Microwave radiation interactions with molecular dipoles

MAE is thus another special case of liquid extraction accelerated by increasing the temperature of the solvent. It differs from traditional solvent extraction (including the Soxhlet method) by the fact that the heat is generated internally within the solvent-solid matrix system wherever the microwaves can interact with solvent molecules, including within any pores in the solid. MAE differs from SAE by the fact that no spectacular localized hot spots are involved. In addition, because of the mechanism involved, different solvents have different properties with respect to microwave radiation and this provides a degree of selectivity for MAE by adjusting the nature of the solvent. The principal molecular properties involved in response to a microwave field are the dipole moment of the solvent and the extent of nonbonding interactions... 

The cooking action in a microwave oven results from the interaction between the electric field component of the radiation with the polar molecules— mostly water—in food. AH molecules rotate at room temperature. If the frequency of the radiation and that of the molecular rotation are equal, energy can be transferred from the microwave to the polar molecule. As a result, the molecule will rotate faster. This is what happens in a gas. In the condensed state (for example, in food), a molecule cannot execute the free rotation. Nevertheless, it still experiences a torque (a force that causes rotation) that tends to align its dipole moment with the oscillating field of the microwave. Consequently, there is friction between the molecules, which appears as heat in the food. 

---