widely used but maybe also widely taken for granted, aroma fixatives are mysterious

I’m going to try and assemble some literature about aroma fixatives.  One goal is to capture search terms about the topic.  I want to unite a lot of literature under one place so people searching for the topic might see many references grouped together.  To do this I’m going to use many quotations so feel free to skim them. I will try to bolden key terms to draw attention to patterns within all the sources.

Fixatives are strange and seem to be poorly understood and poorly described.  Fixatives are acknowledge mostly for lowering the volatility of aroma compounds to better preserve them.  They are then acknowledged for having an aroma that is only perceivable to some and not all, then being used typically sub recognition threshold.  Claims are made that fixatives can lower volatility while simultaneously increasing intensity of the aromas they preserve which leads me to speculate they may do something very unique at the level of perception.  Strange spatial language often gets attached to the effects of fixatives.  Some how they can bring aromas “together” and I suspect that people look to their effect on volatility to explain effects they might actually have on the spatial perception of aromas.  I speculate that fixatives might trigger modes perception where contrast enhancement is altered and odor object recognition might be more recollection heavy which could be said another way as they give us a tendency to complete aromas based on memories making them seem fuller and more complete.

The second time I ever encountered the term fixative was in the utterly brilliant book on chemistry, Molecules (p. 141) by Peter W. Atkins.  Atkins text seemed a unique entry in the bibliography of Neurograstronomy so I took a chance and gave it a read. In regards to fixatives from Atkin’s entry for Civetone C17H30O:

“Musk is used in two ways in perfumery. It is used for its odor, as a component of heavy, musky, oriental perfumes. It is also used as a fixative, sometimes in concentrations so small that its own odor is masked. That is, it is added to more volatile fragrances to retard their evaporation, so that they are experienced as a symphony of odors rather than as a sequence in order of decreasing volatility.”

This reference mentions the idea of the fixative being nearly sub recognition threshold, changing the volatility of the substances, and a spatial concept with a reference to the symphony.

The first time I ever saw the term was in Robert Léauté’s 1989 James Guymon lecture on Alembic distillation:

“The fatty esters give fruitiness to the Cognac; the fatty acids give body and are like fixatives for many other aromatic components; amino-acids are involved in thermic break down reactions.”

In this reference the fatty acid esters are only like a fixative, but we probably can’t put too much emphasis on the intention of that one word.  The specific esters Léauté is talking about are the most basic and abundant esters, ethyl-acetate, which are also only sub recognition threshold.  I can’t imagine the ethyl-acetate in a spirit is significant enough to have any effect on the volatility of the distillate.

Information is not abundant in web searches and among the only things notable are a few patent filings.

From a 1939 patent filing:

… “Ethyl alcohol, however, has the drawback of having a low boiling point and a high vapor pressure, hence when alcohol extracts are used in baking or candy making at temperatures for exceeding the boiling point of alcohol the alcohol used as the menstruum and solvent in the extract volatizes and carries off some of the flavor and aroma-imparting substance thus necessitating the use of considerably more extract than otherwise would be necessary if a solvent having a high boiling point were used.   The esters of glycerine, such as monoacetin and diacetin have been employed as solvents for flavoring extracts and although they are good solvents for most of the aromatic substances used in preparing flavoring extracts, these esters are susceptible to hydrolysis in the presence of water with the liberation of acetic acid which imparts a sour odor to the the flavoring extracts and renders them useless for culinary purposes.   I have found that the ethyl esters of glycerine and particularly the mono-ethyl-ether are valuable solvents in the preparation of flavoring extracts because they are perfectly stable, are miscible with water, they do not hydrolize, they have relatively high boiling points and therefore act as fixatives for the aromatic substances present in the extract, and they show exceptionally good solvent power toward the various essential oils and aromatic chemicals usually employed in the preparation of flavoring solutions. Said ethyl ethers of glycerine are likewise good menstruums when used undiluted or diluted with water for all kinds of vegetable substances such as vanilla beans, tonka beans, Phoenugreek seeds, roasted coffee, roots and herbs, all commonly used by flavoring extract manufacturers.”

Wow! That starts to explain things I’ve seen on so many ingredient labels over the years.  In this case no reference to a threshold is made, there is no spatial reference to the perception of the result. The fixative here is only used for its effect on volatility.

Another patent filing from 1981:

“A fixative for perfume compositions, 1-(2,6,6-trimethylcyclohexyl)-hexane-3-ol, itself odorless to many people, brings about a “rounding-off” of the perfume while increasing its intensity. The compound is made by condensation of citral with pentane-2-on, followed by cyclization and hydration.”   ….”It has been found that an addition of 1-(2,6,6,-trimethyl-cyclohexyl)-hexane-3-ol effectively brings about a “rounding-off” and fixation of perfume compositions while simultaneously increasing the intensity of the fragrance. The quantity used amounts usually to 1 to 25% by weight relative to the total composition, and a very distinct fixing effect can be ascertained particularly from approximately 3%. In principle, more than 25% may also be employed; this depends, however, upon the nature of the total composition and may be easily determined by routine experiments.”   “DETAILED DESCRIPTION OF EMBODIMENTS   The 1-(2,6,6-trimethylcyclohexyl)-hexane-3-ol itself is not to be considered as an odoriferous substance in the conventional sense as this saturated secondary alcohol has been found to be odorless, or only very slightly scented, by a significant number of the test persons who were asked to judge this substance, whereas other persons perceived an intensely radiating wood-like, slightly animalic-urinary odor.   Anosmia, i.e. the incapability to perceive smells, which numerous people have with respect to this new alcohol, may be compared with the effect of various very expensive animalic fixatives, such as musk and amber, whose scent can also be noticed only faintly by certain people, or not at all.   The occurrence of anosmia with respect to a specific substance, particularly in the area of perfumery, appears to always point out to particularly good fixative properties of a substance and holds true at least for all of the fixatives which have heretofore been employed in the manufacture of perfume compositions. These afore-mentioned advantageous properties of 1-(2,6,6-trimethylcyclohexyl)-hexane-3-ol distinguish this substance from the homologous products which have been known for a long time, such as iso-tetrahydromethylionol and n-tetrahydromethylionol, which do not develop any fixative effect and with regard to which also no cases of anosmia have been ascertained.”

Okay I’m getting a sense that people can make fixatives but not well articulate the mechanism of how fixatives work.  They might go beyond physical chemistry and volatility deep into perception.  Here we have emphasis on the sub threshold properties of recognizing the fixative. There is also lots of spatial language describing the fixative effect and somehow an increase in intensity despite a lowering of volatility? Mysterious! The description of this fixative does not emphasize volatility like the previous one does.

Yet another patent application from 2008:

“Compositions are provided that contain fragrance compounds and fragrance fixatives selected from C4-C8 alkyl terephthalates.”   “To slow the evaporation of fragrance compounds, the fragrance compounds are sometimes combined with lower volatility components generally known as fixatives. These fixatives are substances which improve lasting qualities of odorous substances of a fragrance. There is a continuing need for the development of new fragrance fixatives.”   “The present invention is based on the discovery that the use of C4 to C8 alkyl terephthalates in perfumes or fragrances can provide a more enduring fragrance by slowing the evaporation of fragrance compounds.”

Here, there is no mention of a sub threshold property or any spatial language, but rather only a mention of volatility.  But maybe there should be? The sweet-smell of phthalates was part of the olfactory illusion I encountered when drilling red plastic which brought on the illusory aroma of cherries.

From a really wild perfume industry blog post about fixatives from 2009:

“In perfumery there is something called a “fixative” which is used to elongate the life of the aromas as a whole and increase the life and hold the aroma of other oils which would otherwise just fade out.   Some fixatives can be synthetic or natural which include animal and plant derived fixatives. Almost all fixatives are used in minute quantities (except some synthetic ones) because of their powerful effects.   The four main animalistic fixatives, all of but one, hold up as very sweet smelling fragrances are:

*Musk (from the Musk Deer)

*Civet Musk (from the Civet)

*Ambergris (from the Spermwhale)

*Castoreum (from the North American Beaver)”

Really interesting! In this case volatility is the big emphasis, but “minute quantities” might also imply a desire to be sub threshold. The author also uses a few interesting terms.  The four natural fixatives are described as “animalistic” rather than animal derived but then “sweet smelling”.  For starters sweet smelling is a cross-modal metaphor and animalistic, if it is not only describing the source, might imply the olfactory-umami category.

I remember the paper on whiskey, Origins of Flavour in Whiskies and a Revised Flavour Wheel: a Review, by Lee, Paterson and Piggott mentioning a something along the lines of a fixative.  The paper is pretty astounding in its own right and the authors never mention the term fixative specifically but they do describe cyclodextrin bound reference standards for training whiskey assessors:

“The advent of cyclodextrin bound reference standards has enabled communication of information on flavour character in training of assessors, as exploited in the brewing industry.”   “For assessor training, reference compounds are recommended (Table I): formulations with cyclodextrins achieve consistency and parallel conceptualisations.”

The cyclodextrins in this case seem to function as fixatives yet the term is never used. This FAQ [pdf] describes cyclodextrins use as an “encapsulation agent” for food additives and being “virtually odorless” and “slightly sweet tasting” which fits the bill for a fixative.  A bar regular who is a medical research told me that cyclodextrins were known to have a stimulating effect but so far I cannot find any references to the phenomenon. I suspect not finding anything is a matter of language.

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