Three Types of Hydrodistillation
Three are three types of hydrodistillation for isolating essential oils from plant materials:
1. Water distillation
2. Water and steam distillation
3. Direct steam distillation
1.Water Distillation
In this method, the material is completely immersed in water, which is boiled by applying
heat by direct fire, steam jacket, closed steam jacket, closed steam coil or open steam coil. The
main characteristic of this process is that there is direct contact between boiling water and plant
material.
When the still is heated by direct fire, adequate precautions are necessary to prevent the
charge from overheating. When a steam jacket or closed steam coil is used, there is less danger
of overheating; with open steam coils this danger is avoided. But with open steam, care must be
taken to prevent accumulation of condensed water within the still. Therefore, the still should be
well insulated. The plant material in the still must be agitated as the water boils, otherwise
agglomerations of dense material will settle on the bottom and become thermally degraded.
Certain plant materials like cinnamon bark, which are rich in mucilage, must be powdered so that
the charge can readily disperse in the water; as the temperature of the water increases, the
mucilage will be leached from the ground cinnamon. This greatly increases the viscosity of the
water-charge mixture, thereby allowing it to char. Consequently, before any field distillation is
done, a small-scale water distillation in glassware should be performed to observe whether any
changes take place during the distillation process. From this laboratory trial, the yield of oil from
a known weight of the plant material can be determined. The laboratory apparatus recommended
for trial distillations is the Clevenger system.
During water distillation, all parts of the plant charge must be kept in motion by boiling
water; this is possible when the distillation material is charged loosely and remains loose in the
boiling water. For this reason only, water distillation possesses one distinct advantage, i.e. that it
permits processing of finely powdered material or plant parts that, by contact with live steam,
would otherwise form lumps through which the steam cannot penetrate. Other practical
advantages of water distillation are that the stills are inexpensive, easy to construct and suitable
for field operation. These are still widely used with portable equipment in many countries.
The main disadvantage of water distillation is that complete extraction is not possible.
Besides, certain esters are partly hydrolyzed and sensitive substances like aldehydes tend to
polymerize. Water distillation requires a greater number of stills, more space and more fuel. It
demands considerable experience and familiarity with the method. The high-boiling and
somewhat water-soluble oil constituents cannot be completely vaporized or they require large
quantities of steam. Thus, the process becomes uneconomical. For these reasons, water
distillation is used only in cases in which the plant material by its very nature cannot be
processed by water and steam distillation or by direct steam distillation.
Traditional Method of Producing Attar Using Hydrodistillation
Floral attars are defined as the distillates obtained by hydrodistillation of flowers (such as
saffron, marigold, rose, jasmine, pandanus) in sandal wood oil or other base materials like
paraffin. Attar manufacturing takes place in remote places because the flowers must be processed
quickly after collection. The apparatus and equipment used to manufacture attar are light,
flexible, easy to repair, and have a fair degree of efficiency. Keeping in view these facts, the
traditional “deg and bhapka” process has been used for centuries and is used even now with the
following traditional equipment.
• Deg (still)
• Bhapka (receiver)
• Chonga (bamboo condenser)
Traditional bhatti (furnace)
• Gachchi (cooling water tank)
• Kuppi (leather bottle)
Disadvantages of Water Distillation
• Oil components like esters are sensitive to hydrolysis while others like acyclic monoterpene
hydrocarbons and aldehydes are susceptible to polymerization (since the pH of water is often
reduced during distillation, hydrolytic reactions are facilitated).
• Oxygenated components such as phenols have a tendency to dissolve in the still water, so their
complete removal by distillation is not possible.
• As water distillation tends to be a small operation (operated by one or two persons), it takes a
long time to accumulate much oil, so good quality oil is often mixed with bad quality oil.
• The distillation process is treated as an art by local distillers, who rarely try to optimize both oil
yield or quality.
• Water distillation is a slower process than either water and steam distillation or direct steam
distillation.
2.Water and Steam Distillation
In water and steam distillation, the steam can be generated either in a satellite boiler or
within the still, although separated from the plant material. Like water distillation, water and
steam distillation is widely used in rural areas. Moreover, it does not require a great deal more
capital expenditure than water distillation. Also, the equipment used is generally similar to that
used in water distillation, but the plant material is supported above the boiling water on a
perforated grid. In fact, it is common that persons performing water distillation eventually
progress to water and steam distillation.
It follows that once rural distillers have produced a few batches of oil by water
distillation, they realize that the quality of oil is not very good because of its still notes (subdued
aroma). As a result, some modifications are made. Using the same still, a perforated grid or plate
is fashioned so that the plant material is raised above the water. This reduces the capacity of the
still but affords a better quality of oil. If the amount of water is not sufficient to allow the
completion of distillation, a cohobation tube is attached and condensate water is added back to
the still manually, thereby ensuring that the water, which is being used as the steam source, will
never run out. It is also believed that this will, to some extent, control the loss of dissolved
oxygenated constituents in the condensate water because the re-used condensate water will allow
it to become saturated with dissolved constituents, after which more oil will dissolve in it.
Cohobation
Cohobation is a procedure that can only be used during water distillation or water and
steam distillation. It uses the practice of returning the distillate water to the still after the oil has
been separated from it so that it can be re-boiled. The principal behind it is to minimize the
losses of oxygenated components, particularly phenols which dissolve to some extent in the
distillate water. For most oils, this level of oil loss through solution in water is less than 0.2%,
whereas for phenol-rich oils the amount of oil dissolved in the distillate water is 0.2%-0.7%. As
this material is being constantly re-vaporized, condensed and re-vaporized again, any dissolved
oxygenated constituents will promote hydrolysis and degradation of themselves or other oil
constituents. Similarly, if an oxygenated component is constantly brought in contact with a direct
heat source or side of a still, which is considerably hotter than 100° C, then the chances of
degradation are enhanced.
As a result, the practice of cohobation is not recommended unless the temperature to
which oxygenated constituents in the distillate are exposed is no higher than 100° C.
In steam and water distillation, the plant material cannot be in direct contact with the fi re source
beneath the still; however, the walls of the still are good conductors of heat so that still notes can
also be obtained from the thermal degradation reactions of plant material that is touching the
sides of the still. As the steam in the steam and water distillation process is wet, a major
drawback of this type of distillation is that it will make the plant material quite wet. This slows
down distillation as the steam has to vaporize the water to allow it to condense further up the
still. One way to prevent the lower plant material resting on the grid from becoming waterlogged
is to use a baffle to prevent the water from boiling too vigorously and coming in direct contact
with the plant material.
Advantages of Water and Steam Distillation over Water Distillation
• Higher oil yield.
• Components of the volatile oil are less susceptible to hydrolysis and polymerization (the
control of wetness on the bottom of the still affects hydrolysis, whereas the thermal conductivity
of the still walls affects polymerization).
• If refluxing is controlled, then the loss of polar compounds is minimized.
• Oil quality produced by steam and water distillation is more reproducible.
• Steam and water distillation is faster than water distillation, so it is more energy efficient. Many
oils are currently produced by steam and water distillation, for example lemongrass is produced
in Bhutan with a rural steam and water distillation system.
Disadvantages of Water and Steam Distillation
• Due to the low pressure of rising steam, oils of high-boiling range require a greater quantity of
steam for vaporization -hence longer hours of distillation.
• The plant material becomes wet, which slows down distillation as the steam has to vaporize the
water to allow it to condense further up the still.
• To avoid that the lower plant material resting on the grid becomes waterlogged, a baffle is used
to prevent the water from boiling too vigorously and coming in direct contact with the plant
material.
3.Direct Steam Distillation
As the name suggests, direct steam distillation is the process of distilling plant material
with steam generated outside the still in a satellite steam generator generally referred to as a
boiler. As in water and steam distillation, the plant material is supported on a perforated grid
above the steam inlet. A real advantage of satellite steam generation is that the amount of steam
can be readily controlled. Because steam is generated in a satellite boiler, the plant material is
heated no higher than 100° C and, consequently, it should not undergo thermal degradation.
Steam distillation is the most widely accepted process for the production of essential oils on
large scale. Throughout the flavor and fragrance supply business, it is a standard practice.
An obvious drawback to steam distillation is the much higher capital expenditure needed
to build such a facility. In some situations, such as the large-scale production of low-cost oils
(e.g. rosemary, Chinese cedarwood, lemongrass, litsea cubeba, spike lavender, eucalyptus,
citronella, cornmint), the world market prices of the oils are barely high enough to justify their
production by steam distillation without amortizing the capital expenditure required to build the
facility over a period of 10 years or more.
Advantages of Direct Steam Distillation
• Amount of steam can be readily controlled.
• No thermal decomposition of oil constituents.
• Most widely accepted process for large-scale oil production, superior to the other two
processes.
Disadvantage of Direct Steam Distillation
• Much higher capital expenditure needed to establish this activity than for the other two
processes.
Essential Oil Extraction by Hydrolytic Maceration Distillation
Certain plant materials require maceration in warm water before they release their
essential oils, as their volatile components are glycosidically bound. For example, leaves of
wintergreen (Gaultheria procumbens) contain the precursor gaultherin and the enzyme
primeverosidase; when the leaves are macerated in warm water, the enzyme acts on the
gaultherin and liberates free methyl salicylate and primeverose. Other similar examples include
brown mustard (sinigrin), bitter almonds (amygdalin) and garlic (alliin).