Extraction of Olive Oil from Olive Cake using Soxhlet
Apparatus
Banat, F1,2*, Pal, P1, Jwaied, N2 , Al-Rabadi, A2
Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, U.A.E.
2
Department of Chemical Engineering, Jordan University of Science and Technology, Jordan
Email: fbanat@pi.ac.ae
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Abstract
Chemical extraction of olive oil from natural and dried olive cake (obtained from Alyarmouk
press, Irbid, Jordan) using organic solvents like hexane, ethanol, petroleum ether, isopropyl
alcohol and carbon tetrachloride was carried out in a Soxhlet extractor. Olive cake samples
were dried in an oven at 70°C and 105°C for 3.5 hours before being used in the extraction
experiments. Hexane was found to be the best extracting solvent with an oil yield of 12.7%.
Oil yield increased from 10% to 16% when olive cake was soaked with aqueous sodium
chloride solution for 24 hours. Sorbitan monoleate surfactant enhanced the extraction of oil.
Particle size of olive cake and solvent volume had significant effect on oil extraction.
Keyword: Olive oil, extraction, choice of solvent, particle size, solvent-to-solid ratio
1.
Introduction
World production of olive oil has constantly risen over the last ten years and Mediterranean
countries account for more than 90% of all the olive oil produced in the world [1]. Olive oil is
obtaining from olive fruit a natural product containing the wide range of bioactive compounds
and key component of the traditional Mediterranean diet. The olive fruit generally contains
50% water, 20% oil, 20% carbohydrates, organic acids, pigments, phenolic compounds and
minerals as rest [2]. The oil contains high levels of typical phenolics and 70% unsaturated
fatty acids which are believed to be associated with a relatively long life and good health [3].
Olive oil is usually extracted by either mechanical pressing or chemical extraction processes.
Mechanical pressing is used to extract oil from olives on hydraulic presses which resulted in
relatively high pure oil, though it does not extract all of the oil and left the oil residue of about
8-20 weight %. The chemical extraction requires organic solvent for oil extraction. The most
widely used procedure for oil removing from the solid matrix remains conventional Soxhlet
extraction which is straight forward and inexpensive [4]. In this process solvent extract olive
oil by extraction using organic solvent and the oil is separated from the solvent by distillation.
For solvent extraction, the oil recovered is not extra virgin oil and results in crude olivepomace oil. It undergoes a refining process to eliminate impurities and substances using
soda, deodorants and bleaching agents at high temperature. The extraction of oil depends on
moisture content, nature of the solvent, time of extraction, particle size and amount of solvent
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[5, 6]. Hexane is extensively used for oil extraction because of its high stability, low greasy
residual effects, boiling point and low corrosiveness [7]. Jordan contains huge reserves of
energy sources as olive cake. In recent years; rising environmental awareness in the Jordanian
society has brought about concerns regarding the handling and management of the large
amounts of olive cake. Under the current regulatory statutes and disposal practices, these
amounts in many cases pose a disposal problem and constitute a threat to the country’s very
limited water resources [8].
The basic objective of this research was to determine most suitable solvent for olive oil yield
using Soxhlet extractor. To study the effect of oil yield with hexane as solvent dried olive
cake at two different temperatures and pretreated olive cake with sodium chloride were
studied. The effect of surfactants on oil extraction was also touched in this work. The work
extends to examine the effect of various process parameters like extraction time, volume of
solvent, and particle size.
2.
Materials and Methods
2.1.
Materials
All the solvents (hexane, ethanol, petroleum ether, isopropanol and carbon tetrachloride) and
the surfactants (sorbiton monoleate, sorbiton trioleate and polyethylene glycol nonylphenyl
ether (synperonic NP10)) were purchased from Sigma-Aldrich Chemie GmbH, Germany.
2.2.
Methods
2.2.1. Oil extraction using different types of organic solvent
Chemical extraction of olive oil from dried olive cake at 70ºC was carried out using hexane,
ethanol, petroleum ether, isopropanol and carbon tetrachloride to determine the best solvent
for oil extraction. 250 ml of solvent and 10 gram olive cake sample was used in each
experiment to maintain a solvent-to-solid ratio of 25:1. The total extraction process was
completed within one hour. The extracted phase was then distilled to separate the oil from the
solvent.
Of all the solvents, petroleum ether, ethanol and hexane were used to study the oil yield with
olive cake (dried at 70oC) at different intervals of time to understand the oil extraction steps
involved in the process.
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2.2.2. Drying of olive cake
To establish the optimum drying conditions of olive cakes, samples were dried at 70ºC and
105ºC. 10 gram of olive cake samples was then charged to Soxhlet extractor with 250 ml of
hexane as solvent to make a solvent-to-solid ratio 25:1 for 1 hour.
2.2.3. Pretreatment of olive cake with sodium chloride
Dried olive cakes were soaked in aqueous solution of sodium chloride with varying
concentrations (0.1M, 0.5M, 2M and 4M) for 24 hours and then filtered, rinsed with distilled
water and dried in an oven for 3.5 hours at 70°C. 10 gram of each soaked sample was taken
and charged to a Soxhlet extractor with hexane as a solvent for 1 hour. The extracted phase
was then distilled to separate the oil from hexane.
2.2.4. Effect of surfactants
To investigate the effect of surfactants (sorbitane monoleate, sorbitane trioleate and
synperonic NP10) on oil yield, 1.5 ml of surfactant was added to hexane solution. The
solvent-to-solid ratio was maintained at 25:1 for 1 hour of extraction time. The extraction
processes was followed by simple distillation processes to recover the oil.
3. Results and Discussions
Soxhlet extraction is a classical extraction technique to remove oil from biomass particle. It is
important to study the choice of solvent, extraction time and other process parameters for
measuring the % yield of the extracted oil.
The % yield of olive oil (y) is obtained as:
y = (Mass of olive oil extracted)*100/ (Initial weight of olive cake)
2.3.
(1)
Choice of solvents
The selection of solvent for oil extraction from olive cake is an important factor. The solvent
should have good extraction capacity, low viscosity and higher solubility with oil. Proper
penetration of the solvent inside the olive cake is necessary to extract oil effectively with
nearly equal polarity of the solvent and the oil. Generally, organic solvents possess all the
above criteria and are used for oil extraction since long years. Hexane, ethanol, petroleum
ether, isopropanol and carbon tetrachloride were used as organic solvents in this study to find
the best solvent for oil extraction. It was observed that amongst all the organic solvents used,
hexane is the best extracting solvent with a 10.6% yield as shown in Figure1.
3
12
10
% yield
8
6
4
2
0
Hexane
Ethanol
Petroleum
ether
Solvents
Isopropanol
Carbon
tetrachloride
Figure1. Effect of solvents on the yield of olive oil
Figure 2 shows the effect of extraction time over the oil yield on the Soxhlet extractor for
three different solvents (petroleum ether, ethanol and hexane). It was observed that oil yield
increased as time progressed and reached maximum after 1 hour. There are two steps
involved in the oil extraction from biomass. The first step involving the extraction of oil from
the external surface of the particle (solubility-controlled phase) is a very fast physical process
while the second step involving the extraction of oil from the inner particle (diffusioncontrolled phase) is comparatively slower [9]. Further increase in extraction time approached
the oil extraction curve to a limiting value of oil yield. This is an indication that the extraction
process had almost reached completion. For the extraction of oil from olive cake it was found
that the extraction completion time was 1 hour.
15
% yield
10
Petroleum ether
5
Ethanol
Hexane
0
0
15
30
Extraction time, minute
45
60
Figure2. Effect of Extraction time on the yield of oil using different types of solvents
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2.4.
Effect of drying olive cake
Drying caused removal of the moisture content from the olive cake samples and thus the
resistance to oil extraction was reduced. Also, drying before extraction helps rupture the olive
cake walls and thus the solvent can more readily dissolve into the oil. It was observed that by
increasing the drying temperature from 70ºC to 105ºC all the moisture content from olive
cake was removed and the yield of oil was increased from 10.6% to 12.7 % as compared to
7.5% from natural olive cake (Figure3).
14
12
% yield
10
8
6
4
2
0
Natural
Dried at 70°C
Dried at 105°C
Figure3. Effect of drying of olive cake on the yield of olive oil
2.5.
Effect of pretreatment with sodium chloride
Pretreatment of olive cake with aqueous sodium chloride solution had a significant effect on
the yield of oil as shown in Figure4. The oil yield increases with the increase of sodium
chloride concentration and reached a maximum of 16% at 2 (M) sodium chloride solutions.
At lower concentrations, sodium chloride had a strong tendency to rupture the olive cake cells
which causes easier mass transfer of oil and thus extraction of oil increases. A further increase
of sodium chloride concentration leads to decrease in the oil yield. The main reasons
postulated are that during osmotic processing to rupture the biomass cell, the rate of diffusion
of solvent depends on the concentration of the osmotic solution [10].
5
16.5
% yield
15.5
14.5
13.5
0
1
2
3
Salt concentration, molar
4
5
Figure 4. Effect of pretreatment with sodium chloride on olive oil yield
2.6.
Effect of adding surfactants to hexane
Different non-ionic surfactants (sorbiton monoleate, sorbiton trioleate and synperonic NP10)
were used to investigate the effect of surfactant on yield of olive oil. It was observed that the
percentage yield was improved by adding various surfactants, especially to hexane with
sorbitan monoleate. The mobilization of oil molecules depends on the tendency of surfactants
to reduce the capillary forces (i.e. surface tensions) in olive cake and oil tends to adhere to the
structure due to surface tension. The effect of surfactants is to allow the aqueous phase to wet
the solid surface preferentially. The reduction in surface tension of surfactant solution can
displace oil molecules trapped in olive cake that cannot be displaced by pure hexane. As
shown in Figure5, increasing the sorbitan monoleate concentration increases the olive oil
yield significantly.
30
25
% yield
20
15
10
5
0
0
0.005
0.01
0.015
Surfactant concentration, molar
0.02
Figure5. Effect of surfactant concentration on the yield of oil
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2.7.
Effect of solvent-to-solid ratio
The effect of solvent-to-solid ratio on the oil extraction from olive cake is shown in Figure6.
The solvent-to-solid ratio was varied from 5:1 to 25:1 on olive oil extraction studies with
0.855 mm particle size. As the solvent-to-solid ratio increased from 5:1 to 25:1 the oil yield
also increased from 5.4% to 15.6%. With further increase in solvent-to-solid ratio above 25:1
the oil yield remains constant for olive oil extraction process. In an earlier work [11] much
less olive oil yield (9.4%) was obtained for solvent-to-solid ratio of 4:1 from olive cake using
hexane as solvent.
20
% yield,
15
10
5
0
0
5
10
15
20
Solvent-to-soild ratio, ml/gram
25
30
Figure6. Effect of solvent-to-solid ratio on oil yield
2.8.
Effect of particle size
Particle size is an important parameter in extraction of oil process. Smaller the size of solid
particles, the greater the interfacial area between the solid and the liquid and therefore
increases the oil extraction yield [12]. The maximum olive oil extraction of 18.6% was
obtained at 0.212 mm of olive cake having solvent-to-solid ratio of 20:1 for 1 hour. It was
observed that oil extraction yield was gradually increased from 15.3% to 18.6% with decrease
in particle size from 0.855 mm to 0.212 mm. Different researchers have studied the effect of
particle size for the extraction of oil from different biomass [13, 14] and it has been observed
that oil yield increases with decrease in particle size. As the particle size increases it is
difficult for the solvent to reach core of the biomass to leach oil and the oil yield decreases.
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Figure7. Effect of particle size on the yield of oil
3.
Conclusion
Different solvents were used to observe the yield of olive oil using Soxhlet apparatus. Hexane
was found to be the best extracting solvent with an oil yield of 7.5 % from natural olive cake
and 12.7 % from dried samples. Oil yield can be increased by soaking olive cake samples
with aqueous sodium chloride solution with optimum concentration of 2.0 M with 16% oil
yield. Extraction of oil using surfactant solution like sorbitan monoleate with hexane
enhanced the extraction process at high concentrations of surfactant (0.019 molar). Solventto-solid ratio has significant effect on oil yield and with 20:1 hexane-to-olive cake ratio
maximum yield of oil was obtained. As the particle size of the olive cake decreases oil yield
increases.
4.
Acknowledgement
The work has been carried out at Jordan University of Science and Technology.
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