TESTING
In order to make an objective and unbiased assessment of the odour concentration of a specific substance, and by extension its OTV, the European Standard for Olfactometry EN13725 was published in 2003. It defines a method for the determination of the odour concentration of a gaseous sample by dynamic olfactometry with human assessors. In this standard, the odour concentration of a sample is measured by means of an olfactometer. An olfactometer can be seen as a dilution device. A sampling bag containing an odour is attached to the olfactometer, and diluted with neutral odour free air. At the beginning of an analysis, the odour is offered to the assessors at a very high dilution, in which none of the assessors is able to perceive it. The dilution is then gradually decreased, until the assessors are able to notice the odour. There are two methods by which the ability to detect the odour is measured, the Yes/No method and the Forced Choice method. With the Yes/No method, the assessors simply have to press a button once they perceive the odour. With the Forced Choice method, the assessors are presented with two ports from which one port odour free air is emitted and from which the other diluted odour is emitted. The assessors have to indicate from which port they think the odour is emitted and how certain they are about their decision: is it a guess, an inkling or are they certain. In order for the odour in a dilution step to count as being perceived, the assessor has to pick the right port at least two times in a row, and with certainty. Both the Yes/No and the Forced Choice methods result in the European Odour Unit, or OUE One OUE
. is the amount of odorant that, when
evaporated into one cubic meter of neutral gas at standard conditions, is detected by 50% of the assessors. The main objective of the standard
was to provide a common basis for the assessment of odour emissions in the member states of the European Union. The EN13725 mentioned earlier is for example also used for environmental odours, where odour samples are taken directly from a factory chimney. The
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Figure 2: Olfasense GC-Sniffing-MS laboratory.
odour concentration is then measured and the resulting OUE
/m3 is used to determine the
odour emissions from the factory, eventually the efficacy of exiting odour treatment systems (e.g., industrial filters), or combined with dispersion modelling to evaluate possible odour nuisance in the surrounding area. But the same method can also be applied to determine the OTV of specific compounds individually. By definition, one odour unit (OUE
) of a
given compound is equivalent to the threshold concentration in which it is perceived. Hence, by knowing both the odour and mass concentrations it is possible to calculate the OTV. The earlier referred geosmin is an example of a compound with an extremely low OTV, which can be perceived already when it is present at just a few parts per trillion by volume (pptv
). At Olfasense, we
have adapted the EN13725 standard for a more accurate determination of the OTV of target compounds. We are using human panels with a larger number of individuals, which have been calibrated with more than one
reference substance. For a standard odour concentration analysis, a minimum of only four assessors are needed, who are screened based on their sensitivity for one reference substance, n-Butanol. Whereas four assessors can be enough for a standard measurement, more accuracy can be achieved by using more assessors and screening them for more than one reference substance.
Figure 3: Odour wheel and their odorants (sniffing sticks).
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Ground-breaking knowledge of the odour threshold As previously mentioned, precise knowledge of the OTV of a given odourous compound can be a very useful tool, both to prevent unwanted odours and to optimise fragrances, as there is a demand for specialised services in this area from the fragrance industry. Some chemical compounds with unpleasant odours can be necessary in a product formula, or are formed during the production process. Knowledge of the OTV of off-odour chemicals can help research and development teams to define the acceptable concentration limits of these chemicals in their final products. OTV knowledge could also be used to aid in finding a replacement for a specific chemical in a product formula. Furthermore, taking advantage of this capability, we can also design defined synthetic mixtures of volatile compounds that simulate complex odour sources. Once such mixtures have been created, we can analyse the combined effect of their constituents using standardised olfactometric tools, and also analyse their individual components using GC-Sniffing-MS techniques. This allows us to compare the compositional profiles between natural and synthetic sources, for example between a rose species and a synthetic rose odour, and to determine the interactions and contribution of each component to the overall odour. There are a few companies that provide specialised services in OTV determination but, to our knowledge, none of them offer an integration of expertise ranging from individual odorants to the analysis and custom manufacturing of complex mixtures made from
November 2021 PERSONAL CARE
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