Microwave heating is a convenient way to heat materials; it is considered to be a fast, clean, and easy to use technology. The use of microwaves for industrial food unit operations is the subject of research since several years ago. However, the application of microwaves depends, among other variables, on the dielectric properties of the material to be heated; otherwise, the efficiency of the process and the quality of the final product cannot be guaranteed. This paper reviews basic concepts related to microwaves and dielectric properties, and then it presents reported dielectric properties data for selected fluid foods and microwaveheating processes that have been recently studied. These processes are focused mainly on microbial inactivation, enzyme inactivation, chemical, physical, or sensory changes evaluation, or for reheating. The temperature uniformity is also discussed as a key issue for successful application of microwave heating, which is now applied by some companies.

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Lycopene is the primary carotenoid in tomato peels, a processing byproduct, and can be used as a natural color or bioactive ingredient. Unfortunately, extractions are inefficient as lycopene is extremely nonpolar and susceptible to degradation. As a rapid technique, microwave-assisted extraction (MAE) potentially offers efficient lycopene recovery. Thus, the objectives of this research were to: 1) optimize MAE of lycopene from tomato peels and 2) evaluate the effect of treatment on all-trans and isomer yields. Response surface methodology (RSM) was employed to optimize lycopene extraction with solvent ratio solid-liquid ratios, microwave power, and delivered energy equivalents as factors. High performance liquid chromatography with a diode array detector (HPLC-DAD) was used for isomer separation and quantification. Optimum MAE conditions were determined as: 0:10 solvent ratio at 400 W with a yield of 13.592 mg/100 g of extracted all-trans-lycopene. RSM suggested that ethyl acetate was a better MAE solvent for lycopene recovery as compared to hexane, which overall extracted less lycopene. HPLC-DAD indicated that MAE significantly improved all-trans and total lycopene yields, while conventional extraction demonstrated higher proportions of cis-isomer yields. Additionally, electron micrographs showed that significant structural disruption occurred in MAE-treated samples, possibly allowing for the improved lycopene extraction.