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Friday, 19 July 2013

Natural Perfume Materials: what the terms mean

There are quite a few methods by which the aromatic principles of natural materials are made available for use in perfumery and unfortunately some of the terms used are obscure and others are sometimes mis-used.

The purpose of this post is to set out the main terms used, together with definitions of widespread acceptance.  In putting these together I have relied on two main sources: first Arctander’s Pefume and FlavorMaterials of Natural Origin – written in the 1950s and early 60s but still regarded by most perfumers as the definitive work.

Second Brian Lawrence who is published regularly in Perfumer & Flavorist Magazine and elsewhere and is, probably, the definitive modern writer on the subject.

Before we get into the terms for natural materials, it is perhaps useful to examine what we mean by natural in this context and eliminate terms used to indicate synthetic materials.  Very few materials are suitable for use in perfumery exactly as they occur in nature: citrus oils and copaiba balsam are the main exceptions, requiring no processing beyond pressing the peel or releasing the balsam from the tree: in the great majority of cases processing is necessary and in some cases the odorous principles only form during processing (see my post on Bitter Almond Oil for an example of this).  A material is generally considered to be ‘natural’ when that processing is primarily physical, rather than chemical, in nature: these things are what this post is about.
Fragrance Oils are not usually natural

The term Fragrance Oil is widely used to indicate a blend of materials that may be both natural and synthetic, designed to replicate a natural odour.  These are often composed entirely from synthetic ingredients and almost never with entirely natural ones and as such do not fall within the definition of a natural material.  Occasionally they may be passed off as natural, sometimes innocently by traders who have themselves been deceived by a producer, but for the most part they are sold labelled as ‘fragrance oil’ and are usually cheaper than natural equivalents where these exist.

The main types of volatile isolates that are obtained commercially are essential oils, concretes, absolutes, pommades, resinoids, spice/herb oleoresins, extracts, infusions, and tinctures. The definitions of these, and some other important terms are set out here and for convenience I’ve started by listing first the three most common types – Essential Oil, Concrète and Absolute.  All other types follow in alphabetical order for ease of reference.

The most common natural materials

Bergamot in cultivation
(courtesy Wikimedia)
Essential Oil: The isolated aromatic portion of a plant that is borne in that plant within distinctive oil cells. In some exceptional cases the essential oil is formed during processing. Most essential oils are isolated by either hydro-distillation (water, steam or both) or cold pressing with some few being produced by dry (including destructive) distillation.  Water distillation implies direct contact between the plant material and the boiling water, steam distillation implies steam being produced separately and blown through the material – in the combined case the water is heated by injected steam.  Some essential oils are routinely ‘rectified’ after production – a process that may be entirely physical or may involve the introduction of synthetic aroma chemicals to standardise the odour.  Examples include Bergamot, which is routinely rectified to remove bergaptens (furocoumarins) – this is done by fractional distillation.  White Thyme is natural thyme oil that has been corrected by the addition of aroma chemicals to produce standardised oil chemistry – for most this would no longer be considered a natural product.

Concrète (often written as concrete with the same meaning):  an extract of fresh (cellular) plant material made using a hydrocarbon solvent, commonly hexane or petroleum ether. It is rich in hydrocarbon soluble material and devoid of water-soluble components. It is generally a waxy semi-solid, dark colour material free from the original solvent used in extraction, often containing a high percentage of largely odourless plant waxes.

Orris root being dried
One important note here on misuse of this term concerning Orris Butter, which is frequently called Orris Concrète, but is more correctly the essential oil of orris (the roots of certain species of Iris), which happens to be solid.  There is also a true Orris Concrète, from which a true Orris Absolute is made – the latter is a clear, mobile liquid unlike the so-called Orris Concrète or Butter which is solid at room temperature.

Absolute:  A highly concentrated alcoholic extract, usually of a concrète, which contains only alcohol soluble materials. Its primary use is in alcoholic perfumes but normally contains little or no residual ethanol.  Absolutes are also sometimes extracted from pommades (sometimes called Absolute from Pommade or Absolute from Châssis - the latter term is sometimes used to describe an absolute made by extracting the spent flowers already used in enfleurage).  An Absolute from Distilation Water (e.g. Rose Water Absolute) is also sometimes made using the hydrolat left over from making an essential oil as the starting material.  Sometimes the term Absolute is also used to mean the alcohol soluble fraction of a resinoid.

Other terms in alphabetical sequence

Copaifera langsdorfii 
Balsam: A natural exudate obtained from a shrub or a tree (either physiological or pathological). It is characterized by being rich in benzoic and cinnamic acids and their corresponding esters and is insoluble in water but completely or almost completely soluble in ethanol.  Balsams may, upon ageing, form resins and so the boundary between these two may be blurred.

CO2 Extract – these include a range of extraction processes using carbon dioxide as the solvent.  In most cases the solvent used is rendered liquid at much higher temperatures than normal an so the extract produced is often, more correctly, referred to as SFEsupercritical fluid extract – and I’ve said a bit more about it under that heading.

Extract: A concentrate of a dried less volatile aromatic plant part obtained by solvent extraction with a polar solvent.  In practice this term is used quite indiscriminately to mean several of the types of processed material where there result is concentrated – with the solvent removed.  In flavour work the term is used even more widely to include emulsions and diluted materials that may be water soluble (possibly dissolved in water) and of very short shelf-life. To be meaningful this term really requires further qualification or clarification and perfumers should approach ‘extracts’ with appropriate caution.

Note that the French term ‘extrait’ (directly translated this would be extract) is used in English to mean something quite specific: An alcoholic extract of a pomade produced by enfleurage: a tincture of a pomade.   The term is also occasionally hijacked to mean an alcoholic dilution of any material of a particular strength or a blended perfume of a particular strength “extrait strength” is sometimes used to mean much the same as Parfum, that is an alcoholic perfume with 15-30% aromatic ingredients.

Gum: Can be either a natural or synthetic material but, strictly, should be used only for water-soluble materials of very high molecular weight.  In perfumery it can, however, also be used of resins and turpentines.  Under the strict definition gums are odourless and therefore of no use in perfumery.
Commiphora myrrha tree one of the primary sources
from which the oleo-gum-resin
myrrh is harvested.

Gum Resin: A natural exudate obtained from a tree or plant. It is comprised of gums and resins. If the gum resin source also contains an essential oil, it is called an oleo-gum-resin.  Only partially soluble in alcohol, hydrocarbons etc. and may be partially soluble in water where the proportion of gum is significant.

Infusion: A hot extraction of either a plant part or its exudate with either water or an organic solvent. Infusions are not at all popular because it is difficult to control their chemical composition.

Isolate: sometimes clarified as natural isolate, this is prepared material, produced from a precursor of natural origin, most often an essential oil, by any of a range of physical processes including fractional distillation or freezing, chromatographic separation and others.  At one time many perfumery materials were made this way that today are available much more cheaply as synthetics.  Natural perfumers may still take advantage of the fact that many natural isolates continue to be produced commercially for the flavour industry where the premium on natural flavours justifies the increased cost of production.

Oleoresin: The natural tree trunk or bark exudate, which is extremely rich in an essential oil.  The term is occasionally also used of prepared materials.  In either case they consist of essential oil and resin.  Turpentines are oleoresins where the resin portion is acidic.

Pommade:  The product of the enfleurage fat extraction of fresh flowers. Enfleurage was once much more widely used than today but is still the most efficient (highest yielding) method with certain flowers that continue to manufacture perfume in the flower after it is cut, such as tuberose for example.

Resin Absolute: generally applied to materials obtain directly from plant raw-materials by extraction with hot alcohol: once the alcohol has been recovered, what is left is referred to as the Resin Absolute.  As the product is often very thick and sticky, the recovered alcohol may be partially replaced by a high-boiling solvent such as Isopropyl Myristate when it is usually sold as ‘mobilised with N% of XX’.  Notable exceptions include the extraction product of Oakmoss with hot alcohol, which is usually called Oakmoss Resin; that of Orris is likewise called Orris Resin.

Benzoin resin from which Benzoin Resinoid is made
Resinoid:  A solvent extract of a resin-rich material containing natural exudate or dried plant material with a hydrocarbon solvent. Resinoids are generally viscous to semi-solid mixtures. They can be considered as being equivalent to concrètes but made from dead / dried (non-cellular) materials.

Spice/Herb Oleoresin: A solvent extract of a dried spice or herb, which is virtually free from the extracting solvent. It is used more-or-less exclusively by the food and pharmaceutical industries as a replacement for ground spices and spice tinctures.

Supercritical Fluid Extract This is an extract made using supercritical carbon dioxide (CO2) or another suitable supercritical fluid material as the solvent. Supercritical fluid extraction (SFE) of plant material with solvents like CO2,
propane, butane, or ethylene is increasingly being done. SFE allows the processing of plant material at low temperatures, hence limiting thermal degradation, and avoids the use of toxic solvents.  A common downside of SFE is that the resulting material may not be fully soluble in ethanol and in many cases further extraction with ethanol to produce, what is in effect an Absolute from SFE is conducted – these are sometimes sold as CO2 Select Extract or, more intuitively as Ethanol Soluble SFE.

Tincture: An alcoholic or aqueous alcoholic extract of a natural raw material in which the solvent is left in the extract as a diluent. Tinctures are used both in the fragrance and pharmaceutical industries. The amount of alcohol in the tincture, which ranges from 20-95%, is standardized by the manufacturer.  See my post on Ambergris for an example of a tincture used in perfumery