Journal Number 88
September 2003
EDITORIAL
The Science of Scents 2: How People Perceive Perfumes
By Ian St George
Let me say for a start that olfaction is a very complex process; if you want to be dazzled by the
science of the sense of smell, try www.leffingwell.com/olfaction.htm. What I will tell you is
a hugely simplified version.
Smell is one of the chemical senses, the other being taste. They are called that because they sense chemicals. With these senses we sample our environment for chemical information. We sniff the quality of the air we breathe and we test for the presence of food or flowers or another person. Odour molecules must be small enough to be volatile (<300-400 molecular weight) so they can vaporise and reach the nose.
Smell depends on receptors that respond to airborne molecules of odorant chemicals. In humans, these receptors are located in the olfactory epithelium - a patch of tissue about the size of a postage stamp located at the top of the nasal cavity. The olfactory membrane is made up of ciliated sensory neurons (called mitral cells), supporting cells, and between them basal cells that divide regularly, producing a fresh crop of sensory neurons to replace those that die.
The cilia of the mitral cells are immersed in a layer of mucus. Odorant molecules dissolve in the
mucus and bind to receptors on the cilia. Binding activates a sequence of reactions that ends in
an action potential - an electrical impulse - that is conducted back along the olfactory nerve to
the brain.
The brain combines the information in this impulse with other olfactory signals reaching it and interprets the pattern as a particular odour. Humans can discriminate among thousands of different odorant molecules. There are about 1000 related but separate genes encoding different odour receptors.
(Thus as many as one percent of 100,000 human genes may be occupied just with producing
these receptors, as opposed to a mere 3 genes for colour vision).
Each olfactory neuron has only a single type of receptor, but each odorant is capable of binding
to several different receptors. This provides the basis for combined odorants being sensed as a
diversity of smells. It would work like this: assume Odorant A binds to receptors on neurons #3,
#427, and #886. Odorant B binds to receptors on neurons #2, #427, and #743. The brain then
would interpret the two different patterns of impulses as separate odours. Or if the two odorants were mixed, it might interpret an unoaked chardonnay with (A3,427,886+B2,427,743) as "a bouquet of berry fruits".
Eric Scanlen tells me he cannot smell the exquisite perfume of brown Baronia (nor can a third
of people) - yet that fragrance is made up of over 20 different volatile odorants: he must be genetically unendowed with the neurons for some of those chemicals.
Memories
When I smell new haybales I am transported back nearly 60 years to one golden preschool summer twilight when my brother and sister and I were allowed to play late in the paddock while the men rushed the hay in before predicted rain. Everybody has such experiences - long-lost memories triggered by smells - it's called the Proust effect.
From the olfactory neurons the signals move to the limbic system, the part of our brain believed
to be concerned with emotions and motivation. Part of this system is our memory.
We need memory as the yardstick by which to judge a smell. There is no colour chart, and no
tuning fork for smells. We can only say what a smell reminds us of - "This sauvignon blanc has
a strong gooseberry nose", or "Earina autumnalis has the scent of soap". Smell thus relies on
memory and memory is subjective - my wife tells me that to her E. autumnalis smells like urine.
Classifying Fragrances
When we first smell an orchid flower we thus relate it to substances we have already smelled -
Peter de Lange found a Gastrodia "long column" that smelled to him like freesias; Pat Enright found a G. cunninghamii that smelled to him like jasmine; in Australia G. sesamoides is called "Cinnamon bells" because it smells like cinnamon.
Certain fragrances have been related to colours in an attempt to objectify them.
The major colour groups are
- White floral image - a soft fragrance. The white-floral scents incorporate the very
pleasing scent notes of jasmine, tuberose, orange flower, honeysuckle etc. These flowers are usually white and release their scent at night. Most are pollinated by moths which use the scent and whiteness as guides.
- Rosy floral image - a sweet, rosy smell almost like Turkish delight.
The scent of
cyclamen, lily of the valley, sweet pea and rose.
Sunlight and warmth trigger scent
production.
- Yellow scents - strong, citrus-like freesia fragrance.
- Brown scents - of spices; cloves, coriander; caraway and cinnamon.
Typical scent of
a carnation.
- Green scents - a mossy, wet-forest scent.
- Dark smells - mostly unpleasant; musty, stale-socks.
In the perfume industry fragrances may also be compared to musical notes - thus "Aldehydes
are used for their particularly vivid top notes", and "it usually takes from ten to twenty minutes
for the middle notes to develop fully on the skin".
Floral fragrances, mixes of many pure (primary?) odorants, might be thought of as harmonious musical chords (in the industry they are called "accords"). Interestingly colour and music combine in "Green notes add lift and vigour to a fragrance composition".
Judging Fragrance
Linet Hamman wrote that judging fragrance at orchid shows has become "fashionable".
The trouble is that fragrance is a very personal, individual experience - it can never be truly objective.
It is a good idea to try and cover the plant in a neutral container, so its beauty does not influence the judges unwittingly ( when judging the clinical competence of medical students, this trap has been called "the pulchritude factor").
"Another problem in fragrance judging is that orchids are not all fragrant at the same time.
Stimulating the orchids which are fragrant at night to be fragrant in the daytime, and the other
way around is quite a headache!"
As with wine-tasters, some believe you need only to whiff the air above the flower (head-space). Others suggest a waving-and-sniffing action while another school believes you have to stick your nose right into it.
"We have used a basic score-sheet in South Africa: there are five characteristics that the orchid fragrance is judged by:
- Intensity - the strength of fragrance.
- Diffuseness - can you smell it from a distance or only very close?
- Pleasantness - how pleasant (or unpleasant) the fragrance is.
- Elegance - how well rounded and perfumistic the fragrance is; chemical notes or thin fragrances would be marked down.
- Instant appeal - do I like it and how much? "All characteristics are scored out of ten points
(minimum zero, maximum 10), except pleasantness which is scored from -10 to +10.
A maximum of 50 points is possible."
The New Zealand Orchids
What then of the fragrance of NZ orchids? We think at once of the heady Earina autumnalis of course, whose perfume, especially at night, is powerful and (to most of us at least), pleasant. (White? Night? Is it moth--pollinated?)
We might be led to look only at the insect-pollinated NZ orchids, but that would be a mistake,
for Gastrodia cunninghamii is almost obligatory a selfer, yet plants have been found that smell strongly of jasmine. (I hypothesise that, once the key mutation to confer the ability to self-fertilise happens, for ever afterward selfing permits fewer genetic modifications than crossing, so old features that used to be associated with insect--pollination-perfume, labellar decoration, etc-may be stable and persist).
There is an "orchid smell" that Eric Scanlen has referred to - indeed cutting the flower of many species, or crushing their leaves or stems, allows the emission of some fragrance. In the North American Cypripedium acaule the odorants pyridine and methyl anisole have been isolated from stems, leaves and roots, and similar is likely with other species.
Contrariwise Jones and Clements note that the pollinating microdipterans of Pterostylis species "approach the flower flying into the wind as if following a perfume trail", though for most species no perfume is noticeable to humans. Furthermore the insects appear "to become excited with the approach of a squall, almost as if a floral scent was being released". Cryptostylis subulata is similarly pollinated, and so is Nematoceras iridescens. Chiloglottis valida has osmophores on its sepal tips.
I am aware of fragrance detectable to humans in all three Earinas, Gastrodia cuninghamii and
G. "long column", Nematoceras "Trotters" , N. "whiskers" and N. "Craigie lea", Prasophyllum aff. patens, and Thelymitra aff. longifolia.
Eric Scanlen found references in our Journal to several more: Drymoanthus flavus at Invercargill, "slight fragrance" [J6:2]; Thelymitra malvina [J62:2]; T. hatchii Pat Enright [J63:20]; Anzybas carsei oniony-meaty smell Peter de Lange [J70:16 & J78:39]; Thelymitra pulchella violet scent at Macraes Flat Barbara McGann [J70:26]; Thelymitra "tholinigra" faint dung smelt E Scanlen [J85: 101], Earina mucronata (southern form) nose catching like weak ammonia E. Scanlen [J82: 11].
Is that it, or are there more?
References
1. This paper is based on material gleaned from the following websites:
www.orchidssa.co.za/fragrance.htm
www.anucci.com/fyi/tech_glossary.html
www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html
2. Arditti J. Fundamentals of orchid biology. John Wiley, New York, 1992. Scents: PP273-8.
3. Jones DL, Clements MA. A reassessment of Pterostylis R.Br. (Orchidaceae). Australian orchid research
2002; 4: 16.
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