There are no standards regarding qualitative and quantitative composition of genuine essential oils. The composition of essential oil changes continuously during a plant’s life cycle, varies between different parts of the plant and even between different populations of identical species. A number of factors related to climate, habitat, agricultural, environment and technology may lead to these difference in compositions of essential oils.
In the article, we will discuss the following factors in addition to chemotype.
- Soil Quality
- Amount of Water
- Insect Stress and Microorganism
- Type of Biomass
- Agricultural Practices
Before we further discuss in brief some major factors that affect the quality of essential oils, it is important to understand the difference that may arise due to chemotypes. A chemotype is a chemically distinct entity of a plant. Chemotypes belong to the same genus and species but they have different chemical compositions. Chemotype are often defined by the most abundant chemical produced by that individual. Only a few plants have developed chemotypes. Some examples are:
- Thymus Vulgaris: There are several chemotypes of thymus vulgaris, notably ct. linalool, ct. thujanol, ct. thymol, ct. carvacrol.
- Cinnamomum Camphora: ct. cineol and ct. linalool.
- Rosmarinus Officinalis: ct. cineole and ct. verbenone.
- Ocimum Basilicum: ct. eugenol, ct. linalool, ct.estragole.
It is therefore important not only to know the botanical name of the plant from which oil has been extracted, but also its main constituents.
A region’s soil and climatic conditions can produce variations in the proportions of esters, alcohols, and other basic constituents of the oil and thus can affect the aroma, colour and overall quality of essential oils.
Climate plays a vital role in yield and quality of essential oils. Factors like temperature, sunshine, frequency and magnitude of precipitation, wind all can affect the production of essential oil in the plant. For example, as per studies, high temperature coupled with high humidity may lead to higher yield of essential oils. This is due to the fact that such conditions are favourable for growth of microorganism/insects/parasites and thus plant increase the production of essential oil to fight against them.
Condition of soil, specifically its pH, significantly affects the quality of essential oils. A high pH affects the solubility of certain elements like iron, zinc, copper and manganese in the soil and thus quality of essential oil.
Amount of Water
Both conditions, either excess of water or lack of water, can affects the quality and yield of essential oils. Lack of water leads to restriction of growth of the plant which in turn affects the yield and quality of oils. Similarly, excess of water, may lead to damage to the plant due to fungal growth.
Insect Stress and Microorganisms
The composition of essential oil present in a plants changes in case plants are under attack from insects or microorganisms. The change in essential oil compositions acts as a warning signal to other plants and defence against the impending attack.
Type of Biomass
Essential oil is found in many different parts of the plant like leaf, herb, fruit, wood, root and so on. Each part of a plant yields a essential oil of somewhat different composition. And because of this, essential oils are characterised by the part of the plant used for extraction.
Plants that yield several different oils, each extracted from different parts, include Clove (bud/leaf ), Cypress (twig/cone), Juniper (berry/twig), Laurel (leaf/berry), Coriander (seed/leaf ), Cinnamon (bark/leaf ), Angelica (root/seed), Pimenta (berry/ leaf ) and Lovage (root/herb).
For example, Cinnamon bark oil with its high level of cinnamaldehyde possesses a typical fine, powdery sweet-woody aroma. Cinnamon leaf oil, in contrast, includes a fresh-pungent, clove-like note with its extremely high levels of eugenol.
Some of the agricultural practices that affect the quality of essential oils are as follow:
- Harvest Time: The composition of essential oil changes continuously during a plant’s life cycle. Biomass should be picked at their peak. For example, knowledge of the precise time of the onset of flowering often has a great influence on the composition of the oil. In some cases, even the time of the day is also important. One of the best examples is rose oil. The petals should be collected in the morning between 6am and 9am. With rising day temperatures, the oil yield will diminish.
- Spacing of plantings should ensure adequate supply with essential trace elements and nutrients. For example, dill prefers wider row spacing than anise, coriander, or caraway.
- In cases where drying is required, it can be achieved simply by spreading the biomass on the ground where wind movement affects the drying process. Drying can also be carried out by the use of appropriate drying equipment. Drying, too, can affect the quality of the essential oil.
Distillation is one of the most important factor in assuring the quality of the essential oil. Using high-temperature steam at a high vapour pressure, as well as in some cases introducing solvent chemicals to speed up the process, results in an oil of considerably lower quality than if a more ideal, lower steam temperature and pressure were used. However, only a handful of producer adopt such techniques because the largest consumer of essential oils, flavouring industry, standardised the essential oils.
While buying essential oils, it is informative to know that there are a number of factors which may affect the composition and quality of essential oils. There are no exact standards of qualitative and quantitate compositions of genuine essential oils. While buying, please ask your seller about origin, species name and composition of the oil.