Essential oils are aromatic materials of vegetable origin, which are used in perfumery and flavourings. They represent the "essential aroma" of the plant from which they are obtained. The majority of essential oils are produced by the process of steam distillation.
Essential oils occur in many different parts of plants, e.g. roots (vetiver), bark (cinnamon), heartwood (sandalwood), leaves (bay), herb (peppermint), seeds (nutmeg), flowers (cananga and jasmine).
The essential oil of a plant consists of many compounds which generally boil between 150° 300° C. If attempts are made to remove these compounds by dry distillation many will decompose and the oil will be ruined. However, the compounds are steam volatile and can be distilled out of the vegetal materials at around 100° C.
When plant materials are steam distilled chemical changes inevitably occur and the oil obtained will not have an identical aroma to that of the original plant material.
Preparation of material for distillation varies with the material to be distilled. Some material must be distilled immediately after harvesting, whereas others can be (and are best) stored for a day or two before distilling and finally there are materials which can be stored indefinitely before distillation. In general, flowers should be distilled immediately, whereas herbaceous material often benefits from wilting for one or two days before distillation. Woody materials may need to be ground and/or soaked before distillation.
The preparation of the raw material, the packing of the still and the rate/type of distillation can be determined for a particular essential oil crop from the literature or from experimental trials.
There are three basic types of essential oil distillation:
i. "Hydrodistillation" - in this method the charge (which is usually comminuted) is immersed totally in water which is boiled. The stills are of the simplest type (see Figure 1: Simple still hydrpdistillation ) and are used extensively by smallholder producers of essential oils. Often they are heated over an open fire. The disadvantages are that the heat is difficult to control and hence the rate of distillation is variable. Also the possibility exists for local overheating and "burning" of the charge which can lead to a poorer quality oil. Improved distillation control can be obtained by using steam from a separate boiler, which is passed into a jacket around the still or through a closed coil in the bottom of the still, to heat the contents of the still. A further disadvantage of this system is that it requires the heating of a large quantity of water adding to costs and time needed for each distillation. However, it is necessary for certain flower distillations e.g. rose and ylang. It is also necessary for the efficient distillation of certain woody materials e.g. sandalwood and cinnamon bark.
ii. Water/steam distillation this is an improved method, the still contains a grid which keeps the plant material above the water level (Fig. 2. Water/Steam still) The water is boiled below the charge and "wet" steam passes through the plant material. Consequently, if an open fire is used the plant material is protected from direct heat. In Fig. 2 the still is heated by a steam jacket. It is important in both water/steam and steam distillation that the still is packed evenly and not too tightly so that steam can extract from the complete charge efficiently. Over packing of the still can cause the steam to force "rat holes" through the charge and leave other parts of the charge unextracted.
iii. Steam Distillation - the most advanced type of distillation is by direct steam provided from a separate boiler. The still contains a grid plate under which an open steam pipe is fitted (see Fig. 3. Steam distillation unit).
The advantages of this type of "dry" steam distillation are that it is relatively rapid, therefore charging and emptying the still is much faster and energy consumption is lower. The rapid distillation is also less likely to damage those oils which contain reactive compounds, e.g. esters
As a general rule all stills should be insulated ("lagged") to reduce heat losses. Their design and losses . Their design and construction should also facilitate loading and unloading.
Condensers and Separators
The steam containing essential oil vapour leaves the still and passes into a condenser by way of a "gooseneck" (Fig. 4. Btillhead/Gooseneck ). Some sort of gauze or screen is often fitted at the mouth of the gooseneck to prevent plant material being blown over into the condenser.
In the condenser the vapours are cooled and condense. The simple form of condenser is shown in Fig. 5. (see Fig. 5. Coil condenser. The vapours pass through a coiled tube contained in a water bath and condensate is obtained at the bottom of the condenser tube. It is important that condensation is complete or oil may be lost by evaporation.
A more efficient type of condenser is the multi-tubular type shown in Fig. 6 (see Fig. 6. Multi-tubular condenser) in which a series of parallel tubes are mounted inside a cylindrical jacket through which cooling water is passed. This design provides a large surface area for cooling in relation to its volume.
The mixture of water and essential oil leaves the condenser and flows into a separator, called a florentine flask, in which they separate into two layers.
The essential oil will generally be lighter than water, the oil floats to the surface and the distillate water drains away.
Fig. 7. Lighter-than-water separator
It is important that the oil separators should be large enough in volume to minimize turbulence because significant amounts of oil can be lost with the distillate water if the oil is not allowed to separate completely. In addition, the temperature of the distillate may have an important bearing on the efficiency of separation of essential oil and water. The optimum temperature for obtaining the best separation can be found by trial and error. Sometimes when separation of oil is difficult, the distillate water is run back into the still (cohobation) and redistilled.
Fig. 8. Heavier-than-water separator
Condensers and separators should be constructed of materials which do not react with essential oils or water. Mild steel rusts and is not suitable. However, copper has been used successfully for many years tend tinned copper were cooper reacts with the oil). The optimum material for stills, condensers and separators is stainless steel which is resistant and durable but is relatively expensive.
Most essential oils can be stored for long periods under suitable conditions: they should be dry, not in contact with the air or direct sunlight and kept cool.
It is important that essential oils do not come into contact with materials with which they might react, e.g. rubber or plastic bungs.
Glass containers are often used for smaller amounts of oil but larger quantities are invariably stored in metal drums. Mild steel drums lined with epoxy resin are very popular for essential oils. If secondhand drums are to be used, it is important that they are thoroughly cleaned and dried before being filled with essential oil. Plastic containers, e.g. polythene, should not be used because the oil may be absorbed by the plastic and contamination may occur. To ensure that the oil is not wet it should be left to stand for some time before being filtered into its container. Oils generally show no cloudiness when thoroughly dry.
Freshly distilled oils often possess some "still odours" which are unpleasant. These generally disappear after several weeks storage. Some oils gradually improve in storage and acquire a fuller more rounded aroma, e.g. vetiver and patchouli.
Part III - Minor essential oil crops - II. Distillation of essential oils
"Minor oil crops" (Part I - Edible oils, Part II - Non-edible oils, Part III - Essential oils)
FAO AGRICULTURAL SERVICES BULLETIN No. 94
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, Rome, 1992
prepared by B.L. Axtell from research by R.M. Fairman
Intermediate Technology Development Group, Rugby, UK
M-17, ISBN 92-5-103128-2, (c) FAO 1992