Applications of Carbon Dioxide in Food and Processing Industries: Current Status and Future Thrusts

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DOI: 10.13031/2013.23553
Conference: 2007 Minneapolis, Minnesota, June 17-20, 2007
Cite this publication
Abstract
Carbon dioxide is one of the most important chemical compounds of human, animal, and plant lives, and environmental health. Because of several desirable characteristics of carbon dioxide in gas, solid, liquid, and supercritical states such as inertness, non-explosiveness, non-corrosiveness, high volatility, cooling ability, and low-cost, carbon dioxide is being used in a variety of applications in food and processing industries. Applications of carbon dioxide in preserving fruits, vegetables, meats, food grains, and liquid foods; inactivating microorganisms; and extracting oils, flavors, colors, and chemicals are discussed. Carbon dioxide as high pressure gas and supercritical fluid would find a niche in food and processing industries in the future especially in applications involving non-thermal sterilization and supercritical extraction.
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The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the
official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not
constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by
ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is
from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2007. Title of Presentation. ASABE Paper No. 07xxxx. St. Joseph,
Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at
rutter@asabe.org or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
An ASABE Meeting Presentation
Paper Number: 076113
Applications of Carbon Dioxide in Food and
Processing Industries: Current Status and Future
Thrusts
N. Kaliyan, Ph.D. Candidate
Department of Bioproducts and Biosystems Engineering, University of Minnesota,
1390 Eckles Ave, St. Paul, MN, USA - 55108. <kali0071@umn.edu>
P. Gayathri, Ph.D. Student
Department of Biosystems Engineering, University of Manitoba, Winnipeg, Manitoba,
Canada, R3T 5V6. <umpitcha@cc.umanitoba.ca>
K. Alagusundaram, Associate Professor, and National Fellow (ICAR)
Agricultural Engineering College and Research Institute (AEC&RI), Tamil Nadu Agricultural
University (TNAU), Kumulur, Pallapuram (Post), Trichy (Dist.), Tamil Nadu (State), India - 621
712. <k_alagusundaram@yahoo.co.in>
R. V. Morey, Professor
Department of Bioproducts and Biosystems Engineering, University of Minnesota,
1390 Eckles Ave, St. Paul, MN, USA - 55108. <rvmorey@umn.edu>
W. F. Wilcke, Professor, Extension Engineer, and Regional Coordinator for the North
Central Region Sustainable Agriculture Research and Education Program
Department of Bioproducts and Biosystems Engineering, University of Minnesota,
1390 Eckles Ave, St. Paul, MN, USA - 55108. <wilck001@umn.edu>
Written for presentation at the
2007 ASABE Annual International Meeting
Sponsored by ASABE
Minneapolis Convention Center
Minneapolis, Minnesota
17 - 20 June 2007
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the
official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not
constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by
ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is
from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2007. Title of Presentation. ASABE Paper No. 07xxxx. St. Joseph,
Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at
rutter@asabe.org or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
Abstract. Carbon dioxide is one of the most important chemical compounds of human, animal, and
plant lives, and environmental health. Because of several desirable characteristics of carbon dioxide
in gas, solid, liquid, and supercritical states such as inertness, non-explosiveness, non-
corrosiveness, high volatility, cooling ability, and low-cost, carbon dioxide is being used in a variety
of applications in food and processing industries. Applications of carbon dioxide in preserving fruits,
vegetables, meats, food grains, and liquid foods; inactivating microorganisms; and extracting oils,
flavors, colors, and chemicals are discussed. Carbon dioxide as high pressure gas and supercritical
fluid would find a niche in food and processing industries in the future especially in applications
involving non-thermal sterilization and supercritical extraction.
Keywords. Carbon dioxide, Food Industry, Processing, Sterilization, Supercritical extraction.
2
Introduction
Carbon dioxide (CO2) is the third-most abundant gas available in the air. CO2 content of the
atmospheric air ranges from 300 to 600 ppm (by volume) depending on the measurement
location on the earth. CO2 is an important constituent in the life cycle of animals and plants.
Naturally available CO2 gas is used in the photosynthesis process by plants that are the basic
sources of food. The decay (slow oxidation) of all organic materials gives off CO2. In the
respiratory action (breathing) of all animals and humans, CO2 is released during exhalation.
Plants also release some amount of CO2 when they respire during night. CO2 is cycled through
the oceans. CO2 content of the oceans is about 60 times that of the atmosphere (Jones, 1923a
and 1923b). Volcanic eruptions may also release CO2. In addition, CO2 is released by a wide
variety of industries due to processing and due to the use of fossil fuels such as coal. Though
CO2 is a normal constituent of exhaled air, high concentration of CO2 gas is hazardous, even
lethal. A concentration of 3.5% CO2 (by volume) in air will cause deeper breathing, and a
concentration of 25% CO2 (by volume) can cause death of humans (Jones, 1923b). Importantly,
continued increase of ambient CO2 content is believed to be the main cause for global warming
(i.e., greenhouse effect) (Sengul, 2006).
Industrially manufactured CO2 is used in solid, liquid, gas, and supercritical forms in widely
diversified commercial applications such as making explosive gas atmospheres inert, beverage
carbonation, chemical manufacturing, fire fighting, food preservation, foundry-mold preparation,
greenhouses, mining operations, oil well secondary recovery, rubber tumbling, pH depression
for wastewaters, welding, therapeutical work, and medical industry applications (Jones, 1923b;
Standen, 1967; Wine and Morrison, 1986; Pandey, 1997).
Fossil fuel fired plants are responsible for the one third of the CO2 emissions which are thought
to be a major contributor to the current rise in the earth’s surface temperature (Sengul, 2006).
Reducing CO2 atmospheric concentrations by capturing emissions at the source (power plants
and chemical industries) and then storing them in subsurface reservoirs is thought to be a
reliable solution (Sengul, 2006). The captured CO2 could also be utilized for enhanced oil
recovery, enhanced coal bed methane recovery, enhanced gas recovery, food processing
applications, manufacturing minerals and fertilizer (e.g., urea production), promoting algae
growth, and enhanced plant growth in greenhouses (Pauley, 1984; Sengul, 2006). Currently,
technologies are available to purify naturally occurring CO2 into products suitable for the above
applications (Nobles, 1983).
Objectives
Historically, CO2 has been used to maintain the quality, value, and shelf-life of fruits, vegetables,
grains, and muscle foods. The objective of this paper is to review the applications of the CO2 in
food and processing industries. The review was limited to the applications of the CO2 for natural
and processed foods from agricultural-, horticultural-, and animal-based products.
Manufacture of Carbon Dioxide
Carbon dioxide was discovered by J. B. Van Helmont (1577-1644). He observed that CO2 was
formed in the products of combustion and fermentation (Anonymous, 1971). Combustion of
carbonaceous fuels (e.g., methane) in air produces large quantities of CO2 gas. During
fermentation, CO2 evolves as a result of conversion of sugar into alcohol. Fermentation of
molasses, corn, wheat, potatoes and other materials to make alcohol produces CO2. Some CO2
is recovered from fermentation vessels in the manufacture of industrial alcohol. The
3
fermentation gas is purified to remove odors before compressing CO2 into cylinders (Jones,
1923a).
CO2 is widely captured as a by-product during the production of synthetic ammonia (Standen,
1967). CO2 is also produced in the manufacture of hydrogen by the reaction of steam with a
carbonaceous material (Standen, 1967). In limekilns, decomposing calcium carbonate to the
oxide produces CO2. In the manufacture of sodium phosphate by the reaction of sodium
carbonate and phosphoric acid, CO2 is obtained as a by-product (Pandey, 1997). CO2 can also
be obtained from natural CO2 gas wells (Standen, 1967).
Most of the liquid carbon dioxide is manufactured by the combustion of coke. The coke is
burned under boilers, which are used to furnish power for compression of the purified CO2 and
to furnish heat for its separation from the alkaline carbonate solutions that absorb CO2 from the
flue gases (Jones, 1923a). The CO2 coming with the flue gases, of any industry, can be stripped
out by dissolving the gases in a solvent such as ethanolamine at a temperature of 25 to 65oC.
Rushing (1994) reported that production of CO2 at cogeneration plants has the potential to
replace the old sources or answer demand for additional sources. Creating a steam host is a
critical fact of life in the cogeneration industry and carbon dioxide recovery is a very useful
additional operation. The monoethanolamine absorption process is commonly employed to
produce food grade and beverage grade CO2 (Rushing, 1994).
Carbon dioxide is distributed to the industrial applications in three different ways: high pressure
liquid in steel cylinders, low pressure liquid in insulated truck trailers or insulated rail tank cars,
and ice in insulated trailers or rail cars (Anonymous, 1971).
Properties of Carbon Dioxide
Carbon dioxide can be derived in solid, liquid, gas, or supercritical state. Figure 1 shows the
phase diagram of CO2 indicating the pressure and temperature ranges for solid, liquid, gas, and
supercritical forms of CO2. Gaseous CO2 is the most popular and well-known form compared to
the others. CO2 is a colorless gas with a faintly pungent odor and has an acid taste. It is 1.53
times heavier than air at normal temperature and pressure. Some physical and chemical
properties of CO2 are listed in table 1. CO2 has several special properties such as non-oxidizing
quality, inhibitive and partial disinfecting action on certain bacteria, and ability to stimulate taste
sensation, and thus, CO2 has found applications in various food and processing industries
(Jones, 1923a).
Applications of Carbon Dioxide
Gas CO2
Carbonated Beverages
The most common and oldest application of CO2 is in producing carbonated soft drinks and
soda water. To some extent, this application extends to beer and sparkling wine, though CO2 is
naturally produced during fermentation.
The largest use of CO2 gas is in the manufacture of carbonated beverages (Reich, 1945;
Pandey, 1997). In 1920s, about 90% of CO2 produced was used for the manufacture of
carbonated beverages in the U.S. (Jones, 1923a). Carbon dioxide contributes to the
characteristic pungent taste or “bite” to the soft drinks because CO2 has a marked stimulating
effect on the olfactory and gustatory nerves (Jones, 1923a). The carbonation of a beverage
4
helps to prevent mold growth. It also inhibits the growth of bacteria, in some instances, destroys
bacteria depending on the extent of carbonation used. The acid used in the beverage and the
CO2 content satisfactorily preserve carbonated beverages for a long time (Standen, 1967).
Modified Atmosphere Packaging / Controlled Atmosphere Storage
When the concentration of CO2 increases beyond a certain level, it can create a lethal effect on
living beings. This property is effectively used to protect grains, fruits, and vegetables from
insects. It could be a practical solution to replace the chemical fumigants used in grain
industries. Increasing the CO2 level in the surroundings of food material will prolong their shelf-
life with little or no adverse effect on quality. This technique usually named modified atmosphere
packaging (MAP) or controlled atmosphere storage (CAS). Previous literature reviews on this
topic confirm that MAP and CAS have been successfully tested and practiced in many grains,
fruits, vegetables, meats, and some processed food products (Church and Parsons, 1995;
Jayas and Jeyamkondan, 2002). Elevated CO2 atmosphere finds a perfect fit in organic crops
for non-chemical preservation technology (Neven and Rehfield-ray, 2006).
MAP or CAS is a potential chemical-free method of preserving fresh foods. In MAP or CAS, the
gaseous composition of the storage environment around the food is altered by injecting either
CO2 to create a high CO2 atmosphere, or N2 to create a low O2 atmosphere (Ooraikul and
Stiles, 1991; Jayas and Jeyamkondan, 2002). Modified atmosphere packaging is similar in
principle to CAS, except in MAP the control of gas concentration is less precise (Ooraikul and
Stiles, 1991). In MAP, the gas composition is modified initially and it changes dynamically
depending on the respiration rate of the food product and the permeability of film or storage
structure surrounding the food product. In CAS, the gas atmosphere is continuously controlled
throughout the storage period (Jayas and Jeyamkondan, 2002).
Currently, CAS is used for controlling insect pests in stored grains for long-term storage in
Australia, USA, Canada, and several European nations (Jayas et al., 1991). In addition to
providing an effective control of pests, CAS prevents mold growth, preserves grain quality, and
maintains a high germination capacity of the stored grains (Banks, 1981). Disinfestation of
stored grain using CAS involves the alteration of the natural storage gases such as CO2, O2 and
N2 to render the atmosphere in the stores lethal to pests (Alagusundaram et al., 1995a).
Controlled atmosphere storage of grains is a suitable alternative to the use of chemical
fumigants and contact insecticides that are known to leave carcinogenic residues in the treated
product (Bailey and Banks, 1980). CO2 affects complex physiological processes in the insects
and causes desiccation because spiracles remain open and water loss cannot be regulated (Jay
et al., 1971). For treatment of stored grains with CO2 alone, the recommended dosages include
40% CO2 for 17 days, 60% CO2 for 11 days, or 80% CO2 for 8.5 days at >21oC (Annis, 1987).
CO2 at and above 35% levels causes insect death by desiccation, acidification at the cellular
level, and creates a lack of triglycerides for energy metabolism (Donahaye, 1991; Adler, 1994).
The effect of CO2 on mortality of stored products pests was studied by many scientists for more
than three decades. Though the lethal effect of CO2 on pests is well known, its effectiveness is
controlled by several factors: stage of pests, concentration of CO2, and method of introduction
(Jayas et al., 1995). In most insects, the pupal stage has showed more resistance than larva,
egg, and adult (Jayas et al., 1995). Increasing or decreasing CO2 concentration in a stepwise
manner provided better killing effect on insects than maintaining constant concentration levels
(Gunasekaran and Rajendran, 2005). CO2 also has a profound adverse effect on fertility of
insects once they are exposed to CO2 gas (Jayas et al., 1995).
By mixing a small amount of CO2 gas (sub-lethal concentrations of 10 to 20% CO2) with
fumigants such as methyl bromide (AliNiazee and Lindgren, 1969), phosphine (Kashi and Bond,
5
1975), propylene oxide (Navarro et al., 2004), and allyl acetate (Leelaja et al., 2007), the toxic
potential of the fumigants can be increased. This effect was due to the stimulation of the
respiration of insects by CO2 (Hazelhoff, 1928; Jones, 1938; Leelaja et al., 2007). Athie et al.
(1998) showed that mixtures of phosphine and 10 to 20% CO2 reduced the resistance levels to
phosphine in populations of Rhyzopertha dominica F., and Sitophilus oryzae L. In addition,
penetration and distribution of fumigants into the grain bulk are increased due to the much
higher vapor pressure of CO2 gas compared to those of the fumigants (Leelaja et al., 2007).
Controlled atmosphere storage of grains was found to maintain/enhance the quality of grains
(e.g., Sankara Rao and Achaya, 1969; Rajendran et al., 2002). Sankara Rao and Achaya
(1969) reported that CO2 gas storage of oil seeds such as castor, cottonseed, and groundnut
would reduce their degradation during storage due to sorption of CO2 gas by the seeds.
Rajendran et al. (2002) successfully stored basmati rice under high CO2 concentration to control
red flour beetle (Tribolium castaneum). They also found reduced free fatty acid (FFA) formation
in rice due to high CO2 gas storage compared to the control.
Modified atmosphere packaging keeps the fruits and vegetables fresh for months (Moleyar and
Narasimham, 1994). Also, MAP of fruits delays ripening, and controls the development of
toughness in the stored product as well as the spread of diseases (Lipton, 1975; Norman and
James, 1987). In the USA and Canada, both whole and cut fruit and vegetables are stored and
distributed in modified atmosphere packs (Bhattacharyya, 1993). Consumers in more advanced
countries are ready to pay extra for fruit and vegetables that are sold ‘near-fresh’ and are stored
in chemical free environments. Therefore, MAP will not only enhance the food supply in the
domestic markets but also will improve the export earnings.
The study by Olsen and Bartram (1978) showed that CO2 treatment (15 to 17% of CO2) of
apples retained firmness, which permits extension of the packaging and marketing seasons.
Also, apples treated with CO2 and held in controlled atmosphere storage for 5 to 9 months were
8.9 to 13.3 N firmer than those stored under refrigerated conditions.
One role of CO2 in MAP is to suppress the spoiling organisms (Cossentine et al., 2004; Kerry et
al., 2006). For example, fumigating wooden fruit storage bins with 13% CO2 for 21 days resulted
in 75% mortality of diapausing codling moth larvae, and 80% reduction in germination of
Penicillium expansum Link ex Thorn spores (Cossentine et al., 2004). In addition, CO2 storage
not only preserves the quality but also it may enhance the quality of the stored products. For
example, when persimmon (tropical) fruits were stored in high concentration of CO2 and then
exposed to atmospheric condition, their astringency was reduced (Yamada et al., 2002).
In high CO2 and low O2 storage of fruits and vegetables, the respiration rate, ripening rate,
metabolic and biochemical processes are reduced (Jayas and Jeyamkondan, 2002). Modified
atmospheres delay the onset of ripening and increase the firmness in fruits (Jayas and
Jeyamkondan, 2002). MAP conditions for fruits and vegetables vary depending on cultivars, and
growing conditions, locations, and seasons. MAP of apples, pears, tropical fruits (e.g., banana
and pineapple), and vegetables (asparagus, cabbage, and lettuce) requires 1 to 14% CO2
(Jayas and Jeyamkondan, 2002). While CO2 atmosphere is considered a good preservative,
fruits and vegetables are very susceptible to the action of this gas, and careful studies should be
made to quantify the tolerable levels of CO2 for various food products (Reich, 1945; Jayas and
Jeyamkondan, 2002).
Modified atmosphere packaging has also been extended to the muscle foods. MAP technology
in the meat industry has increased phenomenally in the past two decades in various countries,
particularly in Australia, New Zealand, USA, UK and several European countries. MAP greatly
extends the shelf-life of muscle foods under refrigerated storage conditions and maintains color,
texture, slice-ability, and flavor of the product for about 1-2 months (Sahoo and Anjaneyulu,
6
1995). A comprehensive review of MAP for extending the shelf-life of fish and fish products can
be found in Bhattacharyya (1993).
In MAP of meat, control of microorganisms is mainly due to change in intracellular pH due to
soluble CO2 in the lipid bilayer of the microbes (Holley et al., 1994). The shelf-life of meat can be
increased up to 15 times using MAP with CO2 compared to air (Holley et al., 1994).
Sarantopoulos et al. (1998) found that storing chicken leg cuts at 80% CO2: 20% N2 improved
the shelf-life from 7 to 17 days compared to the control samples. When 100% CO2 and a
storage temperature of -1.5oC are simultaneously applied (hurdle concept), the growth of meat
pathogens such as Yersinia enterocolitica, Listeria monocytogenes, and Aeromonas hydrophila
is suppressed (Farber, 1991; Jayas and Jeyamkondan, 2002).
Sankara Rao and Achaya (1969) found that storage of oils under CO2 gas is advantageous
because high CO2 level in oils showed superior storage stability (i.e., peroxide levels of the oils
under CO2 storage were much lower than those of the controls with storage time). In olive oil
production, CO2 emitted by the olive oil paste was effectively used as an oxygen scavenger.
The hermetical sealing of olive paste resulted in high CO2, which reduced the oxidation and
increased the chlorophyll concentration of the end product (Parenti et al., 2006).
Delayed staling of bread, and reduction in water sorption of bread were observed when bread
packed in CO2 compared to fresh bread and bread packed in air (Avital et al., 1990). Del Nobile
et al. (2003) concluded that shelf-life of durum wheat bread could be prolonged from three days
to about 18 days by using MAP (80% CO2: 20% N2).
Inactivation of Microorganisms
In this section, inactivation of microorganisms using sub-critical CO2 gas is presented. Later on,
in a separate section, inactivation of microorganisms using supercritical CO2 (temperature of
31.1oC and pressure of 7.4 MPa) is discussed.
Combinations of carbon dioxide (30% CO2: 5% O2: 65% N2) and ethanol (0.05%) or limonene
(73 ppm) could inhibit the airborne microorganisms and Escherichia coli in ready-to-eat products
stored at ambient pressure and temperature (Chen et al., 2003).
Fraser (1951) was the first researcher who explored the application of high pressure CO2 for
inactivating microorganisms. He found that high pressure CO2 (3.4 MPa and 37oC) inactivated
95 to 99% of Escherichia coli. Presence of CO2 under relatively mild pressures during heat
treatment can increase the rate of microbial inactivation (Ballestra et al., 1996; Ballestra and
Cuq, 1998). In this way, the time and temperature for sterilization or pasteurization can be
significantly reduced. Use of pressurized CO2 in association with mild heat treatment could
provide an alternative to severe thermal processing often responsible for degradation of
nutritional qualities and changes in flavor and texture of sensitive foods.
Ballestra et al. (1996) studied the ability of high pressure CO2 (1.2 to 5 MPa at 25 to 45oC) to
inactivate Escherichia coli. They found that the inactivation mechanisms appeared to be related
to enzyme inactivation. Ballestra and Cuq (1998) found that pressurized CO2 (5 MPa) during
heat treatment increased inactivation rate of Bacilus subtilis spores and Byssochlamys fulva
ascospores (at temperatures above 80oC), and Aspergillus niger conidia (above 50oC).
The U.S. Food and Drug Administration (FDA) has mandated implementation of a Hazard
Analysis Critical Control Point (HACCP) program for processors of unpasteurized fruit and
vegetable juices. The core of the program depends on 5-log pathogen reduction (Kincal et al.,
2005). Therefore, in most of the sterilization studies conducted using CO2, 5-log reduction in the
counts of microorganisms was the target.
7
Hong et al. (1997) found that high pressure CO2 treatment could be a potential non-thermal
technique for kimchi (a group of fermented vegetable foods in Korea) preservation. Reduction of
5-log cycles of Lactobacillus sp. in kimchi was achieved within 200 min under a CO2 pressure of
6.9 MPa at 30oC (Hong et al., 1997).
Erkmen (2000a) reduced the survival count of Brocothrix thermosphacta in meat by about 5-log
cycles with CO2 pressure of 6.1 MPa and treatment time of 150 min. Erkmen (2000b) studied
the effect of high pressure CO2 (1.5 to 7.6 MPa and 35oC) to inactivate Salmonella typhimurium.
He found that at higher pressures, the time required to kill the microorganism was less. This
interesting feature might be due to high solubility of CO2 at higher pressures and better contact
with the cells resulting in short treatment time (Erkmen, 2000b). Erkmen (2001) extended the
application of high pressure CO2 (2.5 to 10.1 MPa and 20 to 45oC) to inactivate Escherichia coli.
Hong and Pyun (2001) treated Lactobacillus plantarum bacteria using high pressure CO2 (7
MPa at 30oC for 10 min). They found that CO2 treatment reduced the viable cells by one to two-
log cycles. They also reported that the death of cells was due to irreversible cellular membrane
damage including loss of salt tolerance, leakage of UV-absorbing substances, release of
intracellular ions, and collapse of proton permeability and uptake of Phloxine B. dye, reduced
glycolytic activity, and inactivation of some constituent enzymes.
Park et al. (2002) found that a combined treatment of high pressure CO2 gas (4.9 MPa) and
high hydrostatic pressure (300 to 600 MPa) completely inactivated aerobes and enzymes in
carrot juice. Park et al. (2003) reported that a combined treatment of high pressure CO2 gas
(0.38 MPa) with high hydrostatic pressure (500 MPa) totally inactivated several food poisoning
bacteria and fungi (Staphylococcus aureus, Fusarium oxysporum, and Fusarium
sporotrichioides).
Kincal et al. (2005) developed a continuous high-pressure CO2 system to destroy micro-flora
and extend shelf-life in orange juice. Their unit was able to cause a 5-log reduction of the
natural flora in spoiled juice, and could attain a 5-log decrease in numbers of pathogenic
Escherichia coli O157:H7, Salmonella typhimurium, and Listeria monocytogenes.
Spilimbergo and Mantoan (2006) studied the effect of high pressure CO2 (6 to 20 MPa) to
inactivate microorganisms in apple juice at temperatures of 20 to 36oC. They concluded that
high pressure CO2 could be used for cold pasteurization of sensitive materials. A continuous
plant for high pressure CO2 pasteurization of liquid foods such as red grape juice, tomato paste
and red orange juice to control Bacillus subtilis is currently available (Parton et al., 2007).
At moderate temperature and pressure, CO2 is able to significantly inactivate bacterial
vegetative cells, molds and yeasts and, at suitable conditions, CO2 can also inactivate
intracellular and pectolytic enzymes (Parton et al., 2007). The principal advantages of high
pressure CO2 are effective inactivation of both airborne and exposed surface bacteria, as well
as easy penetration into porous materials to affect microbes inside the food (Hong et al., 1997).
The principle of high pressure CO2 is based on gas dissolution in a cell by pressurization that,
when rapidly decompressed to atmospheric pressure, causes fatal functional damage to the cell
(Balaban et al., 1991). Ballestra et al. (1996) reported that high pressure CO2 treatment affects
biological systems by causing protein denaturation, lipid phase changes, and rupture of cell
walls and membranes. The literature review by Zhang et al. (2006) shows that high pressure
CO2 is a promising process medium to sterilize medical devices, and liquid foodstuffs.
Inactivation of Enzymes
Lipolytic microorganisms such as psychrotrophic bacteria often are found in fatty foods. These
bacteria can produce lipase enzyme during storage. The lipase enzyme can affect food quality.
8
For instance, increased activity of lipase enzyme can lead to rancidity of fatty foods. Fadiloglu
and Erkmen (2002) used CO2 gas at atmospheric pressure to inactivate lipase enzyme. About
84% of initial activity of the enzyme was lost at initial pH of 7.15 and 50oC within 5 min. This
CO2 treatment under atmospheric pressure can be used as a potential non-thermal technique
for liquid food preservation. Also, this method may be an alternative to pressure treatments
(hydrostatic or CO2) in which extremely high pressures are required for the inactivation of
enzymes (Fadiloglu and Erkmen, 2002).
Perfusion of CO2 through immersion water, for reducing the oxygen content, inhibited the
enzymatic browning in pre-peeled potatoes without any adverse effect on hardening (Kaaber et
al., 2002). This may be due to the increase in acidity of the water (Kaaber et al., 2002). This
CO2 treatment was developed as an alternative to the current sulfate treatment for inhibition of
enzymatic browning of pre-peeled potatoes.
In modified atmosphere packaging or controlled atmosphere storage, CO2 inactivates enzymes
by dissolving in water present in the product (Sanjeev and Ramesh, 2006).
Meat Industry
Carbon dioxide gas is commercially used for immobilizing animals prior to slaughtering.
Animals are led or driven into a tunnel in which CO2 gas is injected. The animal falls
unconscious onto a conveyor and is conveyed directly to the slaughtering area. In this method,
CO2 increases the animal’s blood pressure, which helps recover more blood and results in
better quality meat (e.g., slower rate of pH decline) than other humane slaughtering techniques
(Standen, 1967). Channon et al. (2002) also used CO2 for stunning of pigs. They found that CO2
stunning yielded better meat quality compared to the usual electrical stunning method.
A gas atmosphere containing 20% CO2 was found to be effective in inhibiting the growth of
slime-producing bacteria in meat at high storage temperatures and high relative humidities
(Huffman et al., 1975). In Denmark, 42% of all retail meat is packed under high CO2
atmosphere, following Denmark are U.K. with 29%, France with 15%, and Germany with 5%
(Bhattacharyya, 1993).
Protein Precipitation
Tomasula et al. (1997) developed a continuous process to precipitate casein (protein) from milk
using CO2 at a pressure of less than 14 MPa in a tubular reactor. Optimum casein product was
obtained at 38oC and 5.52 MPa. There are advantages to using CO2 instead of lactic or mineral
acids for casein precipitation. Because of relatively high residual pH of 6.0, the whey would
require less pretreatment for further processing and present less of a disposal problem than
conventionally produced whey with a pH of 4.6 (Tomasula et al., 1997).
High pressure CO2 can be used to precipitate soy protein (Khorshid et al., 2007). About 68.3 %
(by wt.) of soy protein was precipitated at 3.0 MPa of pressurized CO2, at pH of 5.6, and at
constant temperature of 22oC (Khorshid et al., 2007). Compared to the conventional mineral oil
based precipitation technique, high pressure CO2 precipitation is a clean process because this
process produces highly purified food protein which needs no further treatments to purify the
product to be used in food or non-food applications (Khorshid et al., 2007). High pressure CO2
was also used to precipitate protein constituents such as glycinin and β-conglycinin from
soybean (Thiering et al., 2001).
9
Monitoring Food Quality
CO2 gas has not only been used to preserve and process the food products but it has also been
used as an indicator for detecting quality changes in food materials during processing and
storage. Measuring the changes in the CO2 concentration in grain storage bins, presence and
growth of insects and microorganisms such as fungi can be identified (Jayas et al., 1995; Ileleji
et al., 2006). Measuring the CO2 gas produced from grains such as corn and wheat during
storage affected by moisture, temperature, and mechanical damage, the amount of dry matter
loss (i.e., deterioration) of grains can be quantified (Steele et al., 1969; Wilcke et al., 1998;
Gupta et al., 1999; Ileleji et al., 2003). Therefore, relating CO2 content and grain quality would
help find the optimum grain storage conditions such as optimum moisture and temperature, and
manage the stored grain to preserve grain quality (Wilcke et al., 1998).
Miscellaneous Applications
Carbon dioxide has been used as a convenient inert medium for displacing the air from bread
manufacturing machinery, thus eliminating the dust and bacteria carried by the air stream of the
room (Jones, 1923a).
Jones (1923a) reported that packaging food with CO2 can prevent oxidation of flavors and
odors, and vitamins in food products such as orange juice. By introducing CO2 while churning,
oxidation of the butter can be avoided, and the time to become rancid may be delayed (Jones,
1923a). Also, Jones (1923a) reported that presence of CO2 gas can hasten the ripening of citrus
fruits, where CO2 was obtained from combustion gases from kerosene.
In manufacturing of ice cream, CO2 is substituted for air before beating the ice cream mix.
Carbon dioxide increases mechanical strength of the ice cream due to its greater solubility over
air. Also, inclusion of CO2 helps prevent oxidation, accentuates flavors, and has some
bactericidal action on the ice cream (Jones, 1923a).
Carbon dioxide injection into an extrusion cooker was found to help produce structured
extrudates for wheat starch-sucrose mixtures under low-temperature and high-moisture
conditions (Ferdinand et al., 1992). Similarly, Jeong and Toledo (2004) used carbon dioxide at
0.1 to 0.6 MPa pressure to increase the expansion of pre-gelatinized rice flour extrudates during
twin-screw extrusion.
Puffing of dehydrated, diced green bell peppers at a CO2 pressure of 6.48 MPa increased the
bulk specific volume and rehydration ratio by 340 and 143%, respectively, compared to those of
an air dried process (Saputra et al., 1991). Tabeidie et al. (1992) also used CO2 for puffing
potato pieces. They also reported that the CO2 puffing process resulted in higher bulk specific
volume and rehydration ratio than air-dried process.
CO2 has the ability to dissolve in materials and reduce their pH. This characteristic can be used
to prolong the membrane life in reverse osmosis by introducing CO2 (Sanjeev and Ramesh,
2006).
Incorporating CO2 gas in milk before drying can increase the drying rate by increasing the
surface area (Sanjeev and Ramesh, 2006).
10
Liquid CO2
Refrigeration
CO2 doesn’t exist in liquid form at ordinary atmospheric pressure and temperature. It needs
either very high pressure or very low temperature or their combination to keep CO2 in liquid
form. To use liquid CO2, a high operating pressure is necessary which may require features like
high pressure joints leading to increased cost of the system. Though handling high pressure
CO2 raises some safety issues, it has been used widely in food refrigeration. First, liquid CO2
was introduced as a refrigerant in Europe and then in the U.S. (Pratt, 1932). Because of its
inertness, liquid CO2 can be used with any metal and any lubricating oil. Use of CO2 in
mechanical refrigeration systems has outstanding advantages such as safety from fire
explosion, and non-corrosiveness (Jones, 1923a). In addition, the inert nature of CO2 gives
more choices for construction material, and CO2 is low-cost compared to NH3 and
hydrofluorocarbons (HFCs). CO2 in combination with NH3 would be a potential lower capital cost
option for low temperature applications. Today’s refrigeration industries code CO2 as R-744
refrigerant.
Liquid CO2 provides the most readily available method of obtaining rapid refrigeration. CO2
manufacturers have developed refrigerated bulk-liquid CO2 storage systems. Carbon dioxide
refrigeration systems are used for rapid chilling of loaded trucks and rail cars before shipment.
Rapid cooling and low cost of equipment are the two main factors for adopting liquid CO2
refrigeration systems (Standen, 1967).
Curtis et al. (1995) first proposed the rapid cooling of shell eggs in a carton using cryogenic
gases (N2 and CO2), and showed it to be advantageous over traditional refrigerated cooling to
7oC due to shorten cooling time. The time required to equilibrate to 7oC is about 5 days for
traditional cooling and about an hour for cryogenic CO2 cooling (Sabliov et al., 2002). Cold
storage of shell eggs at temperatures below 7oC results in suppressing the growth of Salmonella
enteritidis. Also, compared to the traditional cooling methods, cryogenic carbon dioxide (-78oC)
was found to maintain or in some cases improve egg quality and increase shelf-life.
Quick Frozen Food Industry
The growing popularity of 'Quick Frozen Foods' throughout the world has resulted in the
phenomenal growth of this industry around the world. Frozen sea foods top the global trade list,
followed by 'ready-to-serve' meals, vegetables, poultry-based foods, etc., The USA ranks first in
the world trade of frozen foods. European countries like Germany and UK are also involved in
the trade of frozen foods (Pruthi, 1994).
Quick freezing is a method of increasing the shelf-life of perishable foods by subjecting them to
conditions of temperatures below their freezing point in a short period, say -20oC in 3 to 30
minutes to inhibit oxidative, enzymatic, and microbial changes, which are responsible for the
change of flavor and color of foods and their spoilage. The process involves direct immersion or
indirect contact of foods with a refrigerant and the cold air blasted over the foods being frozen.
Because liquid CO2 provides a better refrigerating effect at lower price, quick frozen food
industries generally use liquid CO2 based cryogenic freezing. It has been found that cryogenic
freezing provides the fastest rate for quick frozen foods (Pruthi, 1994).
Cryogenics
Cryogenics is defined as a branch of engineering specializing in technical operations at very low
temperatures of about -160oC to -50oC. Liquid nitrogen and carbon dioxide (liquid or solid) are
11
the two major cryogens used for food applications. The total usable refrigeration effect available
in CO2, N2 and Freon-25 are 565, 690 and 287 kJ kg-1, respectively (Fellows, 1988; Murthy,
1995). Fennema (1978) defined cryogenic freezing as very rapid freezing achieved by exposing
food to a very cold refrigerant that usually undergoes a change of state while the food is being
frozen. Commercial cryogenic refrigerants are liquid N2, liquid CO2 and dichlorodifluoromethane.
The quality of cryogenically frozen foods equals or exceeds that of conventionally frozen foods,
provided that the period of frozen storage is limited to a few weeks or less and the temperature
of frozen storage is -18oC or lower.
The cryogenic principle is applied for spice grinding and freezing of foods to improve the
organoleptic properties of the processed foods. Grinding of food in the presence of a cryogenic
fluid is referred to as cryo-grinding. Cryo-grinding of spices provides reduced oxidation of spicy
oil, extremely fine grinding, a 30% increase in product quality, lowered microbial load, and
causes no browning of spice (Murthy, 1995). Cryo-grinding is also applied for cocoa, coconut,
chocolate, coffee, tea and dehydrated meat. Liquid CO2 is commonly used for hamburger meat
grinding processes (Standen, 1967). Liquid CO2 is used to freeze the meat before grinding in
order to increase the grinding efficiency.
Cryogenic freezing and preservation of sea-foods, fruits, and vegetables give reduced bacterial
deterioration, reduced oxidation, better retention of original color and texture, longer shelf-life,
lower capital out lay, and easy maintenance (Murthy, 1995). Currently, cryogenic methods are
used for peeling of soft skinned fruits, and homogenization of biological tissues (Murthy, 1995).
Extraction
Liquid CO2 is a selective solvent for typical food flavor constituents like esters, aldehydes,
ketones, and alcohols. Ferreira et al. (1993) used liquid CO2 to extract essential oil from black
pepper. Similarly, Tuan and Ilangantileke (1997) used liquid CO2 to extract essential oil from
star anise fruits (Illicium verum H.). Tuan and Ilangantileke (1997) concluded that extraction of
essential oil with liquid CO2 was better than steam distillation in terms of energy saving, product
yield, and product quality.
Solid CO2 (dry ice)
Dry ice is typically produced as solid blocks (30 kg) for the shipping industry, and as cylindrical
pellets (10-mm in diameter) for grocery and lab uses. Safe handling of dry ice is important
because dry ice can cause burns to the skin by merely touching it (Sanjeev and Ramesh, 2006).
Refrigeration
The first use of CO2 was started with refrigerating ice cream and meats during 1920s in the form
of solid CO2 (Kaloyereas, 1949; Norris Shreve, 1967). The use of solid CO2 in refrigerated
transportation (truck, trailer and rail) has steadily increased from the 1960s (Anonymous, 1971).
Solid CO2 has a greater refrigeration effect than ice made from water because solid CO2
evaporates directly as a gas and it has an extremely low temperature (Anonymous, 1975). Its
low temperature makes it ideal for frozen-food transportation. When it is used for fresh foods,
the CO2 gas in the atmosphere immediately surrounding the product serves to reduce the
oxygen content of the air, thus minimizing possible deterioration resulting from oxidation. Also, it
inhibits the growth and development of bacteria and various molds that cause spoilage
(Anonymous, 1971). Raspberries and strawberries were shipped commercially under solid CO2
refrigeration during 1940s (Reich, 1945). If the berries were cooled to 14.4oC and a CO2
concentration of 30 to 35% was maintained, no changes in flavor and luster were observed
(Reich, 1945).
12
Quick Frozen Food Industry
Dry ice is commonly used for transporting frozen foods. For instance, a van-ceiling-mounted dry
ice "bunker" system is presently operating commercially in the USA at a much lower cost of
transportation of frozen foods than that of the mechanical methods (Pruthi, 1994). Dry ice
assisted rail shipment of frozen-foods is also used for transporting to distant destinations
(Pruthi, 1994).
Meat Industry
As discussed previously, one of the largest uses of dry ice is for the refrigeration of meat
products (Norris Shreve, 1967; Anonymous, 1971; Anonymous, 1975). The grinding of
hamburger meat with the addition of about 5% of dry ice increases the grinding capacity of the
machine and, the appearance and shelf-life of the product (Norris Shreve, 1967).
Miscellaneous Applications
Food products containing fat are apt to become rancid in the package. A small quantity of CO2
in the package can act as an antioxidant. This applies to coffee, nuts, and shredded coconut.
Some coffee packers either put a small pellet of solid CO2 into each cellophane bag/can just
before filling, or they pass CO2 gas into the container (Reich, 1945). A better textured product
was obtained by adding a small amount of solid CO2 in the bread dough, as the dough cooled
during the mixing (Reich, 1945). Dissolving 1.5% (by weight) CO2 into grape juice completely
inactivated microorganisms in a freshly pressed juice (Reich, 1945). Dry ice can be added for
carbonating water and juices prior to consumption (Reich, 1945). Also, dry ice is being used for
making ice cream (Reich, 1945).
In controlled atmosphere storage of grains, dry ice pellets or blocks can be used as a source for
CO2 gas (Alagusundaram et al., 1995b).
Supercritical CO2
Carbon dioxide becomes supercritical when its temperature and pressure are raised above the
critical points. For CO2, the critical temperature (Tc) is 31.06oC and the critical pressure (Pc) is
7.386 MPa (Sihvonen et al., 1999; Rozzi and Singh, 2002). Supercritical CO2 at greater than
31.1oC and pressures exceeding 7.4 MPa exists in a dense liquid state where the CO2 retains
the lower surface tension of a gaseous phase and the increased solubility of a liquid phase. This
supercritical state enables use of CO2 to extract various organic and inorganic molecules, and to
inactivate numerous microorganisms (Novak, 2005).
Supercritical CO2 Extraction
From the viewpoint of pharmaceutical, nutraceutical, and food applications, supercritical CO2 is
a good solvent, because it is relatively inert, non-toxic, non-flammable, odorless, colorless,
inexpensive, recyclable, readily available in high purity, and easy to remove from the product
(i.e., no chemical residue in the product) (Sihvonen et al., 1999; Rozzi and Singh, 2002).
Supercritical CO2 has a low critical temperature compared to other supercritical chemicals such
as propane (Tc = 96.7oC and Pc = 4.247 MPa), which can help prevent thermal degradation of
food components (e.g., carotenoids) while they are being extracted (Rozzi and Singh, 2002;
Uquiche et al., 2004).
Supercritical CO2 has a higher diffusion coefficient and lower viscosity and surface tension than
a liquid solvent, which leads to a more favorable mass transfer (Sihvonen et al., 1999; Rozzi
13
and Singh, 2002). However, supercritical CO2 cannot be used for dissolving polar molecules
(e.g., hydroxyl, carboxyl, or nitrogen) because of its non-polar nature (Sihvonen et al., 1999).
For these situations, addition of a polar co-solvent (e.g., ethanol) with supercritical CO2
enhances extraction efficiency (Li and Hartland, 1996). In addition, solubility of CO2 decreases
with increasing molecular weight (Palmer and Ting, 1995). The major disadvantage is that
application of supercritical CO2 requires high pressure extraction vessels leading to high capital
and operating costs. However, costs of supercritical extraction processes are competitive with
other processes, and sometimes supercritical processes are unique in their ability to produce
solvent-free products and handle high viscosity materials (Brunner, 2005).
Because supercritical fluids have the solvating power of a liquid and better mass transfer
characteristics than traditional solvents, interest in extraction with supercritical fluids has been
growing rapidly in recent years (McHugh and Krukonis, 1994; Zhang et al., 1995). Solvents such
as ammonia, ethylene, toluene, and CO2 show promise for supercritical fluid extraction. Of
these, CO2 offers unique advantages: it is abundant, non-reactive, non-toxic, and harmless
(O’Toole et al., 1986). The natural flavor and aroma of the extracts (e.g., oils) are preserved
during supercritical CO2 extraction since it is carried out at low temperatures (as low as 31oC)
and in an inert CO2 atmosphere (Temelli, 1987; Palazoglu and Balaban, 1998).
Conventional extraction methods such as solvent extraction using Soxhlet apparatus, although
effective for extraction of oils, can lead to degradation of heat sensitive compounds as well as
leave traces of toxic solvents in the solute. This is a concern for food and medicinal extracts.
Supercritical CO2 extraction is a viable alternative to solvent extraction methods due to low
critical temperature (31.1oC) (Tonthubthimthong et al., 2001). Moreover, supercritical CO2
extraction is a popular technique for oil extraction due to its high extraction efficiency, short
extracting time, lower refining requirement, and absence of chemical residues or contamination
in the extracted oils (Bhattacharjee et al., 2007).
Currently, commercial plants exist for decaffeinating coffee and tea, extracting beer flavoring
agents from hops, and separating oils and oleoresins from spices using supercritical CO2
(Palmer and Ting, 1995; Brunner, 2005). Supercritical CO2 replaces water or methyl chloride for
the extraction of these materials.
Palmer and Ting (1995) tabulate references in the literature where supercritical CO2 was used
on a wide range of raw materials for: (i) extraction of fats and oils, (ii) extraction of cholesterol,
(iii) fractionation of fats and oils, (iv) refining fats and oils (e.g., deacidification and
deodorization), (v) flavor/aroma extraction, and (vi) extraction of other food constituents such as
pigments and antioxidants. Selected examples from the above categories, and additional
applications of supercritical CO2 extraction from the recent literature are discussed below.
Supercritical CO2 has been used to extract oils from a variety of raw materials such as peanut
(Santerre et al., 1994), almond (Marrone et al., 1998), pistachio nut (Palazoglu and Balaban,
1998), wheat plumule (Zhang et al., 1998), pecan (Li et al., 1999), egg yolk (Wu et al., 2001),
garlic (Xu et al., 2005), hazelnut (Ozkal et al., 2005a), and cotton seed (Bhattacharjee et al.,
2007). In addition, Zhang et al. (1995) lists the references for supercritical CO2 extraction of
soybean oil, peanut butter, corn oil, evening primrose oil, peppermint and spearmint oil, and
fractional separation of marjoram leaves and onion juice. Rozzi and Singh (2002) provide a list
of studies where supercritical CO2 has been used for extraction of lipids from brown seaweed,
butter oil, corn bran, ground beef, fungi, milk fat, oat bran, pecan, pistachio, pork, rapeseed, rice
bran, safflower, soybean, and sunflower.
In several studies, lipids extracted using supercritical CO2 were found to have superior qualities
to those obtained from traditional solvent extraction methods. For example, Friedrich et al.
(1982) found that soybean oil extracted with supercritical CO2 (20.7 to 69.0 MPa and 50oC) was
14
lighter in color and contained less iron and about one-tenth the phosphorus of hexane-extracted
crude oil from the same beans. Compared to the hexane extract, the supercritical CO2 extract of
fatty and waxy materials from rice bran was lighter in color, and richer in wax content and long-
chain fatty acids C20-C43 (Garcia et al., 1996). Bozan and Temelli (2002) observed that the α-
linolenic acid content of supercritical CO2 extracted flaxseed oil was higher than that obtained by
solvent extraction. Ozkal et al. (2005b) found that fatty acid compositions of apricot kernel oil
extracted with supercritical CO2 and hexane were similar.
In the traditional solvent extraction plants, a great deal of money is spent for removing the
residual solvents in the extracts in order to use the extract in food applications. Recently,
supercritical CO2 extraction based techniques were developed to remove the residual chemicals
from the extracts obtained by traditional solvent extraction methods. For example, Reverchon et
al. (2000) used supercritical CO2 in a packed tower separation process to remove residual
hexane from soybean oil produced by conventional extraction technique in order to use the
soybean oil in food applications. Reverchon et al. (2000) found that supercritical CO2 (with a
CO2 density of 716 kg m-3) reduced the residual hexane content in the soybean oil from 1000
ppm to 20 ppm.
Fat-free or fat-reduced potato chips could be prepared by removing oil using supercritical CO2
(40.8 MPa and 55oC) (Vijayan et al., 1994). Using a supercritical CO2 technique, ω-3 fatty acids
were extracted from brown seaweed (Sargassum hemiphyllum (Turn.) C.) by Cheung et al.
(1998), and from fungi (Cunninghamella echinulata, and Pythium irregulare) by Certik and
Horenitzky (1999). Using supercritical CO2, palm kernel oil was fractionated into shorter-medium
(C8-C14), and longer chain (C16-C18:2) triglycerides in terms of fatty acids for formulating
possible cocoa butter replacer blends (Zaidul et al., 2006).
Rozzi and Singh (2002) reviewed the publications on supercritical CO2 extraction from plant and
botanical samples. A wide range of analytes (e.g., essential oils, phytochemicals, and lipids)
from botanical samples has been extracted using supercritical CO2. These extracts have been
used for analytical purposes, supplementation purposes, and flavor and fragrance purposes
(Rozzi and Singh, 2002). Some examples of extraction of plant and botanical samples follow:
Bixin (a pigment used for food coloring) from annatto seeds (Anderson et al., 1997); lipids,
carotenoids and tocopherols from paprika (Vesper and Nitz, 1997); rosemary aroma from the
extracts obtained by solvent extraction (Lopez-Sebastian et al., 1998); phenolic compounds and
lipids from white grape seeds (Palma and Taylor, 1999); and antioxidants from rosemary
(Ibanez et al., 1999).
Kimball (1987) found that supercritical CO2 was effective in removing limonin, a primary bitter-
component in citrus juices. The processing conditions of pressures between 20.7-41.4 MPa,
temperatures between 30-60oC, and treatment time of four hours debittered California
Washington navel citrus juice reducing the limonin content from about 17.5 ppm to about 7.0
ppm (Kimball, 1987). Goto et al. (1997) used supercritical CO2 to remove terpenes, oxygenated
compounds, waxes, and pigments from citrus oil in a counter current extraction process and
pressure swing adsorption process.
De-alcoholized wine or beer could be made by removing ethanol, where ethanol is removed by
supercritical CO2 in a stripping column (Brunner, 2005). Eckert et al. (1990) used supercritical
CO2 to extract flavor compounds from distillates of beer produced by one-step evaporation.
They concluded that the flavor extracts are suitable for mixing with non-alcoholic beer. Raasch
and Knorr (1990) used supercritical CO2 (10 to 50 MPa pressure, 35 to 60oC, and 5 to 30 min) to
extract aroma from peels and pulp of passion fruit (Passiflora edulis S.). Supercritical CO2 was
found to be the most effective in extracting off-flavor compounds (i.e., medium chain aldehydes)
15
from soybean protein among liquid CO2, supercritical CO2, and supercritical CO2 with 5%
ethanol (Maheshwari et al., 1995).
Sass-Kiss et al. (1998) and Simandi et al. (2000) extracted onion oleoresin (flavor component),
and Uquiche et al. (2004) extracted red pepper oleoresin using supercritical CO2. Supercritical
CO2 extraction produced better quality end products compared to steam distillation of fresh
onion extract (Sinha et al., 1992). The supercritical CO2 extraction of onion resulted in 34
compounds in the extract in addition to the 13 compounds extracted by steam distillation (Sinha
et al., 1992).
Supercritical CO2 extraction can be used to extract natural colors from leaf proteins. These
colors can be used as food dyes (Favati et al., 1988). Favati et al. (1988) extracted carotene
(90% recovery at CO2 pressure of >30 MPa and temperature of 40oC) and lutein (70% recovery
at CO2 pressure of >70 MPa and temperature of 40oC) from leaf protein concentrate.
Supercritical CO2 extraction of annatto (Bixa orellana) seeds gives carotenoid-type food
colorant, used in dairy products (Degnan et al., 1991). Lycopene, a deep-red pigment and a
natural anti-oxidant, can be separated effectively from tomato peels by supercritical CO2
extraction (Topal et al., 2006). Sun and Temelli (2006) extracted natural food colorants (i.e.,
carotenoids) from carrot using supercritical CO2 plus canola oil as a continuous co-solvent.
Rizvi and Mulvaney (1992) used supercritical CO2 to produce puffing in extruded products,
which reduced the temperature and shear needed for conventional thermoplastic extrusion.
Supercritical CO2 acts as a blowing agent to facilitate the formation of cellular structure in
extrudates in place of the expansion of water upon exit of the extruder in conventional extrusion.
Several investigations have also been conducted to determine the application of supercritical
CO2 extraction for dairy processing applications (Samit et al., 1997). A milk fat fraction enriched
with high melting point triglycerides has higher emulsion stability, and thus offering good
potential for use in bakery, chocolate and confectionery products (Shukla et al., 1994).
Supercritical CO2 has been successfully used for fractionating milk fat for these applications.
Kankare and Alkio (1993) found that over 99% of cholesterol from milk fat could be removed
using a supercritical CO2 extraction system equipped with a silica gel column. Ramos et al.
(2000) used supercritical CO2 to fractionate milk fat into four fractions: short-chain triglycerides,
medium-chain triglycerides, long-chain triglycerides, and cholesterol.
Supercritical CO2 (37 MPa and 80oC) was used to remove vitamin A and E from powdered milk
and liquid milk (Turner and Mathiasson, 2000). High quality natural vitamin E in wheat germ was
extracted using supercritical CO2 by Ge et al. (2002). The extraction conditions are CO2
pressure of 34.5 MPa, temperature of 43oC, and flow rate of 1.7 mL/min. The optimum value of
concentration of extracted natural vitamin E was 2307 mg/100 g of wheat germ.
Supercritical CO2 has found applications in analytical purposes. Supercritical CO2 has been
employed for rapidly analyzing fat content and pesticide residues in foods (by supercritical CO2
extraction procedure), and supercritical fluid chromatography (SFC) (supercritical CO2 as a
mobile phase) (Rozzi and Singh, 2002). The liquid chromatographic (LC) separation is replaced
by SFC, when heat-labile non-volatile components need to be quantified. Replacing LC
methods by SFC decreases analysis time, increases sample throughput and decreases liquid
waste (Sihvonen et al., 1999; Rozzi and Singh, 2002).
Recently, supercritical particle formation techniques (rapid expansion of supercritical solutions
[RESS] and supercritical antisolvent crystallization [SAC]) have been developed for
crystallization of bioactive compounds although these techniques have not yet been widely used
in food applications (Sihvonen et al., 1999). The RESS technique can be used for crystallizing of
16
non-polar compounds soluble in carbon dioxide, whereas the SAC technique can be used for
crystallizing polar compounds such as β-carotene (Chang et al., 1991).
A liquid product can be entrapped by adsorption onto solid particles using supercritical CO2,
where the solid material provides a coating for the liquid inside. An example of this is
incorporation of tocopherol acetate in silica gel using supercritical CO2 (Brunner, 2005). This
process is not currently employed in food applications; however, this process can potentially be
used to add selective constituents in foods to enhance quality and value.
Inactivation of Microorganisms
Supercritical CO2 has been successfully used for inactivating a wide range of microorganisms
such as yeasts and bacteria (Dillow et al., 1999; Erkmen, 2002). Since supercritical CO2 is
nontoxic and inexpensive, it has been considered for the sterilization of biomaterials, viruses,
and in pest control. Current sterilization processes (ethylene oxide, gamma irradiation, electron-
beam, steam, and hydrogen peroxide plasma) have been shown to alter the structure and
characteristics of materials, especially to thermally and hydrolytically sensitive polymers and
metal alloys. The supercritical CO2 sterilization process is a viable alternative which addresses
these limitations (White et al., 2006).
In yeast inactivation, supercritical CO2 was found to be more effective than the sub-critical state.
In the supercritical state, it took only 15 min compared to more than one hour in sub-critical
state to inactivate the yeasts (Lin et al., 1992). When high pressure CO2 in a supercritical state
was used, it not only destroyed the yeast but also removed the odor created by the broken
yeast cells (Lin et al., 1991). If the process temperature was less than 35oC, the protein
structure was retained (Lin et al., 1991). Gunes et al. (2005) found that three yeast populations
(Saccharomyces cerevisiae, Candida stellata, and Kloeckera apiculata) in grape juice
decreased to nearly an undetectable level (6.5 to 7.5 log reductions in counts) at 27.6 and 48.3
MPa along with 85 and 170 g/kg CO2 concentrations and a temperature of 35oC.
Watanabe et al. (2003) found that supercritical CO2 (30 MPa and 35oC) was a promising
method for inactivating bacterial spores such as Bacillus species. They also found that
supercritical CO2 treatment was more effective than high temperature (atmospheric pressure
and 85oC) or high hydrostatic pressure sterilization technique (200 MPa and 65oC). Also, the
combination of high pressure CO2 (30 MPa) and high temperature (95oC) treatments was found
to be more effective than high temperature treatment (95oC) alone (Watanabe et al., 2003).
Spilimbergo et al. (2003) estimated that inactivation of spores with supercritical CO2 alone
required a long residence time and high temperature. Therefore, they first treated E. Coli and S.
aureus with pulsed electric field (25 KV/cm and 10 pulses) and then with supercritical CO2 (20
MPa and 34oC for 10 min), which resulted in complete inactivation of these microorganisms. A
combination of pulsed electric field and supercritical CO2 treatment is suitable for the reduction
of microbial cells in foods containing thermo-sensitive components such as protein and plasma
(Spilimbergo et al., 2003).
Spilimbergo et al. (2003) reported that inactivation of microorganisms by supercritical CO2
involves the diffusion of CO2 into and the extraction of vital biomaterials from the cell due to its
lipophilic and hydrophilic properties, and CO2 could diffuse very easily through and into the cell
membrane and increase the membrane fluidity. Once the density of CO2 builds up to a critical
level within the cells, it affects the intracellular pH and extracts constituent to an extent that is
sufficient to manipulate the structure of the bio-membrane or disturb the biological system.
Furthermore, the proposed mechanisms of bacterial inactivation using supercritical CO2 include
cell rupture, acidification, lipid modification, inactivation of essential enzymes, and extraction of
intracellular substances (Spilimbergo and Bertucco, 2003).
17
Supercritical CO2 could also be used to sterilize milk and dairy products (Kaimhira et al., 1987).
Werner and Hotchkiss (2006) studied the CO2-based sterilization of milk to kill bacteria. The
lethal effect of supercritical CO2 was found to be better than sub-critical CO2. Werner and
Hotchkiss (2006) concluded that continuous CO2 treatment is on par with or better than the
commonly used High Temperature Short Time (HTST) treatment of milk. A carbon dioxide
assisted milk pasteurization unit has been developed in the United Kingdom (Anonymous,
1999).
Supercritical CO2 has been used to inactivate microorganisms in meat and meat products.
Sirisee et al. (1998) found that a CO2 pressure of 31.5 MPa and treatment time of 120 to 180
min caused reduction of activity of Staphylococus aureus in ground beef. Choi et al. (2007)
reported that the supercritical CO2 treatment (7.4 to 15.2 MPa at 31.1oC for 10 min) of pork
samples had no effect on muscle pH, tenderness, and water-holding capacity; however, treated
samples had higher lightness values and more pronounced extent of sarcoplasmic protein
denaturation than the control samples.
Novak (2005) developed a micro-porous polypropylene membrane contactor system to sterilize
liquid foods using supercritical CO2. Using this method, a wide range of spoilage and pathogenic
vegetative microorganisms was inactivated in a non-thermal process, while spore-forming
microorganisms were inactivated at temperature of more than 45oC. The supercritical CO2
process was found to be non-toxic to the liquid foods being processed, capable of retaining
fresh juice flavors, economic, and without solvent disposal problems (Novak, 2005).
Future Thrusts of CO2 in Food and Processing Industries
In the future, continued applications of gaseous CO2 in manufacturing carbonated beverages,
and modified atmosphere packaging/controlled atmosphere storage of foods are expected. High
pressure CO2 gas could be used for inactivating microorganisms and enzymes in heat-sensitive
foods. Applications of solid CO2 would be continued in refrigeration and frozen storage of foods.
Applications of liquid CO2 would be continued in the field of very low temperature applications
(i.e., cryogenics).
Due to environmental and occupational concerns of using large amounts of toxic organic
solvents, liquid/supercritical CO2 would serve as an alternate, ideal solvent for the future. Based
on this review, we believe that commercial plants may be founded for supercritical CO2 based
inactivation of microorganisms in solid and liquid foods. According to Palmer and Ting (1995),
commercial developments in the future might be expected to continue in the area of food
flavors, as well as in the extraction and refining of ”natural” food ingredients, functional foods,
alternative medicines, and specialty oils using supercritical CO2. Also, supercritical CO2
applications could be extended to processes such as chemical and enzymatic reactions, re-
crystallizations, supercritical fluid chromatography, and extrusion technology.
Conclusions
Carbon dioxide in solid, liquid, gas, or supercritical state is used in many commercial
applications. A major use of CO2 is for the manufacture of carbonated beverages. The CO2 is
used for refrigerating ice cream and meats. A high CO2 gas atmosphere is used for packaging
fruits, vegetables and muscle foods, and for the control of insects in stored grains. Quick frozen
food industries use CO2 for cryogenic freezing of foods, for cryogenic grinding of spices, and for
frozen-food transportation. Currently, CO2 is used for the supercritical extraction of oils, colors,
flavors, and other selective food components.
18
In addition to continuing the current commercial use of gas, solid and liquid CO2, commercial
applications involving high pressure CO2 and supercritical CO2 can be exploited. As a whole,
CO2 is used to increase the quality, value, and shelf-life of fruits, vegetables, food grains, and
muscle foods. Because of numerous applications, use of CO2 in food and processing industries
will help not only to enhance the food supply in domestic markets but also to improve export
earnings.
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27
Figure 1. Pressure-temperature phase diagram of carbon dioxide (White et al., 2006).
28
Table 1. General properties of carbon dioxide [a].
Property of CO2 Value
Chemical formula CO2
Molecular weight 44.01 g mol-1
Sublimation point -78.5 oC at 0.1 MPa
Triple point -56.6 oC at 0.52 MPa (abs.)
Critical point 31.1 oC at critical pressure 7.4 MPa (abs.)
Critical density 468 kg m-3
Gas density 1.833 kg m-3 at 21oC and 0.1 MPa
1.977 kg m-3 at 0oC and 0.1 MPa
Liquid density 1101 kg m-3 at -37oC and 1.1 MPa
Solid (dry ice) density 1560 kg m-3 at -79oC and 0.1 MPa
Solubility of CO2 gas in water (vol/vol) 1.713 at 0 oC and 0.1 MPa
8.6 at 0 oC and 0.41 MPa
1.0 at 16oC and 0.1 MPa
Viscosity of liquid CO2 0.015 cP at 25oC and 0.1 MPa
Latent heat of vaporization 234.5 kJ kg-1 at 0oC
276.8 kJ kg-1 at -17oC
301.7 kJ kg-1 at -29oC
Latent heat of fusion (triple point) 571.3 kJ kg-1 at -56.6oC
Latent heat of sublimation (dry ice) 199.0 kJ kg-1 at -78.5oC
Heat of formation of CO2 -393546.0 kJ kmol-1 at 25oC
Specific heat of CO2 gas
Cp (at 25oC and 0.1 MPa)
Cv (at 25oC and 0.1 MPa)
0.850 kJ kg-1 K-1
0.657 kJ kg-1 K-1
Specific heat of liquid (at -17oC) 2.048 kJ kg-1 K-1
Thermal conductivity of CO2 gas 0.19 W m-1 K-1
Refrigerant reference number for CO2 R-744
Total usable refrigeration effect 565 kJ kg-1
[a] Sources: Standen (1967); www.wittemann.com; http://en.wikipedia.org/wiki/Carbon_dioxide.
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