introduction to essential oils

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"essential oils are the volatile materials derived by a physical process from odorous plant
material of a single botanical form and species with which it agrees in name and odor
not all oils are essential oils
the above is a good definition of essential oils but there is room for confusion.
correct usage -- "essential oils" should only really be applied to:
distilled oils, as the process of distillation by water and/or steam ensures that only volatile components are collected.
however, the term is also used correctly:
for oils expressed from the peels of citrus fruits,
and oils produced by dry or destructive distillation where tar like materials are obtained by dry heating in the absence of air or in a vacuum. birch tar oil (rectified by steam distillation), copaiba oil
incorrect usage -- it is common now in aromatherapy circles to incorrectly refer to other aromatic materials as essential oils such as the following extracts: (in the list below, the extracts and oils contain substantial amounts of non-volatile substances.)
balsams - natural exudations from plants e.g. benzoin, peru and tolu
resinoids - extracted from a gum or resin by solvents
concretes - hydrocarbon extractions of plant materials containing waxes and color in addition to odorous components
absolutes - alcohol extractions of the concrete
animal extracts - absolutes or alcoholic tinctures of animal excretions or glands and
fixed oils - non volatile oils such as pressed from seeds and fruits - "carrier oils"

expression is the physical process of squeezing the oil out of the plant material.
for essential oil purposes, this process is only used for citrus oils where the oil is held in small sacs on the surface of the outer peel. other oils produced by expression such as nut and vegetable oils are known as fixed oils as they are not volatile. these fixed oils such as, sweet almond, grape seed, peach kernel, olive and peanut oils, etc. are basically odourless and used as carrier oils, massage oils or even for food use.
the term essential oils is extended to citrus oils as their constituents are mainly volatile, so volatile that they are regarded as top note ingredients and for shipping purposes are regarded as flammable ("dangerous goods").

sponge pressing
original method
the original process for the production of citrus essential oils was to cut the fruits into 2 halves, the fruit being scooped out and the hollow peels soaked in warm water. the thick inner pith absorbs water so swelling it plus making it resistant to cracking. the peels were then pressed against a small sponge one by one turning them inside out as they are pressed. the thickened and swollen piths thus apply pressure on the oil sacs rupturing them. the oil leaks or squirts out of the ruptured cells and is absorbed onto the sponge. the sponge is then squeezed out into a collecting bucket.
this method is labour intensive and although not widely used it is apparently still used for the finest sicilian lemon oil. modern mechanical methods have been designed to reproduce sponge pressing but maybe the oil produced is not quite as good as the purist demands.
ecuelle a picquer method
the second method was to place the whole fruits into an inverted bell shaped container, lined with rows of spikes. as the fruits were turned in the container the spikes rasped the oil sacs in the outer peel of the fruits releasing the oil. the oil trickled down the inside walls of the container and collected in the bottom. periodically the bottom was opened to allow the oil and peel debris to flow out. there are larger versions of this process designed for more commercial production.
juice by product
a third method comes under citrus fruit juice production rather than essential oil production. this is where the whole fruit is processed to produce the fruit and the oil is an unwanted by product that is separated and collected for sale. in terms of quantity this is by far the most important source of cheap oils such as brazilian sweet orange oil.

absolutes and concretes
solvent extraction
solvent extraction provides a means of obtaining a higher volatile yield from the plant or, in some cases, a product that cannot be obtained by other extraction processes, (e.g. jasmine or carnation), with the potential current exception of supercritical co2 extraction which remains, in many cases, a very expensive process.
an extracting container ("still") is loaded with perforated trays of blossoms or plant material to be extracted. the blossoms are steeped in the solvent, (usually a hydrocarbon such as hexane), which is circulated through the material washing the oil from deep within the plant cells. the solvent dissolves all extractable matter from the plant, which includes non-aromatic waxes, pigments and volatile aromatic molecules. the solution containing both solvent and dissolvable waxes and color material is filtered and the filtrate subjected to low-pressure distillation using a vacuum pump to recover the solvent for further use. the remaining waxy mass is what is known as the "concrete" which in the case of jasmine may contain as much as 50% of the volatile oil components.
the concentrated concrete may be used in perfumery but its use is limited as the waxes present are insoluble in alcohol (ethanol) the normal solvent for finished perfumes and colognes. it is also generally too expensive for commercial use in soaps and cosmetics. the concrete may be stored as it is more stable in this form than as an absolute, which will be prepared as needed. to prepare the absolute, the waxy concrete is gently warmed and stirred with ethanol. during the heating and stirring process the concrete breaks up into minute globules. since the volatile oil components are soluble in alcohol and the waxes are not, an efficient separation of the two takes place. remaining traces of concrete are removed by chilling the solution to very low temperatures (around -30° f) precipitating remaining waxes. the solution is then cold filtered leaving a wax-free solution of the absolute, color and alcohol. the spent waxes from the concrete minus the absolute may find use in soap perfumery or a general-purpose wax for candles and cosmetics where the remaining soft odor may be useful.
carbon dioxide (co2) extraction
extraction with carbon dioxide is a relatively new process that has been hailed as a major breakthrough for the essential oil industry. co2-distilled oils are superior to steam distillates because of their fresh and typical flavor and aroma which are very close to the botanical raw material.
what is meant by hypercritical? -- this is a term commonly used in conjunction with co2 extraction. any substance can exist in three different states: gas, liquid and solid, depending on its temperature and pressure. in addition, certain substances can be found in the hypercritical state. they are neither liquid nor gas, but rather they are both. the hypercritical fluids disperse as readily as gases and also have solvent properties. carbon dioxide normally only occurs as solid dry ice or a gas, however when subjected to special conditions under high pressure, the gas turns into liquid. this liquid co2 can be used as a very inert, safe "liquid solvent."
carbon dioxide becomes hypercritical at 33°c. hypercritical carbon dioxide then becomes an excellent solvent of essential oils, in an extraction process similar to that used to extract absolutes. the advantage is that the whole operation takes place at a low temperature and, therefore, the essential oil is not affected by heat, as may occur in the distillation process.
the extraction process is almost instantaneous and because the solvent is virtually inert, there are no chemical reactions between the solvent and the aromatic substances.
with this method, more top notes, fewer terpenes, a higher proportion of esters, plus larger molecules, are obtained. the resulting oil contains very little color matter and is said to be more like the essential oil in the living plant. (the normal distillation process seems to form many terpenes and also breaks down some of the acetates (esters) in the plant material.)
no solvent residue remains, since at normal pressure and temperature, the co2 simply reverts to a gas and evaporates. being an odorless normal component of the atmosphere, it is non-polluting and is non-flammable.
co2 extracted oils may cost up to 10 times as much as the steam distilled oils or absolutes. if not for the price disadvantage these materials almost certainly by now would have become the preferred extracts. the price is largely governed by the expensive equipment needed to control the conditions required to keep the co2 as a hypercritical liquid.
alcohol extraction
some trees' resins such as benzoin, frankincense and myrrh may be extracted with alcohol directly to produce a resinoid. these resinoids can be so thick, effectively "rock-hard" solids that they require materials such as benzyl benzoate or diethyl phthalate to be used during the extraction process to "plasticize" them. when the alcohol has been removed under vacuum, the non-volatile plasticzers remain with the extract. note that very few commercial resinoids on the market will have no added plasticizer.
what is meant by "totals"; "select" or "selectives"? what is the difference:
selects - low pressure is used for this method and the resultant product will not have been affected by extreme temperature changes. selects are easier to work with than totals, but do not contain all the soluble components like totals do. selects contain mainly volatiles and essential oils and are superior to steam distillates.
totals - higher pressure is used for this method and the resultant product contains all of the soluble components like those found in the herb. totals are more complete products, although they're thicker and slightly harder with which to work. it's recommended that you warm the total by submerging the container in hot water and letting it liquefy or the total may be added to a small amount of carrier oil with a long shelf life (such as jojoba or fractionated coconut oil) and stirred until it's well blended.

are your oils genuine
you may hear answers from suppliers like - guaranteed 100% natural etc. he may be telling the truth. he may blind you with science and send you a gas chromatogram - as much use as a kiss 'o gram except to the chemist that produced it. the most basic tools that you need to reassure yourself that oils are genuine come from your own observation and logical reasoning.
first: the supplier
(note none of these may be negative factors but each answer builds a picture of reliability.)
are they the producer or merely a trader or broker of oils?
are they represented local to the production area?
what sort of checks on the oils do they carry out?
do they have a laboratory?
how much experience do they have?
what is their ethical stand?
do they supply mainly to perfumery companies, flavor companies or aromatherapists?
case example:
when enquiring of a seller [who supplies mainly for aromatherapy outlets] about his supply of lavender, his first question was - "how much do you want to pay?"
reading between the lines he could supply:
the genuine unadulterated lavender oil,
a lower grade of lavender oil,
semi-genuine, natural but from different species (blends of lavender, lavandin, spike lavender, rosemary, eucalytus, mentha citrata, clary sage etc.), or
largely synthetic (blends of linalool and linalyl acetate and traces of oils).
the choice was mine.
second: the price - is it consistent with a genuine oil?
what country does the material come from; is labour there cheap?
oils from europe will be generally more expensive than those from asia.
think about the area of land required to grow the crop of natural product.
citrus oils: the oil is visible in the skin of the fruit - many fruits to a tree; many trees to an orchard - probably not expensive unless a rare fruit.
grass: dense growth; big leaf area; easy to collect - leaf has strong smell so probably a lot of oil - oil probably cheap.
herb: use flowering tops of herbs - grows densely - doesn't have a lot of flowers so probably more expensive.
flowers: use petals which are very light and generally have a low oil content so probably expensive.
case example:
a supplier received an inquiry for a number of oils, one of which was neroli oil. he said he could supply it but warned that it was expensive. he would check the current price and quote, but to expect it to be in the ball park of $2,000/kg.
a few days later the customer placed an order for most of the oils. noting the neroli being absent from the list, the supplier inquired. the customer replied, "oh never mind, i found a supplier in india that sells genuine neroli oil for $100/kg."
third: the physical appearance - does it look consistent with descriptions that you have read from information sources?
most essential oils are light coloured but very few are completely colorless.
is it the right viscosity?
if it's an absolute, is it dark enough and viscous enough?
fourth: the smell - if you don't know the raw plant material check out the descriptions in books.
does it smell right? - smell it on the top, middle and bottom note.
does it last too long on the smelling strip? - synthetics may have musks added.
caveat emptor* - the buyer will often deliberately hide behind the supplier's assertion that it is natural. the supplier may be hiding behind his supplier's assertion that it is natural. it is usually easier to accept the guarantees, cheap prices and abdicate responsibility rather than have to think, so...
*let the buyer beware!

are your oils fresh
it's difficult to know when we buy essential oils or aroma chemicals whether they are fresh or past their best without specific experience with those materials. unfortunately oils don't usually come in sealed packages with "use by" dates. we can of course read up on them but that is not quite the same.
there are a few rules of thumb that can help you gauge an oil's freshness.
first check your source. are you buying from the manufacturer or a dealer. if a dealer does he appear to have a regular turnover? are essential oils his main business?
essential oils and absolutes should have representative smells of the original plant material. generally the more like the original leaves, herbs or flowers, the more likely it is to be fresh.
citrus oils - citrus oils when they are first pressed have a definite light color of the fruit they came from. i.e.. lemon oil is yellow, orange oil is orange, tangerine oil is tangerine, bergamot is green etc. the colors in the citrus oils are generally not very stable. if the oils have been exposed to light or kept in poor storage conditions their color fades. so most citrus oils after a year or so tend to become almost colorless. note distilled lime oil is colorless even fresh because the color does not volatilize with the oil during the process of distillation.
terpenes (that occur in all essential oils) tend to polymerize in the presence of oxygen and produce sticky resinous masses so look for any signs of gooey masses, they will be translucent brown and will tend to collect around caps or may present themselves as sticky precipitates.
if an oil is very freshly distilled it will have a gassy green "still" note on top. this is particularly characteristic on geranium oil which should have a sweet corn note (dimethyl sulphide). these notes generally dissipate after a month or so. on many oils these "still" notes can be unpleasant so don't be too quick to dismiss an oil that is guaranteed freshly distilled.
woody type oils with high concentrations of sesquiterpenes (or sesquiterpenoids) are more viscous than their mono terpenoid cousins and become increasingly viscous with age. this frequently improves the odor of the oil. such as in patchouli, vetiver, sandalwood etc.
some oils kept in steel drums (even lacquered ones have scratches and allow some contact with bare metal) will pick up iron (fe) from the steel material and this slowly darkens the oil over time. the most susceptible oils are the spicy oils clove and cinnamon which will eventually turn black but many oils and materials will be affected.
most oils will tend to soften in odor as they age as the functional groups (aldehydes, alcohols, esters etc.) on the constituent chemicals break down. strong aldehydes turn into their weaker smelling alcohols. if esters break down they may produce the acids and a sourness may develop.
of course these tips will have many exceptions but these generalities should help you to pick out most common changes in oils that could indicate they are past their best. they are a good starting point to add your own experience with specific oils.