Extraction of oleoresins from fennel seeds and dried onion with
Fennel (Foeniculum vulgare M.) and onion (Allium Cepa L.) are commonly
used spices in food industry, their flavors are incorporated in processing
foods, mainly by adding the hydrodistilled oils, to some extent, by using
the solvent-extracted oleoresins.
Both the hydrodistilled oil and the oleoresin have a different aroma
from that of the natural spices. This is due to the heat and water caused
changes taking place during distillation in the oil and the solvent residue,
removal of more volatiles, etc. in the oleoresin. To obtain the essential
oil or oleoresin, devoid of the above limitations, supercritical carbon
dioxide ( SC-CO2 ) as a solvent has many advantages: colorless,
odourless, tasteless, non corrosive, chemically inert gas which retains
no solvent residue in the extract. The solvation property of the carbon
dioxide can also be modified by adjusting pressure and temperature through
density and dielectric constant changes. Supercritical CO2 extraction
is more selective than the conventional extraction with commonly used organic
solvents. Recently there are a few full-scale plants applying this technology
for the extraction of essential oil and oleoresin from several spices.
Only a few studies exist in which the subcritical or supercritical CO2
extraction of oleoresin or some component from fennel and fresh onion was
performed in small-scale apparatus. The yield of oleoresin was 2.0% when
unripe fennel umbels were extracted by SC-CO2 at 200 bar and
40 oC (Ravid et al., 1983). Liquid CO2 gave a 5.8%
yield in 2 hours at 58 bar and 20 oC when fennel seeds were
used (Naik et al., 1989). Extraction of onion flavor from fresh onion was
investigated using SC-CO2 at 200 bar and 35 oC, combined
gas chromatography - mass spectrometry analysis of SC-CO2 onion
extract showed the presence of 28 sulfur - containing compounds (Nirmal
et al., 199?). Thiosulfinates were extracted from onion juice with SC-CO2
at 240 bar and 35 oC, the SC-CO2 extraction of thiosulfinates
was ca. 60% as efficient as the extraction with diethyl ether, as determined
by GC-MS (Calvey et al., 1994).
The lack of information on the effect of extraction parameters on the
yield and composition of fennel and onion oleoresins, optimisation of the
extraction process for them, and fractionation of oleoresin into essential
oil and fatty oil, makes the development of industrial products difficulty.
At the same time, publications on the sensory evaluation of SC-CO2
extracts are scarcely available.
Aims of the research work
METHODOLOGY OF THE RESEARCH
Investigation of the possibility and feasibility of supercritical CO2
extraction of oleoresins from fennel seeds and dried onion in different
Fractionation of oleoresins into the essential oil rich and fatty oil rich
products in pilot-scale apparatus.
Optimisation of the extraction process for SC-CO2 extraction
of fennel oleoresin in order to minimise the CO2 consumption
and the unwanted components content in both products at the same level
of the yield
Optimisation of the extraction parameters for SC-CO2 extraction
of onion oleoresin to obtain high yield and quality of onion product.
Comparison of the similarity and difference of the yield, volatile compound
composition and sensory characteristics of SC-CO2 extracts,
organic solvent extracts and steam distilled oils of fennel and onion.
Modelling the extraction process in order to predict the important parameters
such as the maximum yield and overall transfer coefficient.
The essential oil of fennel seeds was isolated with a Karlsruber apparatus
The essential oil from onion was extracted in a special steam distillation
apparatus (Sass-Kiss, 1989).
Laboratory-scale type of Soxhlet extraction apparatus was used for
the preparation of oleoresin, and the solvents were hexane and alcohol
( 96 % v/v ).
Small-scale SC-CO2 extraction
Analytical SFE apparatus (Model 7680, Avondale, PA) was used for the
extraction of fennel oleoresin, and a laboratory-scale type SFE apparatus
(SFX2-10, Isco, Inc., USA) for the extraction of onion oleoresin.
Middle-scale SC-CO2 extraction
A middle-scale SC-CO2 extraction has been performed with
the home-made apparatus (extractor, 1.14L).
Pilot-scale SC-CO2 extraction
For the extraction of fennel and onion oleoresins, the home-made SFE
equipment with one extractor ( 5L ) and two separators in series was adopted.
GC analysis of volatile compounds in fennel and onion oleoresins For GC/MS
analysis of volatile compounds in fennel oleoresin from the small-scale
experiment, a Hewlett Packard 5890 Series II gas chromatography equipped
with a 7673 autosampler and a 5889 A MS Engine mass spectrometer was used.
For the qualitative and quantitative analysis of the composition of
volatile components in fennel oleoresin from the large-scale experiment,
gas chromatography was conducted on a Shimadzu GC-A 14 equipped with FID
The composition of volatile components in onion oleoresin was analysed
by Varian 3700 gas chromatograph (Varian Associates, Walnut Creek, CA)
equipped with FID detector.
GC analysis of fatty acid composition of fennel oleoresin The composition
of fatty acids of fennel oleoresin was analysed by Hewlett Packard 5720
A gas chromatograph equipped with FID detector.
HPLC analysis of pigments in fennel oleoresin
The pigments of fennel oleoresin were fractionated isocratically by
reversed-phase HPLC method. A Beckman series of liquid chromatograph equipped
with a Model 165 variable wavelength detector was used.
Determination of sulfur content of onion essential oil and oleoresin Schöniger's
titration method modified by the Department of Analytical Chemistry in
the Technical University was adopted.
Sensory evaluation of fennel and onion products was carried out by
PSA-computerised profile analysis.
A central composite rotatable design of second order was used for the
SC-CO2 extraction of fennel and onion oleoresins.
All related experimental data were submitted to analysis of variance
and response surface, using the STATGRAPHICS computer program (1985).
III. SUMMARY OF NEW SCIENTIFIC RESULTS
The new scientific results presented in this dissertation are as follows:
The result from small-scale extraction of fennel oleoresin showed that
the first-extracted oleoresin contained much more volatile compounds. With
the increase of extraction time, the composition of volatile compounds
altered and the last-extracted oleoresin only contained trans-anethole,
fenchone, methyl chavicol and limonene as principle volatile compounds.
Sensory evaluation indicated that the first-extracted oleoresin exhibited
much more pleasant flavor than the others.
The fennel oleoresin was successfully fractionated into the essential oil
rich and fatty oil rich products in the preliminary pilot-scale extraction.
With an increase of the pressure from 70 to 90 bar in the first separator,
the yield of the fatty oil rich product decreased by 41 %, while the yield
of the essential oil rich product increased as much as 5 times.
As compared with other methods, the SC-CO2 extraction resulted
in higher yield of fennel oleoresin than steam distillation, almost the
same as hexane extraction.
GC analysis of volatile compounds revealed that the compositions of
volatile compounds in fennel essential oil and oleoresins prepared by different
solvents were very different. With the increase of extraction time, the
composition of volatile compounds in the essential oil rich and fatty oil
rich products also changed.
Response surface methodology (RSM) was first used to design the large-scale
extraction and fractionation experiments of fennel oleoresin. The results
from 27 experimental runs indicated that the CO2 amount consumed
for the 80 % relative overall yield with respect to hexane extraction was
at the lowest level when the extraction pressure and temperature were above
285 bar and 63 oC, respectively. The minimum level of total
unwanted components in both separators appeared at the pressure 80-84 bar
and temperature 31-35 oC in the first separator. The composition
of volatile compounds of fennel extracts prepared was not obviously influenced
by extraction and separation parameters.
The result from HPLC analysis implied that in most cases the pigments
in fennel oleoresin were pheophytins and lutein, and their contents in
the fatty oil- rich product increased along with the rise of extraction
pressure and temperature and the decrease of separation temperature in
the first separator. Extension of extraction time also resulted in an increase
of the pigment content. As compared with hexane and alcohol, SC-CO2
exhibited higher solubility for pigments at high pressure and temperature
than hexane, whereas the solubility of the pigments was the highest in
The result from sensory evaluation of fennel essential oil and oleoresins
showed that the SC-CO2 product and disitilled essential oil
were more intensive in odour and taste than alcohol and hexane extracts.
The data from 15 experimental runs for the small-scale extraction of onion
oleoresin were used to develop the polynomial model and fitted the experimental
data very well (R2>0.903). The contribution of each parameter
to the yield was ranked in the following order: extraction pressure > time
>temperature, all of them exerted the significant effect on the yield (p<0.05).
According to the contour plot, the maximum yield of onion oleoresin could
be reached when the extraction pressure and temperature were above 350
bar and 57 oC, respectively. Sulfur content of onion oleoresin
was also influenced by extraction parameters, the low pressure and high
temperature of extraction led to an increase of sulfur content of onion
The yield and sulfur content of onion oleoresin from middle-scale extraction
increased obviously with the rise of extraction temperature, however, the
extraction pressure generated no significant effect on them.
The experimental results from the pilot-scale extraction further confirmed
the conclusion from the small-scale extraction: high extraction pressure
and temperature resulted in significantly positive influence on the yield.
The sulfur content reached maximum level at high temperature and low pressure.
The comparison of the yield and sulfur content, of essential oil and
oleoresins prepared by different methods revealed that alcohol extraction
possessed the highest yield and extraction efficiency of sulfur. The yield
of onion oleoresin by hexane and SC-CO2 extraction was similar
although the extraction efficiency of sulfur by SC-CO2 extraction
was higher than hexane extraction. The yield of onion essential oil obtained
by steam distillation was at the lowest level, nevertheless, its extraction
efficiency of sulfur was higher than hexane extraction, similar to SC-CO2
The composition- of volatile compounds of onion oleoresins produced
by different scales SC-CO2 extractions was characterised by
low concentration of sulfides, di-, tri, tetrasulfides, which are the main
compounds of distilled onion oil. The ratio of area % of I/II; (compounds
appearing in distilled oil / compounds occuring in extracts only) fractions,
except some cases, did not show significant changes by the effect of pressure
Comparising the composition of volatile compounds of distilled oil,
and the oleoresins prepared by SC-CO2, hexan, alcohol, it could
be suggested that the extracts contained much less sulfides, di-, tri-,
tetrasulfides than distilled oil. The similarity of the composition of
II. fraction on the chromatograms showed the analogy of extraction methods,
but the differences characterise the extraction solvents.
The result from sensory evaluation of onion essential oil and oleoresins
indicated that the SC-CO2 extract was of the best quality, distilled
essential oil took the second position and the worst one was alcohol extract.
Based on the experimental data gathered from the SC-CO2 extraction
of fennel and onion oleoresins, a mathematical model was developed using
differential mass balances, and an analytical solution with two parameters
(y¥ , k') was obtained. The model
was checked by the experimental data and a good fit was found. y¥
and k' in the model were estimated by the nonlinear regression program
and statistical analysis of their values implied that y¥
was controlled by the extraction temperature and k' by the extraction pressure
for the SC-CO2 extraction of fennel oleoresin. For the SC-CO2
extraction of onion oleoresin, k' was associated with particle size distribution
and y¥ was influenced by the
extraction pressure and temperature. The reasonable explanation for this
conclusion was also presented.
IV. APPLICABILITY OF THE RESEARCH RESULTS
The experimental data, in particular, the optimum extraction parameters
from the research work can be directly used to guide the practical design
of SC-CO2 extraction technology of fennel and onion oleoresins.
The selection and design of SC-CO2 extractor can be also performed
on the basis of optimum parameters and developed mathematical model. In
order to decrease production cost and improve the product quality, the
desired yield of oleoresin can be predicted by the model.
Sensory analysis program can be successfully applied to evaluate and
compare the quality of SC-CO2 extracts, solvent extracts and
distilled oil. This is much more economic method than instrumental analysis
and it can be extended to other spice products.