Regional Variation of Bitter Taste and Aftertaste in Humans

Abstract

Despite widespread and persistent myths of a tongue map, all 5 prototypical taste qualities are sensed over the entire tongue. However, modern psychophysical data also suggest there may be more nuanced differences in suprathreshold intensity across oral loci, especially for bitterness. Here, we test whether bitter stimuli matched for whole-mouth intensity differ in perceived intensity across regions of the oral cavity in 2 experiments. Experiment 1 consisted of a whole-mouth sip and spit approach and Experiment 2 consisted of a spatial taste test using cotton swabs. In Experiment 1, participants (n = 63) rated overall intensity of 3 bitter solutions at 5 different loci (front, middle, back of tongue; roof of mouth; and lip). Temporal effects were explored using in-mouth and aftertaste ratings. In Experiment 2, participants (n = 48) rated the intensity of quinine and Tetralone solutions after solutions were painted on fungiform, circumvallate, and foliate papillae with a swab. After the spatial taste test, participants completed a questionnaire on self-reported beer intake. Analysis of variance results of both experiments show a significant locus by stimulus interaction, suggesting different bitterants were perceived differently across the various loci. This result was apparently driven by low-intensity ratings for Tetralone on the anterior tongue. Aftertaste ratings in Experiment 1 also revealed significant temporal effects: ratings on the anterior tongue decreased for all bitterants and ratings for quinine decreased at all loci. Reasons for these effects are not known but may suggest differential expression of bitter taste receptors or differences in bitter agonist-receptor binding affinity across tongue regions.

Discrimination of Isointense Bitter Stimuli in a Beer Model System

“…As supplied by the manufacturer, Isolone is a commercial hop extract for use by the brewing industry that is a solution of 30% (w/w) iso-alpha-acid in water. These specific bitterants were chosen in light of prior work investigating temporal and regional differences of bitter stimuli [4]. Initially, we tried to use Tetralone ® (a hop extract of 9.5% (w/w) tetra-hydro-isoalpha-acid in water) as one of the bitterants; however, in pilot attempts to match intensity (not shown), we found Tetralone had a higher hop aroma than the other samples, which would allow participants to differentiate it from the other samples.…”

“…Bitterness is classically considered to be monogeusic (i.e., one singular, indistinguishable percept), and bitter stimuli that lack other side tastes are traditionally labeled with a singular semantic label in English (i.e., bitter) that lacks any additional subgroupings. However, research over the past quarter century suggests that bitterness may actually be multigeusic [1][2][3][4]. Collectively, these prior studies suggest bitter stimuli dissolved in water can be discriminated by humans.…”

“…Research using semantic-free sorting and napping methods suggests that bitter stimuli can be grouped based on their perceived similarities [1,2]. Other psychophysical data in humans also suggest that bitter stimuli can be differentiated based on their temporal [3,8,9] and regional perception [4,10]. However, in a learning task designed to condition participants to associate a taste (i.e., one sweet, three bitter) with a specific color cue, participants were only able to associate the sweet sample significantly above chance; that is, there was no evidence for discrimination between the bitterants in a brief conditioning paradigm [11].…”

“…Three experiments are reported here: an initial intensity scaling study to confirm the samples were effectively intense matched for multiple sensory attributes, a DFC test, and an affective test. Based on data from prior research on the regional [4] and temporal (Higgins, Gipple, and Hayes, under review) perception of bitter stimuli, we added quinine, SOA, or Isolone (a commercial hop extract) to beer. We hypothesized that self-reported beer consumers would be able to discriminate between the beer samples via (a) their DFC and (b) liking ratings.…”

“…We hypothesized that self-reported beer consumers would be able to discriminate between the beer samples via (a) their DFC and (b) liking ratings. First, we hypothesized that Isolone and quinine would have the most different ratings in the DFC test, as these samples were most different in their regional and temporal perceptions [4]. In the affective test, we hypothesized the beer sample made with Isolone would be the most liked sample, as it is a hop extract used by the beer industry and its bitter perceptual qualities would be most similar to the bitterness normally experienced when drinking pale ale-style beers.…”