Polar molecules, microwave heating

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

As an illustration of this principle, a volatile polar molecule is a byproduct, eliminated as a result of the microwave heating (Eq. 4), and the equilibrium is shifted to completion. The second effect of irradiation is activation of the alkylation step itself (Eq. 5). All the reagents can be used in the theoretical stoichiometry. Some indicative results are given in Tab. 5.4 [9]. 

The rate was enhanced up to 2.6-fold for reaction of the 2-isomer and up to 14-fold for the 4-isomer. The product distribution in the final reaction mixtures was always somewhat different when microwave heating was used. The results were explained in terms of efficient interaction of microwaves with a highly polarized reagent molecule adsorbed on the acidic active site. Possible superheating of the active sites was difficult to detect (Sect. 10.3.3). 

There are two major mechanisms by which the microwave electric field is converted to heat within a food. The first, the ionic interaction, comes from the linear acceleration of ions by the field. These ions are primarily from various salts within the product. The second interaction is molecular rotation of polar molecules, primarily water, as well as weaker interactions with carbohydrates and fats. 

In considering the mechanism of interaction of microwave energy and materials, a simplified model of a capacitor with the material between charged plates can illustrate the more important aspects of heating (4). The ability of the material to maintain the charge separation (that is, resist current flow) is closely related to the inverse of the dielectric constant (c ). When materials are subjected to the electric field between the plates, those with permanent dipoles (polar molecules) will orient... 

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