Hydrodistillation
In order to isolate essential oils by hydrodistillation, the aromatic plant material is packed
in a still and a sufficient quantity of water is added and brought to a boil; alternatively, live steam
is injected into the plant charge. Due to the influence of hot water and steam, the essential oil is
freed from the oil glands in the plant tissue. The vapor mixture of water and oil is condensed by
indirect cooling with water. From the condenser, distillate flows into a separator, where oil
separates automatically from the distillate water.
Mechanism of Distillation
Hydrodistillation of plant material involves the following main physicochemical processes:
i) Hydrodiffusion
ii) Hydrolysis
iii) Decomposition by heat
Hydrodiffusion
Diffusion of essential oils and hot water through plant membranes is known as
hydrodiffusion. In steam distillation, the steam does not actually penetrate the dry cell
membranes. Therefore, dry plant material can be exhausted with dry steam only when all the
volatile oil has been freed from the oil-bearing cells by first thorough comminution of the plant
material. But, when the plant material is soaked with water, exchange of vapors within the tissue
is based on their permeability while in swollen condition. Membranes of plant cells are almost
impermeable to volatile oils. Therefore, in the actual process, at the temperature of boiling water,
a part of volatile oil dissolves in the water present within the glands, and this oil-water solution
permeates, by osmosis, the swollen membranes and finally reaches the outer surface, where the
oil is vaporized by passing steam.
Another aspect of hydrodiffusion is that the speed of oil vaporization is not influenced by
the volatility of the oil components, but by their degree of solubility in water. Therefore, the
high-boiling but more water-soluble constituents of oil in plant tissue distill before the lowboiling
but less water-soluble constituents. Since hydrodiffusion rates are slow, distillation of
uncomminuted material takes longer time than comminuted material.
Hydrolysis
Hydrolysis in the present context is defined as a chemical reaction between water and
certain constituents of essential oils. Esters are constituents of essential oils and, in the presence
of water, especially at high temperatures, they tend to react with water to form acids and
alcohols. However, the reactions are not complete in either direction and the relationship
between the molal concentrations of various constituents at equilibrium is written as:
(alcohol) x (acid)
K =
(ester) x (water)
where K is the equilibrium constant.
Therefore, if the amount of water is large, the amounts of alcohol and acid will also be
large, resulting in a decreased yield of essential oil. Furthermore, since this is a time-dependent
reaction, the extent to which hydrolysis proceeds depends on the time of contact between oil and
water. This is one of the disadvantages of water distillation.
Effect of Heat
Almost all constituents of essential oils are unstable at high temperature. To obtain the
best quality oil, distillation must be done at low temperatures. The temperature in steam
distillation is determined entirely by the operating pressure, whereas in water distillation and in
water and steam distillation the operating pressure is usually atmospheric. All the previously
described three effects, i.e. hydrodiffusion, hydrolysis and thermal decomposition, occur
simultaneously and affect one another. The rate of diffusion usually increases with temperatures
as does the solubility of essential oils in water. The same is true for the rate and extent of
hydrolysis. However, it is possible to obtain better yield and quality of oils by: (1) maintaining
the temperature as low as possible, (2) using as little water as possible, in the case of steam
distillation, and (3) thoroughly comminuting the plant material and packing it uniformly before
distillation.
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