Microwave-assisted extraction of volatile substances from natural sources

Abstract

Volatile substances extracted from natural sources like herbs and spices are invaluable resources, useful in daily life as food additives, flavours, fragrances, pharmaceuticals, colours [1]. They are normally obtained by various different methods, e.g. hydro-distillation (HD), steam distillation, Soxhlet extraction, and simultaneous distillation–extraction [2]. Anyway, losses of some volatile compounds, low extraction efficiency, degradation and toxic solvent residue in the extract may be encountered using these extraction methods. Solvent-free microwave extraction (SFME) and microwave hydrodiffusion and gravity (MHG) combine the advantages of microwaves with dry distillation and hydrodiffusion without any added solvent or water. The microwave-promoted internal heating of the in situ water within the plant material distends it and makes the glands and oleiferous receptacles burst. This process frees the volatile compounds that are easily evaporated and collected using dedicated glassware setups [3,4]. To investigate the potential of these new techniques, comparisons have been made with HD for the extraction of essential oils from a number of herbs and spices. Both, SFME and MHG, afforded the expected aromatic profiles with comparable yields in much shorter times (15–60 minutes) than HD (1.5–8 hours). Substantially higher amounts of oxygenated compounds and lower amounts of monoterpene hydrocarbons were present in the essential oils extracted using microwaves in comparison with HD. The use of microwave extraction resulted to give clear advantages compared to conventional hydro-distillation in terms of rapidity, efficiency, cleanliness and saving of energy [3,4].