46 TESTING
At the limits of the sense of smell
Lika van Genderen, Romy Jacobs - Olfasense Netherlands Rita Ribau Domingues - Olfasense Spain
We have all experienced at some time, shortly after the first raindrops fall, that characteristic and evocative smell known technically as petrichor; a combination of words from Greek mythology, ‘petri’ (rock) and ‘ichor’ (the ethereal blood of the gods). Petrichor is often described as the smell of wet stone, but the odour does not come from the stone minerals themselves. Layers of volatiles can accumulate on the stone surfaces, for example those emitted by microbes and plants, and are modified by the atmosphere. These volatiles are often not well perceivable, unless we put our noses up close. Raindrops can drive them out of their stone surfaces and make them better noticeable. One of the organic molecules enhanced by rain, whose smell we colloquially describe as ‘wet earth’, is the so called 4,8a-dimethyl-decahydronaphthalene- 4a-ol. This odorant also known as geosmin comes from the Greek, meaning ‘aroma of the earth’ because of its smell. It is a hydrocarbon belonging to the terpene family produced by some soil bacteria and fungi, mainly belonging to the genera Streptomyces and Penicillium which, among many other chemical compounds, also synthesise antibiotics as fundamental to mankind as streptomycin and penicillin. The ability of our olfactory system to
perceive geosmin is surprising. Some studies indicate that we can detect it when it is present at a concentration of 0.0000065 parts per million, in terms of volume of air (ppm v/v). To give an idea, this is equivalent to diluting the contents of a teaspoon of coffee in about 300 Olympic-size swimming pools. Some researchers have suggested that this heightened sensitivity and fascination with its scent is the result of our evolution, when our ancestors lived on the African savannah and the smell of geosmin served to stimulate the search for water. This olfactory acuity is far superior to
the ability to detect geosmin by analytical techniques in the laboratory, and makes its unwanted presence problematic. In drinking water, it confers an unpleasant musty odour, and can be responsible for some undesirable aromas in wine when grapes have been attacked by certain fungi. But the ‘earthy’ note provided by geosmin has also been used in many perfumes, such as Me Paraissait Une Ombre (Etat Libre d’Orange), La Vamp (Bouge), or Labaie 19 (Le Labo).
PERSONAL CARE November 2021
The olfactory detection limit and its measurement The example of geosmin illustrates the importance of a fundamental parameter in odour science: the concentration at which we begin to perceive the odour of a given compound, known as the olfactory detection limit (often abbreviated as the odour threshold value, or OTV). Knowledge of the OTV of odourous chemical compounds is a very important tool in the design of all kinds of products and consumer goods. For example, to adjust the composition of certain compounds needed to produce an optimised olfactory perception of the final product. Despite this, OTV data available in the
literature show a wide variability, reflecting the diversity of procedures and techniques used in its determination. Factors that can affect the measurement of OTV include the mode of presentation of the odourous compound to the assessors, the influence of extraneous odours in the presentation system, the profile of the chosen assessors, the definition of the odour response, the treatment of the data obtained, the chemical purity of the odourous compound and the uncertainty associated with the results due to working with such low concentrations. At Olfasense, we have developed standardised
Figure 1: Dynamic olfactometry with a TO evolution olfactometer in a Dual Forced Choice setting.
methods for the determination of OTVs for a great variety of compounds, and we have an extensive database of these values. Our laboratories have specialised equipment and protocols for the quantification of odour concentration by dynamic olfactometry. Besides, by means of gas chromatography-mass spectrometry analysis associated with a sniffing port (GC-Sniffing-MS, also called GC-O), the chemical and sensory aspects of odour perception can be integrated. The OTV is determined as the minimum concentration at which 50% of a representative human panel can detect the presence of an odour, without having to be able to characterise the stimulus. This is different from the recognition threshold value, which is the concentration at which 50% of the panel can detect the odour, but also describe it qualitatively.
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