Full text of DOI:10.1111/j.1541-4337.2002.tb00005.x

Supercritical
Fluids and the
Food Industry
N.L. Rozzi and R.K. Singh
Dept. of Food Science and Technology
University of Georgia
Athens,GA 30602-7610
Direct inquiries to author Singh
(E-mail: rsingh@arches.uga.edu)
Abstract: This paper reviews the use of supercritical fluids in various aspects of the food industry, and is divided into six
different areas: modeling of supercritical fluids, separation of extracted material, supercritical carbon dioxide as a solvent
for extraction, supercritical fluids and analytical uses, and supercritical fluids and novel methods of food processing. An
assortment of solutes are covered in the extraction section of the paper, including antineoplastic agents and lipids. The
analytical methods section covers supercritical fluid chromatography, pesticide detection, and lipid analysis. The novel
methods section discusses supercritical fluid extrusion and a new method of eliminating hexane from soybean oil.
Keywords: Supercritical Fluid, Carbon Dioxide, Review
Introduction
1. Supercritical fluids have a higher diffusion coefficient and
lower viscosity than liquids.
2. Absence of surface tension allows for their rapid penetration
into the pores of heterogeneous matrices, which helps enhance
extraction efficiencies.
3. Selectivity during extraction may be manipulated by varying
the conditions of temperature and pressure affecting the solubility
of the various components in the supercritical fluid.
4. Supercritical fluid extraction does not leave a chemical resi-
due.
Supercritical fluid extraction utilizes the ability of certain chemi-
cals to become excellent solvents for certain solutes under a com-
bination of temperature and pressure. The solvent becomes super-
critical when it is raised above its critical point for both tempera-
ture and pressure (Table 1). In the case of carbon dioxide, the crit-
ical point is at 31.06 °C and 7.386 MPa. CO2 is the solvent of
choice for use in supercritical fluid extraction because it is,
“GRAS, nonflammable, noncorrosive and inexpensive (Rizvi and
others 1994)”. In addition, CO2 has a low critical temperature,
which can help prevent thermal degradation of food components
when they are being extracted. The problem with most of the flu-
ids listed (Table 1) besides CO2 is that they are either difficult to
handle and/or obtain in a pure form. The advantages of supercriti-
cal fluid extractions are:
5. Supercritical fluid extractions can use carbon dioxide gas,
which can be recycled and used again as part of the unit opera-
tion (Rizvi and others 1994).
Supercritical carbon dioxide has been researched for potential
applications in many different fields including Food/ Agriculture,
Analytical/Supercritical Fluid Chromatography, and the Petro-
chemical/Chemical Industries. Figure 1, presents the breakdown
of research areas of recent research articles (1999-2000). These
articles were determined by performing a search on Current Con-
tents for articles that contain the terms carbon dioxide and super-
critical fluid, and resulted in 264 articles being obtained. From
Figure 1 it can be seen that 32% of the research articles were from
the Food and Agriculture research areas and an additional 4% of
the research articles were in Pesticide research. Combined, this is
12% greater than the next largest area (Petrochemical). To further
expand on this literature search, a second search was performed
on Food Science and Technology Abstracts using the same terms
as the previous search. In addition, the timeframe for the search
was increased from 1999-2000, to 1995 to 2000. This search re-
sulted in 105 articles being produced. The results of this search
are displayed in Figures 2 and 2a. Figure 2a is a breakdown of
topics found in the Analysis section of Figure 2. Based on these
searches, this paper discusses Modeling of Supercritical Fluids,
Separation of Extracted Materials, Extraction from Botanical Sam-
ples, Extraction of Lipids, Supercritical Fluids and Analytical Uses,
and Supercritical Fluids and Novel Methods of Food Processing.
Table 1—Critical points of selected gasses
Critical
Temperature (K)
Critical
Pressure (MPa)
Fluid
Hydrogen
Neon
Nitrogen
Argon
Oxygen
Methane
Krypton
Carbon Tetrafluoride
Ethylene
Xenon
Carbon Dioxide
Ethane
Acetylene
Nitrous Oxide
Chlorodifluoromethane
Propane
33.25
44.40
1.297
2.6545
3.398
4.860
5.043
4.595
5.49
3.74
5.040
5.87
7.386
4.871
6.247
7.285
4.967
4.247
126.24
150.66
154.58
190.55
209.46
227.6
282.35
289.7
304.17
305.34
308.70
309.15
369.27
369.85
Rizvi and others 1994
© 2002 Institute of Food Technologists
Vol. 1, 2002—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 33

CRFSFS:Comprehensive Reviews in Food Science and Food Safety
modeling supercritical fluid extractions is to model the process it-
self. Sovova (1994) developed a model based on a mass balance
of the process to describe the extraction of natural products with
supercritical fluids. This model takes into account the porosity of
the raw material from which the target compound is being extract-
ed, the density of the solid phase and the length of time used to
extract the target compound. Although each method is vastly dif-
ferent from each other, both provide important information about
the extraction process.
Solubility Parameter
The solubility parameter is the solvating capability of a com-
pressed gas as described by the Hildebrand Solubility Parameter.
The Hildebrand Solubility Parameter is a rough measure of the
ability of a solvent to dissolve a solute. Equation 1 relates the solu-
bility parameter to the density of both the gas and the liquid by:
(1)
Figure 1—Results of Current Contents search using “super-
critical fluids” and “carbon dioxide” as search terms (1999-
2000)
where d is the solubility parameter, Pc is the critical pressure, r is
the gas density and rliq is the density of the liquid (Rizvi and oth-
ers 1994). From this equation, the effect of pressure on the solu-
bility parameter can be seen. When the pressure is at atmospheric
Modeling of Supercritical Fluids
The solubility of substances in supercritical fluids has been de- pressure, the solubility parameter is zero. As the pressure increas-
scribed by many different methods. Two examples of these meth- es the solubility parameter increases, with great changes taking
ods will be included in this paper. One method utilizes: solubility place as the critical pressure is reached (Rizvi and others 1994).
parameters and the other is based on process modeling. Each of
Although this equation can be used with some success to de-
these methods has their benefits and drawbacks. Solubility pa- scribe the solubility of some solutes in supercritical fluids, it does
rameters describe the solubility of the substance in supercritical not work in all cases and must be used with careful attention be-
CO2 under “laboratory conditions” and not during conventional ing paid to the original work of Hildebrand and the warnings giv-
extraction processes. In addition, the solubility parameter is also en there. Some of these warnings are that it is dangerous to em-
influenced by the equation of state, which is used to calculate ploy the solubility parameters for esters, ketones, alcohols and
some variables needed for the solubility parameter. Two equations other polar liquids. This should warn people against using the
of state which can be used are the Peng-Robinson Equation of Hildebrand solubility parameter as the final word and any as-
State and the Sanchez-Lacombe Model. The other approach to sumptions made using the solubility parameters should be
Figure 2—Results of Food Science and Technology Abstracts
search using “supercritical fluids” and “carbon dioxide” as
search terms (1995-2000)
Figure 2(a)—Distribution of target analytes from analysis sec-
tion of figure 2
34 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 1, 2002

Supercritical fluids and the food industry . . .
backed up with experimental evidence (McHugh 1994).
mixing rules. In addition, ␧ and ␴ are equation specific constants.
Allada (1984) proposed a new equation to define the solubility The values for the Peng-Robinson equation of state are –
parameter (Equation 2). This equation uses the, “internal energy of 0.414214 and 2.414214, respectively (Smith 1996).
the gas relative to isothermally expanded ideal gas state as the co-
hesive energy of the gas”, (Allada 1984).
(8)
Equation 8 can be displayed as:
(2)
In Equation (2) dr is the reduced solubility parameter, R is the
universal gas constant, C1 is a conversion factor to make the units
cancel, E is the internal energy of the gas at temperature T and
pressure P. Pr and Tr are the reduced pressure and reduced tem-
perature respectively. The reduced pressure and reduced temper-
ature are related to (P/Pc) and (T/Tc) where P and T are the current
pressure and temperature, and Pc and Tc are the critical pressure
and temperatures respectively. Z is the compressibility factor and
E* is the internal energy of the gas, “isothermally expanded to
‘zero’ pressure where intermolecular separation is infinite and in-
termolecular cohesive force is zero,” (Allada 1984).
(9)
where
(10)
(11)
The values for the compressibility factor and the (E* - E)/TC and
Z can be calculated by using equations (3 to 6)
am is the attraction parameter of the mixture, bm is the mixture’s
van der Waals co-volume, X is the mole fraction in the liquid or
supercritical fluid phase, and i and j represent the different com-
ponents of the mixture. aij comes from the mixing rules presented
by van der Waals, which utilizes only one interaction parameter
(Rizvi and others 1994). Care should be taken when using these
mixing rules though (Equations 10 and 11) since the mixing rules
are for cubic equations of state. Cubic equations of state only ac-
count for dispersion forces and not for hydrogen bonding, which
could be occurring during the extraction. This possible hydrogen
bonding will prevent random distribution of the solute in the sol-
vent, thus causing the need for a different set of mixing rules
(6) (McHugh 1994).
(3)
(4)
(5)
One different set of mixing rule equations which can be used
are equations (12) and (13). These equations incorporate two new
parameters into the mixing rule. These parameters are dij and hij
which represent the interaction and size parameters respectively.
These data parameters are obtained by regressing the P-x data
and fitting it to the equation of state.
where H is the enthalpy of the supercritical fluid and H* is the en-
thalpy of the ideal gas at the same temperature (Allada 1984). The
values for the (H* – H)/Tc and Z data are obtained from the book,
Chemical Process Principles, Part Two, Thermodynamics, by Hou-
gen and Watson (1947). This equation works well for prediction
of the solubility of a substance in different supercritical fluids at
different pressures. This equation (Equation 2) does not, however,
work well for changes in temperature. To account for temperature
effects, it would be necessary to include a solvent-solute interac-
tion represented by a cross-virial coefficient (Allada 1984).
(12)
(13)
The addition of these two parameters greatly improve the fit of
the data for some solutes, but they still need to be investigated for
use with other solvent-solute combinations (McHugh 1994).
Peng-Robinson Equations of State
Equations of state can be used to describe the behavior of sol-
utes being dissolved in supercritical fluids. A favored equation to
describe this action is the Peng-Robinson equation (Smith 1996):
Sanchez-Lacombe Model
Another equation of state is the Sanchez-Lacombe equation of
state (14), which is based on the lattice-fluid theory, which as-
(7) sumes that the polymer has a flexible liquid structure. This limits
the use of this equation to polymers which are noncrystalline, not
cross-linked or slightly crosslinked, and above their glass transi-
tion temperature. This equation has been used to model the solu-
bility of supercritical CO2 in solid, amorphous polymers or molten
polymers. In addition, it also describes the subsequent swelling of
the polymer as the supercritical fluid dissolves in the polymer.
Three assumptions need to be made to use the Sanchez-Lacombe
where Z is the compressibility factor, P is the pressure, T is the
temperature and R is the Universal Gas Constant. This equation is
used to describe the behavior of pure species. It can be modified
to describe the behavior of mixtures which results in Equation 8,
where a and b are mixing values which are related to aI and bI by
Vol. 1, 2002—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 35

CRFSFS:Comprehensive Reviews in Food Science and Food Safety
equation. These assumptions are:
(1) The solubility of the gas in the polymer represents equilibri-
um data.
(24)
(2) The solubility of the polymer in the high-pressure gas phase
is essentially zero.
(3) The amorphous polymer above its glass transition tempera-
ture can be modeled as a liquid.
Using the values for x and y solved in the mass balances, the
curves describing the extraction process can be determined by
the following:
(25)
(14)
where e(t) is the mass extracted relative to the mass of the solute-
free solid phase, xo is the concentration related to the solute-free
solid phase, H is the height of the bed, and x(t,h) is the concentra-
tion related to the solute-free solid phase as a function of time (t)
and height (h). This modeling system can be used to describe the
extraction of natural products using supercritical CO2 (Sovova
1994). This model has been successfully applied by Mira and oth-
ers (1996), Perrut and others (1997), Coelho and others (1997),
and King and others (1997) in modeling the extraction of natural
products from plant sources.
where rr, Pr, and Tr, are the reduced density, pressure and temper-
ature. r represents the number of lattice sites represented by a
molecule. As compared with the Peng-Robinson equation of state,
which has only two mixing rules, the Sanchez-Lacombe equation
has three adjustable characteristics (Kriszka 1988). The variables
in this equation are:
(15)
(16)
(17)
(18)
(19)
Separation of Extracted Material
Extraction is the most common use for supercritical fluids. The
process is simple, with the major process parameters being tem-
perature, pressure and flow rate of the supercritical fluid. Figure 3
presents a basic flow diagram for supercritical fluid extraction,
and Figure 4 illustrates a similar system that is designed for frac-
tionation experiments. The most important aspect of this flow dia-
gram is the separation of the extracted material from the supercrit-
ical fluid. This section discusses multiple methods for the separa-
tion of extracted material from supercritical fluids.
(20)
(21)
(22)
where:
εÿ
=
=
=
=
=
Internal energy per mer
Universal gas constant
Closed-packed volume of a mer
Closed pack volume of the mixture
Molecular weight
R
ÿÿ
V*
M
The values for CO2 of r*, T*, and P* are 1.510 (g/cm3), 305 (K),
and 5670 (atm) respectively (Kiszka 1988).
Process Modeling
Figure 3—Flow diagram of a supercritical fluid extraction
system
The process of extracting natural products by supercritical fluid
extraction can be modeled using the model developed by Sovova
(1994). This model is based on the following assumptions:
(1) The solvent flows axially through a bed of raw material in a
cylindrical extractor.
(2) The solvent is solute-free upon entering the extractor.
(3) The solute is partially exposed to the solvent due to previous
milling (Sovova 1994).
The mass balance for the process is described by Equations
(23) and (24) below, where ␳ is the solvent density, ␧ is the porosi-
ty, y is the solvent-phase concentration related to the solute-free
solvent, x is the concentration related to solute-free solid phase, U
is the superficial velocity of the solvent, h is the axial coordinate,
␳s is the density of the solid phase, t is time, and J(x,y) is the mass
transfer rate.
Figure 4—Flow diagram of a supercritical fluid fractionation
system
(23)
36 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 1, 2002

Supercritical fluids and the food industry . . .
There are multiple methods that can be utilized to separate the purposes, and flavor and fragrance purposes. Some companies
solute from the supercritical fluid, which is acting as the solvent. have even begun to market botanical extracts obtained by super-
There are three basic concepts that can be used to accomplish critical CO2 extraction. One company in particular, Flavex Natur-
this. The first is to change the temperature or pressure so that the extrackte, Rehlingen, Germany, specializes exclusively in SFE pro-
solvent capability of the supercritical fluid changes. The other is to duced products and markets them internationally. Table 2 in-
“wash” the solute out of the supercritical fluid using a solvent that cludes a brief list of different botanicals, which have been investi-
can strip the solute from the supercritical fluid. In addition to the gated since 1997.
separation of the solute from the supercritical fluid, it is possible
to separate multiple solutes within the supercritical fluid using a be extracted from plant materials in addition to flavors and essen-
packed column. tial oils. Krukonis and others (1979) performed a brief extraction
Separation of Antineoplastic Agents: Antineoplastic agents can
The simplest method to remove the solute from a supercritical study on plants which were confirmed by the National Cancer In-
fluid is to change the pressure of the supercritical fluid so that ei- stitute (NCI) to contain antineoplastic agents. One example of
ther the fluid is no longer supercritical or that the solute is no these species is maytansine which is contained in materials
longer soluble in the supercritical fluid. Snow and others (1997), B628318, B628259, and B628201. The materials that were used
Barth and others (1995), and Sugiyama and Saito (1988), all uti- in the study are listed in Table 3 (McHugh and Krukonis 1994).
lized this method to remove their respective solutes from the su-
In this experiment approximately 20 grams of plant material
percritical fluid. In all three cases the individuals performing the was extracted using 200 L of supercritical CO2. This was com-
study routed the solute/solvent combination through a restriction pared with the NCI protocol that utilized a Soxhlet extraction with
valve and into a collection vessel. Inside the collection vessel, the 95% ethanol. The samples were then analyzed in vitro for bioac-
pressure was reduced to room pressure, causing the supercritical tivity using 9KB leukemia cells. The results of this comparison are
fluid to return to its gas state, and the solvent to be released into displayed in Table 4 and are expressed in terms of the ED50 value.
the collection vessel. Inside the collection vessel, there can be a This value is defined as the extract concentration in which 50% of
solvent to receive the solute, so that further analysis can be per- the cell growth is inhibited, and is expressed in mg/l. The lower
formed. Other times the vessel may be left empty so that the solute the ED50 value, the more cytotoxic the extract. From the results in
can crystallize upon leaving the supercritical fluid.
Table 4, it can be seen that all of the extracts exhibited cytotoxic
Another variation of the previous method to remove the solute results. In addition, 2 samples exhibited an ED50 value less than
from the supercritical fluid is to change the temperature of the su- that of the Soxhlet extraction (McHugh and Krukonis 1994). Due
percritical fluid. This may cause the fluid to leave the supercritical to the small amount of raw material provided and the lack of any
region, but it allows for the pressure of the fluid to be maintained. pure material that could be used for solubility testing, this study
This could save money by allowing for the pressurized fluid to be serves mainly as a screening study though, that indicates the po-
recirculated, without having to repressurize it. This means that less tential for using supercritical fluids to isolate antineoplastic agents.
money would have to be spent on the energy to pressurize the If more materials were provided for in this study, determination of
fluid. Lancas and others. (1990) and Vilegas and others (1994) “ideal” extraction conditions, or a method to fractionate the ex-
both utilized this method to remove their respective solutes from tracts could have been developed. To prove this point, a second
the supercritical fluid. This method simply involves the removal of extraction of sample B638318 was performed. This time the ex-
the solute by dropping the temperature of the solvent until the sol- tract was split into 2 fractions, fraction 1 consisted of the materials
ute is no longer soluble and precipitates from the supercritical flu- which were extracted by the first 20 L of CO2 and fraction 2 used
id. This is also referred to as a cryogenic trap. Once the extraction the remaining 180 L of CO2. After analysis of both these samples,
has been completed, the system can be depressurized and the ex- it was found that the second fraction yielded a much lower ED50
tract can be removed from the sample chamber.
value (0.03 vs 0.19 obtained in fraction 1) (McHugh and Krukonis
The solute can also be removed from the supercritical fluid by 1994).
“washing” it from the solvent. This allows for the process to be
Annato: Bixin, a pigment used for food coloring obtained from
maintained at a constant pressure much as the cryogen trap did, the seeds of annatto (Bixa orellana L.) was extracted using super-
but it also allows for the removal of the solute from the process critical carbon dioxide. A range of pressures (40.41 MPa to 60.62
without having to shut the process down. An example of this type MPa) and temperatures (40 and 80 °C) were tested. In addition, 5
of removal is used in the decaffeination of green coffee beans to % chloroform, 10% chloroform, 5 % methanol, 10 % methanol,
remove the caffeine from the supercritical CO2 (McHugh 1994).
and 4% acetonitrile containing 0.05% trifluoroacetic acid were
If more than one chemical is extracted by the supercritical fluid, used as co-solvents in the extraction process. After analysis by
column chromatography can be used, utilizing the supercritical HPLC, it was found that the optimum conditions for extraction of
fluid as the mobile phase. As with HPLC, both normal and re- bixin was 60.62 MPa and 40 °C with 4% acetonitrile containing
versed phase chromatography can be utilized to separate the sol- 0.05% trifluoroacetic acid as the co-solvent (Anderson and others
utes depending on their polarity. The retention times on these col- 1997).
umns can be correlated to the solubility of the solute in the super-
Cloudberry Seed: Cloudberry seeds produce oil that is low in
critical fluid (Sakaki 1994). McHugh and Krukonis (1994) and saturated fats, and contains relatively high amounts of toco-
Sakaki and others (1994) used column chromatography to sepa- pherols and carotenoids. The extraction of Cloudberry seed oil by
rate multiple solutes from supercritical fluids.
supercritical fluid extraction was compared to that of a Soxhlet ex-
traction using diethyl ether as the solvent. The range of extraction
conditions consisted of 9 to 30 MPa at either 40 °C or 60 °C. The
extracts were analyzed by GC-FID and HPLC for their fatty acid
and carotenoid/ tocopherol content respectively. There was no
significant difference in the fatty acid composition of any of the
Supercritical Carbon Dioxide as a Solvent for Extraction
Supercritical Fluid Extraction of Botanical Samples
Supercritical fluids have been used to extract a wide range of extracts; the carotenoid content did not increase at pressures over
analytes from botanical samples. These analytes range from essen- 15 MPa, and the amount of tocopherols extracted decreased with
tial oils to phytochemicals, and can include lipid extraction. These increasing pressure (Manninen and others 1997).
extracts have been used for analytical purposes, supplementation
Grape Seeds: Phenolic compounds and lipids were extracted
Vol. 1, 2002—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 37

CRFSFS:Comprehensive Reviews in Food Science and Food Safety
from white grape seeds and analyzed for phenolics (Palma and microbial activity while the second fraction tested positive for anti-
Taylor 1999) and for the extracts antimicrobial activity (Palma and fungal activity (Palma and others 1999)
others 1999). The optimum extraction conditions for phenolics
Paprika: Lipids, carotenoids and tocopherols were extracted
was determined to be CO2 at a density of 0.95 g/ml and 55 °C from paprika and analyzed by HPLC. Vesper and Nitz (1997)
with a co-solvent of 10% methanol with the most influential vari- compared the use of supercritical CO2 at 35 MPa and 50 °C to the
able on the extraction being the CO2 density, followed by the na- use of hexane and found that hexane had slightly higher caro-
ture of the co-solvent, percentage of co-solvent, and extraction tenoid content while the extracts obtained by SFE had a higher
temperature (Palma and Taylor 1999). In addition, it was found water content (Vesper and Nitz 1997a). After storage studies of
that the extract could be split into two fractions; those compounds these extracts at 40 °C, it was found that the samples extracted by
obtained by pure CO2 and those compounds obtained with CO2 SFE had reduced fatty acid stability compared to the samples ob-
and a co-solvent. The first fraction contained mainly fatty acids tained by hexane extraction (Vesper and Nitz 1997b). Jaren-Galan
and sterols, while the second fraction contained phenolic com- and others (1999) and Skerget and others (1998) analyzed the ef-
pounds. The first fraction was found to have a high degree of anti- fects of temperature and pressure on the composition of the pa-
Table 2—Summary of publications on extraction methods from plant and botanical samples using supercritical fluid extrac-
tion (Pub. Year >1994)
Pressure
Analytical
Sample
Analyte(s)
(MPa)/ T (°C)
Instrument
Reference
Annato
trans-Bixin
40 to 60/40
HPLC
Anderson and others 1997
Black Pepper
Essential Oil
20 to 32/45-65
GC-FID
Tipsrisukond and others 1998
Ferreira and others 1999
Buriti
Carotenoids
Lipids
20-30/40-55
GC-FID
Spectrophotometry
De Franca and others 1999
Caraway Seeds
Limonene
Carvone
7.5-300/32-75
GC-FID
Baysal and Starmans 1999
Carrot
Carotenes
Flavinoids
60.6/40
HPLC
Chandra and Nair 1997
Chamomile
Cloudberry Seed
20.2/40
GC-MS
Scalia, Giuffreda and Pallado 1999
Lipids, ␤-carotene,
Tocopherols
9-30/40-60
GC-FID, HPLC
Manninen and Kallio 1997
Manninen and Laakso 1997a
Manninen and Laakso 1997b
Manninen and others 1997
Clove Bud
Essential Oil
8-20/50-50
20-30/35
8.1/40
GC-MS
Della Porta and others 1998
Illes and others 2000
Coriander Seed
Fennel Seeds
Grapefruit
Essential Oil
GC-FID
Fennel Oil
GC-MS
Simandi and others 1999
Poiana, Sicari and Mincione 1998
Limonene and Terpines
8-25/40
GC-FID
Grape Seeds
Polyphenolic compounds 45.6/35
GC-FID, HPLC
Palma and others 1999
Palma and Taylor 1999
Palma and others 2000
Lavender
Lovage
Onion
Essential Oil
Essential Oil
Essential Oil
9/ 48
GC-MS
GC-FID
GC-FID
Reverchon and Della Porta 1995
Dauksas and others 1998
8-30/10-50
10-30/45-65
Sass-Kiss and others 1998
Sass-Kiss and others 1998a
Paprika
Lipids, carotenoids,
and tocopherols
35/50
15/40
13.7-41.3/40
HPLC, GC-FID
Skerget and others 1998
Jaren, Nienaber and Schwartz 1999
Vesper and Nitz 1997b
Vesper and Nitz 1997a
Illes and others 1999
Peppermint
Essential Oil
6.5-10/25-40
GC-MS
Ammann and others 1999
Aleksovski and others 1999
Olive Products
Tocopherols
Phenol Compounds
35/50
33.4/100
SFC-FID
Electrospray-MS
Ibanez and others 2000
Le Floch and others 1998
Oregano
Essential Oil
8-30/40
GC-FID
GC-MS
Simandi and others 1998
Rosemary
Antioxidants
Essential Oils
10-16/37-47
10-40/60
Ibanez and others 1999
Senorans and others 2000
Lopez-Sebastian and others 1998
Coelho-Luiz and others 1997
Bauman and others 1999
Tena and others 1997
Sage
Essential Oils
Antioxidants
10-40/60
GC_MS
Ronyai and others 1999
Bauman and others 1999
Spearmint Leaf
Star Anise
Essential Oil
Essential Oil
27.6/60
8-20/50
GC-MS
GC-MS
Pino, Garcia and Martinez 1999
Della and others 1998
38 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 1, 2002

Supercritical fluids and the food industry . . .
Table 3—Plant materials analyzed for anticancer agents
Table 4—Cytoxicity of extracts as measured by ED50 values
(mg extract/l C2H5OH)
NCI
Designation Specific Name
Variety
Origin
Supercritical Carbon
Dioxide Extraction
Ambient Temperature
Ethanol Extraction
Plant Material
B628201
B628259
B628318
B634635
B638786
B805592
B806512
Maytenus senegalensis Celastraceae
Maytenus senegalensis Celastraceae
Maytenus senegalensis Celastraceae
Gypsophilia
Citharexylum caudatum Verbenaceae
Tanzania
Kenya
India
B628259
B628318
B634635
B638786
B805592
B806512
30.0
0.25
24.0
4.0
21.0
14.0
11.0
0.11
43.0
>100.0
126.0
6.2
Caryophyllaceae Michigan
Panama
Peru
Peru
Maquire calophylla
Rollinia papilliionela
Moraceae
Annonaceae
Reprinted with permission from McHugh and Krukonis 1994.
Reprinted with permission from McHugh and Krukonis 1994.
the milk fat into 4 different fractions. These factions were short-
chain triglycerides (SCT), medium-chain triglycerides (MCT), long-
chain triglycerides (LCT) and cholesterol. From this research it was
found that PCB’s can be found predominately in the SCT, MCT
and cholesterol fractions of the milk fat (Ramos and others 2000).
Professor S.S.H. Rizvi has been a virtual front runner in the use
of supercritical fluids to fractionate milk fats. In 1992 he pub-
lished an article on the fluid liquid equilibria of anhydrous milkfat
with supercritical CO2 (Yu and others 1992). Following this re-
search, Shukla and others (1994) studied the effect of using milk
fat obtained by supercritical fluid extraction on the physicochemi-
cal and rheological properties of butter. This research demonstrat-
ed the ability to use supercritical CO2 to produce butter with spe-
cific properties such as reduced moisture content, elevated melt-
ing points, and lower cholesterol. This research was expanded
from butter to milk fat which can be used in other products by
Bhaskar and others (1998).
Meats: Supercritical fluid extraction has been utilized to extract
lipids from a variety of meats, including pork and beef. These
methods have been developed for analytical purposes, and are
discussed in detail in analytical section of this paper.
Nuts and Seeds: Lipids have been extracted from a series of dif-
ferent types of nuts and oil seeds. Some examples of these are pe-
cans and rice bran. A reduction in the lipid content of pecans can
increase their storage life (Alexander, Brusewitz and Maness
1997). From this study it was determined that the optimum extrac-
tion conditions for pecan oil was 66.8 MPa and 75 °C. The kinet-
ics of this extraction was also determined to be limited by the sol-
ubility of pecan oil in supercritical CO2 for the initial part of the
prika extract. It was found that the selectivity of CO2 is higher at
lower temperatures (40 °C) and that the optimum pressure for ex-
tracting aromatic compounds is 15 MPa and coloring com-
pounds is 40 MPa (Skerget and others 1998). After further analy-
sis, it was determined that the pigments isolated at lower pressures
consisted mainly of ␤-carotene. Extracts obtained at higher pres-
sures contained a greater proportion of capsorubin, capsanthin,
zeaxanthin, and ␤-cryptoxanthin and little ␤-carotene. It was fur-
ther determined that a two stage extraction could be used to ex-
tract a majority of the compounds present (200% compared to
reference). This extraction first consisted of an extraction at 40 °C
and 13.8 MPa followed by an extraction at 40 °C and 41.3 MPa
(Manuel and others 1999).
Rosemary: Antioxidants from rosemary were extracted and
dearomatized by supercritical CO2 for use in natural supple-
ments. The extraction of rosemary oil was initially modeled by Co-
elho, and others (1997) using the model developed by Sovova
(1994). Ibanez and others (1999) continued this work by utilizing
fractionation to further separate the essential oil extracted from the
rosemary plants into two different fractions. These fractions had
different compositions and antioxidant activities (Ibanez and oth-
ers 1999). Lopez-Sebastian and others (1998) utilized supercriti-
cal CO2 remove the rosemary aroma from the extracts obtained
by solvent extraction. This produces an extract which is more suit-
able for use in food products. In addition, this process also re-
moves almost 100% of the residual solvent left from the extraction
process (Lopez-Sebastian 1998).
Supercritical Fluid Extraction of Lipids
Supercritical carbon dioxide has been utilized to extract lipids extraction, while it was diffusion limited for the second half of the
from an assortment of matrices. Some of these extractions have extraction (Alexander and others 1997). During SFE the possibility
been used to analyze the fat content of different food products. of breakage of the pecan kernels increases with increased mois-
Other extractions have been used to obtain pure lipid extracts or ture content of the kernels (Bellmer and Brusewitz 1999). This ef-
to produce products that contain a reduced amount of certain lip- fect was increased when a short depressurization time was used.
ids or other compounds such as cholesterol. Table 5 presents a A reduction in the moisture content of the pecans to a final mois-
list of some current articles in the field.
ture content range of 3.5% to 11.0% before lipid extraction can
Dairy Products: Dairy products have been subjected to super- reduce the amount of kernel breakage (Bellmer and Brusewitz
critical fluid extraction to fractionate lipids and isolate vitamins for 1999).
quantification. Berg and others (2000), and Turner and Mathias-
Rice bran is a by-product of rice milling that contains 15 to
son (2000) utilized supercritical CO2 to remove vitamin A and E 20% oil by weight which could be of value to the food and phar-
from powdered milk, fluid milk and meat. The extraction condi- maceutical industry (Shen and others 1996). This oil has been ex-
tions for both studies were 80 °C and 37 MPa. Berg and others tracted by SFE. Shen and others (1996) reported that rice bran oil
(2000) furthered this research by using Hydromatrix as a water was extracted with subcritical and supercritical CO2 at a tempera-
absorbent to reduce the adverse effects of the water component of ture range of 0 to 60 °C and pressures of 17 to 31 MPa. This re-
milk on the extraction process. This allowed all of the vitamin A search was furthered by Shen and others. (1997) by the addition
and E to be extracted from a 0.5 gram sample in 80 minutes, of fractionation to separate water and free fatty acids from the ex-
shorter than conventional methods. In addition to vitamins, the tract. Kuk and Dowd (1998) continued this research by compar-
distribution of polychlorinated biphenyls (PCB’s) in the different ing the quality of oil extracted by supercritical fluid extraction as
milk fat fractions was studied using supercritical fluid extraction. compared to that of hexane extraction. It was found that at tem-
Ramos and others (2000) utilized supercritical CO2 to fractionate peratures of 80 °C the quality of oil extracted by supercritical fluid
Vol. 1, 2002—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 39

CRFSFS:Comprehensive Reviews in Food Science and Food Safety
extraction was far superior to that of oil obtained by hexane ex- using SFE, but they had the aid of surfactants which will be dis-
traction (Kuk and Dowd 1998).
cussed later in this article.
Seaweed and Fungi. ␻-3 fatty acids were extracted from brown
seaweed, Sargassum hemiphyllum (Turn.) C., and fungi, Cunning-
hamella echinulata and Pythium irregulare, by SFE for potential
use in functional foods and nutraceuticals. Chung, and others
(1998) reported that they were able to obtain yields of total and
individual ␻-3 fatty acids that were significantly greater than those
obtained by Soxhlet extraction by using extraction conditions of
37.9 MPa and 50 °C. They also found that the concentration of to-
tal polyunsaturated fatty acids increased significantly and total sat-
urated fatty acids decreased significantly with increasing pressure
and solvent density as measured by the ratio of saturated to unsat-
urated fatty acids. Supercritical CO2 was used to extract fungal oil
containing ␥-linolenic acid from Cunninghamella echinulata with
similar results (Certik and Horenitzky 1999). It was found that su-
percritical CO2 extraction at 30 MPa and 50 °C yielded greater
than 110% of the ␥-linolenic acid that was obtained by chemical
extraction. Walker and others (1999) also extracted ␻-3 fatty acids
Supercritical Fluids and Analytical Uses
Supercritical carbon dioxide has been utilized in multiple meth-
ods of analysis. Supercritical carbon dioxide is used as either an
extraction medium, as in rapid analyses for fat content, or as a
mobile phase, as in supercritical fluid chromatography. This sec-
tion covers the use of supercritical fluids in the detection of fat
content, pesticide residues, and supercritical fluid chromatogra-
phy.
Rapid Analysis for Fat Content: Supercritical fluids have been
used to determine the fat content of numerous products ranging
from beef to oil seeds and vegetables. Taylor and others (1997)
compared traditional methods of analysis for fat content to newer
methods developed with supercritical fluid extraction. For the
analysis of fats content in soybeans, sunflower, safflower, cotton-
seed, rapeseed and ground beef, it was found that supercritical
fluid extraction yielded higher recoveries than those obtained by
Table 5—Summary of Publications on Lipid Extraction Studies Using Supercritical Fluid Extraction (Pub. Year >1995)
Pressure
Analytical
Sample
Analyte(s)
(MPa)/ T (°C)
Instrument
Reference
Brown Seaweed
Butter Oil
Fatty Acid Composition
Cholesterol
24.1-37.9/40-50
10-27.6/40-70
GC-FID
GC-FID
Cheung, Leung and Ang 1998
Mohamed, Neves, and
Kieckbusch 1998
Corn Bran
Ferulate phytosterol
esters
13.8-69/40-80
51.7-62/100
SFC
Taylor and King 2000
Cotton Seed
Fatty Acid Composition
GC-FID
Gravametric
Taylor, Eller and King 1997
Certik and Horenitzky 1999
Cunninghamella echinulata
Ground Beef
Fatty Acid Composition
Fatty Acid Composition
20-35/40-70
38/80
GC-FID
GC-FID
King and others 1996
Taylor, Eller and King 1997
Milk Fat
Lipid Profile
Vitamins
6.9-17.2/40-60
GC-FID
HPLC
Bhaskar and others 1998
Ramos, Hernandez
Solid Fat content, PCBs
Pulsed NMR
DSC
and Gonzalez 2000
Turner and Mathiasson 2000
Berg and others 2000
Manganiello, Rios, and Valcarcel 2000
Oat Bran
Pecan
Digalactosyldi-
acylglycerols
40/50-70
SFC
Andersson, Demirbuker and
Blomberg 1997
Fatty acid composition
and product breakage
41.3-66.8/45-75
GC-FID
Alexander, Brusewitz and
Maness 1997
Li, Bellmer and Brusewitz 1999
Pistachio
Pork
Texture and Sensory
Attributes
20.7-34.5/50-70
7.3-34/50-150
Instron Universal
Testing Machine
and Sensory Panel
Palazoglu and Balaban 1998
Cholesterol/Fat Content
Texture and Sensory
Attributes
HPLC
TLC
Sensory Panel
Hunter Colorimeter
Lin and others 1999
Berg and others 1997
Pythium irregulare
Rapeseed
Fatty Acid Composition
13.7-27.5/40-60
51.7/100
GC-FID
Walker, Cochran and Hulbert 1999
Bruhl and Matthaus 1999
Fatty Acid Composition
Tocopherol Content
HPLC
GC-FID
Rice Bran
Fatty Acid Composition
␣-tocopherol sterols
Moisture
17-31/0-60
48.2-62/70-100
GC-MS
HPLC
Shen and others 1996
Shen and others 1997
Kuk and Dowd 1998
Safflower
Soybean
Fatty Acid Composition
Fatty Acid Composition
51.7-62/100
51.7-62/100
GC-FID
GC-FID
Taylor, Eller and King 1997
Taylor, Eller and King 1997
Bruhl and Matthaus 1999
Sunflower
Fatty Acid Composition
51.7-62/100
GC-FID
Taylor, Eller and King 1997
Bruhl and Matthaus 1999
40 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 1, 2002

Supercritical fluids and the food industry . . .
the AOCS approved methods (Taylor and others 1997). Analysis
of the lipid content of dairy products has also seen some advanc-
es in the past year. Dioniosi and others (1999) compared super-
critical fluid extraction to Röse-Gottlieb solvent extraction. Their
research demonstrated a similarity between the two extraction
methods while the supercritical fluid extraction method was faster
and free of organic solvents. Manganiello and others (2000) ap-
plied the use of piezoelectric detection to supercritical fluid ex-
traction. The use of an in-line piezoelectric detector is able to
measure the change in weight of the sample during the extraction
process. This allows for more accurate determination of the final
weight of the sample after all of the fat has been extracted (total
fat). In addition, it can allow for more rapid determination of the
total fat by determining the point when the steady state mass has
been reached without having to re-extract the sample multiple
times to confirm that the steady state mass has been reached
(Manganiello and others 2000).
Figure 5—Flow diagram of a supercritical fluid chromagraphy
system
Rapid Analysis for Pesticides in Foods: Pesticide residues are a
concern among consumers throughout the United States and oth-
er countries. Currently the methods of analyzing food products
and other substances such as contaminated soil and water in-
volve the use of organic solvents such as hexane and dichlo-
romethane to extract the pesticides from the sample matrix. Once
the pesticides have been extracted from the sample matrix, the
samples must be “cleaned” to remove any unwanted com-
pounds, such as lipids, which may interfere GC analysis of the
sample for any pesticides present. The most common method for
cleaning is solid phase extraction (van der Hoff and van Zoonen
1999). Supercritical fluid extraction provides an alternative to us-
ing organic solvents for the extraction of pesticides from their
sample matrix. Some of the advantages which supercritical fluid
extraction provides over the traditional methods of pesticide ex-
traction are that the extraction can be performed in less time, and
utilizes less solvent volume. In addition, supercritical fluid extrac-
tion can be tailored to the solute of interest by changing the tem-
perature and pressure of the extraction process. Supercritical fluid
extraction can also be tailored for pesticides that contain more
polar groups by the addition of polar modifiers to the CO2 such
as methanol (Camel 1998). The use of supercritical CO2 for the
extraction of pesticide residues has been reviewed extensively in
other articles and so it will be only covered briefly in this article.
Three reviews, which stand out in the recent years are: Lehotay
(1997), Camel (1998), and van der Hoff and van Zoonen (1999).
These articles cover the analysis of pesticides in food, water, soil
and animal tissue in depth along with the benefits and drawbacks
of supercritical fluid extraction in the analysis of pesticide resides.
Since the release of the previously mentioned review articles,
more contributions to the body of scientific knowledge have been
published. These articles range from the determination of fenpy-
roximate in apples (Halvorsen and others 2000) to the extraction/
analysis of atrazine and other triazine herbicides from eggs (Pens-
abene 2000).
phase can be varied in supercritical fluid chromatography. By
varying the density, the user is in fact varying the degree to which
the mobile phase is a nonpolar solvent. A polar co-solvent can
also be added to the mobile phase which can allow for the sepa-
ration of more polar compounds as compared to those which can
be separated by carbon dioxide alone (Brunner 1994).
A supercritical fluid chromatograph consists of the same basic
components as HPLC and GC. Both HPLC and supercritical fluid
chromatography systems have a minimum of one pump with the
possibility to upgrade to a system that can handle gradients, an in-
jection apparatus, a column for separation, a detector and possi-
bly a part for collection of different analyte fractions (Figure 5). The
key differences between the two systems are the fact that the
pump on the supercritical fluid chromatography system on aver-
age could handle increased pressures, and the column on the su-
percritical fluid chromatography system has to be heated to ele-
vate the temperature of the mobile phase above its critical point
(Brunner 1994). Much like the similarities between supercritical
fluid chromatography and HPLC, supercritical fluid chromatogra-
phy and GC also have many similarities. Both systems can utilize
capillary columns and FID detectors. In fact, this represents two
different approaches to supercritical fluid chromatography,
packed column and capillary column. Packed column chroma-
tography is similar to HPLC while capillary column is similar to
GC. In packed column chromatography, it is usually necessary to
utilize co-solvents to elute polar compounds from the column,
whereas capillary column chromatography usually uses pure
CO2. The reason for using pure CO2 is that most polar modifiers
are not compatible with FID’s, eliminating the method of detection
(Pyo 2000). In addition, packed column supercritical fluid chro-
matography has advantages over tradition normal phase HPLC in
that it offers faster separations and column re-equilibration, and
higher column efficiencies. Packed column supercritical fluid
chromatography also offers advantages over normal phase HPLC
in that it has more variables which can be manipulated so that the
analysis can be better tailored to the analyte of interest (Poole
2000).
Solvents for Chromatography: Supercritical CO2 has been uti-
lized as a mobile phase in supercritical fluid chromatography
since the early 1980s when it was developed for use as a replace-
ment/ complement to gas chromatography (Poole 2000). Using
supercritical fluids as a mobile phase for chromatography pro-
vides some advantages over conventional GC or HPLC. The sol-
vent power of the supercritical fluid can be manipulated by
changing the temperature and/ or the pressure (density) of the mo-
bile phase. In the case of carbon dioxide, when it is at lower den-
sities, it is similar to that of n-hexane and at higher densities it is
similar to that of dichloromethane (Brunner 1994). In fact, just as
the composition of the mobile phase can be varied during the
separation of compounds during HPLC, the density of the mobile
Supercritical Fluids and Novel Methods of Food
Processing
Supercritical fluids have been utilized in different novel meth-
ods of food processing. Two of these new methods will be dis-
cussed in this paper. These methods are the use of supercritical
CO2 to control puffing in extrusion, and the use of supercritical
Vol. 1, 2002—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 41

CRFSFS:Comprehensive Reviews in Food Science and Food Safety
Conclusions
CO2 to remove excess hexane from soybean oil produced by
conventional extraction techniques.
Based on the information presented in this paper, it can be seen
that supercritical fluids have been used for developing an ever-ex-
panding niche in the food industry. Whether it is used as a solvent
for extraction in analytical methods such as gravimetric determi-
nation of fat content or in large-scale extractions such as decaf-
feinating coffee, supercritical CO2 has proven its usefulness as a
replacement for organic solvents. With increasing concerns about
the use of organic solvents and their disposal, supercritical fluid
extraction is gaining in popularity faster than ever before. The fu-
ture looks promising for the use of supercritical fluids, with new
methods of extraction constantly being developed, as with other
novel uses for the food processing industry.
Controlled Puffing in Extrusion
Rizvi and Mulvaney (1992) patented a new method to produce
extruded products using supercritical CO2 to reduce the tempera-
ture and shear needed for conventional thermoplastic extrusion.
Currently, the high temperatures (130 to 170 °C) and shear pre-
vents the use of thermally liable ingredients such as some flavors,
colors and whey proteins (Alavi and others 1999). This method of
extrusion uses supercritical CO2 as a blowing agent to facilitate
the formation of the cellular structure in extrudates in place of the
expansion of water upon exit of the extruder barrel in convention-
al extrusion. The use of supercritical fluids in the extrusion pro-
cess is based on a four-step process described by Rizvi and others
(1995). The steps in this process include: 1) development of a
dough with gas-holding properties by mixing alone; 2) injection
of supercritical CO2; 3) creation of a controlled thermodynamic
instability by manipulation of the pressure and/ or temperature in
the extruder; and, 4) control of the degree of cell growth during
the setting of the product through appropriate die selection and
post-extrusion drying and cooling processes (Rizvi and others
1995). The use of supercritical fluid extrusion was further studied
by Alavi and others (1999) and Gogoi and others (2000) when
the use of whey protein concentrate and egg whites in production
of extruded products was studied. The results from the study by
Alavi and others demonstrated that the extrudates produced by
supercritical fluid extrusion had a similar bulk density and expan-
sion ratio to steam extrudates, while the products obtained from
supercritical fluid extrusion had a unique composition and uni-
form microcellular composition (Alavi and others 1999). Gogoi
and others then determined that the addition of egg white to the
feed produced a softer skin and fragile, well formed cellular struc-
ture (Gogoi and others 2000). This research can lead to the addi-
tion of heat-sensitive flavorings to extruded products after the
product has been heated or while the product is being expanded
by the previously mentioned method.
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