Thermal taster status (TTS) is a phenomenon in which thermal stimulation of specific areas of the tongue, causes a sensation of a distinct taste in the absence of a gustatory stimulus[3]. Reports vary on what percentage of the general population is a thermal taster, occurrence of thermal tasters in research cohorts of between 20% and 50% has been reported[4].
Not all Thermal Tasters taste alike
Even within the group of thermal tasters, there are subgroups. These groups differ from one another in responsiveness to thermal stimuli in different areas of the tongue and the phantom taste that each type of stimulation arouses. Green and George report that “thermal sweetness” is a common taste occurring in half the thermal tasters in response to warming after the tongue was cooled[1], while Skinner et al. reported 25% of tasters tasting “bitter” while another 25% tasting “sour” in cooling trials[3].
How to assess Thermal Taste
In general, TTS is assessed by applying a thermode with a warming and a cooling stimulus, as each temperature change direction and specific temperatures elicits a different taste sensation in thermal tasters. Thermal taste is classically tested on the tip of the tongue, and some studies report findings from areas lateral of the tip or the back of the tongue.
Several studies on thermal tasters have used Medoc’s Pathway 16*16 mm thermode[2],[3],[4] or the Intra-oral thermode[5],[6].
An example of a testing protocol for TTS could be found in Eldeghaidy et al.’s study in which both warming trials and cooling trials were applied. A warming trial would start at 35°C, cooled down to 15°C and go up to 40°C, and held there for 10 sec., with a ramp of 1 °C/sec[4].
Tongue taste innervation
In Thermal tasters, the anterior part of the tongue, innervated by the chorda tympani nerve, shows a typical reaction to heating and to cooling, while the posterior part of the tongue, innervated by the glossopharyngeal nerve, reacts less typically, Cruz and Green found[7].
Thermal taster status, along with another measure, 6-npropylthiouracil (PROP) taster status, form the taste phenotype[4].
Do fungiform papillae matter?
A hypothesis existed that the fungiform papillae of the tongue would be responsible for thermal taste because of their high density at the tip of the tongue, and their dual role: as they contain both taste buds and mechanoreceptors that are innervated by gustatory and trigeminal nerve fibers. Eldeghaidy et al. found that TTS did not seem to be correlated to fungiform papillae density in contrast to PROP taster status, and thus must have a different mechanism[4].
The taste phenotype as a whole, and the thermal taster status specifically, increasingly allure both neurology researchers and the food and beverage industry alike. Temperature may be actively integrated as a contributor in the totality of the gustatory experience when new taste product are planned to be released to market [8],[9].
References:
[1] Green, B. G., & George, P. (2004). ‘Thermal taste’predicts higher responsiveness to chemical taste and flavor. Chemical Senses, 29(7), 617-628. [2] Yang, Q., Dorado, R., Chaya, C., & Hort, J. (2018). The impact of PROP and thermal taster status on the emotional response to beer. Food Quality and Preference, 68, 420-430. [3] Skinner, M., Eldeghaidy, S., Ford, R., Giesbrecht, T., Thomas, A., Francis, S., & Hort, J. (2018). Variation in thermally induced taste response across thermal tasters. Physiology & behavior, 188, 67-78. [4] Eldeghaidy, S., Thomas, D., Skinner, M., Ford, R., Giesbrecht, T., Thomas, A., ... & Francis, S. (2018). An automated method to detect and quantify fungiform papillae in the human tongue: Validation and relationship to phenotypical differences in taste perception. Physiology & behavior, 184, 226-234. [5] Yang, Q., Hollowood, T., & Hort, J. (2014). Phenotypic variation in oronasal perception and the relative effects of PROP and Thermal Taster Status. Food Quality and Preference, 38, 83-91. [6] Hort, J., Ford, R. A., Eldeghaidy, S., & Francis, S. T. (2016). Thermal taster status: Evidence of cross‐modal integration. Human Brain Mapping, 37(6), 2263-2275. [7] Cruz, A., & Green, B. G. (2000). Thermal stimulation of taste. Nature, 403(6772), 889-892. [8] Small-Kelly, S., & Pickering, G. (2019). Variation in Orosensory Responsiveness to Alcoholic Beverages and Their Constituents—the Role of the Thermal Taste Phenotype. Chemosensory Perception, 1-14. [9] Yang, Q., Dorado, R., Chaya, C., & Hort, J. (2018). The impact of PROP and thermal taster status on the emotional response to beer. Food Quality and Preference, 68, 420-430.