A novel concept on discrete samples has been introduced to analyze the efficient heating processes due to microwave radiations. A preliminary investigation on average power absorption within samples vs sample thickness has been carried out for continuous samples and various regimes have been identified such that these regimes may correspond to minima or maxima of average power. Two discrete sample pieces replace a continuous sample for each of these regimes such that the total thickness of the discrete samples is identical with the thickness of the continuous sample. Electric fields and temperature have been solved for each discrete sample layer and the intermediate air layer. The thickness of air layer plays a significant role to dramatically alter the interference of waves and power absorption within each sample layer. It is observed that the discrete sample layers exhibit larger heating rate than that within continuous sample especially for smaller sample thicknesses. Suitable ratios of discrete sample thickness and the thickness of air layer also show large heating rate for larger sample thicknesses. The discrete samples also play important role to optimize thermal runaway. Based on two factors: 'large heating rate' and 'optimal thermal runaway', the efficient heating strategy has been recommended for highly lossy food substances, and case studies are also demonstrated for low lossy substances. (C) 2008 Elsevier Ltd. All rights reserved.