Kinetics of Escherichia coli destruction by microwave irradiation

The kinetics of destruction of Escherichia coli cells suspended in a solution by microwave irradiation with a microwave oven were studied. During radiation at several powers, the temperature of 0.01 M phosphate buffer (PB), pH 7.0, in a glass beaker increased linearly at a rate of A (degrees Centigrade per second) according to the exposure time. When E. coli cells suspended in PB were exposed in the same beaker, the number of viable cells decreased according to the exposure time and the power used. The survival curve was approximated to a set of three linear parts. For each part, a rate constant of destruction (k) and an extrapolated starting temperature (T0) at several powers were estimated. Thereafter, the relationships between A and k and between A and T0 were studied. When a flat petri dish was used, the A value of exposed PB was lower and bacterial destruction was inhibited; the survival curve was similar to a curve predicted from the A value by using the relationships between the parameters. As the concentration of salt in the solution increased (from 0 to 1.35 M), the A value decreased and bacterial destruction was more suppressed. No remarkable difference between the destruction profiles for microwave exposure and conventional heating, which had the potential to generate an equal A value, was detected. These results showed that the parameter A of an irradiated solution is essential when kinetics of bacterial destruction by microwave exposure are studied and that the destruction profile can be interpreted mostly by means of thermal effects.