If
you have just started your aromatherapy journey, then most probably you are
only familiar with essential oils. However, did you know that there are other
aromatic plant extracts besides essential oils? I’m talking about Absolutes, CO2 Extracts and Hydrosols.
These plant extracts also have benefits akin to essential oils and might have
their own distinct characteristics not found on essential oils.
Let’s
get to know in details the differences between essential oil, CO2 extract,
absolute and hydrosol.
ESSENTIAL OIL
Essential
oils refer to highly concentrated volatile oils that are extracted from plant
materials (e.g. flowers, leaves, fruits, etc.) either by steam distillation
or mechanical expression. “Volatile” means that the essential oil will
easily evaporate at room temperature when exposed to air for a long time. Each
essential oil has its own unique chemical constituents which contribute to its
therapeutic properties. Compared to absolutes and CO2 extracts, essential oils
contain smaller and lighter molecules, that’s why they can be absorbed more
quickly into the body.
There
are two methods to extract essential oils: steam distillation and mechanical
expression.
Mechanical Expression
Mechanical
expression is often used to extract essential oils from the rinds (peels) of
citrus fruits. In this method, the citrus fruits are subjected to high
mechanical pressure (e.g. piercing the fruit peels) in order to extract the
essential oils. Because there is no heat involved during extraction, mechanical
expression is also often called as “cold pressed” method.
Essential
oils produced via mechanical expression have more intense scent than the
distilled versions. However, citrus-based essential oils extracted using this
method usually also contain furanocoumarins
– a non-volatile chemical compound responsible for causing phototoxicity in
citrus essential oils (e.g. lemon, bergamot, etc.).
Steam Distillation
Steam
distillation is the most common method of extracting essential oils from plant
materials. The practice of steam distillation dates back thousands of years in
order to make aromatic waters used in ancient perfumery. The process involves
using low pressure steam to extract essential oil from plants. Compared to
other plant extraction methods (e.g. solvent extraction), steam distillation has
lower production cost, easier process and doesn’t use any chemical solvents.
If
you are a fan of citrus essential oils, then it’s highly recommended that you
choose the steam distilled version over the cold pressed since it doesn’t
contain the phototoxic compound furanocoumarins.
HYDROSOL
There
are two products produced during the steam distillation process: essential oil
and hydrosol. Hydrosols, also known as hydrolats
or flower waters, contain aromatic
compounds from plant materials that are water soluble. Compared to essential
oils, hydrosols have subtle fragrance and more gentle to the skin. In fact,
unlike essential oils which can cause skin sensitization when applied pure on
the skin, hydrosols can be sprayed directly on the skin without the need for dilution.
In
cosmetic formulation, hydrosols are ideal to be used when formulating facial
toners since they’re already water soluble so you don’t need to use a
solubilizer, plus they’re very gentle to the facial skin. Hydrosols are also
perfect for linen spray because they can add a subtle hint of fragrance without
staining your linens (which can be a problem for some essential oils). Take
note also that since hydrosols are water-based, they basically have shorter
shelf life than essential oils.
ABSOLUTE
There
are some aromatic compounds that cannot be extracted via steam distillation
(e.g. plant materials that are heat-sensitive). In this case, a solvent
is used to extract the aromatic compounds from the plant materials. For
absolutes, the solvent used is called hexane
– a colorless liquid that is derived from petroleum. The process involves
soaking the botanicals in hexane to extract the fragrant molecules which are
later separated from the solvent to produce the absolutes.
Compared
to essential oils, absolutes have more intense fragrance, thanks to their high
concentration of heavier, non-volatile (non-evaporating) aromatic molecules
which also contribute to their thicker consistency. Due to their long lasting
scent, absolutes are also more preferred ingredients in making perfumes.
Despite
their popularity in perfumery, there are some people who don’t like to use
absolutes since there could be traces (very tiny amount) of hexane remain in
the plant extracts. Absolutes are also more expensive than essential oils.
CO2 EXTRACT
Another
type of aromatic plant extracts that are obtained by solvent extraction is the
CO2 extracts. Unlike absolutes that use hexane as solvent, CO2 extracts use carbon dioxide. You might ask, CO2 is a
gas, so how could it possibly be used as a solvent? With a specific pressure
and temperature, the carbon dioxide can be transformed into a “supercritical”
state – which is the fluid (liquid) state of CO2. The liquid carbon dioxide is
then used to extract the aromatic compounds from the plant materials. After the
extraction process, the liquid CO2 is brought back to its gaseous state, so
what remains is the CO2 extracts.
Carbon
dioxide extraction is the most expensive plant extraction method but it
produces the highest quality of aromatic extracts compared to steam
distillation. Unlike absolutes, there are no traces of CO2 remain after the
extraction process so CO2 extracts are completely safe and natural. CO2
extracts also have longer shelf life than essential oils and absolutes.
The
chemical constituents of CO2 extracts are also quite different from essential
oils as they also contain heavier aromatic molecules that cannot be extracted
by steam distillation. For example, ginger CO2 extract contains the molecules gingerol – a chemical compound found in
fresh ginger, which is not present in ginger essential oil. CO2 extracts are
said to have closer chemical composition of the original plant.
There
are two types of CO2 extracts: CO2
Select and CO2 Total.
CO2 Selects use lower pressure to extract volatile CO2
soluble aromatic compounds. Consistency wise, CO2 selects are quite similar to
essential oils. On the other hand, CO2
Totals are extracted using higher pressure and contain both volatile and
non-volatile CO2 soluble aromatic compounds. Moreover, CO2 totals have thicker
consistency since they also contain heavier molecules like waxes and resins.
References:
A
Beginner’s Guide to Essential Oils Part 1: Essential Oil Extraction. Retrieved
from https://tisserandinstitute.org/beginners-guide-essential-oils-part-1-essential-oil-extraction/
P.S.
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