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Everyone works poorly when tired. We become tired whenever the work we carry out involves human (physical and/or mental) effort. In the fragrances and flavors (F&F)industry, the routine feature requiring human inputs is the evaluation of aromatic raw materials and fragrance products. Losing sensitivity to odors (olfactory fatigue, wear out) due to continuous exposure to relatively unvarying smell is a common experience occurring in the F&F industry. Dr Sitaram Dixit explains how to prevent and manage lethargy in cases of olfactory fatigue.

God has no doubt endowed us with a robust human olfactory system, however, on facing a continuous stream of constant odorant (fragrance, flavor or an obnoxious odor) it tends to lose its sensitivity by adapting itself,so that the stream of odor tends to smell weak at first and then, later on, goes to produce a total loss of odor perception.


All human senses tend to adapt to varying degrees, with each one susceptible to adaptation according to available sensory inputs. Typically, for instance, if we come out of a dark room, say, a movie theater to bright daylight we would find the outside very bright until our eyes automatically re-adapt to the brightness outside. However, in olfaction, adaptation thankfully is temporary that, which normalizes, simply by breathing non-odorized air. Suppose, we smell a substance and shortly after some moments smell the same substance at a lower concentration than earlier, then either the odor sensation would be absent or would be considerably weaker. This is what we call, “self-adaptation”.

How long does it take a smell to disappear, is a question that many researchers over the past several decades have tried to find an answer to, without success! Every substance does eventually disappear but researchers do not find any clear relation between odor strength and the time taken for the disappearance of the smell sensation. Every person has a different criterion, about the smell sensation disappearance time, versus concentration. Researchers find that the time required to reach total adaptation, is proportional to the square root of concentration. It means that doubling the concentration, requires only about an additional 40% time, to achieve total disappearance of smell sensation. In this case, however, we should consider the existence of biases that could occur as some individuals may adopt a more stringent criterion, to decide on an odor perception in comparison to other individuals, as changing criteria yields varying appearing rates, of olfactory adaptation versus concentration.

The rate, at which an odor sensation disappears, is another question we need to look at. Does it do so rapidly within seconds or are there residual odor left, to adapt out over the next few minutes or does the odor impression diminish slowly and steadily or linearly until no longer present? Expectedly the rate of adaptation is higher when the stimulus is more intense, although biases like assigning an inappropriately initial higher rating could creep in. Studies say that the perceived odor intensity, declines quite rapidly at first and then more gradually until it finally approaches an asymptote, i.e., reflecting a terminal or final, non-zero, perceived odor level. If we interrupt the continuous stream of odorized air with an odorless air or say, with a different substance, then we can bring about a recovery, meaning a return to sensitivity, before adaptation. Increasing adaptation time, increases olfactory fatigue, which would require more long waiting time, to recover sensitivity.

Practically speaking, odor intensity, odorant concentration and the time of exposure, affects odor adaptation and therefore it becomes more relevant, to consider and give importance to all this, while setting up odor evaluation panels. 

  1. To prevent adaptation, choose shorter evaluations that are more effective, than long ones.
  2. Odor intensity perceived, is likely to change during an evaluation, due to adaptation even though, physically the sample is unaffected. Evaluator, therefore, could wrongly state the results of the sample, as having changed in odor quality, when all that has occurred is odor adaptation.
  3. A sufficiently well-planned regimen of evaluation is necessary, so that panelists smell more odorants, especially if the odorants are much above their threshold levels.
  4. During sequencing fragrance evaluations, take care to present the panelist, the weaker odorant stimulus first, followed by the stronger fragrance stimulus and not vice versa.
  5. Presenting odorants of different types or qualities in continuous succession, there is a possibility of cross adaptation.

 Cross Adaptation in olfaction varying with both concentration and time is sometimes non-existent or minimal and when existing, it is neither symmetric nor the effect of one odorant on another dependent or reciprocal. Cross adaptation thus is unsystematic and unpredictable.

Odor intensity increases systematically, with odorant concentration. Researches show, that an adapting stimulus, can affect other stimuli, in the same mode with the effect, primarily apparent on odorants of lower physical concentration.

Hedonic long-term adaptation

Sensory adaptation to odors dissipates quickly, although perceptual or hedonic adaptation remains for a longer time duration. Panelist generally report, that an odorant or fragrance no longer smell as pleasant as another odorant or is not as strong as the other and this adaptation could be due to cognitive adaptive process and not sensory as imagined. 

We observe that substantial hedonic habituation can even occur when we present the same odorant to the panelist in a single session. An obnoxious odor does not smell as bad as it smelt in the beginning. This phenomenon wherein a fragrance or flavor no longer seems as pleasing or strong as earlier is only another form of long-term habituation. Repeated stimulation with different fragrance odors, becomes a contextual experience against which we can evaluate a fresh fragrance or flavor. When one smells a strong fragrance, it will form a reference odor experience, so that even when the strength of the odor remains the same, the next sample will seem weaker in the new context of heightened intensity. Relative odor intensities do not change the frame of reference changes, although these odors are in absolute descriptive terms. Helson calls this the “Adaptation Level Theory of Perception”. The adaptation level is the perceptual/cognitive portion of adaptation that is not sensory, but is a result of our system of processing and coding information, along with the nature of the human classification of sensory inputs on intensity. All these findings, however, are only general observations and not conclusive findings.  

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Practical Implication For Testing Fragrances

In real-life situations, evaluators must carry out dozens of evaluations without either getting bored, nor with any loss of sensitivity. It is imperative to optimize the product testing sequences and 2-5 minutes of rest periods are necessary to ensure recovery of sensitivity, at least at the supraliminal or suprathreshold intensity levels. Panelists, in any case, should not smell a single fragrance continuously, whether using a blotter smelling strip, taking a whiff from a bottle or even from a human arm.

The ideal single smelling time is about 2 seconds only and if the panelist needs to smell it again, it better be only after 15-20 seconds gap. Alternately, the panelist can smell a blank blotter or odorless cloth, etc. The intervening period between two evaluations, breaks the monotony of similar smells. Product wear-out is an indication, of the panelist adaptation level, reflecting the effect of previous experience with new products. Incidentally, product wear-out, is an evaluative cognitive phenomenon and is not a sensory one.

Considering the evaluator panelists are a motivated lot, it is possible to evaluate up to 20 fragrances in accession, taking care to avoid any significant loss of sensitivity. In an optimized testing situation, adaptation is not an inherent limitation, on olfactory sensitivity, but a motivational phenomenon.

To prevent the panelists from experiencing fatigue, it is significant to reduce the numbers of samples, during sensory evaluation sessions. General experience reveals that a fragrance panel could efficiently evaluate 4 pairs (8 samples) per session and flavor panel could evaluate only 3 samples per session in an hour or so.

However, we admit that there exists, a lack of scientific clarity, on the accuracy with respect to various factors like,

  • Experimental conditions 
  • Sensory Evaluation methodology 
  • Duration of the sensory session, etc.

This inadequate scientific information, gives rise to the ambiguity of the word,“FATIGUE”.


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The Components of Fatigue

  • Muscular Fatigue: In sensory evaluation, muscular fatigue can take place at Jaw level, when evaluating the texture of a solid food. e.g., chewing gum or Toffee. However, its importance is generally low. Local muscular fatigue should not be confused with general physical fatigue, which affects the whole body.
  • Psychological Fatigue or Lassitude :The person has no more interest in his/her work due to routine job of smelling/tasting – Similar things’ day in and day out. It is hard for anyone, to concentrate on the job due to this, all resulting in fatigue. 
  • Mental Fatigue: This kind of fatigue occurs in sensory evaluation. When person does not have a scientific training on profiles, or descriptors or uses wrong descriptors, which are irreproducible. When one understands, what and why they are performing the job, mental fatigue will reduce substantially, increasing motivation. We observe this happening, more in perfumery than in flavors.
  • Nervous Fatigue: The fear of getting the evaluations wrong, makes the panelist nervous,who eventually fails to concentrate, on discriminating an odor/flavor. Temporary and reversible increase in the threshold values, relates largely to nervous fatigue. 

Adaption and Sensory Fatigue

Prolonged stimulation makes our senses, lose their sensitivity. We call this phenomenon as sensory adaptation. In olfaction, adaptation can be complete, i.e., a stimulus we perceive in the beginning, may become completely imperceptible, after some continuous exposure. 

Supposing we enter a room having a smoky odor, we will easily be able to perceive it on entering, however after spending some time therein, we will not get that strong smoky smell, thereby meaning our fatigue adapts itself to the smoky background. However, if we come out of the room for few minutes, breath fresh air and then go back inside the same smoky room, we are likely to get the same smoky smell, again as strongly, as the first time earlier. This is a common occurrence in practical life wherein it takes one at least 2-3 minutes to regain the initial sensitivity after fatigue.

Considering all this, we should test odorant samples in pairs, from left to right and then vice-versa or guarantee to follow a random order. We could permit retesting, but not too frequently, else, fatigue could hinder decisions. In any case, the first olfactory impression, is likely the correct one, in tune with the English expression, “First impression is the best impression”. During sensory evaluation of foods, generally complete adaptation takes place, as regaining sensitivity takes time. Practical regular experience show, that panelists perform paired tests, triangular tests, two out of five tests, etc., comfortably without much difficulty.

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Practical Sensory Evaluation

In any sensory evaluation study, when panelists assess the same fragrance, on several occasions or say carry out multiple evaluations, of the same fragrance, perceptual changes, are bound to occur. When using a small expert panel, it is advisable, to have many base sizes of judgments, to ensure stable data. Experience indicates that, even panels consisting of less than 9 expert members, are often too erratic that a single aberrant rating, can yield unstable data results, unduly affecting results. When we use larger consumer panels and several judgments of the same product, we are likely to secure a more reliable data, accurately representing the results of long term, human exposure.

In consumer panels, all too often, a single smell of a fragrance could provide misleading information, as a fragrance liked by a panelist on the first sniff, may be subsequently disliked during normal home use condition. The better way, is to carry out a home-use test, wherein panelists use and evaluate the fragrance for a week or more, along with several other products, in a central location test facility under controlled conditions, unlike one at home.

There is no clear scientific data, on the effects of repeated fragrance evaluation, particularly on product evaluation. In general, replicate ratings, that secures test reliability, should yield same results on different occasions, keeping all other factors constant. Some published reports, instead also attribute odor perceptible changes, to test method inconsistencies.

Repeated smelling of the same fragrance in blind tests, reveals, that the more concrete the sensory attributes (intensity, note) will be, more reliable it is and will repeat themselves with variations. However, for less concrete evaluative attributes (hedonics, imagery) results vary during repetitive evaluations. Thus, in sensory analysis, perfect reliability does not exist. Panelists data varies. Therefore, the best method, to secure reliable ratings data, is to sample large number of panelists, to eliminate errors, due to sampling subpopulations and representations.

 Multiple repetitions of evaluations of the same fragrance also increases bases size, making the data more reliable. However, there are changes in fragrance profile, over replicate evaluations, fortunately though, the fragrance ratings are sufficiently random, to prevent any systematic shift with repeated experience. Evaluative research loses little on any evaluation from multiple panelists rather than testing the same product repeatedly with just a few panelists.

 Although, these findings do not support, superiority of short-term central location testing, over linger-term home use tests, however, it does say, that one rating, secured from a panelist after valid experience with a product, will be as useful, as several such ratings each, secured from the panelist under the same experimental conditions. In fact, selecting meaningful attributes is significantly more important to produce reliable ratings, in addition to increasing repeated product evaluation, instead of just increasing base size. In short, it is better to test once, with 100 panelists, than test it four times with 25 panelists.



  • Duration of sessions in a well-ventilated odor free room.
  • 1-2 hours in the morning.
  • hours in the evening if required.
  •  5-10 minutes break in the fresh air, for every30 minutes.
  • Technically qualified panelists: 3 to 5 in age group 25 to 40 years.
  • Number of samples:  Not more than NINE in each session.
  • Maximum repetitive testing:  3 times with panelists having “Full Concentration” without “Fatigue”. 


  1. Duration of Session: In a well-ventilated convenient aroma free room, 4 or 5 hours with a break of one hour. During this break. Panelist can eat, cakes, bread with butter, pieces of apple, cold coffee or cold tea, fruit juice or drink chocolate. Panelist should strictly avoid spicy or hot food and/or beverages.
  2. Technically qualified panelists: Group of 6 (number can vary, depending on food product) in age group 20-35 years
  3. Number of samples: Not more than five at a time
  4. Maximum repetitive testing: Two times only with panelists having a free mind, without any preoccupation thoughts. (We however disqualify all persons who are smokers, pan eaters, tobacco chewers, etc., from becoming panel members.)

To conclude we can say that, “Fatigue” in sensory evaluation depends on the conditions of sensory evaluation. If panelists are sincere and do not consider sensory evaluation as a chore, there is not much fatigue i.e., there is not much variation in the quality of their evaluation over a period, however, if the panelists consider the sensory evaluation as a routine ritual and real hard work, then surely there is a decrease in the quality of responses. Liking the sensory evaluation processes alone can help manage lethargy, fatigue and boredom in fragrance smelling always.

Author : Dr. Sitaram Dixit

Dr. Sitaram Dixit is an Independent Consultant – F & F, Cosmetics, Personal, Fabric and Household Care Chemical Industry.