 were artificially contaminated with bacterial loads between 100 and 1000 cfu/g.

Nine out of 26 provided quantifiable results with a minimum level of 1.4 MPN/g in kebabs (initial inoculum level: 100 cfu/g) after grilling and a maximum level of 170 MPN/g recorded in sausages (initial inoculum level: 1000 cfu/g) after grilling.

The recommended time/temperature combination of 70 °C for two minutes to produce > 6 logarithm reduction of the most heat resistant bacterium, Listeria monocytogenes, is effective in minimizing risks from foodborne pathogens and is a critical control point for industry, found the study.

On the delivery day the food matrices were analysed by real-time PCR using a commercial kit validated to ISO 16140.

After incubation in Buffered Peptone Water (BPW) for 16 to 20 h at 37 °C, 10μl of pre-enriched sample was used to perform DNA extraction and amplification.

Detection and quantification of Salmonella spp. were performed according to the ISO 6579:2002/Amd1:2007 classical detection method per 25g of meat and the ISO/TS 6579-2 miniaturized Most Probable Number (mini-MPN) method.

Finally, detected Salmonella were serotyped (White–Kauffmann–Le Minor scheme) by slide agglutination with O and H antigen specific sera.

Following first tests on naturally contaminated products, which in most cases contained numbers of Salmonella spp. that were too low (< 1–10 MPN/g) to quantify inactivation, it was decided to use artificially contaminated ones.

Source: International Journal of Food Microbiology Volume 197, 16 March 2015, Pages 1–8

Online ahead of print, DOI: 10.1016/j.ijfoodmicro.2014.12.007

“Survival of Salmonella​ Typhimurium in poultry-based meat preparations during grilling, frying and baking”

Authors: Anna Roccato, Mieke Uyttendaele, Veronica Cibin, Federica Barrucci, Veronica Cappa, Paola Zavagnin, Alessandra Longo and Antonia Ricci.

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