SENSORY THRESHOLDS SENSORY THRESHOLDS

Do You Know Yours?

Do You Know Yours?

n interesting question was raised recently regarding the sensory evaluation of bourbon and rye whiskies with there being a lack of discrimination via sensory descriptive analytes. This topic was raised via the release of a publication by Lahne et al., (2019). Without going into too much detail about any contention of such findings, this begets the question; “Do we know enough about the chemical and sensory complexity of distilled spirits?” The simple answer is no. We do not (yet) have details such as the (accessible) threshold values whereby we can assess the relative amounts of many of the key components in our favorite spirits. Though the current research is helping to clarify the situation, at least with respect to the chemical component aspects, distillers still lag far behind brewers and enologists when it comes to full sensory evaluation of their products.

The training of sensory evaluation teams is a complex topic and often starts with the spiking of products with known amounts of pure chemical components to elevate the actual flavor impression in order for them to learn how to describe and identify such flavors or for aroma purposes only — spiking such components into “sniff” bottles containing water- or spirit-soaked cotton balls. Following such trainee evaluations, the sensory descriptors are discussed and, when possible, more technical, rather than simple hedonic descriptors, are learned, memorized, and applied to subsequent tastings. It is best if all members of a panel are on the same page and know the basic chemical names of components. This makes life easier for the sensory panel team leader. Sensory biases must be avoided, and an understanding of the sensitivities of individuals to specific components needs to be understood as some individuals are sensorially “blind” (having anosmia1) to certain compounds. Individuals thus vary greatly in detecting and identifying many different aroma components. While we use the term or concept of taste to describe food and beverages, taste or rather overall flavor perception is a combination of all the senses; smell (olfactory), taste (gustatory), trigeminal sensations (pain/irritation, warming, cooling — mouthfeel “touch” and viscosity) and even sight (color) and sound (effervescence for carbonated products). Flavor impressions are, however, disproportionately olfactory (smell) based. Moreover, synergistic and antagonistic interactions are at play in how the human senses and brain interpret flavor information with such implications described

1 What is Anosmia? https://jamanetwork.com/journals/jama/fullarticle/2767634 recently by Barwich and Smith (Barwich, 2020; Barwich & Smith, 2022), with the complex nature of flavor and both machine and human sensory evaluations covered by Chambers and Koppel (2013). Further details are not described here. (See Artisan Spirit issues 12, 16, 22, 28, 29, and 39 for more on general sensory and descriptors — the latter concerning the sensory training tool and memory jogger — The Flavor Wheel.) The basis of this article is to go beyond the basics of establishing flavor descriptors and to allow tasters, or taste panel leaders, to better understand the sensitivity of individuals in defining appropriate flavor attributes. Ultimately leading to their learning of the relative concentrations of such components and to be able to quantitatively determine with some degree of accuracy the amounts of various components present in a food or beverage. This will enable a better characterization of the differences and similarities in products and provide better overall profile descriptions of a brand, a new formulation, or competitive product. This then allows for an understanding of the origins and controls of both off-flavor notes and desirable qualities and for better discrimination of those bourbon vs. rye whiskies alluded to above.

Thresholds Definitions

Purposes

A grasp of threshold data allows for the:

• Determination of the level of substances that begin to affect the acceptability of products.

• Selection and training of panelists (and sensory judges).

• New product development.

Determination

Threshold determination is the only method by which:

• Several flavor problems can be investigated.

• The determination as to which aroma/ flavor components, from within a matrix of hundreds of substances found in a product, will influence its flavor.

• The magnitude of such activity for substances found to be flavor active may be estimated.

• Determinations of the point at which known contaminants begin to reduce acceptability.

Panelists Selection And Training

Spiking of spirits and changing concentrations of the spiked components can be used to:

• Determine the relative sensitivity of individuals to a given flavor active substance.

• Detect those with specific anosmia(s).

• Determine low and high sensitivities.

• Train on flavor descriptors to be used by a panelist.

• Note any flavor perception changes associated with increased substance concentration.

• Determine saturation concentrations (see definitions below).

New Product Development

• For the formulation/development of new products the threshold of added desirable substances may be used as a research tool in the formulation of foods and beverages, etc.

Several threshold terms need to be understood. These four key terms are:

1) Absolute (or Detection) Threshold

> The lowest stimulus (energy) capable of producing a sensation.

> Initial detection — “something is different,” but not yet defined/identified. Example: the weakest taste (lowest concentration detectable). Below this, no effect on the sensory system.

> Sometimes also called the stimulus threshold.

2) Recognition Threshold

> This is the level of a stimulus at which the specific “stimulant/component” can be recognized and identified.

> It is usually higher than the absolute threshold. In other words, it is the lowest concentration for positive identification. For example: When tasting pure water +/- sucrose, a transition occurs from “water taste or pure water” to “a very mild taste.” Then as the sucrose concentration increases, a further transition from “a very mild taste” to “sweet” is noted.

> The level at which the second transition occurs is the recognition threshold.

3) Difference Threshold

> The extent of change in the stimulus (energy) necessary to produce a noticeable difference or the smallest change in the concentration of a stimulus which can just be noticed. One changes the variable stimulus by small amounts above and below the standard (sample) until the subject notices a difference.

> This is called the just noticeable difference (JND).

4) Terminal Threshold

> The magnitude of a stimulus above which there is no increase in the perceived intensity of the appropriate quality for that stimulus: above this level, pain often occurs.

> The saturation level! Pain indicates that damage is or might be occurring.

THRESHOLDS = STATISTICS

Intuitively a threshold is the transition point between no detection and detection. The transition point is not, however, constant. Responses are affected by psychological and physiological inputs, so shifts occur. This issue is overcome by treating thresholds as a statistical quantity. The detectability of a stimulus does not jump from 0 percent to 100 percent at some particular value. Rather, the probability of detection increases gradually as the intensity of the stimulus increases, so thresholds are defined as that “Concentration (stimulus intensity) which (elicits a response) that can be detected or recognized 50 percent of the time.” Or that concentration which can just be perceived by 50 percent of the population. Or that difference in concentration which can be resolved 50 percent of the time. See Figure 1. Detailed information on such concepts can also be found in several key works (Lawless & Heymann, 2010; Lawless, 2013; Meilgaard, Civille & Carr, 2016).

Threshold As A Concept For Subject Sensitivity

Thresholds are classically thought of as determinants made in individual subjects. No two human observers are alike; furthermore, a given observer varies in sensitivity from instant to instant.

Therefore, the problem of determining thresholds is a problem of handling variability among and within human observers after the data are collected. A practical consequence of this variability is that those thresholds must be considered only as approximate. They will be found to vary somewhat from one determination to another.

FIGURE 1. Flavor Threshold Models

Figure 1a shows the conventional notion of an absolute threshold whereby the tipping point for detection/change or identification of a component would occur at a specific amount noted by all “normal” subjects (assuming no anosmics). It is an all-or-none mechanism! Figure 1b illustrates the variability in the threshold determination for the personal threshold level. Panelist/subject variation may occur due to ambient conditions, the temperature of samples, variation in sensitivity of the sensory detection systems, physiological (health) issues, and sometimes due to certain biases (not further discussed here). This means the threshold is not a fixed point, instead, it is a value on a stimulus continuum. In essence and theory, the mid-point determines the value which will become known as the personal threshold value and means that this value is the concentration value detected 50 percent of the time. A panel leader will use it to determine generally how sensitive each panelist is to an attribute. Panelists will need to be retested periodically. This value

Threshold Models

In dealing with thresholds and statistics, we can view three models. Figure 1 shows the “Ideal model.” This would indicate the “all or none” situation. At some concentration of any substance, it would become detected and potentially identifiable. However, Figure 1b shows the variability in threshold determination for the personal threshold level. Finally, Figure 1c in the set shows the threshold distribution for a group. The figure legend covers the full detail on these concepts. Details are derived from a course lecture (Spedding and Aiken) with the adapted figures based originally on details from Meilgaard, Civille & Carr (2016).

Individual Taster Calculation

The Best Estimate Threshold (BET) is the geometric mean of the highest concentration depends upon the matrix, so if the product for evaluation changes (or a different distillery is moved to), further training/evaluation will be needed. Finally, the Figure 1c histogram illustrates the issue with respect to group threshold values. Statistically, these group values or Best Estimate Thresholds (BETs) are more reliable than individual BETs. Such values are used as “midpoint anchor” values in the design of threshold test protocols. The group threshold value is determined from the personal BETs (see text and Figure 2) from the characteristic and familiar “Bell curve” data. There is an unequal distribution — shift to the right, which is representative of the number of individuals exhibiting low sensitivity to the stimulus. Unlike many of the subjects here, they miss the determination/detection until a higher concentration of the stimulus compound is presented to them. A few panelists will detect the stimulus change/ identify the component at lower concentrations showing a higher sensitivity than the overall group. Significantly though, due to the shift away from a centralized tendency, the best measure of the group BET is the geometric mean, as it gives less weight to the highest thresholds. not sensed (A), and the next higher concentration (B). Thus, the BET = Square Root (A x B). See Figure 2 for a summary.

Panel Flavor Threshold Calculation: reshold Level = Log -1 Σ =N =1 Log (BET) N

For N Panelists.

The maximum likelihood threshold is the geometric mean of individual thresholds. This can be calculated without the need for accurate determination of each individual threshold. (Statistics smooths out the data!)

How To Determine Flavor Thresholds

1. You need to start with a value that you expect will be at about the threshold level (may need prior experimentation).

2. Use this concentration as a “middle ground” value. Pick two values below and two values above this value (e.g., for acetic acid could use 50, 100, 200, 400, and 800 ppm). Note, values are reduced by half and half again below the 200 ppm concentration and then are doubled and doubled again above 200 ppm. Concentrations and delivery of the “spike” needs to be known/done as accurately as possible. Chemical stocks of high purity and of food grade and safe to use, but may need to be diluted.

3. Set up a row of samples of increasing concentration (with or without a control “zero” to account for the concentration of the component in the base spirit — unless known)

4. Ask panelists to pick the sample in which they can identify (recognize) the compound of interest.

5. Apply the math to determine their (BET) actual (geometric) threshold (at this point in time).

6. Determine a group threshold value.

REFERENCES:

Lahne, J.; Abdi, H.; Collins, T.; Heymann, H. bourbon and Rye Whiskeys Are Legally Distinct but Are Not Discriminated by Sensory Descriptive Analysis. J Food Sci 2019, 84 (3), 629-639. DOI: 10.1111/1750-3841.14468.

Barwich, A.-S. Smellosophy: What the Nose Tells the Mind; 2020. DOI: 10.4159/9780674245426.

Barwich, A.-S.; Smith, B. From Molecules to Perception: Philosophical Investigations of Smell. Philosophy Compass 1903 2022, 17. DOI: 10.1111/ phc3.12883.

Chambers, E. t.; Koppel, K. Associations of volatile compounds with sensory aroma and flavor: the complex nature of flavor. Molecules 2013, 18 (5), 4887-4905. DOI: 10.3390/molecules18054887.

Lawless, H.; Heymann, H. Sensory Evaluation of Food Science Principles and Practices. 2nd Edition, Ithaca, New York. 2010 DOI:10.1007/978-1-44196488-5.

Lawless, H. Quantitative Sensory Analysis: Psychophysics, Models and Intelligent Design. 2013. DOI: 10.1002/9781118684818.

Meilgaard, M.C.; Civille, G.V.; Carr, B.T. Sensory Evaluation Techniques. 5th ed. Boca Raton: CRC Press/Taylor & Francis Group. 2016 DOI:10.1201/b19493.

Meilgaard, M. C. Testing for Sensory Threshold of Added Substances. Journal of the American Society of Brewing Chemists 1991, 49 (3), 128-135. DOI: 10.1094/ ASBCJ-49-0128.

Lee, K.; Paterson, A.; Piggott, J.; Richardson, G. Measurement of Thresholds for Reference Compounds for Sensory Profiling of Scotch Whisky. Journal of the Institute of Brewing 2000, 106. DOI: 10.1002/j.2050-0416.2000.

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Barnes, Q.; Vial, J.; Thiébaut, D.; De Saint Jores, C.; Steyer, D.; Contamin, M.-A.; Papaiconomou, N.; Fernandez, X. Characterization of Flavor Compounds in Distilled Spirits: Developing a Versatile Analytical Method Suitable for Micro-Distilleries. Foods 2022, 11, 3358. DOI: 10.3390/foods11213358.

Miller, G. Whisky

Science: A Condensed Distillation; 2019. DOI: 10.1007/978-3-030-13732-8.

Spedding, G.; Aiken, T. 18Sensory analysis as a tool for beer quality assessment with an emphasis on its use for microbial control in the brewery. In Brewing Microbiology, Hill, A. E. Ed.; Woodhead Publishing, 2015; pp 375-404.