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Thermal inactivation of Listeria monocytogenes during a process simulating temperatures achieved during microwave
P J Coote1, C D Holyoak, M B Cole
1Microbiology Section, Unilever Research, Sharnbrook, UK.
Abstract:
Conventional heating was used to expose cells of Listeria monocytogenes, either in broth or in situ on chicken skin, to the mean times and temperatures that are achieved during a 28 min period of microwave cooking of a whole chicken. Heating L. monocytogenes by this method in culture broth resulted in a reduction in viable cell numbers by a factor of greater than 10(6) upon reaching 70 degrees C. Simulated microwave cooking of L. monocytogenes in situ, on chicken skin, resulted in more variability in the numbers of survivors. Heating for the full cook time of 28 min, however, resulted in a mean measured temperature of 85 degrees C and no surviving listerias were detected. This indicated a reduction in viable numbers of greater than 10(6). To reduce temperature variation, cells were heated on skin in a submerged system in which exposure to 70 degrees C for 2 min resulted in a reduction in viable cell numbers of all strains of listerias tested of between 10(6) and 10(8). These results show that when a temperature of 70 degrees C is reached and maintained for at least 2 min throughout a food there is a substantial reduction in the numbers of L. monocytogenes. The survival of this organism during microwave heating when temperatures of over 70 degrees C are reported is probably due to uneven heating by microwave ovens resulting in the presence of cold spots in the product. The heat resistance of L. monocytogenes is comparable with that of many other non-sporing mesophilic bacteria.
Insights
Microwave cooking can significantly reduce Listeria monocytogenes bacteria, especially when temperatures reach 70°C for at least two minutes. Uneven microwave heating, however, can create cold spots, allowing some bacteria to survive.
Area of Science:
- Food safety
- Microbiology
- Thermal processing
Background:
- Listeria monocytogenes is a foodborne pathogen that can survive inadequate cooking.
- Microwave cooking effectiveness against L. monocytogenes can be variable due to uneven heating.
Purpose of the Study:
- To evaluate the efficacy of microwave cooking temperatures and times against Listeria monocytogenes.
- To determine the impact of temperature variations on bacterial survival during microwave heating.
Main Methods:
- Exposing Listeria monocytogenes in broth and on chicken skin to simulated microwave cooking conditions.
- Measuring bacterial reduction at various temperatures and durations, including 70°C for 2 minutes.
- Investigating the effect of submerged heating to reduce temperature variability.
Main Results:
- Heating in broth to 70°C reduced viable L. monocytogenes by over 10^6 CFU/mL.
- Simulated microwave cooking on chicken skin showed variable survival, but 28 minutes at 85°C eliminated all detectable bacteria (>10^6 CFU).
- Submerged heating to 70°C for 2 minutes reduced bacterial counts by 10^6 to 10^8 CFU.
Conclusions:
- Maintaining a temperature of 70°C for at least 2 minutes throughout food ensures substantial reduction of L. monocytogenes.
- Uneven heating in microwave ovens, leading to cold spots, is a likely reason for L. monocytogenes survival at reported temperatures over 70°C.
- The heat resistance of L. monocytogenes is similar to other non-sporing mesophilic bacteria.