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Health, but also personal values and environment, are driving more and more consumers towards sugar, salt, alcohol, fat reduced products and plant-based foods. Yet consumers often do not like these products because of their poor flavor. This creates a need within the food industry to innovate by testing natural sources for novel compounds that show interesting trigeminal or taste effects. Sensory approaches are proposed to screen and assess the performances of compounds eliciting trigeminal sensations as cooling, warm, pungent, tingling, for e.g. Sensory performances of trigeminal compounds are described using different methods. Considering flavor perception as a functional integrated sensory system, these compounds could modulate taste and improve the overall flavor.
Health, but also personal values and environment, are driving more and more consumers towards sugar, salt, alcohol, fat reduced products and plant-based foods. Yet consumers often do not like these products because of their poor flavor. This creates a need within the food industry to innovate by testing natural sources for novel compounds that show interesting trigeminal or taste effects. Thus, the challenge for flavor companies today is to provide “natural” flavoring solutions to fill the sensory gap of these types of food products, while maintaining the same flavor perception. How can we find these new molecules and characterize their potential interesting sensory effects?
In humans, the olfactory and gustatory systems are the principal chemosensory systems, but the trigeminal somatosensory system also plays a fundamental role in chemosensation and the overall flavor perception of food and beverages [
] proposed that flavor represents a functional sensory system for feeding with inputs from somatosensation, gustation, and olfaction, the key to the system being the meaning of the sensation rather than the organ of transduction.
Considering flavor perception as a functional integrated sensory system could allow flavorists to compensate for the taste of reduced ethanol or salt in a beverage or food product, for example, by using another sensory input such as trigeminal perception. So trigeminal sensation is one of the pillars of the flavor perception. It can potentially contribute to fill some sensory gaps. Sensory approaches will allow to screen and assess the performances of compounds eliciting trigeminal sensations in the oral cavity to modulate taste and improve the overall flavor. These proposals are based on a better understanding of trigeminal and flavor perception, with the final objective to ensure consumer's acceptance.
Trigeminal perception
Trigeminal perception is particularly rich in sensory qualities (Table 1) and is also characterized by the evolution of sensations over time as delay, sensitization, desensitization [
]. Trigeminal sensation is perceived through the sensory endings of the trigeminal (V cranial) nerve. These endings can be activated by physical stimuli (mechanical forces and temperature) and by chemical agents (chemesthesis), and evoke sensations of touch, temperature, and pain [
]. Oral chemesthesis explains the pungent or sharp feel of many different foods and spices such as chili pepper, horseradish, wasabi roots and Szechuan pepper; the coolness of peppermint; the tingle of carbonated drinks; and the irritation produced by substances such as garlic extract [
]. The perception of ethanol is a combination of tastes (bitter and sweet) and odors, but also of oral irritation as pungent, burning, tingling and astringent sensations. Most of the chemical stimuli can produce more than one sensation and can be described more precisely with specific terms (Table 1). The concentration of the compound determines to some extent the perceived sensation. For example, menthol is pure cooling at low concentration while burning is also perceived at high concentration. In recent years, tremendous progress has been made in our understanding of how different chemicals are detected by trigeminal endings as transient receptor potential (TRP) channels [
Table 1Sensory qualities associated to different types of trigeminal stimuli. The dark grey indicates the main sensation at usual concentrations in food product
Table 1Sensory qualities associated to different types of trigeminal stimuli. The dark grey indicates the main sensation at usual concentrations in food product
Interactions between taste and trigeminal perception
The perceptual integration between olfaction and taste and the cognitive mechanisms involved have been widely studied and are addressed [
]. The trigeminal sensation is also often associated to the sensation of smell, but the interactions between taste and trigeminal perception are important and are somehow less described [
] proposed that trigeminal inputs could influence flavor perception by tactile, thermal, painful and/or kinesthetic effects and that this effect has been underestimated for many years. Many psychological studies have emphasized the suppressive or masking effect of chemesthetic compounds on taste perception, but not for all tastes [
] showed that menthol decreased the sweetness and increased the sourness of lemon-flavored yoghurt. Similarly, capsaicin reduced the sweetness in food (soup and flavored mixes), but had no effect on saltiness or sourness [
], suggesting that the saltiness rating could be affected by irritation intensity. Some chemicals such as spilanthol are claimed to enhance saltiness [
] showed that the trigeminal sensations elicited by carbonation or cooling agents could impact the flavor perception of model beverages, depending on the congruency between trigeminal sensation and flavor. Starkenmann et al. [
] showed that trigeminal sensations could also induce new sensations, which help to maintain global taste and flavor perception of products that have reduced sugar, salt, or fat. Green [
] specified how viscosity impacts the perception of sourness. Other investigators demonstrated the importance of temporal synchrony in perceptual interaction mechanisms among olfaction, taste, and texture [
] showed that the evaluation of taste components can also be influenced by the tactile quality of the food. They found that the roughness or smoothness of the foodstuff itself significantly influenced perception: food was rated as significantly sourer if it had a rough (versus smooth) surface.
On another hand, many studies showed that conventional taste compounds such as salts and acids can elicit oral chemesthesis [
] and has the potential to be powerful. Perceptions of the quality and the time course of these trigeminal compounds, however, create difficulties for practical use in flavor optimization. Flavor companies are interested in evaluating products that stimulate trigeminal perception in the oral cavity and can potentially modulate taste (e.g., increased sweetness and saltiness), which improves the overall flavor. The sensory methods presented here allow us to screen and to assess the performances of these products.
Sensory approaches
In this part, we describe the sensory strategy and methods and examples of sensory results obtained with human panels to discover and to characterize uncommon trigeminal-active compounds. The use of human panels allows us to assess the whole trigeminal performance of novel compounds and their potential taste modulation effects. Following the isolation and characterization of novel compounds that potentially elicit an interesting mouth effect, we perform detailed sensory tests with a trained panel to evaluate the parameters of this effect [
]. The sensory aspects currently assessed are determined in two steps.
The first step allows identifying types of trigeminal and modulation taste effects, for example, cooling sensation, enhanced saltiness in an NaCl solution or masking of bitterness and metallic off-note effects in a KCl solution, by comparing model solutions with or without the novel compound. The second step focuses on the sensory characterization of the novel compound by using both quantitative measures, such as detection thresholds, dose-response curves or intensity profile over time, and qualitative measures to define the most appropriate terms to describe the products. The sensory characterization of these new compounds allows comparing their overall performance with that of benchmark products that have previously been characterized.
Our approach, which involves sourcing, screening, and sensory characterization steps, is outlined in Figure 1. Here, we will focus on screening and characterization methods that are based on sensory processes and human panels. As the sensory process with human panels is a low throughput screening method compared with that of a cell-based assay, sourcing must be selective. Product selection is based on knowledge gained from the botanical literature, ethnobotanists, chefs, recipes, and common plant uses. These products must be delivered at a given quantity to allow panel tastings at an acceptable concentration.
Figure 1Sensory Strategy and Methods to discover and assess the sensory performances of trigeminal compounds to enhance overall taste experience whilst reducing where possible the sugar, salt, fat, alcohol content of food/beverage.
Screening new compounds for trigeminal & taste modulation characteristics
The screening of compounds having trigeminal effects and/or potential taste modulation is achieved with a trained panel of 15 subjects. Tastings are organized on a weekly basis using water and water-based model systems (Figure 2). Panelists taste the sample from a black cup with or without using a nose clip to focus on taste and avoid the influence of vision (since some samples are colored). All samples are first tasted in water and then in the water-based model system. For sample tasting in water, the sensory task of the panelist is to describe the sample by selecting appropriate sensory attributes in a predefined list. For the sample evaluation in model systems, the subject must first taste the model system alone as a reference (for example a sucrose solution) and to indicate the perceived intensity of the taste qualities on a linear scale from not intense (0) to very intense (10). The treatment of data for the sample evaluation in water is simply the number of attributes cited by the panelists. For the sample evaluated in different model systems, a student t-test (one-tailed paired test) is performed to compare the systems with the sample and those without it (reference). The example presented in Figure 2 concerns an extract of alligator pepper from Africa tasted at 400 ppm by 16 trained panelists. The left column provides (in brackets) the number of subjects who chose the attribute. The extract has a clear hot/pungent trigeminal effect, which is like the effects produced by capsaicin or piperine. In the right column, the differences in scores between the reference and the sample are indicated for each water-based model system. The number of asterisks corresponds to the significance level of the t-test (∗ = 95%, ∗∗ = 99%, and ∗∗∗ = 99.9% significant difference from the reference). Interestingly, this extract has a significant salt-enhancing effect and a sweet-masking effect. This example demonstrates that some products having trigeminal properties could have an interesting modulation effect on taste. This was also the case for some products that induced a tactile sensation in the mouth and increased sweetness. The next steps are to identify the chemicals that elicits these interesting mouth effects as cooling, hot, pungent, burning, tingling, tactile and/or taste modulation and then to characterize the sensory effects in comparison with benchmark products.
Figure 2Sensory attributes and taste modulation properties of alligator pepper extract tasted in water-based model systems.
Sensory characterization of new trigeminal compounds
In this part, we present examples of compounds having interesting trigeminal effects, which can be used for taste modulation, ethanol enhancing, and more generally to enhance the global flavor of food products or beverages. For some of these compounds, their sensory characteristics are compared to benchmark products.
Mouthfeel effect of (R)-strombine from dried scallop
The mouthfeel of seafood is quite different from that of chicken broth, for example, which is fattier, saltier, and more umami. We investigated the taste-active compounds in dried scallop and discovered that one fraction was clearly described as sweet with fullness in the mouth. (R)-strombine and glycine contribute significantly to this specific taste [
]. The sensory characterization was combined with threshold measurements (ASTM E679 Ascending Concentration Series Method of Limits) in one session per product, with choices of sensory attributes. The panelists were asked to choose an attribute from among sweet, salty, bitter, umami, acid, mouthfeel (Figure 3). (R)-strombine was perceived as salty, umami, and contributing to mouthfeel by most panelists. The multisensory aspect of this compound is interesting, especially for savory applications. The taste component of glycine is unimodal and clearly sweet.
Figure 3Sensory characterization of (R)-strombine and glycine at various concentrations: Number of subjects (Out of 30 panellists) who selected the proposed sensory attributes.
Hot/pungent effect of polygodial from laksa persicaria odorata (lour.) sojak
Laksa Persicaria odorata (Lour.) Sojak, a coriander plant commonly used in Thai cuisine, displays a pungency that is quite different from the pungency or tingling of known compounds such as piperine, capsaicin, gingerol or even spilanthol [
]. It resembles the pungency of galangal acetate, which occurs in Alpinia galangal (Zingiberaceae family). Extraction and analysis of the leaves of laksa led to the discovery of the known polygodial, a compound also present in mountain pepper Tasmannia lanceolata (Poir.) from the family Winteraceae [
Comparison of volatile constituents of Persicaria odorata (Lour.) Sojak (Polygonum odoratum Lour.) and Persicaria hydropiper L. Spach (Polygonum hydropiper L.).
Journal of Agricultural and Food Chemistry.2006; 54: 3067-3071
]. Polygodial is a good example of the potential uses of pungent molecules in different applications. It can be used for example in wasabi paste to enhance the pungent effect of allyl isothiocyanate without the associated rubbery olfactive note, to enhance the freshness of toothpaste [
]. The sensory characterization of polygodial, piperine, and capsaicin was performed by a panel of 25 subjects. Threshold measurements were achieved with the 3-alternative forced choice method (based on the ASTM E679 Ascending Concentration Series Method of Limits) in one session per product. According to average threshold values (Figure 4), polygodial is twice as strong as piperine, but 40 times weaker than capsaicin. The slightly wider distribution range for the perception of polygodial than for the other two compounds means that there are more differences between subjects for this compound. This characterization allows determining relevant concentrations of this compound for dedicated applications.
Figure 4Detection thresholds for polygodial in comparison with those of capsaicin and piperine.
Cooling effect of dihydroumbellulol from the leaves of umbellularia c. nutt
When the leaves of Umbellularia californica Nutt. are crushed, smelled or tasted, a clear trigeminal effect is perceived. As a result of this observation, this plant was studied and umbellulone isolated as a major constituent. Dihydroumbellulol was obtained from the umbellulone [
]. The cooling strength of this compound was then evaluated by a trained panel. The objective of the sensory evaluations was first to confirm the cooling effect of dihydroumbellulol, and then to compare its performance with that of (-)-menthol. Thirty trained panelists evaluated the perceived cooling intensity of the two samples, dihydroumbellulol and (-)-menthol, over time (one sample per session, at 50 mg/L). Each subject received a 30 mL sample and was asked to place it in the mouth and to start a timer at the same time. Subjects spat out the sample after 5 seconds. Panelists evaluated the time at which they started to perceive the cooling sensation (Tbegin), the maximum perceived cooling intensity and its corresponding time (Imax and Tmax), the time when the cooling sensation started to decrease (Tdec), and finally the perceived cooling intensity 3 min after they first tasted the sample (Iend) or the time when the cooling sensation disappeared, if it was less than 3 min (Tend). Each evaluation was rated on a linear scale from not perceived (=0) to very intense (=10). We performed student t-tests (two-tailed, paired) on each parameter (Tbegin, Imax, Tmax, Tdec, Iend, and Tend) to compare data obtained from the two cooling compounds. The probabilities obtained for each of these tests indicate whether the compounds have been perceived as significantly different or not (significance was defined as P < 0.05) for the parameter under consideration. This sensory evaluation (Figure 5) confirmed that dihydroumbellulol has a similar cooling profile over time to that of (-)-menthol, but that overall, it is less powerful. The Imax values showed that dihydroumbellulol is weaker than (-)-menthol (significant difference for Imax) and its persistent trigeminal effect is shorter, with a more pronounced delay. Although this new natural cooling compound is less powerful than menthol, it has a similar cooling profile without the minty note, which makes it interesting for flavoring solutions especially citrus ones.
Figure 5Intensity of cooling effect over time for menthol, dihydroumbellulol and water.
Characterization of compounds to mimic ethanol perception
The perception of alcoholic drinks results from the interactions between sensory modalities, as sweet and sour tastes, smell from different aroma tonalities, and finally trigeminal sensations as warm/burning, pungent/sharp, tingling/numbing, dry/astringent, thick/mouthcoating, …, elicited by ethanol [
Perceptual qualities of ethanol depend on concentration, and variation inthese percepts associates with drinking frequency chemosensory perception.2015; 8: 149-157
]. Some new and already well-known compounds as capsaicin and piperine are compared to ethanol at different concentrations. In Figure 6 are represented the perceived intensities of the global sensation over time for the panel average (time-intensity method, the perceived intensity is recorded over time until 300 seconds) and the subqualities associated (the subject selects the main attributes over time, using CATA, Check All That Apply) method for 2 concentrations of ethanol at 10, and 20% and some selected compounds (C1 and C2) in water. Previously, subjects' sensitivities to different concentrations of ethanol (from 0 to 40%) in water and in a model Gin recipe were assessed using the Quantitative Descriptive Analysis (results not shown here). This allows to better understand ethanol sensitivity of our panel and to select the most relevant attributes to describe ethanol perception and the discriminated concentrations. Interestingly some compounds have quite similar trigeminal sensations over time than ethanol. The proposed solutions to mimic ethanol perception could optimize using mixtures of theses compounds. These results will support the final formulation of ethanol enhancers for alcoholic reduced beverages.
Figure 6Intensity of hot/pungent and tingling/numbing effects over time for compounds vs ethanol.
Trigeminal sensations could be a relevant way to fill the sensory gaps of sugar, salt, alcohol, fat reduced products and plant-based foods in order finally to increase consumer acceptance. Furthermore, it could bring unusual sensations such as tingling with could contribute to the complexity of the product and increases its interest.
We demonstrated that sensory screening and characterization of potential plant extracts/fractions or chemicals by a trained panel is a key analytical step in the discovery and selection of molecules that contribute to the taste of foods. This is a robust and powerful method for identifying interesting trigeminal and taste modulators. Sensory characterization of the hits can be compared with those of benchmark products for further development of new taste modulators, or for use in synergy with other compounds or other applications. Naturally derived compounds that elicit an interesting taste have considerable potential as innovative flavors. However, with the large volume of cheap commodity compounds such as monosodium glutamate (MSG) being the market benchmark, the principal hurdle is the cost of using these types of compounds. In terms of launching commercial products, more potential can be seen in molecules that produce trigeminal effects. Mouth refreshing and fat-, salt-, and sweet, more recently ethanol-enhancing products will always be molecules of interest. Our recent results have clearly shown that it is still possible to discover new molecules in plants that elicit trigeminal effects, which motivates us to continue the search, even though such molecules have been the subject of intense research over many years. Furthermore, this sensory knowledge is a useful tool for flavorists to magnify the taste of existing flavors, to optimize the flavor of healthy products, and to develop new products.
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Overview of chemesthesis with a look to the future.
(Chap.14)in: McDonald Shane T. Bolliet David A. Hayes John E. Chemesthesis: chemical touch in food and eating. First Edition. John Wiley & Sons, Ltd,
2016
Comparison of volatile constituents of Persicaria odorata (Lour.) Sojak (Polygonum odoratum Lour.) and Persicaria hydropiper L. Spach (Polygonum hydropiper L.).
Journal of Agricultural and Food Chemistry.2006; 54: 3067-3071
Perceptual qualities of ethanol depend on concentration, and variation inthese percepts associates with drinking frequency chemosensory perception.2015; 8: 149-157
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