Destruction and injury of Escherichia coli during microwave heating under vacuum

Aims: To study the effect of 2450 MHz microwave radiation under vacuum (vacuum microwave or VM) on survival and injury of Escherichia coli and to search for possible nonthermal effects associated with VM. Methods and Results: Destruction kinetics of E. coli in peptone water were determined in a continuous-flow vacuum system, heated by convection heating in a water bath or with microwaves (VMs). Vacuum was used to control the boiling point of water and to maintain temperature in the bacterial suspensions at specified levels (49-64degreesC). Conclusions: z-Value in the water bath treatment was 9.1degreesC while for VM at 510 and 711 W it was 6.2 and 5.9degreesC, suggesting that E. coli is more sensitive to temperature changes under microwave heating. Arrhenius calculations of the activation energies of the destruction reactions suggest that the mechanism of destruction in VM may be different from that of conventional heat. The number of injured micro-organisms showed no significant differences among treatments. Significance and Impact of the Study: The impact of temperature on E. coli destruction was different when microwaves were the medium of heat transfer, suggesting the existence of factors other than heat contributing to the lethal effect of VM.