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Traditional hydrodistillation (HD), cold pressing (CP) and innovative microwave 'dry' distillation or microwave-accelerated distillation (MAD) methods have been compared and evaluated for their effectiveness in the isolation of essential oil from fresh Citrus peels. The microwave method offers important advantages over traditional alternatives, viz. shorter extraction times (30 min vs. 3 h for hydrodistillation and 1 h for cold pressing); better yields (0.24% vs. 0.21% for HD and 0.05% for CP); environmental impact (energy cost is appreciably higher for performing HD and for mechanical motors (CP) than that required for rapid MAD extraction); cleaner features (as no residue generation and no water or solvent used); increases antimicrobial activities; and provides a more valuable essential oil (with high amounts of oxygenated compounds). It also offers the possibility for better reproduction of the natural aroma of the essential oil from Citrus fruit compared with CP, but more than the HD essential oil. Further, the microwave procedure yields essential oils that can be analysed or used directly without any clean-up, solvent exchange or centrifugation steps. Scanning electron microscopy provides more evidence of the cleanness of microwave extraction, in contrast to the huge perforations on the external surface of the Citrus fruit peel in the case of conventional hydrodistillation. Finally, a mechanism of microwave 'dry' distillation is proposed and discussed.

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Extraction of essential oil from Nigella sativa seed collected at two locations in the Sahara desert, Timimoun (T) and Adrar (A), has been conducted by hydrodistillation (HD) and a microwave distillation process (MD). The composition of the volatile oil was investigated by capillary gas chromatography and gas chromatography-mass spectrometry. Using the hydrodistillation, active compounds such as p-cymene (8.9 and 7.2% in T and A, respectively), 4-terpineol (0.6-8.9%), thymohydroquinone (6.1-12.2%), thymoquinone (1.6-21.8%), carvacrol (12.9-12.9%), carvone (4.4-0.3%) and thymol (1.5-0.7%) were the major components, representing more than 36% (T) and 64% (A) of the oils. For the microwave distillation, p-cymene (28.1% and 32.0% in T and A, respectively), 4-terpineol (3.4-2.0%), thymohydroquinone (0.7-1.1%), thymoquinone (10.8-24.6%), carvacrol (3.0-6.0%), and thymol (0.3-0.3%) represent respectively 46.1% and 66.0% of the T and A oils.

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