Chemistry Reference
In-Depth Information
pleted in 15 min. The authors used about 10.5 times
more water per gram of plant material in super-
heated water extraction than in steam distillation
and stated that steam distillation seemed to have
gone to completion. The yield from superheated
water extraction was 5.1 times that obtained by
steam distillation. This is in accord with results from
our laboratory and others, although the ratio of
yields is larger than obtained elsewhere. Table 23.1
shows the percentages of selected compounds in the
oils obtained by both methods. The major compo-
nent, eucalyptol, is present in roughly the same pro-
portion in both oils, but the monoterpenes (the first
three compounds listed) are much less prominent in
the oil obtained by superheated water extraction.
However, the superheated water extract contains a
higher proportion of most other oxygenated com-
pounds, except for geranyl acetate. These com-
pounds will give the oil its particular fragrance. The
results agree in general with those reported in other
studies.
Using an experimental system modified from, but
similar to, that shown in Fig. 23.2, plant essential oils
also have been extracted at Leeds, although the work
has not been published yet. The cell was filled with
packing material and the oil pumped through a tube
so that it entered 20 mm above the bottom of the cell.
It then trickled up over the packing material and col-
lected at the top of the cell. The flow rate of oil was
one-tenth of that of water. Initial experiments were
carried out on cold-pressed lemon oil, which con-
sisted largely of terpenes, mainly limonene, with
about 1% of oxygenated compounds. The extract,
however, consisted only of the oxygenated flavour
and fragrance compounds. Similar results were
obtained with other oils. These experiments could
form the basis of a method for deterpenating essen-
tial oils, as discussed below.
7 Process Development
The advantages in yield and quality, as well as the
environmental and clean-product benefits, give rise
to interest in developing processes from these small-
scale experiments. There are also energy savings in
principle. A greater mass of superheated water for a
given mass of plant material is required than for
steam distillation. However, only 505 kJ kg
-1
is
required to heat liquid water from 30°C to 150°C,
compared with the 2550 kJ kg
-1
required to convert
water at 30°C to steam at 100°C [3]. Moreover, it is
relatively easy to recycle the heat in a superheated
water process by passing the water leaving the
extraction cell through a heat exchanger to heat the
water flowing to the cell. About 80% of the energy
realistically can be recovered in this way. By contrast,
it is difficult to recover heat in steam distillation,
because a relatively large amount of cooling water is
needed to condense the steam and only part of this
can pass into the boiler. The amount of heat that can
be recovered in stream distillation is of the order of
5%. These advantages mean that in a well-designed
process, in spite of the greater amount of water
required, less energy is needed. Extraction by super-
heated water, therefore, has the potential advantages
over steam distillation (an equally environmentally
friendly process) of better yields, a higher quality
product and less energy use.
Process development is, however, at an early stage.
Not much work is published yet but the challenges
are being tackled. The main problems to be solved
are separation of the desired components at the end
of the process from both the extracting water and
from highly water-soluble material also present in
the plant material and therefore water extract. The
extract often is coloured brown, like tea, and often
contains a precipitate in which the oxygenated com-
pounds are absorbed. On a small scale these are
back-extracted using an organic solvent such as
pentane for analysis, but this would not be accept-
able in an environmentally friendly process.
One suggestion is to back-extract with supercriti-
cal carbon dioxide, which also is a clean solvent.
Table 23.1
Comparison of percentages of selected components
in the essential oil obtained from marjoram by steam distilla-
tion and superheated water extraction at 150°C
Superheated water
Steam distillation
extraction
a
-Pinene
2.5
0.1
b
-Pinene
4.4
0.5
b
-Myrcene
5.0
0.3
Eucalyptol
67
65.0
Linalool
2.1
4.7
a
-Terpineol
6.0
10.1
Geraniol
3.6
6.1
Geranyl acetate
3.6
2.6
Data taken from Ref. 17.
