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American Journal of Biochemistry and Biotechnology 4 (4): 345-353, 2008
ISSN 1553-3468
© 2008 Science Publications
A Review on Supercritical Fluid Extraction as New Analytical Method
1 1 1 2
Abbas, K.A., A. Mohamed, A.S. Abdulamir and H.A. Abas
1Department of Food Technology, Faculty Food Science and Technology,
University Putra Malaysia (UPM), Malaysia
2Department of English, Faculty of Arts, University of Imam Al-sadiq, Iraq
Abstract: This review study summarized the Supercritical Fluid Extraction (SFE) as new analytical
method. The advantage and disadvantage of CO (SC-CO )as well as special applications of SFE in
2 2
food processing such as removal of fat from food, enrichment of vitamin E from natural sources,
removal of alcohol from wine and beer, encapsulation of liquids for engineering solid products and
extraction and characterization of functional compounds were also highlighted. The study also covered
the application of SFE in food safety such as rapid analysis for fat content, rapid analysis for pesticides
in foods as well as the recent applications of SFE in food such as supercritical particle formation and
nan particle formation and ssupercritical drying. The outcome finding confirmed that SFE was found
to be advance, fast, reliable, clean and cheap methods for routine food analysis.
Key words: Supercritical fluid, applications, features
INTRODUCTION temperature. Especially in the vicinity of the critical
point a small isothermal raise in pressure increases the
SFE is used on a large scale for the extraction of fluid density remarkably while the effect diminishes
some food grade and essential oils and pharmaceutical with increasing distance from the critical values. Thus,
products from plants. It is relatively rapid because of the solvent strength of a fluid can be controlled more
[1]
the low viscosities and high diffusivities associated easily than that of a liquid solvent . Recently, the
with supercritical fluids. The extraction can be selective supercritical fluid has approved higher diffusion
to some extent by controlling the density of the medium coefficient and lower viscosity and surface tension than
and the extracted material is easily recovered by simply a liquid solvent, which leads to a more favorable mass
[3]
depressurizing, allowing the supercritical fluid to return transfer .
to gas phase and evaporate leaving no or little solvent The demand for new analytical techniques in food
[1]
residues . However, carbon dioxide is the most and environmental sciences is strongly related to the
common supercritical solvent. higher demand of information on processing, quality
Since the end of the 1970s, supercritical fluids has control, adulteration, contamination, environmental and
been used to isolate natural products, but for a long food regulations and the need for faster, more powerful,
time the applications concentrated on only few cleaner and cheaper analytical procedures are required
products. Now the development of processes and by chemists, regulatory agencies and quality control
equipment is beginning to pay off and industries are laboratories to meet these demands can be achieved by
getting more and more interested in supercritical SFE. Thus, this review paper has been justified.
[2]
techniques .
In SF the physicochemical properties of a given Properties and fundamentals of supercritical fluids
fluid, such as density, diffusivity, dielectric constant Solvent strength: The density of a supercritical fluid is
and viscosity can be easily controlled by changing the extremely sensitive to minor changes in temperature
pressure or the temperature without ever crossing phase and pressure near the critical point. The densities of the
[1]
boundaries . The phase diagram of a single substance fluids are closer to that of organic liquids but the
can be seen in Fig. 1. solubility of solids can be 3-10 orders of magnitude
The dissolving power of a supercritical fluid higher. The solvent strength of a fluid can be expressed
depends on its density, which unlike that of liquid by the solubility parameter, which is the square root of
solvents is highly adjustable by changing pressure or the cohesive energy density and is defined rigorously
Corresponding Author: Abbas, K.A., Department of Food Technology, Faculty Food Science and Technology, University
Putra Malaysia (UPM), Malaysia
345
Am. J. Biochem. & Biotech., 4 (4): 345-353, 2008
CO is not a very good solvent for high molecular
2
weight and polar compounds. To increase the solubility
of such compounds in supercritical carbon dioxide,
small amounts (ranging from 0 to 20 mol %) of polar or
non-polar cosolvents called modifiers may be added.
The co solvent interacts strongly with the solute and
[1,2]
significantly increases the solubility .
Co-solvents or modifiers: A more common practice in
SFE is to change the polarity of the supercritical fluid
and increase their solvating power towards the analyte
of interest by employing polar modifiers (co-solvents).
For example, the addition of relatively small
percentages (1-10%) of methanol to carbon dioxide
expands its extraction range to include more polar
analytes. The modifiers can also reduce the analyte-
Fig. 1: Phase diagram for a single substance. P , critical
[1] c matrix interactions improving their quantitative
pressure; T , critical temperature [1]
c extraction .
There are two main procedures to study with co-
from first principles. A plot of the solubility parameter solvents or modifiers in SFE are; the first one, and the
for carbon dioxide versus pressure would resemble that most common, accounts for a mixing of the modifier
[3]
a plot of density versus pressure . This confirms that with the CO flow while the second mixes the modifier
the solvation strength of a supercritical fluid is directly 2
with raw material in the extraction cell. This procedure
related to the fluid density. Thus the solubility of a solid is always associated to a static extraction step in which
can be manipulated by making slight changes in the modifier, in intimate contact with the sample
temperatures and pressures. Another attractive feature matrix, is able to substitute the analyte molecules bound
of supercritical fluids is that the properties lie between in active centers of the matrix and release them into the
that of gases and liquids. A supercritical fluid has [5]
supercritical fluid phase .
densities similar to that of liquids, while the viscosities
and diffusivities are closer to that of gases. Thus, a Extraction with supercritical fluids: Supercritical
supercritical fluid can diffuse faster in a solid matrix extraction has been applied to a large number of solid
than a liquid, yet possess a solvent strength to extract matrices. The desired product can be either the extract
[4, 2]
the solute from the solid matrix . or the extracted solid itself. The advantage of using
supercritical fluids in extraction is the ease of
Phase behavior: An understanding of the phase separation of the extracted solute from the supercritical
behavior is important since the phase behavior observed fluid solvent by simple expansion. In addition,
in supercritical fluids considerably differ from the supercritical fluids have liquid like densities but
behavior observed in liquids. One such behavior is the superior mass transfer characteristics compared to
retrograde region. For an isobaric system, an increase in liquid solvents due to their high diffusion and very low
the temperature of a solution increases the solubility of surface tension that enables easy penetration into the
the solute over certain ranges of pressure (consistent porous structure of the solid matrix to release the
with the typical liquid systems) but decreases the solute [2,6]
solute .
solubility in other pressure ranges. This anomalous Extraction of soluble species (solutes) from solid
behavior wherein the solubility of the solute decreases matrices takes place through four different mechanisms.
with a temperature increase is called the retrograde If there are no interactions between the solute and the
behavior. Thus, the following generalizations may be solid phase, the process is simple dissolution of the
made regarding the solute solubilities in supercritical solute in a suitable solvent that does not dissolve the
fluids. Solute solubilities approach and may exceed that [2]
solid matrix . If there are interactions between the
of liquid solvents. Solubilities generally increase with solid and the solute, then the extraction process is
increase in pressure. An increase in the temperature of termed as desorption and the adsorption isotherm of the
the supercritical fluid may increase, decrease or have no solute on the solid in presence of the solvent determines
effect on the solubility of the solute depending upon the the equilibrium. Most solids extraction processes, such
[1]
pressure . as activated carbon regeneration, fall in this category. A
346
Am. J. Biochem. & Biotech., 4 (4): 345-353, 2008
third mechanism is swelling of the solid phase by the
solvent accompanied by extraction of the entrapped
solute through the first two mechanisms, such as
extraction of pigments or residual solvents from
polymeric matrices. The fourth mechanism is reactive
extraction where the insoluble solute reacts with the
solvent and the reaction products are soluble hence
extractable, such as extraction of lignin from cellulose.
Extraction is always followed by another separation
process where the extracted solute is separated from the
[4]
solvent .
Another important aspect in supercritical extraction
relates to solvent/solute interactions. Normally the Fig. 2: A simplified drawing of a process-scale
[1]
interactions between the solid and the solute determine supercritical fluid extractor .
the ease of extraction, i.e., the strength of the adsorption
isotherm is determined by interactions between the the system. The product is then collected via a valve
[1,2]
adsorbent and the adsorbate. However, when located in the lower part of the separator(s) .
supercritical fluids are used, interactions between the
solvent and the solute affect the adsorption Advantages and drawbacks of supercritical CO2
characteristics due to large negative partial molar (SC-CO ): There are a large number of compounds that
2
volumes and partial molar enthalpies in supercritical can be used as a fluid in supercritical techniques, but by
[4,6] far the most widely used is carbon dioxide. From the
fluids .
The thermodynamic parameters that govern the viewpoint of pharmaceutical, nutraceutical and food
extraction are found to be temperature, pressure, the applications it is a good solvent, because it is non-toxic,
adsorption equilibrium constant and the solubility of the non-flammable, inexpensive, easy to remove from the
organic in supercritical fluid. Similar to the retrograde product and its critical temperature and pressure are
behavior of solubility in supercritical fluids, the relatively low (Tc=31.1°C, pc=72 bar) make it important
adsorption equilibrium constants can either decrease or for food and natural products sample preparation, is the
increase for an increase in temperature at isobaric ability of SFE using CO to be operated at low
2
conditions. This is primarily due to the large negative temperatures using a non-oxidant medium, which
partial molar properties of the supercritical fluids. In allows the extraction of thermally labile or easily
addition to the above factors, the rate parameters like oxidized compounds. It is environmental friendly and
the external mass transfer resistances, the axial generally recognized as safe by FDA and EFSA. These
dispersion in the fluid phase, and the effective diffusion properties make it suitable for extracting, for example,
of the organics in the pores also play a crucial role in thermally labile and non-polar bioactive compounds
the desorption process. A thorough understanding of but, because of its non-polar nature, it cannot be used
these governing parameters is important in the for dissolving polar molecules. The solubility of polar
modeling of supercritical fluid extraction process and in compounds and the selectivity of the process can be
the design, development and future scale-up of the increased by adding small quantities of other solvents,
[4] such as ethanol, in the fluid that named as co-solvent or
process .
modifier. On one hand, it decreases the processing
SFE process: A simplified process-scale SFE system is times, increases yields and makes it possible to use
shown in Fig. 2 and a typical batch extraction proceeds milder processing conditions, but on the other, it
as follows. Raw material is charged in the extraction complicates system thermodynamics and increases
[3]
tank which is equipped with temperature controllers capital costs .
and pressure valves at both ends to keep desired The use of high purity SFE-grade CO is not
2
extraction conditions. The extraction tank is pressurized required but impurity and moisture in industrial grade
with the fluid by means of pumps, which are also CO can accumulate and may interfere with further
2
needed for the circulation of the fluid in the system. analytical operations (gas or liquid chromatography).
From the tank the fluid and the solubilized components Thus, an on-line fluid cleanup system may be used to
are transferred to the separator where the solvation remove trace contaminants. An important drawback of
power of the fluid is decreased by increasing the SC-CO2 and most of the other supercritical fluids is that
temperature, or more likely, decreasing the pressure of predominantly, a non-polar extraction fluid, such as
347
Am. J. Biochem. & Biotech., 4 (4): 345-353, 2008
CO, is used. Therefore, a logical trend to widen the Enrichment of vitamin E from natural sources: SFE
2
application range of this technique is the study of new offers several advantages for the enrichment of
methods to decrease analyte polarity to make them tocochromanols over conventional techniques such as
more soluble in non-polar supercritical fluids. In this vacuum distillation, in particular a lower operating
sense, chemical in situ derivatization has been applied temperature. As starting material one can use various
to improve the selectivity of the extraction towards a edible oils or their distillates. Most promising as feed
specific group of compounds[3,5]. So the solvent power [9]
of SC-CO can be summarized by a few rules: materials are CPO) and SODD . CPO contains several
2 tocotrienols and tocopherols at a total concentration of
approximately 500ppm. SODD may contain (after
• It dissolves non-polar or slightly polar compounds. several conventional concentration steps) about 50%
• The solvent power for low molecular weight tocopherols. Both materials can be used for the
compounds is high and decreases with increasing [8,9]
molecular weight. production of enriched fractions of tocochromanols .
• SC-CO has high affinity with oxygenated organic Although it is possible to recover tocochromanols
2 directly from CPO, it is better to produce esters of the
compounds of medium molecular weight. triglycerides in order to be able to more easily separate
• Free fatty acids and their glycerides exhibit low these compounds from the tocochromanols. In this
solubilities. method, the triglycerides are subject to an esterification
• Pigments are even less soluble. with methanol to form fatty acid methyl esters, which
• Water has a low solubility (<0.5% w/w) at
o are easily extractable with CO . That means that the
temperatures below 100 C. 2
• Proteins, polysaccharides, sugars and mineral salts tocochromanols, together with other unsaponifiable
are insoluble and; matter such as squalene and sterol are enriched in the
• SC-CO2 is capable of separating compounds that bottom phase of an extraction column. This attempt is
are less volatile, have a higher molecular weight described in more detail by. For a discussion of
[2] enriching tocochromanols, phase equilibrium data have
and/or are more polar as pressure increases . [10]
to be considered first . FFA and tocochromanols
Special applications of supercritical fluids to food exhibit a much higher solubility in CO than the
2
processing: As mentioned before carbon dioxide is the triglycerides. Hence, these components are enriched in
most common supercritical fluid in the food industry. the gaseous phase, expressed by a distribution
Due to the non-toxicity and low critical temperature, it coefficient being higher than one. The distribution
can be used to extract thermally labile food components coefficient of the triglycerides is smaller than one,
and the product is not contaminated with residual whereas that for the carotenes is much smaller than one,
solvent. Further, the extract’s color, composition, odor, meaning that these components stay in the liquid oil
texture are controllable and extraction by supercritical phase. Thus, tocochromanols can be extracted as the top
fluid carbon dioxide retains the aroma of the product. phase product in a separation column, whereas
Supercritical fluid extraction provides a distinct carotenes remain in the bottom phase product together
advantage not only in the replacement but also extracts with triglycerides. For recovering the carotenes together
oils that are lower in iron and free fatty acid. Some with the tococromanols the above mentioned
application of SFE in food is mentioned below: esterification to volatile (CO soluble) methyl esters
2
Removal of fat from foods: Edible oils and their makes possible to recover tocochromanols and
components has been the target of supercritical fluid carotenes (together with squalene and sterols) as bottom
[9,11]
processing since the early 70s. Although product from this natural source .When the
triacylglycerides are only fairly soluble in SC-CO , the glycerides (in case of the esterification) or the FFAs
2 from deodorizer distillates have been removed, then
advantages of organic solvent-free processing have there is a feed material available for obtaining enriched
stimulated research and development in various areas. fractions of tocochromanols and carotenes of much
One of these is the removal of fat from food. The higher concentration. In this feed material,
process has been fully designed for commercial tocochromanols and carotenes (in case of palm oil) are
application, using the aforementioned standard design. the main components and have to be separated from
The process has the advantage of producing fat-free or other unsaponifiable substances present, such as
fat-reduced potato chips. According to the expected squalene and sterols. Of these compounds, squalene has
taste the amount of remaining fat in the potato chips can
[7,8] the highest solubility in SC-CO , all phytosterols have a
easily be controlled . 348 2
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