Quality judgement of an essential oil should be based on the combined data obtained by the above analyses.
(1) Sensory Evaluations
Such an ability can be gained after years-long tedious but systematic olfactive training. To an experienced nose, the evaporation pattern of an essential oil smeared on a smelling strip, over a period of time, gives information about its source, age, main components and even its authenticity.
Smelling must be carried out at intervals immediately after dipping to 1, 2 and 6 hours, and after standing overnight over a period not less than 18 hours.
Comparison with an authentic sample of acceptable quality will help the assessor to make a correct judgement. As obvious, sensory evaluations are subjective and may vary from person to person. Therefore, such assessments are, in general, realized by a panel of experts and in all cases their assessments should be verified and documented by experimental proof.
(2) Physical Tests
The following physical tests are conducted:
a) Moisture content
b) Specific gravity
c) Optical rotation
d) Refractive index
e) Residue on evaporation
f) Freezing or congealing point
g) Solubility in dilute alcohol
a) Moisture content determination of an essential oil can be determined by Karl-Fischer titration, gas chromatographic, spectroscopic or electrometric methods. Drying of an essential oil is accomplished by the addition of or filtering through desiccating agents such as anhydrous sodium sulphate. A simple way to check the presence of moisture in an essential oil is carried out by mixing 0.5 ml essential oil with 1ml carbondisulphide. A clear solution indicates the absence of moisture.
b) The specific gravity of an oil is the weight of a given volume of the oil at a specified temperature compared with the weight of an equal volume of water at the same temperature, all weightings being taken in air. A pycnometer is used for this determination.
c) The optical rotation of an oil is the angle through which the plane of polarization is rotated when polarized light passes through it. Results are expressed as dextrorotatory (+) or levorotatory (-) according to whether the plane of polarization is rotated clockwise or anticlockwise, respectively, as determined by viewing towards the light source. This technique provides useful data on the optical purity of the oil.
d) The refractive index of an oil with reference to air is the ratio of the sine of the angle of incidence to the sine of the angle of refraction of a beam of light passing from air into the oil.
It varies with the wavelength of the light used in its measurement. Abbe type refractometers are widely used for the determination of the refractive index.
e) The residue on evaporation is the percentage by weight of the oil that remains after evaporation when determined in a heat-resistant glass evaporating basin.
f) The freezing or congealing point is the highest temperature observed during the solidification of a supercooled liquid.
g) Solubility in dilute alcohol: All essential oils are soluble in absolute alcohol and many are soluble in dilute alcohol. It is, therefore, possible to determine the number of volumes of dilute alcohol required for the complete solubility of one volume of oil. Sometimes, however, the solutions obtained are not clear, then, they are defined as “opalescent”.
(3) Chemical Tests
Chemical tests include determination of aldehydes, acid value, ester value (for calculation of esters and combined alcohols), ester value after acetylation (for calculation of free alcohols), carbonyl value, phenol content, etc.
Determination of the following constituents are specifically mentioned in some monographs for quality assessment of the below mentioned essential oils: anethole in anise and star anise oils; carvone in caraway, dill and spearmint oils; cineole in cardamom, rosemary and sage oils; linalool in coriander and basil oils.
Acid value is a numerical value equivalent to the number of milligrams of potassium hydroxide required to neutralize the free acids present in 1 g of the oil.
Carbonyl value is numerically equivalent to the number of milligrams of potassium hydroxide that is equivalent to the amount of hydroxylamine required to oximate the carbonyl compounds present in 1 g of the oil.
Ester value is the number of milligrams of potassium hydroxide required to neutralize the acids liberated by the hydrolysis of the esters present in 1 g of the oil.
Ester value after acetylation is numerically equivalent to the number of milligrams of potassium hydroxide required to neutralize the acids liberated by the hydrolysis of 1 g of the acetylated oil.
(4) Instrumental Techniques
These include chromatographic and spectroscopic techniques or their combinations (often described as hyphenated techniques) such as the following:
Column Chromatography (CC)
Thin Layer Chromatography (TLC)
Gas-Liquid Chromatography (GLC or GC)
High Pressure Liquid Chromatography (HPLC)
Medium Pressure Liquid Chromatography (MPLC)
Supercritical Fluid Chromatography (SFE)
Size Exclusion Chromatography (SEC)
Ultraviolet and Visible Spectrophotometry (UV/VIS)
Infrared Spectrophotometry (IR)
Mass Spectrometry (MS)
Nuclear Magnetic Resonance Spectroscopy (NMR)
- 13C-NMR Spectroscopy
- Site-Specific Natural Isotope Fractionation NMR (SNIF-NMR)
Gas Chromatography/Mass Spectrometry (GC/MS)
Liquid Chromatography/Mass Spectrometry (LC/MS)
Gas Chromatography/Fourier-Transform Infrared Spectrophotometry (GC/FT-IR)
Gas Chromatography/Fourier-Transform infrared Spectrophotometry/Mass Spectrometry (GC/FT-IR/MS)
Gas Chromatography/Atomic Emission Detector (GC/AED)
Gas Chromatography/Isotope Ratio Mass Spectrometry (GC/IRMS)
Multidimensional Gas Chromatography (MDGC)