Chemistry Reference
In-Depth Information
refinements are not shown here. Other studies
referred to below use very similar experimental
systems. For kinetic experiments, to study extraction
rates, the vial is replaced at known intervals. The
extract then is back-extracted into a solvent such as
pentane and analysed and quantified by gas chro-
matography.
Figure 23.3 shows the results obtained at 150°C
for the extraction of rosemary with liquid water, pre-
sented in the form of the percentage of each of eight
compounds of interest obtained after a given time
compared with the amount that can be obtained
after a long extraction. The monoterpenes extracted
are camphene and limonene, whose extraction pro-
files are indistinguishable (shown as filled circles),
and a-pinene (shown as open circles). These extract
relatively slowly. The ester isobornyl acetate is
extracted a little more quickly. A more rapidly
extracted group is formed by the cyclic ether 1,8-
cineole (open inverted triangles) and the alcohol
borneol (closed inverted triangles). The ketones are
removed most rapidly. Of the latter, verbenone
(closed triangles), which has a double carbon-carbon
bond conjugated with the ketone bond, is extracted
more rapidly than camphor (open triangles). Very
small and unquantifiable amounts of heavier hydro-
carbons were extracted.
All the extraction curves are found to be expo-
nential in form. This indicates that the extraction
rate is controlled by partitioning the compounds
between the water and the plant material. Release
from the plant material and diffusion through it
appear to be very rapid in this extraction process, in
contrast to extraction by liquids and supercritical
carbon dioxide. Other experiments showed that the
total amount of compounds obtained by superheated
water extraction was greater than that obtained by
steam distillation. This could be because penetration
of the plant material is better with hot water under
pressure. Analysis of the rate curves obtained
showed that if extraction was stopped after 70% of
the total material had been extracted, then 90% of
the essential oil extract would be oxygenated flavour
and fragrance compounds.
6 Extraction of Other Plant Materials
A number of studies now have been made on a small
scale, including work on laurel [16,21], marjoram
[17], clove buds [18], peppermint [19] and various
medicinal herbs [20]. These reach similar conclu-
sions to those given in the previous section and in
some cases comparisons with steam distillation and
other methods have been made [15, 17-20]. As an
example of these comparisons, the work on marjo-
ram (
Thymus mastichina
) will be outlined briefly [17].
Steam distillation was carried out over 3 h, whereas
superheated water extraction at 150°C was com-
Fig. 23.3
Recovery curves for
compounds extracted from rosemary
with liquid water at 150°C. (From Ref.
15.)
