Effect of Microwaves on Escherichia coli and Bacillus subtilis
1Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
Abstract:
Suspensions of Escherichia coli and Bacillus subtilis spores were exposed to conventional thermal and microwave energy at 2,450 MHz. The degrees of inactivation by the different energy sources were compared quantitatively. During the transient heating period by microwave energy, approximately a 6 log cycle reduction in viability was encountered for E. coli. This reduction was nearly identical to what is expected for the same time-temperature exposure to conventional heating. Heating of B. subtilis spores by conventional and microwave energy was also carried out at 100 C, in ice and for transient heating. The degree of inactivation by microwave energy was again identical to that by conventional heating. In conclusion, inactivation of E. coli and B. subtilis by exposure to microwaves is solely due to the thermal energy, and there is no per se effect of microwaves.
Insights
Microwave inactivation of Escherichia coli and Bacillus subtilis spores is solely due to thermal energy. No per se microwave effect was observed, with inactivation matching conventional heating methods.
Area of Science:
- Microbiology
- Food Science
- Biophysics
Background:
- Conventional thermal processing is a standard method for microbial inactivation.
- Microwave heating is an alternative method with potential for rapid heating.
Purpose of the Study:
- To quantitatively compare the microbial inactivation efficacy of microwave energy versus conventional thermal energy.
- To determine if microwaves have a direct (per se) effect on microbial inactivation beyond thermal effects.
Main Methods:
- Suspensions of Escherichia coli and Bacillus subtilis spores were subjected to microwave (2,450 MHz) and conventional thermal heating.
- Inactivation levels were measured quantitatively.
- Heating was performed under various conditions, including transient heating, at 100°C, and in ice.
Main Results:
- Microwave heating of E. coli resulted in a 6-log cycle reduction in viability during transient heating, comparable to conventional heating.
- B. subtilis spores showed identical inactivation levels when heated by microwave or conventional energy under all tested conditions (100°C, ice, transient heating).
Conclusions:
- The inactivation of Escherichia coli and Bacillus subtilis by microwave energy is exclusively attributed to the thermal energy generated.
- There is no evidence of a direct, non-thermal (per se) effect of microwaves on these microorganisms.
Related Concept Videos
01:28Antimicrobial Effectiveness
01:23Stringent Response in E. coli
01:18Bacterial Gastroenteritis