VOLATILE OILS - Minia

VOLATILE OILS

Introduction:

• Volatile oils are the odorous principles found in various plant parts.

• They are called volatile oils or ethereal oils as they evaporate when

exposed to air at ordinary temperatures.

• They are also known as essential oils as they represent the “essences”

or the odorous constituents of plants.

• They are aromatic complex mixtures of different groups of organic

compounds called terpenes.

• Terpenes consist of either carbon and hydrogen only

(hydrocarbons) or carbon, hydrogen, and oxygen

(oxygenated compounds) such as alcohols, phenols, esters,

ketones, aldehydes, etc.

• The odour and taste of volatile oils are mainly attributed to their

oxygenated compounds, e.g. Cinnamon oil (cinnamic

aldehyde), Clove oil (eugenol), etc.

Volatile oils differ from fixed oils in the following:

1. Volatile oils are fragrant, while fixed oils are not.

2. Fixed oils are nutritive (high calories), while volatile oils are non-nutritive.

3. Volatile oils can be distilled, while fixed oils cannot.

4. Fixed oils consist of glyceryl esters of fatty acids, while volatile oils consist of

terpenes.

5. Fixed oils can leave permanent grease spots on a filter paper, while volatile oils

cannot.

6. Fixed oils can be saponified with alkalies, while volatile oils cannot.

7. On exposure to light and air, fixed oils become rancid, while volatile oils undergo

oxidation and resinification.

Distribution and localization of volatile oils:

• Volatile oils occur mainly in higher plants; there are about 17500 aromatic

species.

• Myrtaceae, Lauraceae, Rutaceae, Lamiaceae, Asteraceae, Apiaceae,

Poaceae, Zingiberaceae, and Piperaceae are common volatile oils-

producing families.

Volatile oils accumulate in all types of vegetable organs, such

as:

- Flowers (Chamomile, Rose, Jasmine)

- Leaves (Citronella, Eucalyptus)

- Barks (Cinnamon)

- Woods (Rosewood, Sandalwood)

- Roots (Vetiver)

- Rhizomes (Ginger)

- Fruits (Anise, Star anise, Fennel)

- Seeds (Nutmeg, Cardamom)

Volatile oils are localized in different secretory structured, such

as:

◦ Oil cell (e.g. Lauraceae or Zingiberaceae)

◦ Glandular trichomes (e.g. Lamiaceae)

◦ Secretory cavities (e.g. Myrtaceae or Rutaceae)

◦ Secretory canals (e.g. Apiaceae or Asteraceae)

Functions of volatile oils in plants:

• Allelopathic, particularly as germination inhibitors.

• Protection against insect, fungi, bacteria, and sometimes animals.

• Attraction of different pollinating insects.◦

General uses of volatile oils:

1. In manufacturing of perfumes, cosmetics, and cleaning products.

2. In preparation of disinfectant agents with antiseptic properties.

3. As antimicrobials for food preservation.

4. Flavoring agents for foods, candies, gum, confectionary, toothpastes, and

various pharmaceutical products.

5. As insect repellent and green pesticides.

6. Antibacterial, antifungal, and antiviral agents.

7. Carminative, antispasmodic, and bronchodilator (smooth muscle relaxant).

8. In aromatherapy and massage therapy (forms of complementary medicine).

Physical properties of volatile oils:

1. Almost with fragrant odours.

2. Easily volatilized at room temperature.

3. Liquids at room temperature except Anise oil (solid until 15 °C and melts at 16 °C)

and Rose oil (solid until 18 °C and melts at 19 °C).

4. Colorless liquids but some oils are colored, e.g. Chamomile oil (blue in color).

5. Their density is lower than water, with the exception of Clove and Cinnamon oils

are heavier than water.

6. They show high refractive indices and optically active.

7. Soluble in most organic solvents.

8. Sparingly soluble in water; however, they are soluble enough to impart a distinct

fragrance to water.

Stability of volatile oils during storage:

➢The exposure of volatile oils to heat, light, and air during storage will negatively

affect their chemical composition.

➢Under such conditions, various components of volatile oils can undergo

isomerization, oxidation, dehydrogenation, polymerization or thermal

rearrangements.

➢These chemical changes of volatile oils will result in deterioration of their taste

and fragrance (due to oxidation), viscosity (due to polymerization or

resinification), and therapeutic use (due to skin sensitization and dermatitis).

Stability of volatile oils during storage:

➢Some components of volatile oils can be converted

into toxic derivatives or isomers under these

conditions.

➢Accordingly, volatile oils should be stored in well-

closed amber glass, cobalt blue or aluminum

bottles. Bottles should also be filled to the neck to

expel all oxygen and stored at low temperatures

(e.g. refrigerators).

1- Distillation methods:

a) Water distillation (hydrodistillation):

1-The dried plant material is immersed in

water saturated with NaCl.

2- Water is heated until boiling, and then the

steam together with the V.O are condensed

and received as two layers, one for water and

the other for the V.O.

3- Cool and separate.

Important notes:

1- Clevenger apparatus is used in the water distillation method. It is available

both for oils lighter than water and oils heavier than water.

2- Water distillation is suitable only for thermally stable V.O. Under the drastic

conditions of distillation, unstable V.O. will undergo degradation.

3- This method is used for dried plant material, which keep their odour for

long time (oils are secreted from internal secretory structures). Thus,

preparation of the volatile oil is under drastic conditions (boiling).

4- NaCl is added to minimize the solubility of the distilled oils in water.

Disadvantages:

1- Some compounds may be subjected to hydrolysis, e.g. esters.

2- Complete exhaustion of the plant material is difficult as this

process is insufficient for high boiling point compounds.

Clevenger apparatus for Clevenger apparatus for

oils heavier than water oils lighter than water

b) Steam distillation:

1- It is used only for fresh plant material (especially for those containing

oils in external secretory structures).

2- The plant material is cut and transferred to a distilling chamber

(suspended in a wire basket or perforated trays).

3- Steam is forced through the plant material and the oil is obtained as

before.

4- Mostly used in the large scale production of V.O.

c) Water and steam distillation:

1- The plant material (fresh or dried) is immersed in water.

2- Water is heated and at the same time, the steam generated elsewhere is

piped into the container containing the macerated material.

3- The steam is condensed and the oily layer is separated as before.

4- This method provides some sort of heat control to prevent overheating and

decomposition of V.O.

5- Useful in case of hard plant material like seeds or barks.

d) Steam distillation by microwaves:

1- A new technology that include steam distillation by

microwaves under vacuum.

2- The plant is heated selectively by microwave

radiation in a chamber inside which the pressure is

reduced sequentially. The V.O is distilled away with the

water vapours from the plant itself (fresh plants do not

require the addition of water).

3- This method is fast, consumes little energy, and

yields oils of high quality.

General notes on distillation methods:

◦ Sample pretreatment is necessary (cutting, crushing, etc.).

◦ Using very high temperatures will not only affect the oil constituents (hydrolysis

or decomposition) but may also affect the other plant components.

◦ Complete distillation is essential to get all the V.O. constituents.

◦ Iron containers should not be used for distillation processes of oxygenated

compounds because the oil will darken in colour. Therefore, stainless steel or

copper lined with tin containers should be used.

General notes on distillation methods:

◦ Oil recovery: the collected distilled water and V.O are received in the oil

separator, where the oil can be easily separated. However, the water will keep

some of the obtained volatile oils either in solution or suspension, leading to

loss of some volatile oils; therefore, Cohobation is recommended, where the

water can be freed from the oil by saturation with NaCl or by extraction with an

organic solvent.

2- Expression methods:

- Before the discovery of distillation, all essential oils were extracted by pressing.

- These methods are used for preparation of V.O occurring in the rind of fruits, e.g. peels of orange, lemon, bergamot, etc. They are also the most suitable method for sensitive oils, such as that in the rind of Citrus fruits. They include:

a) Sponge method:

◦ 1- It is used on small scales and mainly for citrus fruits.

◦ 2- The rind is squeezed in front of a sponge and then transferred to a vessel in which it

is left for separation.

b) Scarification method:

◦ The plant material, e.g. entire fruits, is transferred to a funnel of tinned copper in which

the upper surface bears numerous strong pins to penetrate the outer layers of the

fruits. The oil is collected and left to separate from the juice or the moisture.

◦ c) Rasping process:

◦ 1- A very old technique in which the peel containing the oil glands is removed

by rasping with graters.

◦ 2- The raspings are strongly pressed in bags of horse-hair and the oil is then

collected.

◦ d) Machines:

◦ The most widely used expression method nowadays. Large amounts of plant

materials are pressed by machines and the oil is obtained after centrifugation.

3- Extraction methods:

◦Volatile oils which decompose by heat or present in trace amounts in their plant materials are usually prepared by this process. Extraction methods include:

a) Extraction with volatile organic solvents:

◦ 1- The plant material is extracted with certain organic solvents of low polarities, such as hexane, petroleum ether or benzene in a Soxhlet apparatus.

◦ 2- The resulting extract is evaporated under reduced pressure leaving a residue of V.O and other less polar compounds of the plant, e.g. pigments and waxes. The obtained residue is known as floral concrete.

◦ 3- The absolute oil (or the absolutes) is obtained by dissolving the floral

concretes in absolute alcohol thus precipitating the albuminous materials,

waxes, and pigments, followed by concentrating the alcoholic solution. The

absolute is the most pure and expensive form of the oil.

◦ Advantages of extraction with volatile solvents over distillation:

a- Much lower temperature is used (not more than 50 °C).

b- The prepared oil keeps its natural odour.

Soxhlet apparatus

b) Extraction with non-volatile solvents:

b-1) Maceration:

The plant materials, usually fragrant flowers, are allowed to remain in contact

with inodorous fixed oils or fats, which absorb the odorous principles. The total

oily portion is then separated by filtration.

b-2) Digestion:

This process is similar to maceration but moderate heat is applied to aid

extraction.

b-3) Enfleurage:

◦ - This process was once used for extraction of volatile oils

of delicate flowers, e.g. Jasmine.

◦ - Two thin layers of inodorous fat (beef tallow or lard) are

spread and flowers are then spread lightly between them.

◦ - After complete extraction, the flowers are removed

(defleurage) and replaced with new flowers.

◦ - With prolonged contact, the fat layers become saturated

with the volatile oil of the flowers.

◦ - These fatty layers, called pomade, are extracted with

alcohol (stirred with alcohol that dissolves the oil leaving

the fat).

◦ - The oil is separated by fractional distillation from alcohol.

The resulting oil, which is free from both fat and alcohol, is

called absolute or enfleurage.

b-4) Pneumatic method:

◦ The same principle of enfleurage method but includes passing a current of

warm air through the flowers. The air loaded with the oil is passed through a

spray of melted fat in which the oil is adsorbed and separated as before.

c) Extraction with a supercritical fluid (supercritical gas

extraction):

◦ At the supercritical pressure & temperature, gases have two important

characters: flow ability like a gas & extraction ability like a solvent. Thus,

supercritical gases/fluids can defuse through plant tissues and extract essential

oils.

◦ Carbon dioxide (CO2) is the most used supercritical fluid. Extraction conditions

for supercritical CO2 are above the critical temperature of 31 °C and critical

pressure of 74 bar.

4- Enzymatic hydrolysis:

◦ The powdered plant material is moistened with water at 40 °C for 24 h. The

V.O. is released by enzymatic hydrolysis followed by steam distillation.

Example 1: Preparation of methyl salicylate from winter green (Gaultheria

procumbens) oil

Example 2: Preparation of the volatile oil of black mustard.

Example 3: Preparation of the volatile oil of Geum urbanum (F: Rosaceace).

Example 4: Preparation of the volatile oil of bitter almond (Benzaldehyde).

Amygdalin + Emulsin Benzaldehyde + Hydrocyanic acid + Glucose

◦To remove the toxic HCN from the obtained hydrolysate, the oil

distillate is treated with calcium hydroxide solution, where calcium

cyanide is formed. On addition of ferrous sulphate, calcium

ferrocyanide and calcium sulphate are produced. These substances

are non-volatile, and thus will remain

Determination of the percentage (yield) of V.O. in plants (% v/w):

This can be done by using Clevenger apparatus according to the following

equation: % of V.O= Volume of oil (ml) x 100 / weight of material (g)

Terpenes:

◦ Volatile oils contain complex mixtures of several groups of organic compounds

called terpenes.

◦ Terpenes are natural products that are biosynthetically derived from isoprene.

Terpenoid compounds consist of a number of isoprene units.

◦ The isoprene unit is a branched-chain, five-carbon unit containing two

unsaturated bonds (2-methyl-1,3-butadiene).

◦ According to the total number of carbon atoms, terpenes are

classified into:

◦ Terpenes found in volatile oils are often monoterpenes and sesquiterpens.

They occur as acyclic or cyclic (monocyclic, bicyclic or tricyclic) hydrocarbons

and as oxygenated derivatives, such as alcohols, aldehydes, ketones,

phenols, oxides, peroxides, and esters.

◦ Structure determination of terpenes:

A) Analytical methods:

◦ 1- Elemental analysis: a technique used to determine the molecular formula

of the compound.

◦ 2- The degree of unsaturation: determined either by quantitative catalytic

hydrogenation or with organic per acids as follows:

B) Chemical methods:

1- Functional group determination:

◦ a- Alcohols are insoluble in KOH and Na2CO3.

◦ b- Phenols are soluble in KOH and insoluble in Na2CO3.

◦ c- Carboxylic acids are soluble in both KOH and Na2CO3.

2- Unsaturation:

by formation of additive crystalline products with Br2 or HBr.

3- Ozonolysis:

a valuable method for cleavage of ethylenic bonds.

C) Spectral techniques:

1- UV analysis: for aromatic and polyunsaturated compounds.

2- IR: for determination of functional groups.

3- NMR: for full structural assignment by 1H and 13C NMR methods…etc.

4- Mass spectrometry (MS):

◦ a- High Resolution MS (HRMS) for determination of the molecular formula.

◦ b- Electron Impact MS: for confirmation of the structural identity from the fragmentation pattern

of each component.

5- GC/MS: a combination of gas chromatography and MS spectral analysis for structure

assignment (preliminary investigation).

6- X-ray crystallography: to give the final structure with its absolute configuration (the most

precise method).

◦ Determination of the percentage of

various volatile oil constituents:

◦ The composition of V.O. is usually investigated

by GC and GC/MS analyses.

◦ The percentage of each component in the V.O.

can be calculated from the obtained GC

chromatogram according to the following

equation:

◦ AUP of the compound x 100/ Total AUP of

all oil components

Thank you