ACS Distance Education UK
The boom in the industries of natural health and tourism has resulted in an increased demand for locally produced herbs.
Herbs can be supplied as either fresh produce for culinary, medical or essential oil extraction; or as dried matter for culinary and craft uses.
Some possibilities that could be readily grown:
Benzoin Styrax sp.
Bergamot Monarda didyma
Cedarwood Cedrus atlantica
Chamomile, Roman Anthemis nobilis
Chamomile, German Matricaria chamomilla
Chamomile, Moroccan Ormenis mixta
Clary Sage Salvia sclarea
Eucalyptus Eucalyptus globulus & other species
Geranium Pelargonium graveolens
Juniper Juniperus communis
Lemon Citrus Limon
Lavender Lavandula angustifolia
Sweet Marjoram Origanum majorana
Orange, Bitter Citrus Aurantium var amara
Orange, Sweet Citrus Aurantium var sinesis
Peppermint Mentha piperita
Rose Otto Rosa centifolia
Rosemary Rosmarinus officinalis
Sandalwood Santalum album
Tea Tree Melaleuca alternifolia
Sweet Thyme Thymus vulgaris
Ylang Ylang Cananga odorata
More On Selected Herbs
Tea Tree Oil
Traditional harvesting was carried out by hand. Cutters walked along existing established trees and carefully placed leaves and branches into carry bags so as not to burst the oil glands. These bags were then carried to the distillation plant. More modern plantations generally go for regimental planting arrangements which allow easy mechanical harvesting. These machines drive "over" the rows of plants and remove a predetermined section of branches that protrude outward. This is collected, then taken to the distillation plant.
Some farms have sown seeds on a broad acre format or in rows, and then when plants have reached a predetermine height, the whole plantation can be harvested. Next season the entire "crop" is replanted by seed. For commercial growers the situation of optimum harvest time is a little complicated. They must consider not only the total biomass and amount of oil present in the leaf at the time, and the amount of material that can be processed at any one time, but also the speed with which the tree will regrow.
The percentage of oil in fresh tea tree material varies significantly in natural cultivars from 0.4% to 1.18%, and selected cultivars have shown a range from 1.2% to as high as 1.6% of leaf and stem. Poorly designed distillation plants can cause high losses. Some operators have experienced losses up to 50% oil loss.
The essential oil is contained principally in microscopic glands in the calyx and to some degree in the lip of the corolla of the flower. Traditional hand harvesting, although high in labour, could ensure a high grade of uniformity of plant material which was to be distilled. Modern mechanical harvesting tends to produce a variable product for distillation (ie includes flowers, leaves and small stems). As soon as possible after harvesting the plant material should be taken to the distillation factory for immediate processing. Lavender oil has a raw odour immediately after harvesting but is best let to mature to develop optimum aroma. It can be used four months after distilling, but is best is left for a few years.
High yielding (but low Camphor content) Lavandula angustifolia cultivars (English Lavender) are most commonly grown for oil production.
METHODS OF EXTRACTING ESSENTIAL OILS
The main techniques used for extracting essential oils include:
PRODUCING ESSENTIAL OILS
A means of adding value to herbs is to extract the essential oil from the plant material. Much of the flavour and fragrance of herbs is due to their aromatic compounds and when these are extracted by distillation, the resulting product is a volatile oil-like material. Distillation techniques have been recorded to have been used as long ago as 3000 B.C. Other methods for extracting herbal essences are effleurage, maceration and pressing, however for most herbs distillation is the preferred process of extraction.
The yield of essential oil is usually very low in relation to the amount of plant material used. Depending on the quality of the herb and the distillation method used, yields of between 0.005% and 5% may be obtained. Different plants contain their aromatic compounds in different parts of the plant. Often the leaves and flowering tops are distilled, eg. Lavender and Sage. Fruits, seeds, wood, leaf and stem, roots and bark are all used to obtain essential oil, depending on the plant.
The two most common distillation techniques used to extract essential oils are:
The herb material is placed in a closed vessel (usually made of glass or metal) with water and heated rapidly. As the water boils and becomes steam the aromatic compounds in the herb are released and turned into a vapour. The two vapours mix as they rise up, then enter a condensation chamber where they are rapidly cooled. Cooling cause the vapours to become liquid again, with water reforming and essential oil forming on top of the water. The two parts are now separate, the water can be drained off and the essential oil collected.
Similar to water distillation, but the herb is placed on a mesh surface in the closed vessel and steam is pumped into the vessel. The steam 'boils' the herb very quickly and draws of the vaporised oil. Again, the two vapours are rapidly cooled, separated and the essential oil collected. Modern essential oil producers may perform this process under a partial vacuum. This reduces the boiling point of the steam allowing a 'cooler' distillation which does less harm to the more fragile fragrance components.
In water distillation, interaction of the essential oil with water can cause an acid reaction, altering the quality of the oil. Steam distillation creates less interaction with water, so it is preferable for this reason. The greater control of heat in steam distillation is also advantageous. Therefore, it is the method normally used commercially. For many people, however, the equipment used to produce steam may not be available or accessible.
The distillation is affected by temperature, the amount of pressure within the chamber, the amount of time needed and the oils present in the plant and the amount of plant material required. These many variables are part of the reason that there is such a wide range of prices for essential oils.
Material including flowers, leaves or fruit material is placed in steeped oil at room temperature, and for some plant materials, hot oil (between 60-70 degrees Celsius). The heated oil releases the essential oils. Plant material needs to be in full contact with the heated oil throughout the process and immersed no longer than five minutes at a time. Common oils used in this technique include either olive, apricot, almond or avocado.
Rose petals are extracted by cool temperature maceration. Commercially, the petals are removed at the end of 48 hours and the oil is recharged with fresh petals. It is advised no to blend petal colours in the same oil maceration.
This technique involves transferring the volatile oil to a thin layer of carrier grease (usually clarified lard or mutton fat for traditional techniques) at room temperature. Glass is smeared with a thin layer of the grease on which the petals are placed. Another greased sheet of glass is placed on top of these petals, this is repeated up to about five layers. It is important that not too many layers are used as crushing the petals is to be avoided - you want the petals to release the essential oils freely, not be pressured to release unwanted oils. Petals are replaced each day for up to a week for maximum effect. The result is "pomade", which is used for further extraction by alcohol.
It is commonly used for flowers which have unstable essential oils. The flowers must be able to retain their fragrance for between 12-24 hours after cutting in order for this to succeed. Understandable this is a very expensive technique with high labour costs. Flowers extracted this way include: daphne, jonquils, tuberoses, orange blossom, freesias and carnations. The final product called Floral Absolute, is a concentrated mixture of essential oils and very expensive.
In this technique, oil is mechanically released from ruptured oil glands. Plant material is either crushed or pressed to release fragrant/essential oils which are then collected (these days by centrifuges). All the citrus related oils are extracted by this method.