Enhanced thermal effects under microwave heating conditions

This chapter brings into perspective issues related to the thermal and nonthermal effects of microwave heating. To distinguish thermal and nonthermal effects, most studies rely on experimental evaluation of existing differences in microbial or enzyme destruction under somewhat identical heating conditions, using some form of conventional heating and a microwave oven. Studies supporting additive microwave contribution to lethal effect over that caused by heat have been mainly criticized for lack of reliable temperature measurements or uncontrolled heating. Surprisingly, studies supporting only thermal effects don't suffer from such criticisms, although they are carried out under similarly questionable conditions. Results of several studies carried out in our laboratory under continuous flow microwave and conventional heating systems are provided to demonstrate meaningful comparison of microbial destruction (E. coli, L plantarian, and S. cerevisae) and enzyme inactivation (pectin methyl esterase) under the two heating modes. To quantify the thermal and/or nonthermal effects of microwave on microbial destruction and enzyme inactivation, traditional kinetic parameters in the form of D and z values were evaluated based on come-up and come-down time corrected temperature data obtained under experimental conditions. Modeling studies indicated that the time-temperature distribution in helical coils under continuous flow microwave heating systems closely matched a plug-flow distribution. D values for microorganisms and enzymes computed assuming such a distribution, effective residence times and microbial survivor data were almost an order of magnitude lower for microwaves than those obtained under comparable water bath heating conditions. Such consistent results obtained for microorganisms and enzymes in different systems clearly demonstrate that lethal contributions under microwave heating conditions cannot be solely explained on the basis of time-temperature distribution, thus implying additional contributory effects.