Other Applications of Microwave Heating

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

Microwaves are electromagnetic radiation placed between infrared radiation and radio frequencies, with wavelengths of 1 mm to 1 m, which corresponds to the frequencies of 300 GHz to 300 MHz, respectively. The extensive application of microwaves in the field of telecommunications means that only specially assigned frequencies are allowed to be allocated for industrial, scientific or medical applications (e.g., most of wavelength of the range between 1 and 25 cm is used for mobile phones, radar and radio-line transmissions). Currently, in order not to cause interference with telecommunication devices, household and industrial microwave ovens (applicators) are operated at either 12.2 cm (2.45 GHz) or 32.7 cm (915 MHz). However, some other frequencies are also available for heating [1]. Most common domestic microwave ovens utilize the frequency of 2.45 GHz, and this may be a reason that all commercially available microwave reactors for chemical use operate at the same frequency. 

As was mentioned above, every efficient application of microwave energy to perform chemical syntheses requires reliable temperature measurement as well as continuous power feedback control, which enable heating of reaction mixtures to a desired temperature without thermal runaways. Moreover, power feedback control systems that are operated in the most microwave reactors enable a synthesis to be carried out without knowing the dielectric properties or/and conductive properties of all the components of the reaction mixture in detail. On the other hand, temperature control during microwave irradiation is a major problem that one faces during microwave-assisted chemical reactions. In general, temperature in microwave field can be measured by means of ... 

Flavors added to microwave food systems have a greatly expanded role compared to flavors added to products prepared by conventional heating. The flavors must provide not only the characterizing flavor (i.e., lemon, butter, vanilla, etc.), but also the typical roasted, toasted, and baked flavors which do not develop in microwave heated products. New flavors designed for use in microwave products must mask the raw uncooked flavor characteristics and other undesirable flavor notes frequently found in many microwave bases. Microwave flavors must also deliver pleasant aromas into the room during the microwave process. Development of these flavors for microwave application is dependent upon a fundamental understanding of microwave heating on flavor performance in food systems. 

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