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Lethal effect of microwaves on spores of Bacillus spp
Jinn-Chyi Wang1, Shu-Hui Hu, Chin-Yang Lin
1Department of Food Science and Technology, Tajen Institute of Technology, 20, Wei-Shin Road, Shin-Erh Village, Yen-Pu Hsing, Ping Tung, Taiwan. jicy.wang@msa.hinet.net
Abstract:
Strains representing four types of common heat-resistant spores of Bacillus spp., B. cereus CCRC 14655, B. coagulans CCRC10606, B. licheniformis CCRC14693, and B. subtilis CCRC14199, were heated with microwaves at different power levels and under different conditions in salt solutions, starch solutions, and containers. The results of this study showed that B. licheniformis spores had the highest microwave tolerance at a power level of 100% for different incubation times. B. coagulans spores showed the lowest microwave tolerance in salt solutions with water activity values of 0.6, 0.7, 0.8, and 0.9, and B. licheniformis spores were the most resistant in the tested salt solutions at different incubation times. An analysis of the effect of the viscosity of the medium revealed that the bacteria had the lowest microwave resistance in a medium containing <0.8% starch in solution. The microwave resistance levels of the test microorganisms were the lower in glass containers than in polypropylene containers and aluminum foil-enclosed pouches. Of the four species of bacilli, B. licheniformis had the highest microwave tolerance (P < 0.05) under all conditions.
Insights
Bacillus licheniformis spores exhibit the highest microwave resistance among common heat-resistant Bacillus species. Factors like salt concentration, viscosity, and container type significantly influence microwave tolerance in bacterial spores.
Area of Science:
- Food Microbiology
- Food Processing Technology
- Microbial Spore Resistance
Background:
- Heat-resistant bacterial spores, particularly from Bacillus species, pose challenges in food preservation.
- Understanding spore resistance to non-thermal processing methods like microwave heating is crucial for effective sterilization.
Purpose of the Study:
- To evaluate the microwave tolerance of four common heat-resistant Bacillus spore types.
- To investigate the influence of medium composition (salt, starch) and packaging on spore resistance.
Main Methods:
- Microwave heating of Bacillus cereus, B. coagulans, B. licheniformis, and B. subtilis spores.
- Varying microwave power, incubation times, salt solutions (aw 0.6-0.9), starch solutions (<0.8%), and containers (glass, polypropylene, aluminum foil).
Main Results:
- Bacillus licheniformis spores demonstrated the highest microwave tolerance across all tested conditions.
- Bacillus coagulans spores showed the lowest tolerance in salt solutions.
- Lower microwave resistance was observed in media with <0.8% starch and in glass containers.
Conclusions:
- Bacillus licheniformis is the most microwave-tolerant species among those tested.
- Medium viscosity and water activity, along with container material, significantly impact Bacillus spore resistance to microwave heating.