Microwave-assisted extraction: An introduction to dielectric heating

Abstract

Low environmental impact and high efficiency in chemical processing and manufacturing can be attained with the use of “light” instead of conductive heating. At microwave (MW) frequencies (300 MHz–300 GHz), the electromagnetic field can be dissipated by matter in the form of heat via polarization mechanisms, magnetic interactions, and ionic conduction. The conversion is extremely rapid, and heat is volumetrically diffused into the solvents, leading to an incredibly fast temperature increase that can speed up extraction processes in a way no other technique can match. In closed systems, microwave-assisted extraction can be performed at higher temperatures, and extraction times can be reduced drastically. In fact, enhanced plant matrix swelling increases the diffusion rate and promotes faster extraction kinetics. However, the user must take into account that the temperature distribution in a solvent exposed to microwave radiation is strictly dependent on the electromagnetic field distribution in the reactor, on the dielectric properties of the material, and on the presence of mass transport phenomena such as natural convection and stirring. Shorter extraction times and shorter cooling times usually avoid material degradation and afford high-quality extracts.