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Properly cooked pork chops may still contain listeria, salmonellaProperly cooked pork chops may still contain listeria, salmonella

After storage in refrigerator or at room temperature, leftover pork chops may have higher levels of pathogenic bacteria.

September 28, 2017

4 Min Read
Properly cooked pork chops may still contain listeria, salmonella

While many individuals prefer to consume their pork cooked to medium, a new study published in Risk Analysis: An International Journal revealed that cooking pork chops to an acceptable temperature does not completely eliminate pathogens but provides these cells with the opportunity to multiply during storage and harm consumers when the leftovers are consumed.

The study, "Impact of Cooking Procedures & Storage Practices at Home on Consumer Exposure to Listeria monocytogenes & Salmonella Due to the Consumption of Pork Meat," found that only pork loin chops cooked well-done in a static oven (the researchers also tested cooking on a gas stovetop) completely eliminated the listeria and salmonella pathogens.

Other levels of cooking, i.e., rare and medium, while satisfying the requirements of a product temperature greater than or equal to 73.6°C (165°F) and decreasing the pathogen levels, did leave behind a few surviving cells that then had the opportunity to multiply during food storage before being consumed.

It is generally believed that heat-treating meat is to 70°C (158°F) for two minutes achieves a 1 million-cell reduction in Escherichia coli, salmonella and listeria and that, thus, the meat will be free of pathogens and safe to eat. However, a report by the European Food Safety Authority revealed that more than 57% of salmonella outbreaks in 2014 were in the household/kitchen, and 13% were associated with inadequate heat treatment.

“The results of this study can be combined with dose-response models and included in guidelines for consumers on practices to be followed to manage cooking of pork meat at home,” said lead author Dr. Alessandra De Cesare, a professor at the University of Bologna in Italy.

In order to assess the pathogen levels in cooked pork, the researchers -- from the University of Bologna, the Institute of Food Engineering for Development and the Istituto Zooprofilattico delle Venezie -- tested 160 packs of loin chops. The samples were experimentally contaminated with 10 million cells of L. monocytogenes and salmonella to assess the reduction in pathogens after cooking, in accordance with U.S. Food Safety & Inspection Service (FSIS) and British Retail Consortium (BRC) specifications (ensuring a reduction of at least 100,000 and 1,000,000 cells, respectively). The samples were contaminated on the surface to mimic contamination via slaughter and cutting.

The samples were divided into groups to be cooked either over gas in a nonstick pan or in a static oven. In each setting, the pork chops were cooked to rare, medium and well-done. For each cooking combination, 40 repetitions were performed, for a total of 240 cooking tests.

The researchers also interviewed 40 individuals between the ages of 20 and 60 to determine their household consumer habits regarding doneness preferences. Prior published research was referenced to define meat storage practices and the probability that consumers store their leftovers at room temperature or in the refrigerator or discard them immediately. Growth rate data for the pathogens at each temperature were obtained using the software tool ComBase.

The only cooking treatment able to completely inactivate the pathogens was well-done in the static oven, which achieved a reduction of 1 million to 10 million cells, the researchers said. Statistical analyses of the data showed significant differences related to level of cooking and cooking procedure. However, the researchers explained that factors such as moisture, water activity, fat levels, salts, carbohydrates, pH and proteins can affect the cooking treatment and effectiveness and, as a consequence, the bacteria survival rate.

These results emphasize the need to consider the form of pork (such as whole muscle versus ground) being cooked, in addition to the final temperature necessary to inactivate pathogens, the researchers suggested.

The results show that a reduction of 1-10 million of pathogen cells was reached when applying all of the tested cooking treatments, with product temperatures always reaching 73.6°C or greater. However, according to the simulation results using the obtained cell growth rates, the few surviving cells can multiply during storage in both the refrigerator and at room temperature, reaching dangerous levels for vulnerable and regular consumers.

After storing leftovers, there was a probability for the concentration of pathogens to reach 10 cells, ranging between 0.031 and 0.059 for all combinations except oven well-done. Overall, the mean level of exposure to listeria and salmonella at the time of consumption was one cell for each gram of meat. The results obtained from this study can be implemented in consumer guidelines for practices to follow when cooking pork meat at home.

This research was funded by the European Union Seventh Framework Programme, Grant No. KBBE 222738 BASELINE.

Risk Analysis: An International Journal is published by the nonprofit Society for Risk Analysis, an interdisciplinary, scholarly, international society that provides an open forum for all who are interested in risk analysis, which is a critical function in complex, modern societies. Risk analysis includes risk assessment, risk characterization, risk communication, risk management and risk policy affecting individuals, public- and private-sector organizations and societies at a local, regional, national or global level.

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