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

Подождите немного. Документ загружается.

ESSENTIAL OILS

Contents

Properties and Uses

Isolation and Production

Properties and Uses

J Grassmann and E F Elstner, Technical University

of Munich, Munich, Germany

Background

0001 Essential oils have been used in folk medicine for a

very long time, but their mode of action in healing

was not clear. Recently, these oils have become in-

creasingly important, such as in aromatherapy, which

is controversial, but also in traditional medicine and

nutrition. For this reason, an increasing number of

researchers are becoming interested in the biochem-

ical effects of these oils.

0002 This article gives an overview of the definition,

abundance, components, biosynthesis, and biological

functions of essential oils in the plant kingdom, but

the main aim is to describe the use and biochemical

effects as well as the pharmacological properties of

these oils. A short chapter deals with the stability and,

in this context, with the problem of aging of essential

oils.

Essential Oils: Definition and

Abundancies

0003 Essential oils (EOs) are complex mixtures of more or

less lipophilic substances obtained by distillation or

pressing of plants or parts of them. Most of the com-

ponents are terpenoids (isoprenoids), mostly mono-

and sesquiterpenes. Furthermore, this main group

is followed by aromatic compounds (phenylpropa-

noids) derived from the shikimate–chorismate

pathway, polyketide derivatives, paraffinic hydrocar-

bons, alcohols and ketones, fatty acids, and finally

sulfur- and nitrogen-containing compounds. The

compartments of highest concentration of EOs in

the plant are special oil cells, which are eventually

converted into oil containers. They are also found

intercellularly, with the oil compartments being

divided into schizogenic and lysigenic oil containers.

Furthermore, milk tubes and plant-surface gland

hairs may also contain EOs. The plant families

containing EOs include the Apiaceae, Asteraceae,

Cupressaceae, Lamiaceae, Myrtaceae, Pinaceae, Poa-

ceae, and Rutaceae.

0004Isoprenes and monoterpenes play crucial roles in

atmospheric chemistry (tropospheric ozone forma-

tion) where annual global fluxes between 250 and

450 million tonnes are estimated to arise from the

vegetation, mainly from trees. This important func-

tion in the atmosphere is due to their fast reaction

with reactive oxygen species (ROS) such as the OH

radical (see also section on Antioxidative Effects).

Terpenoids also seem to play an important ecological

role in plant–herbivore interactions, since their

biosynthesis is induced by insect feeding, eventually

triggered by an elicitor from oral secretion of the

feeding insect.

Technical Procedures: Extraction from

Plants or Plant Organs

0005The technical procedures for obtaining EOs from

plant material comprise steam distillation, extraction

and, as mostly in the case of citrus oil, pressing of

peels. These procedures are dealt with in a special

chapter of this encyclopedia. (See Essential Oils: Isol-

ation and Production.)

Physical Properties, Chemical Structures,

and Reactivities

0006Fundamental work on EOs was started as early as

1884, by O. Wallach (1847–1941). EOs are in a

liquid state at ambient temperatures, but they are

volatile (with boiling temperatures between 50 and

320



C), thus distinguishing them from mineral or

fatty oils. Because of their volatility, EOs have a char-

acteristic odor. They produce a transparent spot, on

adsorbent paper, that disappears within 24 h. Most of

the EOs are almost colorless, have a high refractive

index, and are optically active (see below). They are

soluble in most organic solvents, but have a very

limited solubility in water. Their density is lower

than that of water, with the exception of the oils

of sassafras, clove, and cinnamon. An important fea-

ture is their property of being steam-distillable, thus

ESSENTIAL OILS/Properties and Uses 2177

enabling physical separation from other lipophilic

plant constituents such as ‘fixed oils’ or membran-

eous components. In the following the most import-

ant groups of compounds of essential oils will be

listed.

Most Important Components

0007 Today, several thousand different components are

known, by far the most prominent being the terpe-

noids, followed by phenylpropanoids and other less

important ingredients of plant oil compartments.

0008 -Monoterpenes The monoterpenes (MTs) are

widely distributed and sometimes contribute over

90% of the total oil content. They can be divided

into regular and irregular monoterpenes and iridoids.

Most components of essential oils have regular struc-

tures. Irregular members such as chrysanthemic acid

and its derivatives are found in EOs of Asteraceae.

Other examples of irregular MTs are the artemisanes,

santolinanes, and lavandulanes or the artemisiaketon,

which is present in Achillea millefolium.

0009Iridoids are another group of the MT family, in

which the bicyclic ingredients of the Valerianaceae-

like baldrinal, valeranone, or valtrate compounds

from the olive tree (oleacin, oleuropein) or from

yellow gentian (gentisin, sweroside, gentiopicrin) are

the best known.

0010Since the ‘regular’ MTs are the most common com-

ponents in essential oils, this chapter will deal only

with this group. They can be subdivided into MT

hydrocarbons, MT alcohols and their esters, MT

aldehydes, MT ketones, and MT ethers such as

cineol or dill ether. A special MT is the endoperoxide

ascaridol.

0011MT hydrocarbons This group comprises several im-

portant compounds present in various essential oil

compartments of different plant groups and include

well-known compounds such as pinene, limonene,

and terpinene. These compounds are frequently found

as isomers, e.g., a- and g-terpinene, as optical isomers,

for example (þ)- and ()-limonene or both, for

example ()-a-phellandrene and ()-b-phellandrene

or (þ)- and ()-a-pinene together with ()-b-pinene.

β-Myrcene

α-Terpinene γ-Terpinene ρ-Cymene (+)-Limonene (−)-α-Phellandrene (−)-β-Phellandrene

(+)-α-Pinene (−)-α-Pinene

(+)-3-Carene (−)-Camphene

(−)-β-Pinene

trans-β-Ocimene cis-β-Ocimene

fig0001 Figure 1 Examples of acyclic and cyclic structures of regular monoterpene hydrocarbons in essential oils.

2178 ESSENTIAL OILS/Properties and Uses

A selection of important structures in this class is

shown in Figure 1.

0012 MT alcohols Important members of this class are

the widely distributed compounds geraniol, nerol,

linalool, citronellol, terpineol, and borneol. They

also comprise both acyclic and cyclic forms. Several

important structures are shown in Figure 2.

0013 MT aldehydes and MT ketones Again, both acyclic

compounds (geranial, neral, citronellal) and cyclic

forms (carvone, menthon, pulegone) are found.

A mixture of the isomers geranial and neral is

called citral. Bicyclic MT ketones are the well-

known (þ)- and ()- camphor, fenchone, and thujone

(Figure 3).

0014 MT ethers and endoperoxides Menthofuran, dill

ether, cineol, and ascaridol are representatives of

this class of compounds and are shown in Figure 4.

0015 -Sesquiterpenes With more than 50 different struc-

tural types, sesquiterpenes constitute another import-

ant group of compounds within the EOs. They arise

from farnesylpyrophosphate, which in turn is formed

by head-to-tail condensation of geranylpyrophos-

phate and isopentenylpyrophosphate, forming a C15

carbon skeleton (see below). They are especially

abundant in essential oils of the plant family of the

Zingiberaceae and sesquiterpene lactones in the

Asteraceae. b-Carophyllen-1, 2-epoxide is found in

Geraniol Nerol

Linalool

(−)-β-Citronellol

(−)-α-Terpineol

Terpinen-4-ol

fig0002Figure 2 Examples of acyclic and cyclic structures of mono-

terpene alcohols in essential oils.

(S)-(+)-Carvon

(R)-(−)-Carvon

(−)-Menthon

(+)-Pulegone

Geranial

(Citral a)

Neral

(Crital b)

(+)-Citronellal

(−)-Campher

(+)-Campher (+)-Fenchone

(−)-Thujone

fig0003 Figure 3 Examples of acyclic and cyclic structures of monoterpene aldehydes and ketones in essential oils.

ESSENTIAL OILS/Properties and Uses 2179

clove and sage oil, in which it can be used for quality

control (Figure 5).

0016 -Aromatic compounds of the phenylpropane group

Aromatic compounds (ACs) are minor components in

EOs. They essentially comprise phenylpropanoids,

propenyl- or allylphenols and coumarins. Typical

members from this group, like anethol or estragol,

are found in Apiacean oils (fennel, anise) but also in

Rutacea. Eugenol and vanillin are two typical ACs of

this type (Figure 6).

0017-Other compounds EOs also may contain degrad-

ation products of membraneous unsaturated fatty

acids such as ‘green odor’ components (cis-ortrans-

hexenal, hexanal, and hexanol) and several lactones.

Furthermore, compounds from terpene degradation,

for example C

-norisoprenoids, and sulfur- and

nitrogen-containing compounds (e.g., pyridine de-

rivatives in spearmint oils) are found. Several volatile

terpenoids are present in plants as glycosides and

contribute to the overall, typical fruit flavors.

Biosynthetic Pathways

0018The two cell compartments of plant isoprenoid

biosynthesis, plastid and cytosol, have two different

ways to synthesize terpenoids: the classic mevalonate

pathway and the alternative 1-deoxy-d-xylulose-5-

phosphate (DOXP) pathway, which occurs in the

plastids and leads to, among others carotenoids,

1,8-Cineol Menthofuran Dill ether Ascaridol

fig0004 Figure 4 Examples of structures of monoterpene ethers and

endoperoxides in essential oils.

(E,E)-α-Farnesene

α-Caryophyllene

(Humulen)

β-Caryophyllene

(E)-β-Farnesene

fig0005 Figure 5 Examples of structures of sesquiterpenes in essential oils.

(E)-Anethol

OCH

Estragol

(Methylchavicol)

OCH

Eugenol Vanillin

OCH

fig0006 Figure 6 Examples of structures of aromatic compounds of the phenylpropane group.

2180 ESSENTIAL OILS/Properties and Uses

monoterpenes, and isoprene. In the cytoplasm, the

mevalonate pathway takes place and forms sesquiter-

penes, for example.

The Mevalonate Pathway

0019 The biosynthesis of monoterpenes in the cytoplas-

matic space proceeds via the well-known mevalonic

acid pathway, starting from acetyl CoA by various

condensations including activation by ATP and re-

duction by NADH. A key step is the two-step reduc-

tion of hydroxymethyl-glutaryl-CoA (HMG CoA)

into mevalonate under CoA release by the enzyme

HMG CoA reductase. This step has become ex-

tremely important, since it represents the main possi-

bility of interaction with several types of inhibitors

with this pathway and is used for the treatment of

hypercholesterolemia in humans. After the condensa-

tion of two C5 bodies (activated isoprenes: isopentyl

pyrophosphate (IPP) and dimethylallyl pyrophos-

phate (DMAPP), with pyrophosphate as

a thermodynamically preferred leaving group

(splitting into two orthophosphates, E

approx.

35 kJ mol

1

), the product is geranylpyrophosphate

(GPP), the first C10 backbone of the monoterpenes.

The formation of acyclic monoterpenes from GPP is

obvious, and the generation of monocyclic and bicyc-

lic monoterpenes involves monoterpene cyclases.

These enzymes catalyze an isomerization–cyclization

mechanism generating the a-terpinyl cation. Loss of a

proton leads to the formation of hydrocarbons and

addition of water to alcohols. Generation of oxygen-

ated terpenoids also can be carried out by classic

oxidases that catalyze allylic hydroxylation, oxida-

tion of the alcohol to an a, b-unsaturated ketone,

and possibly reduction of the double bond. Thus,

the a-terpinyl cation is the origin for a large number

of different monoterpenes.

0020 The precursor of sesquiterpenes, farnesyl pyropho-

sphate, is the product of the addition of one molecule

IPP to GPP. Similar to monoterpene cyclizations, re-

arrangements, isomerizations, or oxidations lead to a

wealth of different structures.

The DOXP Pathway of IPP Biosynthesis

0021 Recently, another pathway, starting not from

acetyl-CoA but from pyruvate and glyceraldehyde-

3-phosphate (GAP), was established for plastidic-

derived terpene derivatives in the middle of the

1990s, especially by the Lichtenthaler group in

Germany. This pathway, in contrast to the above

mevalonate pathway, is not inhibited by HMG CoA-

reductase inhibitors. It includes, after a thiamin-

dependent condensation of pyruvate and GAP

(CO

-release), DOXP. An intramolecular C-C-

skeleton rearrangement catalyzed by DOXP

reducto-isomerase generates 2-C-methyl-d-erythrose-

phosphate (MEP). In 2000, it was shown that the

following steps involve a cytidylyl transfer (catalyzed

by MEP cytidylyl-transferase) generating 4-(cytidine

-diphospho)-2-C-methyl-d-erythritol (CDP-ME),

which is phosphorylated by CDP-ME kinase, leading

to 2-phospho-4-(cytidine 5

-diphospho)-2-C-methyl-

d-erythritol. This compound finally undergoes cycli-

zation to form 2-C-methyl-d-erythritol-2, 4-cyclodi-

phosphate (MECDP), a reaction catalyzed by MECDP

synthase. MECDP is then converted into DMAPP, but

the reactions leading to this product are still unknown.

Biological Functions in the Plant Kingdom

0022There are good reasons to assume ecological func-

tions for most of EOs such as acting as allelopathic

agents, repellants, or attractants in plant–plant or

plant–pathogen/herbivore interactions. Observations

in the Californian Chapparal and corresponding ex-

perimental approaches have made clear that certain

evergreen shrubs such as Salvia leucophylla or Arte-

misia leucophylla retain a 1–2 m plant-free area in the

grassland around them by inhibiting the growth of

annual species such as Avena fatua, Bromus rigidus,

B. mollis, B. rubens, Festuca megalora,orErodium

circutarium. As reason for this phenomenon volatile

terpenes present in the epidermal glands of these

shrubs such as cineol, camphor, a- and b-pinene,

camphene, and thujone have been identified acting

as germination inhibitors. The terpenoids are emitted

during hot periods and condense in the soil surface

around the shrubs.

0023Another function is seen in defense and wound

sealing in pine tree species where a coordinated in-

duction of the biosynthesis of MTs and resin acids is

observed. MTs are thought to function as a solvent

and vehicle for resin acids whereby the MTs evapor-

ate from the wound region, and resin acids undergo

oxidative polymerization and crystallization in this

region, closing the wound and thus preventing access

to pathogens.

0024Furthermore, volatile MTs released in response to

herbivore activity or insect feeding are ‘designed’

as deterrents or antibiotics by plants, but they may

act as a guide for predators or host-seeking parasites,

as shown for parasitoid wasps, which find their egg

host, a herbivorous caterpillar: they inject their

eggs into the herbivore and thus propagate their life

cycle at the expense of their egg hosts. Plants in

extreme environments often have to tolerate extreme

temperatures. The direct correlation between

isoprene concentration and thermotolerance has led

to the speculation that these compounds increase

ESSENTIAL OILS/Properties and Uses 2181

thermotolerance by interacting with membranes of the

plants. Such an interaction is also assumed for physio-

logical and pharmacological functions in man (see

below). In horticulture, MTs already may have or re-

ceive some practical function, acting as flowering pro-

moters or as nematicides.

Uses and Effects

Use of EOs

0025 The use of EOs is mainly restricted to the perfume

and flavoring industries; above all, citrus oils are used

in perfumery and to flavor soft drinks or sweets.

Recently, aromatherapy has become increasingly im-

portant, with the treatment of functional disorders or

even diseases by using a pleasant odor, which is often

achieved by evaporating EOs. This kind of therapy,

however, is controversial, mainly because most stud-

ies are questionable, but the use of a pleasant smell

can help a person to relax and may help them to feel

better. In pharmacy, oils are mostly applied as com-

bined preparations like bath salts, embrocations, and

inhalants; another application is external antiseptics.

Examples of oils used for pharmaceutical use include

eucalyptus oil (and its main component, cineol), pep-

permint oil, and menthol. Peppermint oil is also often

used to alleviate headaches by rubbing the oil into the

temples, and its effect may be due to its cooling effect

on the skin. In folk medicine, there are various appli-

cations for essential oils and their components, as

shown in Table 1 (see also section on Pharmaco-

logical Properties).

0026 Some terpenoids like pinenes from turpentine, ger-

aniol, (þ)-citronellal, or eugenol are also used as a

starting material for the synthesis of other substances.

The applications described indicate that the most

commonly used essential oils are citrus oils like

sweet orange, lemon, or grapefruit oil, followed by

peppermint and eucalyptus oil.

Biochemical Effects

0027 EOs are mixtures of many compounds belonging to

several biochemical classes (see above). Thus, a dis-

tinction has to be made between the properties of

single isolated compounds and EOs generally, i.e.,

oil mixtures. To make things even more complicated,

although an isolated compound may exhibit toxic

properties if tested on its own, it may prove inactive

in its ‘natural environment’ as an oil mixture.

Uptake, Metabolism, and Elimination in Man

0028Essential oils and their components are readily

absorbed via the respiration tract in the case of inhal-

ation; resorption through skin and uptake in the

stomach after oral application are less effective but

still possible. After being absorbed, the terpenoids are

readily distributed throughout the body.

0029The terpenoids are metabolized in the liver, e.g., by

hydroxylation or glucuronidation, and then elimin-

ated through the kidneys. Small quantities reach the

bile or mother’s milk unchanged, so, during breast-

feeding, the baby may take up terpenoids. Some

terpenoids, such as cineole and menthol, are also

eliminated unchanged through the respiration tract

or skin. Thus, even if they are taken orally, these

terpenoids can exert their effects in the bronchial

tract, which explains their preferential use in diseases

of the respiratory tract, e.g., bronchitis.

0030There are very few studies on the pharmakokinetics

of essential oils or their components, so it is not clear

which concentrations they really can reach in blood

or plasma. Some studies have examined plasma con-

centrations of a-pinene, limonene, and/or cineol, and

have reported values of 5–20 mM, but mostly, such

studies have been carried out with too few partici-

pants. Since, as already mentioned, the components

of essential oils are very lipophilic, they can be en-

riched in adipose tissues or membranes and therefore

reach much higher concentrations in those environ-

ments. If they are distributed in biological mem-

branes, they can exhibit both stabilizing and

destabilizing effects.

Pharmacological Properties

0031EOs exhibit antiseptic, sedative, spasmolytic, and ir-

ritating properties. Some oils have local anesthetic

and antiinflammatory effects, and recently, anticarci-

nogenic and serum-cholesterol-lowering activities of

terpenoids have been discussed. Some terpenoids are

also capable of inhibiting HIV.

0032The antibiotic and antiseptic properties are against

bacteria, fungi, and yeast. The most antiseptic oils are

savory, cinnamon, thyme, clove, lavender, and euca-

lyptus. Thymol, a compound found in thyme oil, for

example, is 20 times more antiseptic than phenol. It

has been shown that the different enantiomers of

limonene and a-pinene, respectively, exhibit different

antibacterial and antifungal activities. This result

draws attention to the fact that receptors may play a

tbl0001 Table 1 Pharmacological properties of some essential oils

Action Essentialoil

Expectorant Anise, fennel, eucalyptus, pine needle

Spasmolytic Anise, fennel, lavender, caraway,

peppermint

Antiinflammatory Sage, camomile

2182 ESSENTIAL OILS/Properties and Uses

role in the antiseptic properties of essential oils. An-

other possible mechanism of antimicrobial activities

involves membrane disruption by lipophilic com-

pounds, and it has been shown that reducing the

lipophilicity by the introduction of a hydroxyl

group reduces the antimicrobial activity of certain

diterpenoids.

0033 In vitro, many oils like camomile, clove, mint, and

thyme exhibit spasmolytic activity. Some essential-oil

drugs like mint or European vervain have a positive

effect on gastrointestinal spasms; they are also

capable of stimulating gastric secretion and are there-

fore known as digestives. The oils of fennel, dill, and

caraway have positive effects on stomach upset and

flatulence.

0034 EOs coming into contact with the skin and mucous

membranes may lead to irritations such as reddening

and a sensation of hot or cold. These irritating prop-

erties are probably due to an increase in capillary

blood flow or to an effect of terpenoids on the cal-

cium channels, as shown by the cooling effect of

peppermint oil. These effects are also the basis for

the use of EOs in ointments, embrocations, and gels.

0035 Eucalyptus oil is thought to increase the motility of

the ciliated epithelia in the bronchia, thus explaining

the expectorant effect of this oil.

0036 Some terpenoids, such as limonene, geraniol,

menthol, and carvone, have been shown to possess

anticancer activity, increasing tumor latency and de-

creasing tumor growth. These effects may result from

the ability of the terpenoids to inhibit the activity of

HMG-CoA-reductase. This is probably also the

reason for the serum-cholesterol-lowering capability

of some terpenoids.

0037 The anti-HIV activity can be attributed to different

mechanisms; the terpenoid carnosolic acid, for

example, has been shown to inhibit HIV-1 protease,

whereas the terpenoid nigranoic acid inhibits the

reverse transcriptase.

Toxicity

0038 The first point to mention in this section is that one

should not confuse essential oil plants and essential

oils: even if the plant is harmless, the essential oil

derived from it may be toxic. Fortunately, acute

toxicity by oral intake is generally very low with an

ld50 of 2–5gkg

1

or above.

0039 There are more toxic oils such as boldo (LD50

¼ 0.13 g kg

1

), chenopodium (LD50 ¼ 0.25 g kg

1

thuja (LD50 ¼ 0.8 g kg

1

), pennyroyal (LD50 ¼

0.4 g kg

1

), and mustard oil (LD50 ¼ 0.34 g kg

1

)

and their components, such as thujone (LD50 ¼

0.2 g kg

1

), carvone (LD50 ¼ 0.16 g kg

1

), carvacrole

(LD50 ¼ 0.8 g kg

1

) and pulegone (LD50 ¼

0.47 g kg

1

). Thujone is a strong neurotoxin and is

found in thuja and sage. Pulegone can be oxidized

to menthofuran in the liver and can cause liver

damage.

0040If taken up erroneously in high quantities, intoxi-

cation and even death may result. This is especially

the case for camphor (LD50 ¼ approx. 1.5 mg kg

1

which causes epileptic convulsions, and also for EOs

from plants such as clove, wintergreen, and parsley.

0041There is very little knowledge about chronic tox-

icity, but it seems that the use of essential oils as food

flavorings is essentially safe. Especially in aromather-

apy, the interactions of products – which may be

harmless if used alone – may lead to unexpected

side-effects.

0042There have been several observations concerning

skin toxicity (irritations or sensitation after topical

applications, e.g., cosmetics or purfumes), including

phototoxicity (e.g., photodynamic compounds in the

Apiaceae ¼ Umbelliferae family; see below). Allergic

reactions caused by EO application are mainly due to

the formation of isomers, cyclic compounds, or

peroxides during storage under inadequate condi-

tions such as high temperature, light, and access to

air and oxygen (see below).

0043Carcinogenic effects have been observed in rodent

models, in which EOs from sweet flag have been

found to cause intestinal tumors, and hepatic tumors

induced by sassafras, sweet basil, or terragon. Car-

cinogenic activity is probably due to various allyl-

phenols and alkenylbenzenes (arene structures), and

because of the different metabolic pathways, the tox-

icity has been calculated to be 13 million times higher

in rodents than in humans.

Antioxidant Effects

0044The pathogenesis and symptoms of inflammatory pro-

cesses are accompanied and/or initiated by the

production of ROS. These include, for example, the

OH radical, superoxide, hydrogen peroxide, hypo-

chlorite, and peroxynitrite, and are generated,

amongst others, by several types of inflammatory

cells, e.g., neutrophils and alveolar macrophages, to

defend the organism against invading microorgan-

isms. Continuous production of these ROS accom-

panied by a weak antioxidant defense in the

organism may lead to tissue and finally organ damage.

Recently, many researchers have regarded antioxida-

tive properties as an important fact in the mode of

action of several drugs, food ingredients, or food

additives. (See Antioxidants: Natural Antioxidants;

Synthetic Antioxidants; Synthetic Antioxidants,

Characterization and Analysis; Role of Antioxidant

Nutrients in Defense Systems.)

ESSENTIAL OILS/Properties and Uses 2183

0045 EOs have been shown to have significant antioxi-

dant properties in different biochemical model reac-

tions simulating different pathological situations. It

has been shown that several essential oils show a high

antioxidant capacity regarding lipid peroxidation,

which simulates the situation in biological mem-

branes during oxidative stress. Since lipid peroxida-

tion can cause cell death, EOs may be able to protect

these cells. EOs also have a positive effect on reac-

tions of stimulated neutrophilic granulocytes, which

appear in great numbers during inflammatory pro-

cesses. Therefore, the antioxidative capacity of

essential oils may be an explanation for their antiin-

flammatory properties, acting by several different

mechanisms on different organizational platforms.

0046 Another pathological situation related to oxidative

stress is the oxidation of low-density lipoprotein

(LDL), which is regarded as an important factor

during atherogenesis. Recently, it has been shown

that after incubation of human plasma with compon-

ents of essential oils, these can be enriched in LDL

and protect it from copper-induced oxidation.

EOs and Food

0047 As mentioned previously, citrus oils are used in per-

fumery and to flavor soft drinks or sweets. The use of

spices to flavor foods is an indirect use of essential

oils, since often, the characteristic taste of herbs is

based on their content of EOs oils. EOs are also

antibacterial against some species involved in meat

spoilage, so the use of spices in food may not only

improve the taste but also prevent spoilage. Essential

oils of sweet basil, bay, cinnamon, clove, and thyme,

for example, show antimicrobial activity against

a wide range of foodborne Gram-positive and -nega-

tive bacteria.

0048 Lipids containing unsaturated fatty acids are im-

portant food components, but they are very suscep-

tible to autooxidation, which leads to undesirable

products. To retard this process, different antioxi-

dants like butylated hydroxytoluene and butylated

hydroxyanisole are used. Owing to safety consider-

ations (some of these synthetic antioxidants may be

carcinogenic), natural antioxidants are under investi-

gation as to whether they can replace synthetic anti-

oxidants. Since essential oils have been shown to have

a high antioxidant capacity (see section on Antioxi-

dant Effects), particularly regarding lipid peroxida-

tion, these substances may be good alternatives to

synthetic antioxidants.

Stability of EOs

0049 During storage of EOs photoisomerization, photo-

cyclization, generation of peroxides and free acids,

thermoisomerization, transition-metal catalyzed

decay, and other reactions may initiate the decompos-

ition of essential oils, which leads to changes in color

and consistency. Since this alters the properties of the

oil – the generation of peroxides in tea tree oil, for

example, is responsible for the irritating activities of

this oil – it is important to store essential oils securely

under a nitrogen atmosphere, away from light and

heat, and perhaps to add antioxidants.

0050For the determination of peroxides and thus the

‘age’ of an essential oil, conventional methods such

as iodine titration and peroxide determination via

photometrical procedures have been described,

but the best and most convenient method is based

on chemiluminescence by following the light emiss-

ion of the peroxides formed in the presence of

sulfite. This method is much faster and produces

much less toxic waste than the traditional iodine

titration.

See also: Antioxidants: Natural Antioxidants; Synthetic

Antioxidants; Synthetic Antioxidants, Characterization

and Analysis; Role of Antioxidant Nutrients in Defense

Systems; Essential Oils: Isolation and Production

Further Reading

Baratta MT, Dorman HJD and Deans SG (1998) Chemical

composition, antimicrobial and antioxidative activity of

laure, sage, rosemary, oregano and coriander essential

oils. Journal of Essential Oil Research 10: 618–627.

Baratta MT, Dorman HJ, Deans SG et al. (1998) Antimi-

crobial and antioxidant properties of some commercial

essential oils. Flavour and Fragrance Journal 13:

235–244.

Bruneton J (1999) Pharmacognosy, Phytochemistry, Medi-

cinal Plants, 2nd edn. Paris: Lavoisier.

Cowan MM (1999) Plant products as antimicrobial agents.

Clinical Microbiology Reviews 12: 564–582.

Kreuzwieser J, Schnitzler J-P and Steinbrecher R (1999)

Biosynthesis of organic compounds emitted by plants.

Plant Biology 1: 149–159.

Leung AY and Foster S (1996) Encyclopedia of Common

Natural Ingredients, 2nd edn. New York: John Wiley.

Lichtenthaler HK (1999) The 1-deoxy-d-xylulose-5-phos-

phate pathway of isoprenoid biosynthesis in plants.

Annual Review of Plant Physiology and Plant Molecular

Biology 50: 47–65.

Lis-Balchin M (1997) Essential oils and ‘aromatherapy’:

their modern role in healing. Journal of the Royal

Society of Health 117(5): 324–329.

Loza-Tavera H (1999) Monoterpenes in essential oils: Bio-

synthesis and properties. Advances in Experimental

Medicine and Biology 464: 49–62.

Pare

PW and Tumlinson JH (1999) Plant volatiles as a

defense against insect herbivores. Plant Physiology

121: 325–331.

2184 ESSENTIAL OILS/Properties and Uses

Isolation and Production

B Sankarikutty and C S Narayanan, Council

of Scientific and Industrial Research, Trivandrum,

India

This article is reproduced from Encyclopaedia of Food Science,

Background

0001 Essential oils can be given a simple definition as the

predominantly volatile and odorous fraction isol-

ated by some physical process from vegetable ma-

terials. The term ‘essential oil’ was coined by

Paracelsus and other alchemists to represent the

quintessence or the total odor and flavor of each

plant species. In plants, essential oils impart their

distinctive and often diagnostic odor. Essential oil

from a selected single plant species is made up of

organic compounds whose nature and relative pro-

portions are dependent on a number of agricultural

factors, viz. environment, climate, soil conditions,

time of harvesting and postharvest handling prior to

isolation. (See Flavor (Flavour) Compounds: Struc-

tures and Characteristics.)

0002 Essential oils isolated and identified from the vast

number of plant species amount to over 3000, and of

these, several hundreds have been produced commer-

cially. The price of any particular commercial essen-

tial oil is dependent on the percentage oil yield from

the plant species, its availability (production rate),

and finally, the most important factor, its demand

(application).

0003 Essential oils are isolated from various plant com-

ponents such as leaves, fruit, bark, root, wood, heart-

wood, gum, balsam, berries, seeds, flowers, twigs,

and buds. The plant components are suitably pro-

cessed to yield essential oils, which are mostly devoid

of cellulose, glycerides, starches, sugars, tannins,

salts, and minerals. The yield of essential oils

from plants varies widely, and the broad range is

0.05–18.0%. Information on isolation, production,

and markets of some selected commercial essential

oils are presented in Table 1. The essential oil is

present in various plant components in oil sacs and

is isolated by comminution and the action of heat,

water, and solvents. Distillation, selective solvent

extraction, and mechanical expression are the three

general basic methods employed for essential oil isol-

ation with improvements or modifications incor-

porated for each as available. Thus, essential oils

include concretes (flower ‘concretes’–hexane extrac-

tives), absolutes, resinoids, distillates, etc. (See Herbs:

Herbs and Their Uses.)

Distillation Techniques

0004The most popular physical method for essential oil

isolation is distillation. Prior to distillation, plant ma-

terials are in most cases dried and are then suitably

ground so that the oil sacs are broken and a maximum

of surface area is exposed for efficient oil release.

Various distillation techniques widely adopted for

essential oil isolation are briefly discussed below.

Direct-heating Distillation (Hydrodistillation)

0005Suitably ground plant materials are placed in a boiler

with water completely covering them. As heat is

slowly applied, steam alone will be initially formed,

and the distillate will be clear. With continued

heating, the essential oil starts to distil over with the

steam, and the distillate becomes milky white. Distil-

lation is continued until the distillate becomes clear,

with no more oil distilling over from the material.

Even though the essential oils have relatively high

boiling points, codistillation like this brings about a

satisfactory recovery of the oil because, in accordance

with Dalton’s law, a mixture boils when the sum of

the vapor pressures of the individual components

equals the atmospheric pressure. However, this

method is somewhat slow and requires close manual

attention and separation of the oil and aqueous

phases of the condensate.

Steam Distillation

0006This method is widely used and is faster than direct

distillation. Steam under pressure is passed through

the prepared plant material, and the volatile oils are

condensed along with the water. Steam can also be

bled directly into the boiler. To avoid thermal decom-

position of lower-boiling-point constituents, steam

pressure is increased only gradually. The distillation

temperature with water or steam at atmospheric

pressure is usually slightly under 100



C and can be

reduced further by drawing a partial vacuum on the

still.

0007-Hydrodiffusion In this method, the steam enters

the still at the top and passes down through the

charge. Oil and water vapor condense on coils fixed

below the bottom grid within the still. The oil and

water are separated in the usual way. The plant design

results in considerable energy savings and minimum

degradation of the volatile oil. This method is found

to be very advantageous in the distillation of seed oils.

Vacuum Distillation

0008This method is faster than steam distillation. Vacuum

distillation is generally adopted to rectify an oil and

very rarely to distil an oil directly from the plant

material.

ESSENTIAL OILS/Isolation and Production 2185

tbl0001 Table 1 Isolation, production, and markets of a selection of commercial essential oils

Essential Botanical source Plant part used;

method of oil isolation

Principal producing

countries/areas

Approximate volume

of production (tonnes

per year)

Major consuming

markets

Amyris Amyris balsamifera

L. First

Wood; steam

distillation

Haiti 70 France, Germany,

USA

Anise Pimpinella anisum L. Dried fruit; steam

distillation

Spain, Russia,

Poland

40–50 France

Anise, Star Illicium verum Hook f. Dried star-shaped

fruit; steam

distillation

China, Vietnam 60–70 France

Basil (Re

union) Ocimum basilicum L. Herbaceous tops;

steam distillation

union, Comoros,

Madagascar,

South Africa

10–12

Basil (sweet) Ocimum basilicum L. Herbaceous tops;

steam distillation

Egypt *2 USA, Europe,

Switzerland,

Japan

Bay, Turkish

(Laurel)

Laurus nobilis L. Leaves; steam

distillation

Turkey 1–2 Germany,

Netherlands,

USA, UK

Bay, West Indies Pimenta racemosa

Mill

Leaves; steam

distillation

Dominica 40–45 USA, UK,

Switzerland

Bergamot Citrus aurantium L.

ssp. bergamia

Fruit peel (rind);

expression

Italy, Ivory Coast 175–225 USA, France,

Netherlands

Caraway Carum carvi L. Ripe seeds; steam

distillation

Western Europe 10–30 North America

Cardamom Elettaria

cardamomum

Maton

Seeds; steam

distillation

India, Sri Lanka,

Guatemala

10–20 Western Europe,

Scandinavia, USA

Cassia Cinnamomum cassia

Blume

Leaves, twigs and

inner bark; steam

distillation

China 140–150 USA, Japan,

Western Europe

Cedarwood,

Southern Red,

East African

Juniperus virginiana Wood; steam

distillation

USA, China, Kenya

700–1 400 USA, Western

Europe, Japan

Juniperus procera Wood; steam

;

distillation

Cedarwood, Texas Juniperus mexicana

Schiede

Wood; steam

distillation

Cinnamon Cinnamomum

zeylanicum Nees

Dried inner bark;

steam distillation

Sri Lanka 100 Western Europe,

USA, India

Citronella, Ceylon Cymbopogon nardus

Rendle

Herb; steam

distillation

Sri Lanka *2 000 USA, Europe,

Japan, Mexico

Citronella, Jawa Cymbopogon

winterianus Jowitt

Herb; steam

distillation

Indonesia, China,

Guatemala, India

*2 000 USA, Europe,

Japan, Mexico

Clove Eugenia

caryophyllata

Thunb

Unopened bud;

steam distillation

Madagascar,

Indonesia,

Tanzania,

Sri Lanka

2 000 USA, Western

Europe, Japan

Coriander Coriandrum sativum Seeds; steam

distillation

Russia 20–40 USA

Davana Artemisia pallens

Wall.

Flowers; steam

distillation

India *2 Western Europe,

USA

Eucalyptus Eucalyptus globulus,

E. citriodora,

E. dives ‘A’ & ‘C’

Leaves; steam

distillation

China, Portugal,

Spain, South

Africa, Brazil,

Australia

*1 800 Western Europe,

USA, Japan

Fennel, bitter Foeniculum vulgare

var. vulgare

Seed; steam

distillation

Germany, Spain,

India, China

10–20 USA, Japan

Fennel, sweet Foeniculum vulgare

var. duler

Seed; steam

distillation

Spain 10–20 France

Garlic Allium sativum L. Bulbs; steam

distillation

Egypt 2–3 France, Spain

Continued

2186 ESSENTIAL OILS/Isolation and Production