2.9.2 Extraction of essential oils

The extraction of essential oils from plant material can be achieved by a number of different methods and these are shown in the generalised flow diagram (Figure 4). There are five main methods of extraction:

• Expression
• Hydro- or water-distillation.
• Water and steam distillation
• Steam distillation
• Solvent extraction

For each method there may be many variations and refinements and the extraction may be conducted under reduced pressure (vacuum), ambient pressure or excess pressure. The choice of extraction method will depend on the nature of the material, the stability of the chemical components and the specification of the targeted product.

Flowers are generally solvent-extracted and not steam distilled with the exception of rose, ylang and orange blossom. In some applications an isolate or essential oil fraction is preferred to the total oil. Terpeneless oils (eg bay oil) and folded citrus oils (a concentrated oil produced by removing unwanted compounds from the whole oil) are well-known examples of fractionated essential oils with better quality than the whole oil. Sometimes, fractionation is used to reduce undesirable notes. This is the case for antheole-containing essential oils from anise, star anise and fennel. Other processing steps may be applied to reduce instability of certain oils (eg lemon oil which is known to be unstable in soft drinks due to the level of citral). The production of some special oils, oleoresins, absolutes and concretes requires much greater technologically-advanced facilities, labour skills and safety systems. These processing facilities are generally beyond the capability of the small individual producer. The high capital cost and highly-skilled labour skill requirements of a supercritical carbon dioxide extraction plant also limits the widespread application of this extraction process except for large flavour, fragrance or pharmaceutical manufacturers.

Expression is used exclusively for the extraction of citrus oil from the fruit peel, because the chemical components of the oil are easily damaged by heat. Citrus oil production is now a major by-product process of the juice industry.

Distillation is still the most economical method of extracting essential oil from spices and aromatic plant material. The main advantage of distillation is that it can generally be carried out with some very simple equipment, close to the location of plant production. Even in relatively remote locations large quantities of material can be processed in a relatively short time. Distillation is less labour intensive and has a lower labour skill requirement than solvent extraction. Adopting the simplest or cheapest extraction method however, may prove to be false economy because of low yield, poor or highly variable oil quality and low market value.

Water distillation is the simplest of the three distillation methods. The plant material is mixed directly with water in a still pot. A perforated grid may be inserted above the base of the still pot to prevent the plant material settling on the bottom and coming in direct contact with the heated base of the still and charring (Figure 7). Water distillation is probably the simplest and cheapest method of extracting essential oils, but the quality of the oil has the greatest potential to be modified due to the effects of direct heating and the water contact.

• The water present in the still must always be more than enough to last the duration of the distillation, other wise the plant material can over-heat and char.
• It is very easy for still ‘off-notes’ to be generated, since some components of the oil are more susceptible to chemical change and oxygenated componets tend to dissolve in the still water so their complete extraction is not possible.
• The plant material must be kept agitated as the water boils otherwise it may settle in the bottom of the still and become damaged by the heating. Chopping or grinding the material into fine particles may help to keep the material dispersed in the water.
• Some plant materials like cinnamon bark contain high levels of mucilages and as these are leached out the viscosity of the water increases and there is a high risk of charring.
• The stills tend to be small and therefore it will take a long time to accumulate much oil and each batch may be highly variable containing better quality oil mixed with poor quality.
• Water distillation is a slower extraction process than the other two distillation types and therefore less energy efficient.
• The only advantage of water distillation is the cost of the equipment tends to be extremely low and the designs of the stills, condensers and oil separator are simple so they can operate and be maintained in very remote locations.

Figure 7. Diagramatic representation of water distillation unit where the plant material is suspended in the water.

The water distilled oils are commonly darker in colour and have stronger still ‘off-note’ odours than oils produced by the other methods, and therefore tend to be of the lowest value. The disadvantages of the water distillation method would generally outweigh the advantages except for local market use.

In steam-and-water distillation the basic still design is very similar to that of water distillation (Figure 8). The plant material is packed into the still pot sitting on a grill or perforated plate above the boiling water. The capacity of the still pot volume is reduced but it may be possible to achieve a high packing density because the plant material is not suspended in the water. The advantages of steam and water distillation over water distillation are as follows:

• Higher oil yield.
• Oil component less susceptible to change due to wetness and thermal conductivity of the still from the heat source.
• The effect of refluxing is minimised.
• Oil quality more reproducible.
• Faster process so more energy efficient.

Figure 8. Diagrammatic representation of a steam and water distillation unit with a baffle to prevent direct water contact with the plant material on the perforated grid.

Steam distillation is the process of distilling plant material with the steam generated outside the still in a stand-alone boiler (Figure 9).

Figure 9. Diagrammatic representation of steam distillation unit

As in the steam-and-water distillation system the plant material is supported on a perforated grid above the steam inlet. The advantages and disadvantages of steam distillation are as follows:

• The amount of steam and the quality of the steam can be controlled.
• Lower risk of thermal degredation as temperature generally not above 100 °C.
• Most widely used process for the extraction of essential oils on a large scale.
• Throughout the flavour and fragrance supply industry it is the standard method of extraction.
• There is a much higher capital requirement and with low-priced oils the pay back period can be over 10 years.
• Requires higher level of technical skill and fabrication and repairs and maintenance require a higher level of skill.
• Many variations of the process exist, e.g. batch, hydrodiffusion, maceration distillation, mobile stills and continuous distillation process.

When designing a distillation system a number of issues must be considered:

Site

• Availability of adequate water
• Energy source: electricity, boiler fuel
• Easy transport access
• Skilled and unskilled labour
• Close proximity to plant material
• Access to fabricators and machine shop for repairs
• Environmental zoning, plant waste and waste water discharge

Distillation Charge (the amount of plant material that can be processed in a single cycle)

• Size of the still
• Plant species and oil content
• Daily volume and condition of plant material and frequency of supply
• Distance of the plant material production to still and how it will be transported
• Required pretreatment (chopping, crushing, powdering, maceration)
• Time taken to charge and discharge the still
• Storage capacity of plant material prior to distilling incase of poor weather
• Disposal of waste plant material after distillation

Still

• Design determined by distillation method; seek professional advice
• Ideally constructed of stainless steel
• Size determined by capacity of boiler
• Distillation time affected by height of the charge, flow rate and pressure of steam
• Easy to charge and discharge

Boiler

• Should produce enough steam to adequately remove the oil from the plant material
• Low pressure (saturated steam) or pressurised (dry steam)
• Best to measure output of home made or commercial boiler (condensing steam for set time) to determine capacity
• Seek professional advice on design and access for repairs and maintenance

Condenser

• The role of the condenser is to change the oil and water vapour back to a liquid
• Two main types: coiled tube or multitube
• Multitube difficult to make, needs running water, but has good heat transfer, efficient water use and no pressure build up
• Coiled tube easy to make, just needs a tank of water and sparse use of running water, but has poor heat transfer, risk of high pressure build up during distillation and poor use of water

Oil Separator

• Design of separator depends on density of the oil (if <1, oils are lighter than water and float; if >1, oils heavier than water and sink). Only a few wood and root oils are heavier than water
• Large enough capacity to allow the oil particles to form droplets and readily separate from the water (recommended at least a 4 minute retention time in the separator before out flow)
• Controlling temperature can be used to improve separation
• Seek professional advice on design as poor separation affects all the effort of distillation to extract oil

Storage

• System to filter separated oil
• Storage in suitable containers that exclude light
• Method to remove dissolved water (filtered bed of anhydrous sodium sulfate or chilling)
• Removal of residual still notes and dissolved oxygen (bubbling stream of nitrogen or allow oil to breathe and topping drum to over flowing to remove all air)

Where possible the still vessel condenser and separator should be fabricated from stainless steel. In developing countries access to specialist fabricators, and equipment and skills for maintenance and repair, should be of primary consideration in the design of the distillation system. Seeking professional advice is also critical.

• Having an efficient separator in an important item of the distillation process. To put a lot of effort into distilling the oil is self-defeating if the separator recovery process is less than adequate. The majority of essential oils float on water i . e . their specific gravity is less than 1, but there are a few of the wood and root oils that are heavier than water. Separation of oils whose density is close to that of water or where the oil contains one major component whose density is greater than 1, while the other components have a density less than 1, is more difficult. The design and operation of the separator needs to be specific to the oil being extracted [4]

In addition to steam distillation, specific crops, particularly the high priced spices, are now also extracted by solvents and carbon dioxide as this provides standardized extracts of high quality, free from contaminants. The following examples demonstrate different management needs for specific spice crops.

There are two main methods of vanilla extraction, a percolation method and the oleoresin method. The percolation method consists of circulating a solvent solution of ethanol and water 35-50:65-50 (v/v), over and through the beans, under vacuum. This process may take 48-72 hours and at completion a vanillin solution about four-fold strength can be obtained. The oleoresin method consists of pulverising whole beans and then circulating ethanol over the beans under vacuum at about 45°C. The excess alcohol is removed by evaporation. This process takes about 8-9 days, but by using this process, approximately 10-fold strength vanillin may be obtained. Commercially natural vanillin is sold as a dilute ethanol extract. Post-extraction processing includes filtration or centrifuging to aid clarification and aging for a year follows this. The industrial process to recover the essential oil from pepper involves the flaking of the black pepper using roller mills and placing this material in a distillation basket. Dry steam is passed through the ground pepper and the oil present in the pepper is vaporized. This mixture is cooled in the condenser and the oil collected using an oil/water separator. It is possible to make essential oils to different flavour specifications by blending oils from different grades of pepper. Oleoresin offers considerable advantages over whole or ground spice in that they are uniform in composition and strength. Contaminants are absent and thus oleoresins can be added to any food after adjusting the flavour concentration. Oleoresin extraction represents the total flavour and aroma of pepper by the extraction of ground pepper using solvents like ethanol, acetone, ethyl dichloride or ethyl acetate. Clove bud oil is obtained by hydro- or steam distilling whole or broken buds and is now commonly extracted in the country of origin. Leaf oil is extracted on-site, again using hydro- or steam distillation. A typical charge of 1000kg of dry leaves yields <25kg of crude oil, over a 4-6 hour distillation. The distilling method and distillation time affect both the oil yield and characteristics: e.g. combined water and steam distilling produces more oil than steam alone. Using water and steam for four hours gave the highest oil yield of 3% while steam alone produced oils lower in eugenol. Nutmeg oleoresins, obtained by solvent extraction from the dried spice of nutmeg, are used in colourings and flavourings in the food industry and are a direct competitor of the dried spice. Nutmeg butter that contains between 25-40% of fixed oil is obtained by expression, usually by hot manual pressing. Nutmeg butter is a highly aromatic, orange-coloured fat. Poor quality nutmegs are used for nutmeg butter production. The seed of nutmeg contains 8%-15% of essential oil obtained by steam distillation. Nutmeg oil is a colourless, pale yellow or pale green liquid with an odour and taste of the spice. During distillation exposure to the vapours should be kept to a minimum due to the toxicity of the essential oil compounds.Solvent extracting dried and ground fruits of Capsicums, Chilli peppers and Paprika obtains oleoresins. Super critical fluid or gaseous extraction methods are becoming more common because of the problem of solvent residues in food. The fruits must be free of other plant material and have a moisture level less than 8% and fineness of grinding is critical to oleoresin yield and quality. Supercritical fluid extraction produces oleoresins whose pungency and organoleptic characteristics are close to the original fruit. Oleoresins reflect the parent material and vary in pungency and colour. Oleoresins have replaced powders in the food processing industry. Cinnamon bark produces two oils, a superior type derived from the inner bark and a lower quality from broken quills, chips and bark. Hydro distillation or steam distillation of chips, featherings and quillings produce cinnamon bark oil or Chinese cassia oil. Bark to be distilled for oil should be kept dry as dampness encourages mould or fermentation that directly affects oil composition. The leaves left after trimming the cut stems, as well as those obtained from pruning, provide the raw material for production of cinnamon leaf oil. The leaves are usually allowed to dry for a few days before distillation. Cinnamon and cassia oils, which have variation in quality because of geographical origins of the source material, are both normally rectified to provide oils of a more uniform composition. Allspice–Pimenta leaves may be fresh, or dried and stored for 2-3 months prior to steam distilling, with little effect on oil yield or phenol content. Yield from dried leaves is 0.5-3% and fresh leaves 0.3-1.25%. Modern stills taking a 1-2 t charge are used, with an approximate 4-hour distilling run. The oil is stored in stainless steel drums. Berry oil is obtained by crushing berries immediately prior to loading, and steam distilling for about 10 hours with coh o ab itation (repeated distillation). The total oil yield is 3.3-4.5%. The organoleptic properties of the berry oil vary with production area and chemotype variation exists in the species. The oil content of ginger rhizomes is dependent on cultivar and a volatile oil content up to 4.4% is quoted. Poor post-harvest handling or distilling can substantially reduce the important pungent compounds. Oleoresins and absolutes are also extracted from ginger rhizomes.