A Complex Relationship among Chemical Concentration, Detection Threshold, and Suprathreshold Intensity of Bitter Compounds

School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway, Burwood, Victoria 3125, Australia.
Chemical Senses (Impact Factor: 3.16). 04/2007; 32(3):245-53. DOI: 10.1093/chemse/bjl052
Source: PubMed
ABSTRACT
Detection thresholds and psychophysical curves were established for caffeine, quinine-HCl (QHCl), and propylthiouracil (PROP)
in a sample of 33 subjects (28 female mean age 24 ± 4). The mean detection threshold (±standard error) for caffeine, QHCl,
and PROP was 1.2 ± 0.12, 0.0083 ± 0.001, and 0.088 ± 0.07 mM, respectively. Pearson product–moment analysis revealed no significant
correlations between detection thresholds of the compounds. Psychophysical curves were constructed for each bitter compound
over 6 concentrations. There were significant correlations between incremental points of the individual psychophysical curves
for QHCl and PROP. Regarding caffeine, there was a specific concentration (6 mM) below and above which the incremental steps
in bitterness were correlated. Between compounds, analysis of psychophysical curves revealed no correlations with PROP, but
there were significant correlations between the bitterness of caffeine and QHCl at higher concentrations on the psychophysical
curve (P < 0.05). Correlation analysis of detection threshold and suprathreshold intensity within a compound revealed a significant
correlation between PROP threshold and suprathreshold intensity (r = 0.46–0.4, P < 0.05), a significant negative correlation for QHCl (r = −0.33 to −0.4, P < 0.05), and no correlation for caffeine. The results suggest a complex relationship between chemical concentration, detection
threshold, and suprathreshold intensity.

Full-text

Available from: Russell Keast, Jan 04, 2016
A Complex Relationship among Chemical Concentration, Detection
Threshold, and Suprathreshold Intensity of Bitter Compounds
Russell S.J. Keast
1
and Jessica Roper
2
1
School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway,
Burwood, Victoria 3125, Australia and
2
RMIT University, PVC Science Engineering &
Technology Applied Sciences, Melbourne, Victoria, 3001, Australia
Correspondence to be sent to: Russell S.J. Keast, School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway,
Burwood, Victoria 3125, Australia. e-mail: russell.keast@deakin.edu.au
Abstract
Detection thresholds and psychophysical curves were established for caffeine, quinine-HCl (QHCl), and propylthiouracil (PROP) in
a sample of 33 subjects (28 female mean age 24 ± 4). The mean detection threshold (±standard error) for caffeine, QHCl, and
PROP was 1.2 ± 0.12, 0.0083 ± 0.001, and 0.088 ± 0.07 mM, respectively. Pearson product–moment analysis revealed no
significant correlations between detection thresholds of the compounds. Psychophysical curves were constructed for each bitter
compound over 6 concentrations. There were significant correlations between incremental points of the individual psychophys-
ical curves for QHCl and PROP. Regarding caffeine, there was a specific concentration (6 mM) below and above which the
incremental steps in bitterness were correlated. Between compounds, analysis of psychophysical curves revealed no correlations
with PROP, but there were significant correlations between the bitterness of caffeine and QHCl at higher concentrations on the
psychophysical curve (P < 0.05). Correlation analysis of detection threshold and suprathreshold intensity within a compound
revealed a significant correlation between PROP threshold and suprathreshold intensity (r = 0.46–0.4, P < 0.05), a significant
negative correlation for QHCl (r = ÿ0.33 to ÿ0.4, P < 0.05), and no correlation for caffeine. The results suggest a complex
relationship between chemical concentration, detection threshold, and suprathreshold intensity.
Key words: bitter taste, caffeine, individual differences, propylthiouracil, threshold
Introduction
Taste receptors locat ed on taste cells in the surface regions of
our oral cavity are activated when chemicals enter our
mouths. An electrical impulse is initiated and transferred
via afferent fibers to cortical levels of the brain where it is
decoded and we experienc e a perception associated with
the chemical. A taste quality is experienced when the chem-
ical concentration in the oral cavity reaches a level that not
only activates a receptor, but the signal sent from the recep-
tor is strong enough to elicit a perception. For example,
a chemi cal may be in solution at a concentration that the sam-
ple population could not detect. As the concentration of the
chemical increases, a detection threshold will be reached, the
level at which the chemical in solution may be discriminated
from water. As the concentration of the chemical increases
further, the recognition threshold is reached, the point at
which the quality (e.g., bitter) can be identified. As the con-
centration of the chemical increases still further, the intensity
of bitterness mutually increases to a theoretical asymptote
where concentration increases no longer cause subsequent
increases in intensity (Keast and Breslin 2003) (Figure 1).
Intuitively, you may expect an individual with low detec-
tion threshold (sensitive to the chemical) to experience higher
intensities at higher concentrations of the chemical com-
pared with a second individual with higher detection thres h-
old (insensitive to the chemical). An example of this intuitive
model is observed with phenylthiocarbamide (PTC), if you
have a low detection threshold for PTC (sensitive) you will be
sensitive throughout the entire psychophysical functi on for
that compound (Bufe et al. 2005). However, such relation-
ships are not the norm (Bartoshuk 2000; Mojet et al. 2003),
presumably, due to both genetic and environmental factors
influencing bitter taste and the complex nature of the orga-
nization of the oral peripheral and central cognitive system
involved in bitter taste processing.
There is a large family of approxim ately 30 putative bitter
taste receptors (TAS2R’s) (Adler et al. 2000; Chandrashekar
et al. 2000) located on bitt er taste cells (Mu eller et al. 2005).
There are also many postreceptor transduction mechanisms
including a-gustducin (McLaughlin et al. 1992), a phospho-
lipase b subtype (Rossler et al. 1998), and transient receptor
Chem. Senses 32: 245–253, 2007 doi:10.1093/chemse/bjl052
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potential ion channels (Perez et al. 2002) to name a few. Any
one bitter compound may access multiple transduction mech-
anisms. For example, caffeine is capable of translocating
through cellular membranes and accessing second messen ger
systems associated with bitter taste (Peri et al. 2000), and
quinine-HCl (QHCl) can also activate nonreceptor mechanisms
associated with bitter taste cells (Kinnamon and Cummings
1992; Rosenzweig et al. 1999; Caicedo et al. 2003). Although
there are multiple mechanisms on or within the bitter taste cell,
the bitter quality we perceive is controlled by the taste cell,
not the receptors; TAS2R’s expressed on sweet taste cells
confer appetitive quality to what should be an aversive chem-
icals (Mueller et al. 2005).
An electrical signal leaves the taste cell and is transferred via
afferent fibers to the subcortical areas nucleus of the solitary
tract, followed by the second-order synapse in the thalamus,
before terminating in several regions of the insula (important
in detection and suprathreshol d intensity), frontal operculum
cortex, and the orbital frontal cortex (important in hedonics).
As the signal progresses upstream toward the cortical regions
of the brain, greater selectivity of activation is observed and
the neurons in the orbital frontal cortex may respond to only
one taste quality. The cortical and subcortical regions of the
brain integrate the signals and introduce plasticity into the
gustatory system with feed-forward and feedback pathways
in operation (Katz et al. 2002; Jones et al. 200 6).
Presumably, differences in the quality and quantity of the
multiple cellular mechanisms associated with bitter taste cells
manifest in the large individual variation observed in bitter
taste perception (Yokomukai et al. 1993; Bartoshuk et al.
1998; Delwiche et al. 2001; Keast and Breslin 2002b).
Even though there is large variation in bitter taste percep-
tion, there is some commonality to bitter taste elicited by mul-
tiple chemicals, and these associations have been supported
in human psychophysical studies (McBurney 1969; Lawless
1979; Delwiche et al. 2001; Keast and Breslin 2002a). The
most studied of all bitter chemicals that have commonality
of bitterness are propylthiouracil (PROP) and PTC, primar-
ily, because there is known heritable variability in bitter taste
perception that is related to haplotypes of the TAS2R38 gene
(Duffy et al. 2004; Bufe et al. 2005). Other bitter compounds
such as caffeine and QHCl have also been extensively studied,
and commonality in suprathreshold bitterness has been estab-
lished by phenotypic variation and genetic modeling (Hansen
et al. 2006). However, there is no commonality between PROP
bitterness and the bitterness elicited by QHCl and caffeine
(Delwiche et al. 2001; Keast et al. 2003; Hansen et al. 2006).
In the present study, the objective was to assess the rela-
tionship between chemical concentration, detection thresh-
old, and suprathreshold intensity within and between 3
bitter compounds. Caffeine and QHCl were selected as they
share commonality in suprat hreshold bitterness perception
and therefore may have commonality at detection thresholds
level. PROP was selected as it elicits bitterness independent
of caffeine and quinine, and the bitterness of PROP has been
linked to a single receptor, TAS2R38.
Materials and methods
Subjects
Subjects (n = 33, 23 ± 4 years old, 28 female) between the
ages of 18 and 38 were University students in Melbourne,
Australia. All subjects agreed to participate and provided in-
formed consent on an approved Institutional Review Board
form. The participants, all nonsmokers, were asked to re-
frain from eating, drinking, or chewing gum for 1 h prior
to testing.
Subject training
Participants were initially trained in the use of the general
Labeled Magnitude Scale (gLMS) following the published
standard procedures (Green et al. 1993, 1996) except the
top of the scale was described as the strongest imaginable
sensation of any kind (Bartoshuk 2000). The gLMS is a psy-
chophysical tool that requires participants to rate perceived
intensity along a vertical axis lined with adjectives: barely
detectable = 1.5, weak = 6, moderate = 17, strong = 35, very
strong = 52, and strongest imaginable = 100; the adjectives
are placed semilogarithmically, based upon experimentally
determined intervals to yield data equivalent to magnitude
estimation (Green et al. 1993, 1996). The scale only shows
adjectives, not numbers, to the participants, but the experi-
menter calculates numerical data from the scale.
Participants were trained to identify each of the 5 taste
qualities by presenting them with exemplars. Salty taste
Figure 1 Schematic illustration of the relationship between chemical con-
centration, detection threshold, and suprathreshold intensity using gLMS.
The left-hand side of the bold black y axis represents chemical concentration
from 0 molar (0 M) solution to a saturated solution. The right-hand side of the
bold black y axis represents the perceptual relationship to increasing concen-
tration. The far right vertical axis represents the gLMS scale from no percep-
tion to a theoretical terminal threshold.
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was identified as the predominant taste quality from 150 mM
NaCl, bitterness as the predominant quality from 0.50 mM
QHCl, sweetness as the predominant quality from 300 mM su-
crose, sourness as the predominant quality from 3 mM citric
acid, and umami the predominant quality from a mixture of
100 mM Monosodium glutamate and 50 mM inosine mono-
phosphate. To help subjects understand a stimulus could elicit
multiple taste qualities, 300 mM urea (bitter and slightly sour)
and 50 mM NH
4
Cl (salty, bitter, and slightly sour) were
employed as training stimuli. Sucrose and NaCl were pre-
sented at 3 concentrations (50, 200, and 400 mM) to ensure
subjects could rank the solutions from least to most intense.
All subjects were able to identify and rank taste solutions.
Stimuli and delivery
Caffeine and 6-PROP were purchased from Sigma Chemical
(St Louis, MO) and were Sigma-ultra grade. QHCl was pur-
chased from Fluka Chemika (Buchs, Switzerland).
All solutions were prepared with deionized (di) filtered
water and were stored in glass bottles at 4–8 C and were
brought to room temperature (20 ± 3 C) prior to testing.
Filtered di water was used as the blank stimulus and the rins-
ing agent in all experiments.
All testing too k place in specialized sensory testing facility
comprising 7 individual computerized booths. Each subject
was isolated from other subjects by vertical divide rs, and
there was no interaction between subjects.
Detection threshold determination for caffeine and
QHCl, and n-PROP
A triangle forced-choice initially ascending procedure was
used to determine detection threshold of caffeine, QHCl,
and PROP for each subject. The range of concentration used
is shown in Table 1: caffeine concentrations were modified
from ‘‘ISO 3972 method of investigating sensitivity of taste,’’
QHCl concentrations were 0.2 log concentration steps, and
PROP concentrations were 0.125 log concentration steps.
Starting at the dilution step 3, solutions (10 ml) were pre-
sented in 30-ml plastic medicine cups in groups of 3. Subjects
were instructed to hold the sample in their mouth for 3 s, then
expectorate. Within each set of 3 solutions, 2 were water
blanks and the 3rd was the bitter compound, and subjects
had to identify which one was different (triangle test). The
order of presentation was randomized and cou ld ha ve been
any of 3 possible orders (A, blank, and B,stimulus): AAB,
ABA, and BAA. If sub jects failed to correctly identify the
odd sample, the concentration was increased one step. If sub-
jects correctly identified the sample on 2 occasions, the con-
centration was decreased one step. The level at which the
sequence changed from ascending to descending or descend-
ing to ascending was termed a reversal. Four reversals were
required, and the best estimate threshold for each subject was
the geometric mean of the concentration where the last miss
occurred and the next higher step. There was an interstim-
ulus interval of approximately 60 s, during which time the
subject was required to rinse with di water at least 4 times.
Any one session included only one bitter compound and each
session could take 30 mins to complete. The detection thresh-
old method was repeated in a separate session to check re-
producibility of detection thresholds, meaning a minimum of
6 sessions in total for each subject.
Construction of psychophysical curve for caffeine, QHCl,
and n-PROP
The concentration ranges for constructing a psychophysical
curve for the bitter stimuli are shown in Table 2. For caffeine
and QHCl, subjects were presented with numbered trays that
Table 2 Concentrations of caffeine, QHCl, and n-PROP used to generate
psychophysical curves
Caffeine [mM] QHCl [mM] PROP [mM]
000
3 0.05 0.05
6 0.1 0.25
12 0.15 0.75
24 0.2 1.25
48 0.25 2.5
0.3 5.5
Table 1 Concentrations and dilution steps used to determine subject
detection threshold for caffeine, QHCl, and n-PROP in water
Caffeine [mM] QHCl [mM] PROP [mM] Dilution step
0.28 0.00064 0.01 1
0.33 0.0009 0.014 2
0.42 0.0013 0.019 3
0.52 0.0017 0.025 4
0.66 0.0025 0.033 5
0.80 0.0035 0.045 6
1.03 0.005 0.059 7
1.3 0.007 0.079 8
1.57 0.01 0.1 9
1.84 0.014 0.14 10
2.11 0.02 0.19 11
2.38 0.028 0.25 12
2.65 0.04 0.33 13
The concentration series for caffeine was adapted from ISO3970, ‘method
of investigating sensitivity of taste,’ the concentration series for QHCl was
prepared with successive 0.15 log dilutions with filtered di water, and the
concentration series for PROP was prepared with successive 0.125 log
dilution steps.
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contained 7 randomized solutions (10 ml) of one bitter stim-
ulus (6 concentrations from the psychophysical curve and
one di water control). For PROP, the only difference was
solutions were presented in ascending concentration order,
rather than randomized order (Bartoshuk 2000). The 6 con-
centrations for each bitter stimulus ranged from below
‘‘weak’’ on the gLMS to maximum practical tasting limit.
Each point on an individual psychophysical curve was tested
at least 3 times.
Stimulus delivery
An aliquot of 10 ml of each solution (n = 7) was presented in
30-ml polyethylene medicine cups (Dynarex, Orangeburg,
NY) in randomized order (except PROP see above) on a
numbered tray. Subjects rinsed with di water at least 4 times
over a 2-min period prior to testing. Each subject taste d and
then rated each solution for sweetness, sourness, saltiness,
bitterness, and um ami, prior to exp ectorating. All subjects
rinsed with di water 4 times during the interstimulus interval
of 90 s. The gLMS was used as the rating method. Each sam-
ple was tasted only once per session, and there were 3 ses-
sions in total as a test of reliability of rating.
Psychophysical curves were constructed for the bitter com-
pounds for each individual subject. These curves provided
the opportunity to investigate perceived bitterness correla-
tions as a function of individual sensitivities among bitter
compounds at 6 different concentration levels and threshold
concentrations. First, the intensity ratings were adjusted for
bias in scale use.
Standarization of gLMS ratings with sweetness and
weight ratings
The gLMS standardization was a modified version of Delwiche
et al. (2001). Briefly, subjects rated the sweetness and total
intensity of 10-ml samples of 5 concentrations of sucrose
(50, 100, 150, 250, and 400 mM). Between each sample, sub-
jects rinsed 4 times with di water. Subjects also rated the
heaviness of 5 visually identical weights (opaque, sand-filled
jars at levels 52, 294, 538, 789, and 1028 g). All ratings wer e
made on the gLMS. Subjects were asked to rate the intensity
of taste or heaviness, and all judgments were made within the
context of the full range of sensations experi enced in life. All
stimuli were presented twice in blocks of ascending order.
Subjects first rated the heaviness of weights and then the in-
tensity of sucrose solutions.
There was a significant correlation between sucrose sweet-
ness and heaviness ratings (r
2
= 0.49, P < 0.05). Because these
sensory modalities were assumed to be unrelated, the signif-
icant correlation indicated that the gLMS ratings were prone
to individual scale-use bias and required standardization
across subjects.
To determine a standardization factor, each subject’s aver-
age intensity for heaviness was divided by the grand mean for
heaviness across weight levels and subjects. Each individual’s
bitter intensity ratings for caffeine, QHCl, and PROP were
multiplied by his or her personal standardization factor for
scale-use bias.
Statistical analysis
Data used for correlation analysis were the detection thresh-
old concentrations and the individual bitterness intensity
ratings (gLMS) at stated concentration levels. Correlation
analysis (Pearson prod uct–moment coefficients) was per-
formed using SPSS version 12.0.1. Subjects who are termed
insensitive to the bitter compounds tested have a higher de-
tection threshold and lower intensity rating than sensitive
subjects (lower detection threshold, higher intensity rating).
When this data is analyzed, what is a positive correlation will
have a negative sign. Therefore, in order to assess correla-
tions between the detection threshold concentrations and
suprathreshold intensities, positive r values were converted
to negative and vice versa.
PASS statistical software (2005) was used to determine the
power of this study. Assuming r = 0.35, n = 33, and a < 0.05,
the power of the study is 0.65. Ideally, a power of 0.8 should
be achieved, and with n = 33 and a < 0.05, the r value = 0.45.
The study was large enough to assume a type II error is
within acceptable range.
Results
Detection threshold
The mean detection threshold and standard error for caf-
feine, QHCl, and PROP was 1.2 ± 0.12, 0.0083 ± 0.001,
and 0.088 ± 0.07 mM, respectively. The relationship between
detection thresholds for caffeine and QHCl among subjects
was investigated using Pearson product–moment correlation
coefficient. There was no correlation between detection
thresholds for caffeine, QHCl, and PROP (n = 33, r =
ÿ0.006 to ÿ0.24, P = 0.97 0.18) (Figure 2).
Suprathreshold intensities
Psychophysical curves were constructed for caffeine, QHCl ,
and PROP, and there was much individual variation in bit-
terness perception (Figure 3A,B, and C). Even though bitter-
ness intensity varied among subjects, as the concentration of
QHCl and PROP increased, there was ordinal increases in
bitterness intensity across subjects and, as expected, Pearson
coefficient correlations revealed a significant relationship
between all points on a bitter compound’s psychophysical
curve ([QHCl; r = 0.61–0.88, P < 0.001) (PROP; r = 0.65–
0.924, P < 0.001]). Analysis of variance results showed
significant differences between all incremental steps on the
psychophysical curves (P < 0.05). This indicates that when
a subject is given increasing concentrations of quinine or
PROP (above detection threshold), there is an ordinal in-
crease in bitterness intensity relative to intensity ratings
across all subjects (a subject who was insensitive to the bitter
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taste of the stimulus remains insensitive in relation to the
other subjects for the concen trations tested). The strong cor-
relation was also evident for caffeine but only at the higher
concentrations 12–72mM (r = 0.61–0.96, P < 0.001).
Whereas, at 6 mM caffeine, there was a strong correlation
with 3 (r = 0.63, P < 0.001) and 12 mM caffeine (r =
0.61, P < 0.001) and weaker correlations with higher caffeine
concentration (r = 0.43–0.46, P < 0.05). The bitterness inten-
sity ratings of the subjects at lowest concentration of caffeine
(3 mM) did not correlate with any of the concentrations
above 6 mM (r = ÿ0.06–0.2, P = 0.2–0.9). This indicates
a low concentration and high concentration mechanism re-
sponsible for the perceived bitter taste of caffeine.
There were no significant correlations with subjects’ inten-
sity rating of caffeine and PROP (r = ÿ0.06–0.1, P = 0.82–
0.5) or QHCl and PROP (r = 0.07 0.3, P = 0.72–0.07),
which is similar to other studies invest igating correlations
of bitter compounds with PROP bitterness (Delwiche
et al. 2001; Keast et al. 2003; Hansen et al. 2006). Therefore,
sensitivity to the bitterness of PROP does not predicate that
the subject will be sensitive to the bitterness of caffeine or
QHCl. At the 3 highest concentrations of caffeine and QHCl
tested, there were significant correlations (r = 0.56–0.36, P <
0.05). This supports previous research indicating perceptual
and genetic similarities between the bitterness of caffeine and
QHCl (Delwiche et al. 2001; Hansen et al. 2006).
Detection threshold and suprathreshold intensity among
compounds
Table 3 shows Pearson product–moment correlation coefficient
for detection threshold concentration and suprathreshold
Figure 3 Psychophysical curves of the sample population mean and exam-
ples of an insensitive and sensitive subject for (A) caffeine, (B) QHCl, and
(C) PROP. Included in each graph is a sensitive (highest curve) and insensitive
subject (lowest curve) for that compound as well as the mean psychophysical
curve. The y axis is a numerical measure of bitterness intensity from the gLMS.
The x axis has 2 labels, the upper label in the log millimolar concentration for
the particular compound and the lower label is the actual millimolar concen-
tration. Error bars represent standard errors.
Figure 2 Detection threshold correlation. Detection threshold concentra-
tions for caffeine, QHCl and n-PROP on a 3-dimensional plot. All concentra-
tions arein millimolar, the y axis is caffeine, x axis is QHCl, and the z axis is PROP.
Each point represents the threshold concentrations for 1 of the 33 subjects.
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intensities for the individual bitter compounds across sub-
jects. There was no significant correlation between detection
threshold and suprathreshold intensity ratings for caffeine.
Surprisingly, there was a negative correlation between
threshold of QHCl and suprathreshold intensity ratings of
QHCl. This indicates that subjects who were sensitive to
QHCl (low-threshold concentrations) generally found higher
concentrations of QHCl less bitter, whereas subjects who
were insensitive to QHCl (high-threshold concentrations)
perceived higher concentrations of QHCl more bitter. There
were positive correlations between PROP threshold and
suprathreshold intensity rating (except at the lowest concen-
tration on the psychophysical curve).
Discussion
The relationship between the concentration of a chemical
and the perception of that chemical (intensity and liking)
is complex (Amerine et al. 1965; Bartoshuk 2000; Mojet
et al. 2005). The results from this study do not diminish
that complexity; indeed, they add to complex relationship
between chemical concentration, detection threshold, and
suprathreshold intensity. As the concentration of a chemical
increases from detection threshold to suprathreshold, there
was a significant positive correlation for PROP, a significant
negative correlation with QHCl, and no correlation for caf-
feine. The complexity may be due to multiple perceptual and
peripheral mechanisms of bitter taste, and these multiple
mechanisms may be activated at different concentrations.
Figure 4 illustrates the positive and negative correlations
among chemical concentration, detection threshold, and
suprathreshold intensity observed in this study. As the sta-
tistics infer, Figure 4 is a generalization of results from this
study and not all subjects will follow the model.
6-n-Propylthiouracil
In this study, PROP observed the intuiti ve model of sensitiv-
ity throughout a concentration range with sensitivity at low
concentration predicting sensitivity at higher concentrations
(Figure 4). However, in a comprehensive review of variation
in taste perception, Bartoshuk (2000) has previously stated
relying on detection thresholds for PROP may cause misclas-
sification of subjects’ ‘‘taster’’ status in the suprathreshold
range. In support of Bartoshuk’s observation, classifying
PROP taster status on detection thresholds would have
resulted misclassification of 4 of the 33 subjects at supra-
threshold intensity, even though there was a significant cor-
relation between detection threshold and suprathreshold
sensitivity for PROP. The ability to taste PROP has been
linked to the bitter receptor gene hTAS2R38 (Duffy et al.
2004), and there is a very close association between absolute
detection threshold and hTAS2R38 haplotypes (Bufe et al.
2005). As there is one known receptor linked to perception
Table 3 Pearsons product–moment correlation between threshold and 6 suprathreshold intensity ratings for caffeine, QHCl, and 6-n-PROP
Stimulus No. 1 2 3 4 5 6
Caffeine 0.001, NS 0.15, NS ÿ0.2, NS ÿ0.09, NS ÿ0.08, NS ÿ0.05, NS
QHCl ÿ0.08, NS ÿ0.38* ÿ0.4* ÿ0.36* ÿ0.37* ÿ0.33*
PROP 0.26, NS 0.43** 0.43** 0.46** 0.4* 0.43**
NS, not significant. Concentrations of chemicals for stimulus number 1–6 are shown in Table 2.
*P < 0.05.
**P £ 0.01.
Figure 4 Schematic illustration of the association between chemical con-
centration, detection threshold, and suprathreshold intensity for PROP and
QHCl. The bold black solid vertical line represents the chemical concentration.
The thin solid vertical lines represent the gLMS intensity rating relative to the
chemical concentration. The bottom of each thin solid line represents the de-
tection threshold. The top of each solid line represents an intensity of ;20 on
the gLMS scale. The vertical dashed line below the solid line represents the
concentrations of chemical in solution without eliciting a noticeable differ-
ence from water. The left-hand side of the chemical concentration axis illus-
trates results observed for PROP, with sensitivity at detection threshold
consistent over the concentration range tested. This is illustrated by an equal
perceived intensity range relative to chemical concentration regardless of an
individual’s sensitivity to PROP. The right-hand side of the concentration axis
illustrates the results observed for QHCl with subjects rating between 0 and
20 gLMS as either a compressed perceived intensity range relative to chemical
concentration or an expansive perceived intensity range (far right) relative to
the chemical concentration.
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of PROP, it is not surprising to find a significant relationship
between detection threshold and suprathreshold intensity.
However, even for the PROP, there is speculation that ad-
ditional genetic or environmental co ntrols govern bitter taste
perception as the PROP concentration increases (Bufe et al.
2005).
Quinine-HCl
In this study, there was a negative correlation between QHCl
detection threshold concentra tion and suprathreshold inten-
sity. Figure 4 illustrates that a subset of the sample popula-
tion have a compressed perceived intensity range relative
to chemical concentration, whereas a second subset of the
population have an expansive perceived intensity range rel-
ative to the chemical concentration. There have been few
reports of such negative correlations between threshold
and suprathreshold sensitivity within taste, although Mojet
et al. (2005) reported similar negative correlations for salt
and umami qua lities. The psychophysical data for QHCl
suggests at least 2 perceptual mechanisms, an independent
factor regulating threshold detection, which covaries with
mechanisms associated with suprathreshold intensities. Mul-
tiple perceptual mechanisms of QHCl are supported by mul-
tiple peripheral mechanisms, including the ability of quinine
to block K+ channels (Kinnamon and Cummings 1992), and
in addition to shari ng genetic factors associated with varia-
tion in perception with caffeine, QHCl has a putative spe-
cific genetic factor regulating only its bitterness perception
(Bachmanov et al. 1996; Hansen et al. 2006).
Caffeine
Caffeine results were the most intr iguing of the compounds
tested. There was no correlation between detection sensitiv-
ity and sensitivity to caffeine at any point of the psychophysical
function. Moreover, there was a specific concentration (6 mM)
where perceived bitt er taste could be differentiated—the
lower concentrations elicited bitterness that was correlated
among sub jects, the same for the higher concentrations.
However, the bitterness elicited by £6 and 6 mM concen-
trations did not correlate with each other. Overall, there were
3 perceptual shifts associated with caffeine concentration,
which may indicate 3 different bitter taste mechanisms: one
for detection threshold (very low concentrations, £1 mM);
one for ;1to<6 mM concentrations of caffeine; and one
for >6 mM co ncentrations of caffeine. Multiple perceptual
mechanisms for caffeine bitterness is supported by multiple
independent putative mechanisms: caffeine can translocate
through cellular membranes and has the ability to interfere
with second messenger systems (Peri et al. 2000); the bitter-
ness of caffeine has been associated with the bitterness of
QHCl (this study and Delwiche et al. 2001); and there is a pro-
posed small (2%) genetic link between PROP and caffeine
(hTAS2R38) (Hansen et al. 2006).
Detection threshold and suprathreshold intensity among
compounds
In this study, there was no correlation between the detection
thresholds of all 3 compounds; therefore, sensitivity to bitter
compounds at threshold level was not common across sub-
jects. This suggests that caffeine, QHCl, and PROP have in-
dependent mechanisms responsible for their detection at low
concentration. This was not surprising for PROP, as previ-
ous research has established no common bitterness with caf-
feine and QHCl at suprathreshold level, a result that was
replicated in the present study. Previous research has shown
an association between caffeine and QHCl at suprathreshold
intensities (Delwiche et al. 2001; Hansen et al. 2006), a findin g
that was also replicated in the present study. However, at
lower concentrations, there was no correlation indicating
that the co mmonality in bitterness between caffeine and
QHCl may be due to a bitter taste mechanism activated at
higher concentrations of the 2 compounds.
Organization of the bitter taste system
If a single receptor was responsible for detection and supra-
threshold intensity, you would expect a strong correlation
between chemical concentration, detection threshold, and
suprathreshold intensity, and this was observed with PROP
(Figure 4). However, if there are multiple taste transduction
mechanisms that are activated at varying concentrations of
the chemical, there may be no association between detection
threshold and suprathreshold intensity, and this was ob-
served with caffeine. A negative association may occur if a
high-affinity receptor process was activated at very low con-
centrations of the chemical, but high enough to reach a detec-
tion thres hold; then, as the concentration was increased,
a lower affinity receptor mechanism was activated and was
responsible for a perceived quality. If a subject had a larger
quantity of 1 of the 2 receptor types, we may expect a negative
association be tween detection threshold and suprathreshold
intensity, and this was observed with QHCl (Figure 4).
The variation and lack of correlation in bitter taste percep-
tion may be due to multiple factors. Recent advances in our
knowledge of the peripher al organization of the taste system
strongly indicate that taste receptor cells are quality specific
(Mueller et al. 2005; Huang et al. 2006). In addition to this,
not all bitter taste cells contain all bitter taste receptors,
but subsets of receptors are located on bitter taste cells
(Chandrashekar et al. 2000). Variation in receptor subsets
of receptors on bitter taste cells may influence bitter taste
perception. For example, sweet and umami taste are acti-
vated by heterodimers of the TAS1R family, and it is not
inconceivable the same dimer system could occur with the
TAS2Rs on bitter taste cells. If a bitter taste cell lacks one
part of a dimer, activation of that cell would not occur. There
may also be single-nucleotide polymorphisms in TAS2Rs
that result in differences in bitter taste perception (Bufe
et al. 2005). Moreover, each TAS2R may have multiple
Complexity of Bitter Taste 251
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Page 7
binding sites that are low or high affinity, and as the concen-
tration of a compound increases the lower affinity receptor
or active site of the receptor is activated (Galindo-Cuspinera
et al. 2006).
Within an individual, the strength of an afferent signal may
be magnified relative to other individuals. There may also be
interindividual variations in the signal processing in the
human brain, although our understanding of gustatory pro-
cessing in the brain is still in its infancy (Small 2006).
Conclusions
There is a complex relationship between chemical concentra -
tion, detection threshold, and suprathreshold intensity of
bitter compounds. The sensitivity of a person to detect very
low concentrations of a compound is not necessarily associ -
ated with their sensi tivity to the same compound when it is
perceivably bitter. Moreover, in some situations, threshold
sensitivity to a compound may be inversely related to the in-
tensity of perceived bitterness of that compound. Such com-
plexity has practical implications as threshold determination
methods are increasingly (and incorr ectly) used to infer
suprathreshold intensity of specific compounds, for example,
taste dilution analysis, (Frank et al. 2001; Ottinger et al.
2003). More broadly, this paper also continues to support
that attempts to link threshold measures to food sensations
and intake are at best misguided.
The bitter taste system may have distinct perceptual stages,
one for threshold and at least one for suprathreshold inten-
sities, and these perceptual stages may relate to distinct oral
peripheral mechanisms. As the concentration of a compound
increases, receptors that have a lower affinity for the com-
pound may become involved in the process of taste transduc-
tion, resulting in perceptual phases that can be differentiated
using psychophysical methods of evaluat ion.
Acknowledgements
We thank Professor Sing Kai Lo for his advice on the statistics
undertaken in this study. Financial support was received from
the School of Exercise and Nutrition Science, Deakin University.
We thank all the subjects who took part in this study.
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    • "Sensory threshold is commonly used as a phenotypic indicator of the taste function (sensitivity). The absolute or detection threshold (DT) is the lowest level that a stimulus is perceiv- able [36,37]. This is the concentration when the person can detect there is something other than water in solution, but cannot identify a quality. "
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