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Odor detection threshold
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The odor detection threshold is the lowest concentration of a certain odor compound that is perceivable by the human sense of smell. The threshold of a chemical compound is determined in part by its shape, polarity, partial charges, and molecular mass. The olfactory mechanisms responsible for a compound's different detection threshold is not well understood. As such, odor thresholds cannot be accurately predicted. Rather, they must be measured through extensive tests using human subjects in laboratory settings.
Optical isomers can have different detection thresholds because their conformations may cause them to be less perceivable for the human nose. It is only in recent years that such compounds were separated on gas chromatographs.
For raw water treatment and waste water management, the term commonly used is Threshold Odor Number (TON). For instance, the water to be supplied for domestic use in Illinois is 3 TON.[1]
Values
[edit]- The threshold value is the concentration at which an aroma or taste can be detected (air, water and fat).
- The recognition threshold or arousal threshold of olfactory neurons is the concentration at which you can identify an odor (air, water and fat).
- The odour activity value is the concentration divided by the threshold.
- The flavor impact is the value the rate of change in perception with concentration.
- The flavor contribution of an aroma component in a mixture to the total profile can be calculated from the total odor units and the number contributed by that aroma chemical.
Odor detection value
[edit]Odor threshold value (OTV) (also aroma threshold value (ATV), Flavor threshold) is defined as the most minimal concentration of a substance that can be detected by a human nose. Some substances can be detected when their concentration is only few milligrams per 1000 tonnes, which is less than a drop in an Olympic swimming pool. Odor threshold value can be expressed as a concentration in water or concentration in air.
Two major types of flavor thresholds can be distinguished: the absolute and the difference threshold. The odor detection threshold and the odor recognition threshold are absolute thresholds; the first is the minimum concentration at which an odor can be detected without any requirements to identify or recognize the stimulus, while the second is the minimum concentration at which a stimulus can be identified or recognized.[2]
The odor threshold value of an odorant is influenced by the medium.
Examples of substances with strong odors:
- Grapefruit mercaptan (OTV = 0.1 ppt)[3]
- Sotolon (OTV = 1 ppt)[3]
- (Z)-8-tetradecenal (OTV = 9 ppt in water)[4]
- Geosmin (OTV = 10 ppt[5])
- strawberry furanone (OTV = 40 ppt)[3]
- Trimethylamine (OTV = 0.37 - 1.06 ppb) [3]
- p-vinylguaiacol (OTV = 10 ppb)[6]
Variables
[edit]Threshold in a food is dependent upon:
- The threshold of the aroma in air.
- Concentration in the food.
- Solubility in oil and water.
- Partition coefficient between the air and the food.
- The pH of the food. Some aroma compounds are affected by the pH: weak organic acids are protonated at low pH making them less soluble and hence more volatile.
- Number and functionality of odorant receptors in the observer's nose.
The concentration of an odor above a food is dependent on its solubility in that food and its vapor pressure and concentration in that food.
Variation by individual
[edit]A 2014 study found no significant differences between men and women, and between non-pregnant and pregnant individuals, despite the existence of anecdotal reports of hyperosmia among the latter.[7]
People with Multiple Sclerosis have been found to have higher olfactory thresholds. In scientific research, this is often represented by a lower threshold score, i.e. reversing the scale. Olfactory function is more impaired in patients with primary progressive MS than that in relapsing-remitting MS.[8]
Variation among species
[edit]Some species can detect odors that others cannot. It is widely believed that animals such as dogs and rodents have a superior sense of smell overall, however a 2017 paper disputed that, saying that "the absolute number of olfactory neurons is remarkably consistent across mammals".[9]
See also
[edit]- Dimethyl sulfide – One of the molecules responsible for the odour of the sea
- Geosmin – Chemical compound responsible for the characteristic odour of earth
- Olfactometer – Instrument used to detect and measure odor dilution
- Taste detection threshold – Minimum concentration of a flavoured substance detectable by the sense of taste
References
[edit]- ^ Lin, S (1977). "Tastes and Odors in Water Supplies - A Review" (PDF). Department of Registration and Education. 127: 1.
- ^ L.J. van Gemert (2003) Flavour thresholds
- ^ a b c d Leffingwell & Associates. "Odor & Flavor Detection Thresholds in Water (In Parts per Billion)". Retrieved May 10, 2022.
- ^ PSabine, Widder; Symrise GmbH & Co. KG. "8-tetradecenal as fragrance and flavoring substance". Google Patents. Retrieved July 20, 2017.
- ^ Polak et al find an average threshold of 9.5 ppt for the negative enantiomer of Geosmin, for the positive enantiomer it's 78 ppt. The range for both enantiomers is between 4 ppt to > 100 ppt across 50 human subjects. Polak, E.H.; Provasi, J. (1992). "Odor sensitivity to geosmin enantiomers". Chemical Senses. 17 (1): 23–26. doi:10.1093/chemse/17.1.23.
- ^ H.H. Baek, K.R. Cadwallader (1999) Contribution of Free and Glycosidically Bound Volatile Compounds to the Aroma of Muscadine Grape Juice Journal of Food Science 64 (3), 441–444 doi:10.1111/j.1365-2621.1999.tb15059.x
- ^ Cameron, E. L. (2014). "Pregnancy does not affect human olfactory detection thresholds". Chemical Senses. 39 (2): 143–150. doi:10.1093/chemse/bjt063. PMID 24302690.
- ^ Schmidt, Felix A.; Maas, Matthew B.; Geran, Rohat; Schmidt, Charlotte; Kunte, Hagen; Ruprecht, Klemens; Paul, Friedemann; Göktas, Önder; Harms, Lutz (July 2017). "Olfactory dysfunction in patients with primary progressive MS". Neurology: Neuroimmunology & Neuroinflammation. 4 (4): e369. doi:10.1212/NXI.0000000000000369. PMC 5471346. PMID 28638852.
- ^ "Not to be sniffed at: Human sense of smell rivals that of dogs, says study". TheGuardian.com. May 11, 2017.
Odor detection threshold
View on Grokipediafrom Grokipedia
The odor detection threshold (ODT) is the lowest concentration of a volatile chemical in air that is perceivable by the human olfactory system, typically defined as the concentration at which 50% of an exposed population can reliably detect the presence of the odorant without necessarily identifying it.[1] This threshold serves as a key psychophysical measure in olfaction research, quantifying the sensitivity of the nose to airborne stimuli and distinguishing detection (mere perception) from recognition (identification of the odor's quality).[2] ODT values are expressed in units such as parts per million (ppm), parts per billion (ppb), or odor units (OU), and they vary widely across chemicals—for instance, highly potent odorants like mercaptans have ODTs in the ppb range, while less sensitive ones like alcohols may require ppm levels.[1]
Measurement of ODT relies on standardized psychophysical protocols to minimize bias and account for sensory adaptation. Common methods include the triangle odor bag technique, where participants sample three bags (one containing the odorant diluted in odor-free air and two blanks) and identify the different one, with dilutions progressing until detection fails; this approach yields geometric mean thresholds with standard deviations around 0.66 log units for many compounds.[1] Another widely used method is the ascending series in forced-choice formats, such as the odor squeeze bottle procedure, which presents pairs of samples to avoid fatigue and provides thresholds varying by up to 100-fold depending on protocol details like dilution factors or presentation order.[1] These techniques are applied in fields like environmental monitoring, where ODTs help assess air quality impacts, and industrial safety, such as detecting leaks in odorized gases like natural gas.[3]
Individual and contextual factors significantly influence ODT variability, with thresholds spanning orders of magnitude even within populations. Age-related decline in olfactory function raises ODTs in older adults; for example, in a community-based study of adults aged 68–99, the mean n-butanol detection score was 8.2 (on a scale where higher scores indicate better sensitivity to lower concentrations), decreasing to 6.9 in those 85 and older.[4] Health conditions like anosmia (complete loss of smell, resulting in effectively infinite thresholds) or exposure to pollutants can further elevate thresholds by impairing nasal mucosa or neural signaling.[5] Chemical properties, including molecular structure (e.g., higher potency for aldehydes and thiols) and atmospheric lifetime, correlate with ODT sensitivity, suggesting evolutionary tuning to environmentally relevant volatiles.[1] Across datasets of over 350 compounds, inter-individual standard deviations reach 0.82 log units, underscoring the need for large-sample testing in applications like food science and perfumery.[1]
Certain odorants, particularly thiols like methanethiol and hydrogen sulfide, exhibit exceptionally low detection thresholds due to evolutionary adaptations that enhance sensitivity to potential environmental hazards, such as decaying organic matter or toxic gases, allowing early warning of dangers like food spoilage or poisonous emissions.[30]
Compilations of odor thresholds in the literature, such as the U.S. EPA's 1992 reference guide for hazardous air pollutants and the AIHA's Odor Thresholds for Chemicals with Established Occupational Health Standards (3rd edition, 2013), highlight significant discrepancies between pre-2000 and post-2000 studies. Earlier reports often showed broader ranges (e.g., H₂S from 0.0001 to 1.5 ppm) owing to inconsistent testing protocols and smaller panels, whereas more recent investigations employing standardized psychophysical methods like ASTM E679 yield tighter, more reproducible values (e.g., H₂S around 0.0004-0.001 ppm).[29][28][22]
Fundamentals
Definition
The odor detection threshold is defined as the lowest concentration of an odorant in the air that is perceivable by the human olfactory system, specifically the point at which 50% of a tested population can reliably detect its presence through smell.[6][7] This threshold represents a key psychophysical measure, quantifying the sensitivity of the nose to airborne chemicals without requiring identification of the odor's source or quality.[1] In olfaction, this detection threshold corresponds to the absolute threshold, which marks the minimal stimulus intensity for initial perception of an odorant.[8] It differs from the difference threshold, also known as the just noticeable difference, which assesses the smallest incremental change in odorant concentration that can be discerned once an odor is already present.[9] Additionally, the detection threshold is distinct from the recognition threshold, the higher concentration at which an odorant can be correctly identified by name or characteristic, and from intensity thresholds that gauge perceived odor strength rather than mere detectability.[8][10] The concept of the odor detection threshold emerged from the broader field of psychophysics, pioneered by Gustav Fechner in the 19th century, which sought to relate physical stimuli to perceptual responses through threshold measurements.[9] In the early 20th century, Hans Henning advanced olfactory psychophysics in his seminal work Der Geruch (1916), where he explored odor quality classification via a prism model.[11] These studies laid foundational groundwork for distinguishing detection limits from qualitative aspects of olfaction, influencing subsequent sensory research.[12]Significance
The odor detection threshold represents the initial gateway in the olfactory process, where the minimal concentration of an odorant sufficient to elicit a sensory response activates specialized olfactory receptors on sensory neurons, triggering signal transduction that converges in the olfactory bulb for initial processing before transmission to cortical areas involved in odor perception and discrimination.[13] This threshold thus bridges peripheral chemosensory detection with central neural mechanisms, enabling the brain to interpret environmental chemical cues and form the basis for more complex olfactory experiences such as identification and hedonic evaluation.[14] As the foundational measure of olfactory sensitivity, it underscores how variations in threshold levels can influence overall perceptual acuity across individuals.[15] Beyond its neurophysiological role, the odor detection threshold holds profound significance for daily safety and well-being, allowing humans to identify spoilage in food, leaks of hazardous gases, or other environmental threats that could pose immediate risks to health.[4] Effective detection at these thresholds supports survival instincts by alerting individuals to potential dangers, such as toxic fumes or contaminated substances, thereby preventing exposure and associated illnesses.[16] Disruptions in this capability, often due to age or disease, can diminish quality of life by increasing vulnerability to unseen perils and reducing the sensory enjoyment derived from safe, familiar scents.[17] In applied sciences, particularly sensory evaluation, odor detection thresholds inform critical advancements in product design across the food, fragrance, and consumer goods sectors, where they guide the selection and concentration of aroma compounds to ensure detectability and appeal without overwhelming the senses.[18] These thresholds enable precise formulation strategies that align with human perceptual limits, enhancing flavor profiles in foodstuffs or scent compositions in perfumes to meet market standards and consumer preferences.[19] By prioritizing threshold data, industries can develop more effective and innovative products that leverage olfactory science for improved sensory outcomes.[20]Measurement Methods
Psychophysical Techniques
Psychophysical techniques for assessing odor detection thresholds rely on human sensory evaluation through olfactometry, utilizing dynamic olfactometers to generate and deliver precisely controlled dilutions of odorants in clean airstreams, thereby minimizing variability from static headspace methods and enabling measurements down to parts-per-trillion levels.[21] These instruments mix odorous samples with odor-free air via syringe pumps or mass flow controllers, vaporizing liquids into a carrier gas like nitrogen and distributing uniform concentrations to multiple sniffing ports for simultaneous panel testing.[21] The core procedure follows forced-choice paradigms, such as the triangular test outlined in ASTM E679, where panelists receive three presentations per trial—two blanks (odor-free) and one containing the diluted odorant—and must select the odd sample.[22] Testing begins with low concentrations in an ascending series, with each dilution step typically increasing by a factor of 2 or 3 (e.g., starting at 1:1000 dilution), and continues until the panelist correctly identifies the odorous sample in at least two consecutive trials at higher levels, bracketing the point of 50% detection probability known as the dilution-to-threshold (DT). Yes/no forced-choice variants may also be used, prompting panelists to indicate presence or absence, but triangular methods reduce guessing bias by requiring active discrimination.[22] Statistical analysis derives the threshold from individual and group responses, defining it as the concentration yielding 50% correct detections above chance. For each panelist, the best-estimate threshold is the geometric mean between the last undetectable and first detectable concentrations; the group threshold aggregates these via the geometric mean formula: where represents each panelist's detection concentration and is the panel size, ensuring logarithmic scaling appropriate for perceptual data.[22] ASTM E679 provides guidelines for panel composition, recommending 6-8 screened panelists free from olfactory impairments, selected through preliminary sensitivity tests to represent typical population variance, with annual recertification.[22] Protocols emphasize replication, typically 4-6 trials per panelist, to enhance reliability and account for intra- and inter-individual variability in responses.[22]Instrumental Approaches
Instrumental approaches to odor detection thresholds employ automated technologies to complement human psychophysical measurements, providing scalable and reproducible data that approximate or correlate with sensory thresholds. These methods leverage chemical separation, sensor arrays, and ionization techniques to identify and detect volatile odorants at low levels, often achieving sensitivities comparable to human olfaction in controlled settings.[23] Gas chromatography-olfactometry (GC-O) integrates gas chromatography for separating complex mixtures of volatile compounds with human sensory evaluation to pinpoint odor-active components and their detection thresholds. In this technique, effluent from the chromatographic column is directed to a sniffing port where trained panelists detect odors as they elute, allowing correlation of retention times with sensory responses to determine threshold concentrations for individual odorants. This hybrid method excels in deconvoluting mixtures, such as those in food or environmental samples, by linking instrumental peaks to perceptual thresholds, with detection limits often reaching parts per billion for potent odorants.[24][23] Electronic noses (e-noses) simulate olfactory systems using arrays of gas sensors, such as metal oxide semiconductors (e.g., tin dioxide-based devices), to capture response patterns from volatile organic compounds indicative of odors. These sensors undergo changes in electrical resistance upon exposure to odorants, generating a multivariate signal that pattern recognition algorithms— including principal component analysis (PCA), artificial neural networks (ANNs), and support vector machines (SVMs)—process to quantify thresholds and classify odor intensities. E-noses achieve classification accuracies exceeding 90% in applications like food spoilage detection, enabling threshold estimation through calibration curves derived from known odor standards.[25][23] Calibration of these instrumental methods typically involves validation against human sensory panels using standardized olfactometric protocols, such as dynamic olfactometry with n-butanol references, to establish equivalence in threshold detection. For instance, e-noses and GC-O systems are trained on panel-derived data to minimize discrepancies, with reported correlations showing error rates below 20% in controlled environmental monitoring scenarios. These approaches offer advantages in continuous, real-time monitoring, reducing operator fatigue and enabling long-term deployment in applications like wastewater treatment, where human panels are impractical.[23][25]Threshold Values
Examples for Common Odorants
Odor detection thresholds vary widely among common odorants, reflecting differences in molecular structure and human olfactory sensitivity. For instance, hydrogen sulfide (H₂S), often associated with rotten egg smells, has a detection threshold of approximately 0.00047 ppm in air, while methanethiol (a mercaptan) is detectable at around 0.00067 ppm, and vanillin at about 0.2 ppm in aqueous solutions. These values illustrate the scale of sensitivity, where some odorants can be perceived at parts per billion or lower concentrations.[26][27] The following table summarizes representative detection thresholds for selected common odorants, drawn from standardized compilations. These are typically reported as the concentration at which 50% of a panel detects the odor (ODT₅₀), measured in air unless noted otherwise.| Odorant | Detection Threshold (ppm) | Notes/Source |
|---|---|---|
| Hydrogen sulfide (H₂S) | 0.00047 | Rotten egg odor; AIHA compilation (Nagata, 2003)[28] |
| Methanethiol (CH₃SH) | 0.00067 | Mercaptan family; AIHA compilation (various studies)[28] |
| Ammonia (NH₃) | 5.0 | Pungent; EPA reference guide (1992)[29] |
| Acetone | 20 | Sweet; AIHA compilation (May, 1966)[28] |
| Vanillin | 0.0002 (air); 0.2 (water) | Vanilla-like; AIHA and PubChem data[26][27] |