Natural flavourings from green chemistry for foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 267

Table 12.5 Comparison between supercritical CO 2 and subcritical water for the
extraction of essential oils from plants

Supercritical CO 2 Supercritical CO 2 -Water

Environmentally friendly Yes Yes

Capital cost High Medium

Operating cost High Low

Extraction conditions Mild Moderate

Co-extraction of cuticle waxes Yes* No

Drying stage Ye s No

Note: * Not under liquid CO 2 conditions .

causes heating via two simultaneous mechanisms, dipolar rotation and ionic
conduction.
The solvents generally used cover a wide range of polarities from hexane to
water, and normally a solvent is chosen that has a high dielectric constant and
strongly absorbs microwave energy to produce optimal heating. However, the rate
of heating is strongly dependent on the nature of both the solvent and the solid
matrix. In contrast to conductive heating methods, microwaves heat the whole
sample simultaneously, and when used for extraction, microwaves interact with
the polar molecules present in glands, trichomes or vascular tissues. Localised
heating leads to the expansion and rupture of cell walls, followed by the release of
essential oils into the solvent or steam.
Microwave energy can be used in a number of ways to assist the extraction of
essential oils, as an energy source for solvent extraction (Pare 1994) and steam
distillation (Tigrine-Kordjani et al. 2006), or to carry out 'dry-distillation' where
the microwave energy provides the internal heating of the water within the plant
tissue with no additional water being added (Chemat et al. 2004; Lucchesi et al.
2004a). All these techniques are available at laboratory extraction scale. So far no
large-scale extraction has been installed, although continuous large-scale
microwave processing is commonplace in the food industry.
Of the three techniques, solvent-free microwave extraction (SFME) offers the
greenest solution as it allows substantial savings in operating costs and resources
particularly energy, water use and time, and appears as a good alternative for the
extraction of essential oils from aromatic plants. SFME has been compared with
conventional hydro-distillation for the extraction of essential oils from herbs,
spices, fl owers and berries. Leaf herbs such as Cinnamon ( Cinnamomum iners )
(Phutdhawong et al. 2007), basil (Ocimum basilicum L.), garden mint ( Mentha
crispa L.) and thyme ( Thymus vulgaris L.) were compared by Lucchesi et al.
(2004b) who found that the essential oils extracted by SFME for 30 minutes were
both quantitatively and qualitatively similar to those obtained by conventional
hydro-distillation for 4.5 hours. SFME resulted in an essential oil with slightly
higher amounts of oxygenated compounds.

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