OLFACTION – OL WHAT?
1. human anatomy 2. aging & olfaction 3.English language of odors
4. Variation in citronellol concentration in S. Africa
5. Array of different essential oils in "Rose" geranium
6. Changes in levels of menthol & isogeraniol during vegetative stages of P. sp.
7. & 8. Floral scented P. gibbosum (flowers & leaf)
9. & 10. Coconut scented P. grossularioides (flowers & leaf)
11. & 12. P. reniforme (flowers & leaf) 13. & 14. P. exstipulatum (flower & leaves)
15. ‘Chocolate Peppermint’
Olfaction or the sense of smell is of importance when it comes to talking (or writing) about
scented geraniums/Pelargoniums. We have noted that the scents produced by various plants
can vary from time to time based on various environmental factors. More exhaustive studies show
that among the scented geraniums there are very significant differences in which scented compounds
are produced by different species. Local population studies (particularly in P. capitatum in South
Africa) have shown that there are great differences among the populations as to which set of scented
chemicals and what quantity of each of them is produced (Fig. 4). The primary commercial cultivars
of the "rose geranium" yield very different arrays of essential oils and produce them in different quantities
(Fig. 5). The essential oils are not produced in equal quantities on all plant parts – the leaves being
much more productive than the stems. In addition younger plants produce less oil than older plants.
Various aspects of the climate and weather, including sunlight, rainfall, temperature, humidity and time
of harvest (Fig. 6) affect the quantity and quality of rose oil produced by commercial cultivars.
Furthermore, such physical conditions as soil fertilization, shade, weeds, diseases and irrigation also
have an effect on the oil being produced.
But we haven’t noted the extent to which individual humans vary in their ability to detect scents.
Humans (and many other animals) have special sensory cells in the skin (epithelium) lining their
nasal cavities. The sensory cells are connected by nerves to the part of the brain called the
olfactory bulb which is located above the nasal cavity and below the frontal lobes of the brain
(Fig. 1). Volatile, air-borne, scented chemicals get attached to the olfactory receptors and a
signal is sent to the olfactory bulb where it is interpreted as a scent. The relationship between
the scent molecule and the receptor is thought to be very precise, like a lock and key, and only
the appropriate scent receptor and scent molecule will trigger a signal to the olfactory bulb.
It has been suggested that there are a thousand genes that code for odor reception in animals.
Each person has only a few of these genes functioning – so the human ability to recognize different
scents is much less than many other animals. Studies have determined that humans have as many
as 368 different olfactory receptors. However, not everyone has the same set of receptors which
are under genetic control. Each person has his own distinct/unique combination of receptors. So
we then end up with a situation where we are trying to recognize the same scent but we don’t have
the same genetic equipment to recognize the scented chemicals.
Humans are estimated to have about 10 square centimeters of olfactory epithelium while a dog may
have 170 square centimeters of such epithelium – in addition to as many as 100 times more receptors
on each square centimeter of epithelium. Hunting dogs’ have the ability to smell one million to ten
million more times acutely than humans, while bloodhounds are 10 to 100 million times more acute
in their ability to detect scents. Mice may have 1000 different olfactory receptors putting them at the
top of the list of creatures with different abilities to detect different odors.
Studies have shown that humans’ peak ability to detect scents is between the ages of 20 and 40.
In the general population this ability then declines with time (Fig. 2). So while 20-40 year olds can
identify 50 to 75% of odors, 50 to 70 year olds can identify only 30-45% of odors. (D. Purves et
al., 2001, Neuroscience 2nd ed.)
Women’s sense of smell is more acute than men’s (Fig. 2). The decline in ability to detect different
scents is more pronounced in people with Parkinson’s disease or Alzheimer’s syndrome. Some
commonly prescribed drugs, especially anti-cholesterol and anti-hypertension drugs are known to
reduce olfactory ability.
Classical studies of people of German and Japanese cultures show significant differences in their
ability to identify various scents. The European languages have very few abstract words to identify
different scents (unlike their capacity to identify different colors with abstract terms). Europeans
most often describe scents in relation to some event in life. On the other hand certain ethnic groups
on the Malay Peninsula have numerous abstract words for recognizing different scents (Fig. 3).
However, it is possible to develop the sense of smell and an appropriate vocabulary through training
as is the case with perfumers and sommeliers.
Different chemical compounds can be detected in different concentrations so there is no one threshold
level at which odors become detectable.
As an aside, about one in a thousand humans cannot detect butyl mercaptan – the scent produced by
skunks. And one in 10 cannot detect hydrogen cyanide (a poisonous gas) or ethyl mercaptan (the
chemical added to natural gas to make it detectable).
But to reiterate. Any scented geranium/Pelargonium produces essential oils that give that plant its
distinctive odor. Chemical studies show that each plant produces a variety of different scent compounds
at the same time. (The quantity and proportions of each compound varying through time and in reaction
to environmental and genetic factors.) Some of these compounds are judged to be pleasant while others
are less so. The number of chemical compounds produced by each plant may include as many as 120
different substances. When a human nose/brain registers these constituents each of us gets different
messages depending on which receptors we have in our repertoire. So, it is not surprising that some of
us like a given plant’s smell while others of us find nothing of interest (maybe no smell at all) and yet
others of us find the plant to smell unpleasant or even repulsive.
Think of the implications of all this on the beverage (wine, brandy, etc.) and scent (perfume, cologne,
deodorant, soap) industries.