Objective: To assess the impact of sodium chloride, pH and acid type on destruction effect of microwave heating on Listeria monocytogenes biotype 4a in liquid medium.
Design: Experimental study
Procedure: Bacterial suspension (1.5 x 107 CFU ml-1) in nutrient
broth with different concentrations of sodium chloride (0.5, 1.5
and 3.0 percentage) was prepared at various pH values (4.5, 5.5 and 6.5) induced by hydrochloric acid, and acetic acid. The suspensions were irradiated with microwave at successive five ¬second intervals and bacterial colonies were counted with standard plate count method. Radiation was continued until the microbial count reached to zero. Each experiment was repeated three times. Statistical analysis: One way ANOVA repeated measures.
Results: Various concentrations of sodium chloride had no significant effect on destruction of Listeria monocytogenes by microwave
heating (p>0.05). At pH=4.5, the destruction effect of microwave
radiation was significantly greater than those at pH values of 5.5 and 6.5 (p<0.0l). The destruction effect of microwave was not affected by acidulent type at pH=4.5 (p>0.05).
Conclusion: Several factors affect both microwave heating and heat resistance of microorganisms. The effect of each factor such as sodium chloride may be affected by interactions among other factors, resulting in opposite effects of the considered factor. The
present study shows that sodium chloride in liquid media at levels equal to those offood (0.5-3.0 percentage), had no significant effect
on destruction of Listeria monocytogenes by microwave heating. The heat sensitivity of bacteria increases by acidic condition.
Although the destruction effect of microwave radiation increased significantly at pH=4.5, it was not affected at pH levels equal to those of food (5.5 -6.5). At equal pH levels, organic acids increase the destruction effect of heat more than inorganic acids. At the present study, however, no significant difference was observed in liquid media containing acetic acid or hydrochloric acid at pH=4.5.
J. Fac. Vet. Med. Univ. Tehran. 58, 4:395-400, 2003.