Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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Deep-Fat Frying

0026 Deep-fat frying, a quick elaboration procedure of

food, improves the palatability of food as well as

providing flavor and nutritive value. This culinary

technology can be defined as a complex process of

controlled dehydration and browning with hot oil as

the heat transfer medium.

0027 Many factors are at work:

0028 The process itself: temperature, time, frying

method, and vessel material.

0029 The type of fat: chemical and physical properties

of the fat, additives, and contaminants.

0030 The food size and form: rate surface/volume,

battered, floured, direct action (on thermolabile

nutrients improving nutrient interactions, nutrient

interchange between the foodstuff and the frying

fat).

0031 During deep-fat frying a wide variety of chemical

reactions take place. The steam from a moist food

will cause hydrolysis of triacylglycerols, resulting

mainly in the formation of free fatty acids and dia-

cylglycerols (monoacylglycerols and glycerol are pro-

duced in small quantities during controlled frying),

while the air released into the frying system will initi-

ate a cycle of oxidation involving the formation of

free radicals. These oxidation processes will involve

fatty acids in intact triacylglycerols as well as the

products of triacylglycerol hydrolysis. The fatty acid

residues of triacylglycerols formed can react to form

polymers and other complex reaction products. On

the other hand, the food being fried can influence the

type and quantity of breakdown products in the

frying medium.

0032 The substrate can influence the process in three

ways:

0033 By releasing natural antioxidants or prooxidants

into the frying oil. These products can also be

absorbed by the frying substrate.

0034 By the absorption of lipid oxidation products on

to the substrates

0035 By the catalytic effect of various functional groups

present in the food.

0036 Frying with sunflower oil Sunflower oil is one of the

most important vegetable oils employed for deep-

frying. It has been used in the cooking of food such

as French fries and frozen prefried foods at home, in

fast-food restaurants, and in the industry. The frying

process can also be used to produce cardio-health

foods because during frying the exchange of fat

between the frying medium and the food occurs.

Thus, the level of linoleic acid in the food can be

deeply increased when frying is performed in sun-

flower oil. Moreover, after frying some minor com-

ponents from the oils, such as tocopherols, are

present in the fried food.

0037The degradation must be ascribed to a lower turn-

over of fresh oil during frying, but in industrial

cooking, where a high turnover of fresh oil is used,

sunflower oil can be used successfully. The thermal

oxidation of sunflower oil used in frying (as with

other oils or fats) can be evaluated by measuring the

percentage of total polar content by column chroma-

tography. The level of 25% polar content has been

accepted as critical for oil discarding. Frying potatoes

in sunflower oil following domestic conditions shows

that polar content increases in oil and fried potatoes.

However, when frying is performed with frequent oil

turnover, the amount of polar material in sunflower

oil is substantially below the critical level (Figure 3).

0038The amount of polymers and dimers of triacylgly-

cerols formed in sunflower oil through deep-fat frying

is also lower in the frequent turnover of fresh oil than

in the null turnover one. The benefit of the high

turnover of sunflower oil has also been found

in the frying of frozen foods (potatoes, fish fingers,

croquettes, pastries, etc.) under domestic conditions.

0039HOSO seems to be a rather stable oil and has been

recently studied. Experiments using HOSO to fry

potatoes following a domestic frying model suggests

20 40 60 80

Critical level

Polar content (g 100 g

−1

oil)

Fryings

fig0003Figure 3 Sunflower oil and high-oleic-acid sunflower oil behav-

ior in frying potato. Polar fraction (mg 100 mg

1

oil) change. Open

circles, sunflower oil used in discontinuous frying of French fries

without replenishment with fresh oil. Open squares, sunflower oil

in discontinuous frying of French fries with frequent replenish-

ment with fresh oil. Open triangles, high-oleic-acid sunflower oil

in discontinuous frying with frequent replenishment of French

fries with fresh oil. Modified from Cuesta C and Sa

nchez-Muniz

FJ (1998) Quality control during repeated fryings, Grasas y

Aceites 49: 310–318 and Sa

nchez-Muniz FJ, Cuesta C, Lo

pez-

rela S, Garrido-Polonio MC, and Arroyo R (1993) Evaluation

of the thermal oxidation rate of sunflower oil using various frying

treatments. In: Applewhite TH (ed.) Proceedings of the World Con-

ference on Oilseed Technology and Utilization, pp. 448–452. Cham-

paign, IL: AOCS Press.

SUNFLOWER OIL 5677

that frequent replenishment of used HOSO with fresh

HOSO permits the frying of sets of fresh potatoes at a

very high rate (Figure 3). Moreover, HOSO performs

much better than conventional sunflower oil in the

frying of prefried frozen foods. This occurred using

the frequent and the null turnover techniques, com-

mented on before. The triacylglycerol and fatty acid

composition and the ratio of antioxidant to unsatur-

ated fatty acids explain the different stability and

frying behavior of these two variants of sunflower oil.

0040 Among natural sunflower oil antioxidants, toco-

pherols seem to be more relevant. In this way, it is

interesting to note that, in continuous frying oper-

ations, a regular turnover of fresh oil maintains the

level of antioxidant in the oil.

0041 However, data from some publications suggest that

addition of the antioxidant exerts little influence in

extending frying life of the oil used in frying but it can

extend the shelf-life of the food by reducing the rate

of oxidation of the absorbed oil. Nevertheless, data

from the AIR (The Agriculture and Agro Industry

Research) project have shown that peroxidative deg-

radation of frying oils occurred when there was a lack

of tocopherol content.

Oleochemistry

0042 Margarines Margarines were originally obtained

from lard and other fat sources, and were widely

consumed through the world until the 1970s and

1980s. However the association of saturated fats

with coronary heart disease led to the development

of new margarine products of vegetable origin. These

new margarines, with high-polyunsaturated-fatty-

acid content, include soft margarines and diet

spreads. Sunflower oil is one of the oils employed in

the formulation of fat blends to achieve an adequate

product. Typical fat blends for margarines are shown

in Table 5.

0043 In margarines the level of polyunsaturated fatty

acids should not be less than 45% of cis, cis linoleic

acid or less than 50% of total polyunsaturated

fatty acids; the sum of saturated fatty acids and

trans fatty acids should not exceed 25% and the

level of cholesterol should not exceed 15 mg kg

1

Considering these facts, in order to reach 45–50%

in polyunsaturated fatty acids, it will be necessary to

use 70–80% sunflower seed or soybean oils.

Other Uses

0044The sunflower meal, obtained after the oil is ex-

tracted from the seed, has a high protein percentage

(28% for meal from hulled seeds to 42% for dehulled

seed meals) and is used primarily in food rations for

livestock and poultry. Compared to soybean, sun-

flower meal contains lower energy and lysine but

higher fiber and methionine content.

0045Variants of sunflower seeds with low oil content

(nonoilseed sunflower) are primarily used as a snack

food, an ingredient in baked foods, salads, and

candies, and for feeding birds and small animal pets.

During the last decades a flowering market has grown

round these seeds.

0046Finally, sunflower hulls present a potential use as

litter for livestock or poultry, and as a source of

roughage in animal feeding. Building or insulation

boards and, fireplace logs are other interesting poten-

tial applications.

0047However, the relatively higher price of sunflower

oil in respect to other seed oils explains why it is not

widely used for industrial purposes. Nevertheless,

some paints, varnishes, and plastics take advantage

of its semidrying properties without the yellowing

problems associated with oils rich in linolenic acids.

In some eastern European countries the oil is also

employed by detergent and soap manufacturers. Like

other vegetable oils, it has potential value for use in the

production of adhesives, agrochemicals, surfactants,

additional plastics and plastics additives, fabric

softeners, synthetic lubricants, coatings, and other

products.

0048HOSO has potential value in industrial products

where oleic acid and related chemicals are currently

utilized. This new oil is of initial interest in food use

(primarily in frying of snack foods to enhance the

shelf-life of retail products), and as an ingredient in

infant food formulas which require stability, bland

flavor, and specific fatty acid composition. The high

level of oleic acid together with the high oxidative

stability is valuable for cosmetic and pharmaceutical

manufacture.

Dietary Importance and Health Effects

Sunflower Oil as Energy Source

0049While protein and carbohydrates provide 17 kJ g

1

(4 kcal g

1

), long-chain triacylglycerols provide

38 kJ g

1

(9 kcal g

1

). The fatty acids are used for

tbl0005 Table 5 Fat blends for polyunsaturated fatty acid margarines

PUFA1(%) PUFA 2(%)

Hydrogenated vegetable oil

(melting point 55



Hydrogenated vegetable oil

(melting point 46–48



Hydrogenated vegetable oil

(melting point 40



Sunflower oil 80 80

PUFA, polyunsaturated fatty acids.

5678 SUNFLOWER OIL

energy production in the cell mitochondria and per-

oxisomes. Present recommendations for fat are about

30% of total energy intake (80–90 g day

1

in a 2200–

2450 kcal/diet). Due to a large proportion (more than

50% of total fat, in Mediterranean countries) that

comes from cooking oil, the fatty acid composition

of a diet depends largely on the culinary oil used.

0050 Recent knowledge suggests that a unit of energy

from fat may be more fattening than a unit of

energy from carbohydrate. This fact can be related

to the limited body capacity to convert carbohydrate

into fat and the faster adaptation to oxidize excess

carbohydrate than excess fat. Moreover, body cap-

acity for storing carbohydrate as glycogen is limited,

whereas in adipose tissue, fat storage is virtually

unlimited.

Sunflower Oil as Essential Fatty Acid Source

0051 Linoleic and linolenic acids are the two first (parent)

members of o-6 (n-6) and o-3 (n-3) fatty acid fam-

ilies, respectively. Both are essential and must be sup-

plied by the diet because humans and many animals

have lost the ability to synthesize them. Moreover,

other fatty acids of the n-6 family such as g-linolenic

acid (18:3, n-6), dihomo-g-linolenic acid (20:3, n-6),

and arachidonic acid (20:4, n-6) cannot be formed.

0052 The major n-6 fatty acid in the western diet is

linoleic acid. Although other oils are very rich

in linoleic acid, sunflower oil appears to be the pri-

mary source of this fatty acid. The importance of

linoleic acid consumption in humans and animals is

detailed elsewhere. (See Essential Fatty Acids.)

0053 Deficiency of this fatty acid is called essential fatty

acid (EFA) deficiency; it is well documented in the rat

and can be produced in several animals, including

humans. The disease is characterized by skin symp-

toms such as dermatosis. Growth is retarded, repro-

duction is impaired, and there is degeneration or

impairment of function in many organs of the body.

EFA deficiency is characterized by changes in the fatty

acid composition of many biological membranes,

depending on how much vegetable oil, such as sun-

flower oil, is consumed in the diet; the intake of this

fatty acid generally varies from 4–10%. Overt EFA

deficiency is only seen when it provides less than

1–2% of dietary energy (about 2–5gd

1

for adults).

Sunflower Oil as Antioxidant Source

0054 As commented on before, sunflower oil contains

valuable amounts of minor compounds such as a-

tocopherols, g-tocopherols, and phytosterols. The

correct equilibrium between polyunsaturated fatty

acids and antioxidants protects membranes from oxi-

dative stress. Among other properties, phytosterols

act not only as free radical scavengers but also as

antipolymerization agents in frying oils.

Influence of Polyunsaturated Fatty Acids

-6 on

Cholesterolemia and Lipoproteinemia

0055It is currently recommended that dietary total fat,

saturated fatty acids, and cholesterol should be re-

duced while the intake of unsaturated fats should be

increased. As a result of these considerations the con-

sumption of unsaturated oils has been recommended

and margarines rich in polyunsaturated and mono-

unsaturated fatty acids, called health margarines,

have been developed. Linoleic acid has been con-

sidered for many years to be the unique fatty acid

that had a hypocholesterolemic effect at serum and

lipoprotein particle level.

0056Recent investigations suggest that oleic acid also

presents such an effect on low-density lipoprotein

particles. However, a large intake of linoleic acid

decreases high-density-lipoprotein cholesterol while

oleic acid has no effect on high-density-lipoprotein

cholesterol levels.

Influence of Polyunsaturated Fatty Acids

-6 on

Other Physiological Processes

0057n-6 fatty acids have been proven to exert other im-

portant roles in cytokine production and monocyte

quimiotactism, and in eicosanoid production.

0058Platelet aggregation, inflammation, and cellular

immunity are processes modulated by metabolic

products derived from n-6 polyunsaturated fatty

acids. Because sunflower oil contains large amounts

of linoleic acid (the fatty acid mother of other n-6

fatty acids), the intake of this oil should be adequate,

avoiding low and very high intakes that would pro-

duce an EFA deficiency or n-6 fatty acid saturation.

0059Like other fats, sunflower oil may modulate all these

previous cited processes because of its ability to

change the fatty acid composition of membrane

phospholipids. As a consequence, membrane fluidity

may change and also, when cytokines act on target

cells, the amount and types of prostaglandins and

leukotriens originated are modified. Dihomo-g-

linolenic and arachidonic acids derive from linoleic

acid, and are precursors of eicosanoids. These fatty

acids can be metabolized via different enzymatic

pathways, giving rise to thromboxanes, prostacyclins,

prostaglandins, leukotriens, and hydroxy-fatty acids

and oxygenated metabolites, which have been proved

to modulate inflammation, cellular immunology,

thrombosis, cardiovascular disease, and cancer.

0060The recommendation for the polyunsaturated fatty

acid contribution to total energy is 7.5%, with a total

of 6% as linoleic acid. Other scientific societies rec-

ommend that linoleic acid should contribute 3–6% of

SUNFLOWER OIL 5679

total energy in the diet. However, because cardiovas-

cular disease and cancer have been defined as a

consequence of oxidative damage, vegetable oils con-

sumed in correct amounts would add benefits in re-

ducing such degenerative diseases, because they

contain antioxidant and flavonoids and an adequate

linoleic-acid-to-vitamin-E ratio. Immune response

can be improved by consuming unsaturated fatty

acids; however large quantities of n-6 fatty acids

could suppress cellular immunity.

Seealso: Antioxidants:NaturalAntioxidants; Essential

Fatty Acids; Fatty Acids:Metabolism;Analysis;Trans-

fattyAcids:HealthEffects; Gums:PropertiesofIndividual

Gums;FoodUses;DietaryImportance; Lipoproteins;

Margarine:TypesandProperties;Methodsof

Manufacture;CompositionandAnalysis;Dietary

Importance; Marine Foods:ProductionandUsesof

MarineAlgae

Further Reading

Abraham G and Hron RJ (1992) Oilseeds and their oils. In:

Hui YH (ed.) Encyclopedia of Food Science and Tech-

nology, vol. 3, pp. 1901–1910. New York: John Wiley.

Anonymous. Article on high-oleic oils posted on the inter-

net (netlink: www.high-oliec.de/ and www.High-Oleic.-

com)

Anuario de EstadI

stica Agraria (1997) pp. 148–150.

Madrid: Ministerio de Agricultura, Pesca y Alimenta-

cio

Boskou D (1999) Non-nutrient antioxidants and stability of

frying oils. In: Boskou D and Elmadfa I (eds) Frying of

Food. Oxidation, Nutrient and Non-Nutrient Anti-

oxidants, Biologically Active Compounds and High

Temperatures. Lancaster: Technomic, pp. 183–204.

British Nutrition Foundation (1992) Unsaturated Fatty

Acids: Nutritional and Physiological Significance. The

Report of British Nutrition Foundation’s Task Force.

London: Chapman & Hall.

Cuesta C and Sa

nchez-Muniz FJ (1998) Quality control

during repeated fryings. Grasas y Aceites 49: 310–318.

Dupont J (1999) Vegetable oils. In: Macrae R, Robinson

RK and Salder MJ (eds) Encyclopedia of Food Science,

Food Technology and Nutrition, vol. 7, pp. 4686–

4714.London: Academic Press.

Eierdanz H (1993) Oleochemistry: processes and products.

In: Applewhite TH (ed.) Proceedings of the World Con-

ference on Oilseed Technology and Utilization, pp.

217–220. Champaign, IL: AOCS Press.

European Community Regulation. CE no 656/95 and CE

no 2568/91.

Gundstone F (2002) Sunflower seed and its products.

Inform 13: 159–163.

Gupta MK and NuSun (2000) Healthy oil at a commodity

price. Lipid Technology 12: 29–33.

Fick GN (1989) Sunflower. In: Ro

bbelen G, Downey RK

and Ashri A (eds) Oil Crops of the World. Their Breed-

ing and Utilization, pp. 301–318. New York: McGraw-

Hill.

Firestone D (ed.) (1998) In: Physical and chemical charac-

teristics of oils, fats and waxes, 5th edition, Champaign,

IL: AOCS Press.

Gray J (1998) Nuts and seeds. In: Sadler MJ, Strain JJ,

and Caballero B (eds) Encyclopeia of Human Nutrition,

vol. 3, pp. 1422–1429. London: Academic Press.

Macrae R, Robinson RK and Sadler MJ (eds) (1993)

Encyclopaedia of Food Sciences, Food Technology and

Nutrition. London: Academic Press.

Oil World Annual 2001, vol 44, number 45. (netlink:

www.oilworld.de/).

nchez-Muniz FJ, Cuesta C, Lo

pez-Varela S, Garrido-

Polonio MC and Arroyo R (1993) Evaluation of the

thermal oxidation rate of sunflower oil using various

frying treatments. In: Applewhite TH (ed.) Processing

of the World Conference on Oilseed Technology and

Utilization, pp. 448–452. Champaign, IL: AOCS Press.

Unilever Food Espan

a (2000) Statistics. Personal communi-

cation. Bilbao, Spain: Agra.

Weiss TJ (ed.) (1983) Sunflower oil. In: Food Oils and Their

Uses, 2nd edn, pp. 43–44. Chichester: Ellis Horwood.

SUPERCRITICAL FLUID EXTRACTION

E Reverchon, University of Salerno, Via Ponte Don

Melillo, Italy

Background

0001 Many food and nutrition ingredients are traditionally

produced by organic solvents extraction. This process

is relatively simple to perform and is firmly estab-

lished in industrial practice; however, it has several

drawbacks: it is a polluting process, since organic

solvents can be released to the atmosphere, they

remain in starting material, and they have to be

eliminated from the extracted products. Owing to

the increasing demand for nonpolluting processes

and product purity, alternative processes are in-

tensively coming under scrutiny. The most prom-

ising alternative to organic solvent processing

is supercritical fluids extraction (SCFE); several

SCFE produced compounds are already commer-

cialized.

5680 SUPERCRITICAL FLUID EXTRACTION

0002 A supercritical fluid (SCF) is, as a rule, a gas under

room conditions that is heated and compressed above

its critical temperature and pressure. When it is under

supercritical conditions, the SCF is in the gas state

and maintains its related properties, but, depending

on the operating pressure, it can assume variable

densities from gas-like to liquid-like. Therefore,

SCFs are sometimes called ‘variable geometry’ solv-

ents. This characteristic makes them suitable for se-

lective extraction processes. Indeed, it is possible to

modulate the solvent power of a SCF to perform

intensive or selective extraction processes. After the

extraction, owing to pressure reduction, the SCF is

immediately and completely eliminated from the raw

material and from the extracts.

0003 Table 1 lists some SCFs proposed for extraction

processes, together with their critical temperatures

and pressures. It is evident that only some of them

can be applied to food extraction, since their critical

temperatures are often too high for the stability of

thermolabile compounds that are characteristic of

food.

0004 The large majority of the extraction applications

proposed for SCFE use carbon dioxide (CO

)asthe

SC solvent because CO

is cheap, nonflammable,

nonpolluting, and available in large quantities. Its

critical temperature and pressure are readily obtain-

able in an industrial process, and the SCF process

temperatures that can be set (for example from 35

to 50



C) are compatible with the thermal stability of

most food-related ingredients. The general behavior

of SC-CO

is similar to a nonpolar solvent, i.e., it has

strong affinity with lipophylic molecules. Also, since

supercritical CO

is used during the process, but not

produced during extraction, it does not contribute to

the so-called ‘greenhouse effect.’ Thus, the only draw-

back of SC-CO

extraction is the requirement of a

pressurized plant to perform the process.

0005 SCF extraction has been successfully performed in

many cases on the laboratory scale, but only a few

industrial-scale applications have been developed

until now. However, the potential of SCFE is very

high, and many industrial applications are expected

to be developed in the next years.

Extraction Processes

0006From the point of view of the process arrangement,

SCFE can be divided into three major areas,

according to the physical state of the compounds to

be extracted or of the raw material to be treated:

0007Extraction from solid matrices is used to extract

one or more compounds from a solid matrix, for

example, to extract oil from seeds, essential oils,

etc.

0008Extraction from liquid mixtures is used when

liquid compounds are to be extracted from a liquid

mixture. It is used, for example, to eliminate

hexane from seed oil and alcohol from wine, and

for milk fat fractionation.

0009Spray extraction is used to extract solid com-

pounds from a liquid mixture. In this case, the

previous process arrangements cannot be used. It

can be used to extract lecithin from seed oils.

0010This classification underlines the similarities among

the various extraction processes and can be useful for

choosing the process arrangement to apply SCFE to

products different from those described in the scien-

tific and technical literature.

Extraction from Solid Matrices

0011This is the most studied SCFE application, since the

most frequently required process is the extraction/

elimination of one or more compound families from

a solid (natural) matrix. The base extraction scheme

is represented in Figure 1. It consists of an extraction

vessel filled with the raw matter to be extracted. As a

rule, the raw matter is dried and finely ground to

facilitate the extraction process. It is loaded in a

basket located inside the extractor to allow rapid

charge–discharge of the extraction vessel. Metallic

frits at both ends of the extractor allow the SCF to

go through the bed of solid material to extract the

required compounds. The SCF at the exit of the ex-

tractor flows through a depressurization valve to a

tbl0001 Table 1 List of supercritical fluids proposed for the extraction

process

Supercritical fluid T

(



C) p

(bar)

Carbon dioxide 31.1 73.8

Nitrogen protoxide 36.5 70.6

Propane 96.7 42.4

Ethanol 243.4 72.0

Isopropyl alcohol 235.3 47.6

Trifluoromethane 26.0 46.9

SCF in

out

fig0001Figure 1 Extraction scheme for extraction from solid matrices.

SUPERCRITICAL FLUID EXTRACTION 5681

separator in which, because of the lower pressure, the

extracts are released from the gaseous medium and

collected. More sophisticated extraction schemes

(such as that shown in Figure 1) contain two or

more separators. In this way, it is possible to fraction-

ate the extract in two or more fractions of different

composition by setting suitable temperature and pres-

sure values in the separators.

0012 Other possible variations of the SCFE scheme

are multistage extraction and cosolvent addition.

The multistage extraction strategy can be used when

several compound families are to be extracted from

the same matrix, and these are characterized by their

different solubilities in SC-CO

. It takes advantage of

the fact that the solvent power of SC-CO

strongly

increases with pressure at a fixed temperature. There-

fore, it is useful to perform a first extraction operating

at a low CO

density (e.g., 0.29 g cm

3

,90bar,50



followed by a second extraction step at a high CO

density (e.g., 0.87 g cm

3

, 300 bar, 50



C). The most

soluble compounds are extracted during the first

stage (e.g., essential oil) and the less soluble com-

pounds are extracted in the second one (e.g., antioxi-

dant compounds).

0013 A liquid cosolvent can be added to SC-CO

increase its solvent power towards polar molecules.

Indeed, as previously mentioned, SC-CO

is a good

solvent for lipophilic (nonpolar) compounds, whereas

it has a low affinity with polar compounds. Various

authors have added small quantities of liquid polar

solvents (for example, ethyl alcohol) that are readily

solubilized by SC-CO

owing to their low molecular

weight but, when in solution, modify the solvent

power of the SC solvent. The only drawback with

this strategy is that the cosolvent is a liquid and

is collected in the separator together with the ex-

tracted compounds that have to be processed later

for solvent elimination. Therefore, one of the ad-

vantages of the SCFE, i.e., the complete solvent

elimination, is lost.

Selection of the Operative Conditions

0014 The selection of the operating conditions depends of

course on the specific compound or compound family

to be extracted. Molecular weight and polarity

have to be taken into account case by case, but some

general rules can be applied. First, the SCFE tempera-

ture for food compounds is fixed between 40 and



C, e.g., in the vicinity of the critical point and as

low as possible to avoid thermal degradation. The

increase of temperature reduces the density of SC-

(for a fixed pressure), thus reducing the solvent

power of the SC solvent, but it increases the vapor

pressure of the compounds to be extracted and, there-

fore, their tendency to pass in the fluid phase.

However, the most relevant process parameter is the

extraction pressure that can be used to tune the

selectivity of the SCF. The general rule is: the higher

the pressure, the greater the solvent power and the

lower the extraction selectivity. Frequently, the solv-

ent power is described in terms of the SC-CO

density

under the operating conditions. CO

density can vary

from about 0.15 to 1.0 g cm

3

at the operating con-

ditions useful for SCFE and is connected to both

pressure and temperature. Its variation is strongly

nonlinear, so the proper selection requires the use of

accurate tables of CO

properties (for example, the

International Thermodynamic Tables of the Fluid

State CO

0015The other crucial parameters in SCFE are CO

flow

rate, the particle size of the matrix, and the duration

of the process (extraction time). The appropriate

selection of these parameters leads to complete

extraction of the desired compounds in a shorter

time. These are associated with the thermodynamics

(solubility) and the kinetics of the extraction pro-

cess in the specific raw matter (mass-transfer

resistance).

0016The proper selection depends on the mechanism

that controls the process (the controlling mechanism

is the slower mechanism and determines the overall

process velocity). For example, particle size plays an

important role in extraction processes controlled by

the mass-transfer resistance, since a reduction in the

mean particle size reduces the length of diffusion of

the solvent. However, if particles are too small (for

example, smaller than 0.2 mm), they can cause prob-

lems of channeling inside the extraction bed: part of

the solvent flows through channels formed inside the

extraction bed and does not contact the material to be

extracted, thus causing a loss of efficiency and yield in

the process. Moreover, the production of very small

particles by grinding could lead to a loss of volatile

compounds.

Applications of Solid Matter Extraction

0017Many solid matrices have been processed by SCFE.

Some relevant categories of products are listed in

Table 2. Some well-established industrial processes

tbl0002Table 2 Some food and food ingredients processed by solid

matter SCFE

Essential oils (spices, citrus, etc.)

Seed oils, fats, and waxes

Fatty acids, fatty acid methyl esters

Natural colorants, pigments

Vitamins

Antioxidants (licopene, tocopherols, etc.)

Alkaloids (caffeine)

5682 SUPERCRITICAL FLUID EXTRACTION

use SCFE to produce hops extracts and several food

nutritional substances that also offer some aspects of

therapeutic protection to the human body (nutraceu-

ticals). One of the largest industrial applications is

coffee decaffeination, which is performed on green

coffee beans presoaked in water. Green coffee beans

are used to prevent coextraction of aroma com-

pounds that are formed during roasting. The use of

water in the preliminary treatment allows the release

of caffeine from the natural substrate where it is

linked in the form of sodium salts. Very large plants

that operate at pressures between 280 and 300 bar at



C are used in this process with internal volumes

larger than 10 m

. Caffeine is recovered during the

SCFE with a purity of around 96% by weight; the

final caffeine content in coffee beans can be lower

than 0.05% w/w.

0018 Two examples of SCFE are now described in detail:

essential oil isolation, which represents a good

example of extraction, followed by fractional separ-

ation, and seed oil extraction.

0019 Essential oil isolation Essential oils are mainly

formed by hydrocarbon and oxygenated terpenes

and by hydrocarbon and oxygenated sesquiterpenes.

They are commonly extracted from seeds, roots,

flowers, herbs and leaves using the so-called hydro-

distillation. This is a very simple process but has

many drawbacks: thermal degradation, hydrolysis,

and solubilization in water of some compounds that

alter the flavor and fragrance profile of many essen-

tial oils extracted by this technique. (See Essential

Oils: Isolation and Production.)

0020 From the point of view of SCFE, essential oil isol-

ation is an example of extraction followed by frac-

tional separation. Indeed, this extraction process can

be optimally performed at mild pressures (from 90 to

100 bar) and from 40 to 50



C, which correspond to

relatively low CO

densities. However, even under

these operating conditions, essential oil components

are extracted together with cuticular waxes, i.e., par-

affinic compounds located on the surface of vegetable

matter with the aim of controlling its perspiration.

Paraffins have a relatively low solubility in SC-CO

but are located on the surface of the vegetable matter,

whereas essential oil compounds are readily soluble

in SC-CO

but are partly located inside the vegetable

structure. Therefore, extraction of waxes is con-

trolled by their solubility, and essential oil extraction

is controlled by mass-transfer resistance through the

vegetable structure. As a result of these different inter-

actions, the two compound families are coextracted

even under the optimum extraction conditions. Since

coextraction is unavoidable, it is possible to take

advantage of the fact that at the same pressures, but

at low temperatures (from 5toþ5



C), waxes are

practically insoluble in CO

, whereas essential oil

compounds are still very soluble. Therefore, fraction-

ation of the extract is possible by operating a first

separation, for example at 0



C and 90 bar and oper-

ating a second separation at 20



C and 30 bar. In the

first separator, the selective precipitation of waxes is

obtained, and in the second, essential oil is recovered.

This concept has been applied up to the industrial

scale, and to date, two companies operating in Ger-

many and in Italy use this process to isolate essential

oils.

0021It is worth noting that it is not possible to perform

the CO

extraction directly at 0



C and 90 bar, since

the vegetable matter contains many other compound

families (antioxidants, colors, etc.) that are very sol-

uble under these process conditions.

0022XSeed oil extraction Vegetable oil from seeds is

traditionally produced by liquid-solvent extraction;

as a rule, hexane is used to extract the oil from ground

seeds. The process is very efficient, but its major

problem is hexane elimination after extraction.

Three distillation units in series have to be used,

operated under vacuum. The drawbacks of this

process are the high costs, the possible thermal deg-

radation of the oil, and the incomplete hexane elim-

ination (from 500 to 1000 p.p.m. residue).

0023Therefore, several authors have proposed substi-

tuting the traditional process by SC-CO

extraction

of oil from seeds. Indeed, tryglicerides forming seed

oils are readily soluble in SC-CO

at 40



C and at

pressures higher than about 280 bar. The main par-

ameters to be taken into account for this process are

particle size, pressure and residence time. Small par-

ticles (1 mm mean diameter or less) and high

pressures (300–500 bar) can strongly reduce the

extraction time. After the extraction, the SC-CO

tryglicerides solution is sent to a separator working

at a subcritical pressure. This operation reduces the

solvent power of CO

to almost zero and allows the

recovery of oil. The complete elimination of gaseous

from oil is also carried out in the separator. The

SCFE of several seed oils has been successfully per-

formed up to the pilot scale. An alternative process

has also been proposed, in which the operation is

performed at a fixed pressure, and only temperature

variations are used to recover the oil.

0024A comprehensive mathematical model of oil ex-

traction has also been validated in which the oil-

bearing seed structures, their characteristics, and the

experimental data from several authors have been

considered to obtain a general validity representation

of the process. (See Vegetable Oils: Oil Production

and Processing.)

SUPERCRITICAL FLUID EXTRACTION 5683

Extraction from Liquid Mixtures

0025 When liquid compounds have to be extracted from a

liquid mixture, it is not possible to use the fixed-bed

extractor. In this case, a packed tower operating in

continuous mode has to be used. The packing is

formed by an inert material characterized by a large

surface area that facilitates the contact between the

liquid and the supercritical fluid that are sent to

the tower in the countercurrent. Typical packings are

Rashig rings, perforate saddles, etc. Note that while

the extraction from solids is a discontinuous oper-

ation, the packed tower is capable of a continuous

steady-state operation, which allows large quantities

of liquid mixtures to be processed in a relatively small

apparatus and in a short time.

Selection of Operating Conditions

0026 The typical extraction apparatus used in this case is

shown in Figure 2. Two pumps deliver the liquid

solution and SC-CO

to the packed column. SC-

flows along the column from the bottom to the

top, whereas the liquid solution is usually added to

the top, though it is also possible to feed it at an

intermediate position along the column.

0027 The process is based on the different solubilities of

liquids to be treated in SC-CO

. The ideal case is

obtained when only the compounds to be extracted

are soluble in SC-CO

, whereas all the other liquid

components are completely insoluble. However, this

case is rare, and as a rule, the limited solubility of the

other liquid compounds forming the mixture has to

be taken into account. For this reason, the appropri-

ate pressure and temperature of the process have to be

chosen to select the conditions under which there is a

maximum difference in solubility among the com-

pounds to be extracted and all the other compounds

in the mixture. Also, in this case, CO

density is

frequently used as a criterion to find the conditions

of maximum selectivity.

0028The difference in density between the liquid and

SCF is in this case another parameter to be taken into

account: to allow the countercurrent operation, the

SCF density has to be lower than that of the liquid.

0029The traditional operation of packed columns re-

quires that the liquid flow rate will be greater than

the minimum amount that ensures complete wetting

of the packing. The feed ratio is also selected to avoid

the massive entrapment of the liquid in the fluid phase

(flooding). Of course, these conditions have to be

respected also when a supercritical fluid is used as

the fluid processing medium. The classical calculation

in terms of the number of theoretical equilibrium

stages required for separation can also be applied.

0030A possible variation of this processing scheme can

involve the adoption of a temperature profile along

the column with the aim of optimizing the separation

temperature with respect to the composition of the

mixtures at different levels inside the column.

0031The extraction of liquid mixtures is controlled

by the relative solubilities in SC-CO

of the various

compounds forming the mixture, that is, the thermo-

dynamic limitation of the process. Mass transfer

between the two phases represents the kinetic limita-

tion. The distance from the equilibrium condition

represents the driving force for the separation along

the column.

Applications of Liquid Extraction

0032The fractionation of a liquid mixture by SCFE has

been proposed for various applications. A selection of

the most relevant examples is given in Table 3.Two

examples will be discussed: hexane elimination from

vegetable oils, representing a new approach to seed

oil extraction, and citrus essential oil deterpenation,

used to extend the shelf-life of these products.

0033Hexane elimination from traditionally extracted seed

oils This process represents a different approach to

seed oil extraction by SC-CO

. Indeed, the supercrit-

ical extraction applied directly to seeds has the draw-

back of high costs owing to the use of large

pressurized vessels and the discontinuous operation.

0034If the approach is to treat the liquid–hexane oil

mixture after traditional hexane extraction, it is

SCF in

out

Liquid in

fig0002 Figure 2 Extraction scheme for extraction from liquid mixtures.

tbl0003Table 3 Applications of liquid fractionation by SC-CO

Oil deacidification

Solvent elimination (hexane from vegetable oil)

Wine dealcoholation

Milk fat fractionation

Deterpenation of citrus oils

Recycling of frying oils (fatty acid methyl esters and peroxide

elimination)

5684 SUPERCRITICAL FLUID EXTRACTION

possible to use a smaller apparatus and a continuous

operation that allows large quantities of raw mixture

to be processed.

0035 Hexane and triglycerides are both soluble in SC-

, but at a temperature of 40



C, hexane is com-

pletely miscible at pressures higher than about 100

bar, whereas, at the same temperature, tryglicerides

show a nonnegligible solubility in SC-CO

only for

pressures higher than 250 bar. Therefore, these com-

pounds can be separated if the operation pressure is

considerably lower than 200 bar.

0036 For example, the fractionation of soybean oil–

hexane mixture has been successfully performed

using a 1.8-m-tall packed tower, charged with stain-

less steel perforated saddles with a very large specific

surface. The extraction has been performed at 120

bar and 40



C using different starting concentrations

of soybean oil in hexane up to 70:30 hexane–oil

concentration by weight, which corresponds to the

composition of the mixture at the exit of the trad-

itional liquid extraction vessels. With a single passage

in the packed tower, it has been possible to obtain a

bottom product formed by soybean oil containing

down to 20 p.p.m. of hexane residue and to recover

pure hexane at the top of the column.

0037 Citrus essential oils deterpenation Citrus essential

oils are produced by cold pressing the peel of citrus

fruits. These contain very large quantities of hydro-

carbon terpenes (mainly limonene) up to 99% in

some cases, and small quantities of oxygenated ter-

penes. Whilst limonene undergoes degradation and

produces an unpleasant flavor, oxygenated terpenes

make a large contribution to the characteristic citrus

flavor and fragrance. Therefore, in order to prolong

the shelf-life of these products and concentrate their

fragrance, hydrocarbon terpene elimination (deter-

penation) processes have been proposed. The most

commonly used processes to deterpenate peel oils

are vacuum distillation, extraction with alcohol, and

partitioning between two solvents. The disadvantages

of these processes are contamination with organic

solvents and thermal degradation of the products.

0038 The alternative to traditional processes is SCF

deterpenation in a packed tower operating in con-

tinuous mode, taking advantage of the different solu-

bilities of hydrocarbon and oxygenated terpenes in

SC-CO

to fractionate the mixture. This separation is

more difficult to perform than that described in the

previous paragraph for soybean oil, since both hydro-

carbon and oxygenated terpenes are very soluble in

SC-CO

, and their solubilities are very similar.

0039 Successful processing has been proposed, operating

at 40



C and at pressures near the critical value of

(from 75 to 90 bar) to obtain the maximum

possible selectivity (the maximum difference in solu-

bility between two compound families can be

obtained in this range). Nevertheless, no operating

conditions exist at which the less soluble oxygenated

terpenes are not soluble in SC-CO

. Thus, fraction-

ation is possible only using very tall columns and with

a fractionation efficiency lower than 100%.

0040For these reasons, some authors have proposed

another SC processing scheme, based on the adsorp-

tion of the citrus oil on an adsorbent (silica gel) and

the fractional desorption by SC CO

. The process is

very similar to SCFE; it has the advantage that it is an

enhanced separation owing to the different strength

of the adsorption of hydrocarbon and oxygenated

terpenes on silica-gel active sites.

Spray extraction

0041A liquid mixture can also be formed by a liquid that

contains one or more solid solutes. The extraction of

these compounds from the liquid cannot be performed

using a packed tower, since the solid matter will pre-

cipitate inside the tower. It cannot be recovered, and

the internal packing is made dirty by this material.

Therefore, in the case of solid-containing liquid mix-

tures, a supercritical spray extraction process has to

be adopted. The preconditions to apply spray extrac-

tion are that the liquid solvent has to be very soluble

in SC-CO

, whereas the solids have to be insoluble in

the SC fluid. These conditions can be frequently

obtained, since many organic solvents are readily

soluble in SC-CO

, even under mild operating condi-

tions (e.g., a CO

density lower than 0.6 g cm

3

), and

many high-molecular-weight solids have negligible

solubilities in SC-CO

, especially at low CO

dens-

ities.

0042The supercritical spray extraction process consists

of a precipitation vessel previously charged with SC-

(and in which the supercritical fluid continues to

flow), in which a spray of the liquid solution is con-

tinuously injected. If the process conditions have been

properly selected, the liquid is rapidly dissolved in the

SC fluid, whereas the solid precipitates at the bottom

of the precipitation vessel. A possible representation of

the process is shown in Figure 3; two pumps deliver

the liquid solution and SC fluid, respectively. A pre-

cipitation vessel is used to collect the solid, and a

liquid solvent recovery vessel located downstream

the precipitator and operated at lower pressure

(for example, 30 bar, 25



C) is used to recover the

liquid.

Selection of the Operative Conditions

0043One of the key steps in this process is the spray

formation. The aim of this operation is to produce a

SUPERCRITICAL FLUID EXTRACTION 5685

very large liquid surface forming small liquid

droplets. The large surface area between the liquid

and SCF strongly enhances the solubilization of the

liquid phase in the supercritical medium and reduces

the time required for the complete elimination of the

liquid phase to a few seconds or less. For this reason,

the process is performed under operating conditions

at which the liquid is completely soluble in SC-CO

Knowledge of the solubility data on the liquid solv-

ents and of the solids in SC-CO

is mandatory for this

process for the appropriate selection of process tem-

perature and pressure.

0044 Also, in the case of SC spray extraction, there is the

interaction between thermodynamics constraints and

mass-transfer mechanisms that control the process

performance. The enhanced mass transfer that char-

acterized SC fluids is again a distinctive advantage in

their adoption as extraction media, together with the

rapid and complete separation by simple depressur-

ization between the SC solvent and the liquid.

0045 A limitation of this process is the possible formation

of a ternary liquid–solid–SC-CO

mixture. Indeed, the

liquid can induce a nonnegligible solubility of the solid

compounds in SC-CO

. In this case, the liquid can act

as a cosolvent from the point of view of the solid

solubilization. When this phenomenon occurs, the

part of solid retained in the fluid phase obviously

does not precipitate and is lost in the liquid recovered

in the separation vessel. However, this liquid can be

used to solubilize the solid compound again before

the SC spray extraction. The limit case is represented

by the complete entrapment of the solid in the fluid

phase that produces the process failure.

Applications of Spray Extraction

0046Until now, spray extraction has been used in a limited

number of processes, but it has a great potential for

future applications. Several possible applications are

listed in Table 4.

0047Two examples will be described in detail: lecithin

extraction from seed oil and citric acid purification.

The first example is a classical spray extraction with

the solid solute that is completely insoluble in SC-

. Citric acid purification introduces a multistep

spray extraction in which different compounds are

precipitated in sequence.

0048Lecithin extraction from soybean oil Lecithin is

formed by a mixture of phospholopids and is a valu-

able nutraceutical compound. Its major source is

soybean (1.1–3.2 wt%). It is an important natural

emulsifier used in food and pharmaceutical industries

and can be extracted from soybean oil. However, as

long as the oil is extracted, the liquid becomes very

viscous because of the concentration of lecithin. (See

Phospholipids: Properties and Occurrence.)

0049Some authors have proposed using a spray extrac-

tion process and using SC-CO

as the spray extraction

solvent, since seed oils are soluble in SC-CO

(though

at pressures higher than about 280 bar, as previously

specified), and phospholipids are completely insol-

uble in this medium.

0050Thus, the proposed process consists of a precipita-

tion vessel pressurized with SC-CO

(at 400 bar and



C, for example), in which CO

flows continu-

ously and an injection system that sprays the leci-

thin-containing oil into the precipitator in the form

of small droplets. The oil solubilizes very rapidly in

SC-CO

and is extracted and recovered in a separator

operating at low pressure. Lecithin precipitates in the

precipitation vessel as a solid powder and is collected

when the vessel is discharged.

0051Citric acid purification Citric acid is an important

compound that is used in food and beverages as an

acidulant. It is generally produced by fungal fermen-

tation, and the purification process includes several

treatments with chemical reagents. The spray extrac-

tion process has been proposed to purify citric acid

after extraction from the fermentation broth using an

organic solvent (e.g., ethanol or acetone). Two kinds

SCF in

out

Liquid in

fig0003 Figure 3 Apparatus used in spray extraction.

tbl0004Table 4 Applications of SC-CO

spray extraction

Citric acid extraction and purification

Phospholipids from soybean oil and egg yolk

b-Carotene from oxides

Protein purification

Di- and triglyceride fractionation

5686 SUPERCRITICAL FLUID EXTRACTION