Food Biochemistry and Food Processing (2 edition)

BLBS102-c40 BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come

40 Separation Technology in Food Processing 769

separation operations and the final composition of the extracts

obtained.

Supercritical CO 2 Fluid Technology

Changes in food processing practices and new opportunities

for innovative food products have spurred interest in supercrit-

ical CO 2 fluid extraction. Supercritical CO 2 fluid technology

has been widely used to extract essential oils, functional fatty

acids, and bioactive compounds, and also been applied in re-

cently developed extraction and fractionation for carbohydrates

(Glisic et al. 2007, Shi et al. 2007a, Montan ̃es et al. 2008, Mitra ́

et al. 2009, Montan ̃es et al. 2009, Sanchez-Vicente et al. 2009, ́

Shi et al. 2010a, 2010b). Supercritical CO 2 fluid extraction is

a novel separation technique that utilizes the solvent properties

of fluids near their thermodynamic critical point. Supercritical

CO 2 is being given a great deal more attention as an alternative to

organic chemical industrial solvents, and increased governmen-

tal scrutiny and new regulations restricting the use of common

industrial solvents such as chlorinated hydrocarbons. It is one

of the “green” separation processes that provides nontoxic and

environmentally friendly attributes and leaves no traces of any

toxic chemical solvent residue in foods.

When CO 2 fluid is forced into a pressure and temperature

above its critical point (Fig. 40.1), CO 2 becomes a supercritical

fluid. Under these conditions, various properties of the fluid are

placed between those of a gas and those of a liquid. Although

the density of a supercritical CO 2 fluid is similar to a liquid and

its viscosity is similar to a gas, its diffusivity is intermediate

between the two states. Thus, the supercritical state of a fluid

has been defined as a state in which liquid and gas are indis-

tinguishable from each other or as a state in which the fluid

is compressible (i.e., similar behavior to a gas), even though

possessing a density similar to that of a liquid and with similar

Temperature (°C)

Supercritical

region

Tc=31.1°C

Pc=7.38MPa

Pressure (MPa)

Liquid phase

Solid phase

Gas phase

TC

PC

Triple point

Figure 40.1.Supercritical pressure–temperature diagram for

carbon dioxide.

solvating power. Because of its different physicochemical prop-

erties, supercritical CO 2 provides several operational advantages

over traditional extraction methods. Because of their low viscos-

ity and relatively high diffusivity, supercritical CO 2 fluids have

better transport properties than liquids. They can diffuse eas-

ily through solid materials and therefore give faster extraction

yields. One of the main characteristics of a supercritical fluid is

the possibility of modifying the density of the fluid by chang-

ing its pressure and/or its temperature. Since density is directly

related to solubility (Raventos et al. 2002, Shi and Zhou 2007, ́

Shi et al. 2009a), by altering the extraction pressure, the solvent

strength of the fluid can be modified.

The characteristic traits of CO 2 are inertness, nonflamma-

bility, noncorrosiveness, inexpensive, easily available, odorless,

tasteless, and environmentally friendly. Its near-ambient criti-

cal temperature makes it ideal for thermolabile natural products

(Mendiola et al. 2007). CO 2 has its favorable properties and the

ease of changing selectivity by the addition of a relatively low

amount of modifier (co-solvent) such as ethanol and other polar

solvents (e.g., water). CO 2 may be considered the most desirable

supercritical fluid for extracting natural products for food and

medicinal uses (Shi et al. 2007b, Kasamma et al. 2008, Shi et al.

2009b, Yi et al. 2009). Other supercritical fluids, such as ethane,

propane, butane, pentane, ethylene, ammonia, sulphur dioxide,

water, chlorodifluoromethane, etc., are also used in supercritical

fluid extraction processes.

In a supercritical CO 2 fluid extraction process, supercritical

CO 2 fluid has a solvating power similar to organic liquid solvents

and a higher diffusivity, with lower surface tension and viscosity.

The physicochemical properties of supercritical fluids, such as

the density, diffusivity, viscosity, and dielectric constant, can be

controlled by varying the operating conditions of pressure and

temperature or both in combination (Tena et al. 1997, Shi et al.

2007a, 2007b, 2007e, Kasamma et al. 2008, Shi et al. 2009b).

The separation process can be affected by simply changing the

operating pressure and temperature to alter the solvating power

of the solvent. After modifying CO 2 with a co-solvent, the ex-

traction process can significantly augment the selective and sep-

aration power and in some cases extend its solvating powers to

polar components (Shi et al. 2009a).

Supercritical CO 2 fluid extraction is particularly relevant to

food and pharmaceutical applications involving the processing

and handling of complex, thermo-sensitive bioactive compo-

nents, and an increased application in the areas of nutraceuticals,

flavors, and other high-value items such as in the extraction and

fractionation of fats and oils (Reverchon et al. 1992, Rizvi and

Bhaskar 1995), the purification of a solid matrix, separation of

tocopherols and antioxidants, removal of pesticide residues from

herbs, medicines, and food products, the detoxification of shell-

fish, the concentration of fermented broth, fruit juices, essential

oils, spices, coffee, and the separation of caffeine, etc. (Perrut

2000, Gonz ́alez et al. 2002, Miyawaki et al. 2008, Martinez et al.

2008, Liu et al. 2009a, 2009b).

This technology has been successfully applied in the ex-

traction of bioactive components (antioxidants, flavonoids, ly-

copene, essential oils, lectins, carotenoids, etc.) from a variety

of biological materials such as hops, spices, tomato skins, and