Selective heating and enhanced boiling in microwave heating of multicomponent (solid–liquid) foods

Selective or enhanced heating of a solid in a solid–liquid mixture in microwaves that can lead to enhanced boiling and possibly bumping was studied. A soup containing broth and particulate foods was heated in a cylindrical container in a standard domestic microwave oven. Heating patterns were obtained using a coupled three dimensional thermal‐electromagnetic model, as well as an infrared camera, and boiling was visually observed. Selective heating of the solid particulates over the broth depends on their dielectric and thermal property differential. Enhanced and early boiling on the surface resulting from selective heating also depends on factors that aid in bubble formation and growth such as the surface roughness and porosity (gas pockets) in the solid. For quick prediction of the potential for enhanced heating in product design, a characteristic number was developed that can predict situations that are more prone to enhanced heating than the surrounding liquid. PRACTICAL APPLICATIONS: When microwave heating of a soup or a soup‐like multicomponent mixture of solid and liquid foods, for example carrot or corn pieces in a broth, the solid can end up heating faster than the surrounding liquid, leading boiling to be initiated preferentially at the solid surface, as opposed to in the bulk broth. Depending on the solid material and its surface properties, such a boiling can be further enhanced and sudden, leading to possible bumping during the heating process. A simple design parameter, based on easily available dielectric and thermal properties of the solid and liquid, was developed to quickly predict the possibility of such an enhanced heating of the solid surface (that could eventually lead to bumping). When designing microwaveable products like soups, this simple parameter can be of particular help, reducing the time and resources for experimentation by providing a quick estimate of the probability of enhanced boiling and thus bumping.