Chem. Senses 26: 41–47, 2001
PROP (6-n-Propylthiouracil) Tasting and Sensory Responses to Caffeine,
Sucrose, Neohesperidin Dihydrochalcone and Chocolate
Agnes Ly and Adam Drewnowski
Nutritional Sciences Program, University of Washington, Seattle, WA 98195-353410, USA
Correspondence to be sent to: Adam Drewnowski, Nutritional Sciences Program, University of Washington, 305 Raitt Hall, Box 353410,
Seattle, WA 98105, USA. e-mail: adamdrew@u.washington.edu
Abstract
The genetically determined ability to taste 6-n-propylthiouracil (PROP) has been linked with lowered acceptance of some
bitter foods. Fifty-four women, aged 18–30 years, tasted and rated PROP-impregnated filter paper and seven solutions of
PROP. Summed bitterness intensity ratings for PROP solutions determined PROP taster status. Respondents also tasted five
sucrose and seven caffeine solutions, as well as seven solutions each of caffeine and PROP that had been sweetened with
0.3 mmol/l neohesperidin dihydrochalcone (NHDC). Respondents also rated three kinds of chocolate using 9-point category
scales. PROP tasters rated caffeine solutions as more bitter than did non-tasters and liked them less. PROP tasters did not rate
either sucrose or NHDC as more sweet. The addition of NHDC to PROP and caffeine solutions suppressed bitterness intensity
more effectively for tasters than for non-tasters and improved hedonic ratings among both groups. PROP tasters and
non-tasters showed the same hedonic response to sweetened caffeine solutions and did not differ in their sensory responses
to chocolate. Genetic taste markers may have only a minor impact on the consumption of such foods as sweetened coffee or
chocolate.
Introduction
The ability to taste some bitter compounds is genetically
determined (Snyder, 1931). Two compounds, phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP), taste
bitter to some people and are tasteless to others (Blakeslee
and Fox, 1932; Fox, 1932; Fischer, 1964). In past studies
assignment of PROP taster status was based on the bimodal
distribution of taste detection thresholds obtained using
PROP solutions (Drewnowski and Rock, 1995; Drewnowski
et al., 1997a). Other studies simply relied on the distribution of taste responses to PROP-impregnated filter paper
(Snyder, 1931; Blakeslee and Fox, 1932).
The genetic ability to taste PTC/PROP has been linked to
greater perceived bitterness of some bitter compounds (Hall
et al., 1975; Bartoshuk, 1979). However, the data have not
always been consistent. Using threshold detection methods
some studies have failed to find a link between PTC/PROP
sensitivity and the sensitivity to caffeine, quinine HCl and
urea (Leach and Noble, 1986; Mela, 1989; Schifferstein and
Frijters, 1991; Smagghe and Louis-Sylvestre, 1998). Others
(Hall et al., 1975) found that PTC tasters rated low caffeine
concentrations (0.0056–0.03 mol/l) as more intensely bitter
than did non-tasters. In a time–intensity study (Neely and
Borg, 1999), tasters gave higher aftertaste intensity ratings
to a low concentration of caffeine (0.018 mol/l) than did
non-tasters.
Whether PROP tasters and non-tasters differ in their
© Oxford University Press 2001. All rights reserved.
perception of sweetness intensity is another unresolved
issue. In some studies sucrose and neohesperidin dihydrochalcone (NHDC), an intense sweetener, tasted sweeter to
tasters than to non-tasters (Bartoshuk, 1979; Gent and
Bartoshuk, 1983). In contrast, other studies found no relationship between PROP tasting and the perceived sweetness
of sucrose solutions (Drewnowski et al., 1997a,b). Other
studies compared taste responses of PROP tasters and nontasters to compounds that simultaneously exhibit both
sweet and bitter qualities (Bartoshuk, 1979). PROP tasters
perceived low concentrations of saccharin as both sweeter
and more bitter than did non-tasters.
One question is whether genetic ability to taste PROP
influences sensory response to bitter phytonutrients. Plantderived phenolic compounds, including flavonoids in
grapefruit, isoflavones in soy products and catechins in tea,
are almost always bitter. In past studies PROP tasters rated
naringin solutions and infusions of Japanese green tea as
more bitter than did non-tasters and liked them less (Akella
et al., 1997; Drewnowski et al., 1997c). PROP tasters rated
soybean tofu as less acceptable than did non-tasters and
reported lower preference for grapefruit juice (Akella et al.,
1997; Drewnowski et al., 1997c). Chocolate contains caffeine and has four times the amount of a bitter polyphenol
catechin than tea (Arts et al., 1999). However, bitter
beverages and chocolate are almost always sweetened.
42
A. Ly and A. Drewnowski
The bitterness of foods can be masked by the addition of
sweeteners. The mutual suppression of bitter and sweet
tastes may be due to neural interactions as opposed to
competition at the receptor level (Lawless, 1979). In one
study of bittersweet mixtures of PTC and sucrose, tasters
of PTC showed a stronger suppression of sweetness than
did non-tasters (Lawless, 1979). There is little additional
information on the impact of genetic taste factors on taste
mixtures and mixture interactions. We therefore examined
whether the addition of an intense sweetener, NHDC, would
differentially affect taste responses to PROP and caffeine
solutions by PROP taster status. We also examined whether
PROP tasters and non-tasters differed in their sensory
responses to white, milk and bittersweet chocolate. Respondents were also asked about their food choices and the
consumption of chocolate and coffee beverages.
Materials and methods
Respondents
Respondents were 54 young women between the ages of 18
and 30 years, recruited through advertisements and flyers
posted in the University of Washington. The sample was
75.9% Caucasian, 18.5% Asian/Pacific Islander, 1.9%
African-American, 1.9% Native American and 1.9% listed
as ‘other’. Women reporting use of tobacco or medication
that would alter taste or smell were excluded from the study.
All respondents were weighed and measured and body mass
indices (BMI, kg/m2) were calculated. Compensation was
provided for completing each taste session. All research
protocols were approved by the Human Subjects Review
Board of the University of Washington.
Taste stimuli
PROP-saturated filter paper
PROP filter papers were prepared by dipping Whatman no.
1 filter paper into a concentrated solution of PROP heated
close to boiling point, as described in previous studies
(Drewnowski et al., 1997a; Kaminski et al., 2000). Filter
papers were dried and cut into 1 inch squares that were then
stored in glassine envelopes.
PROP solutions
Seven solutions of PROP (Pfalz & Bauer, Waterbury, CT)
were prepared in distilled water using progressive serial dilutions of the most concentrated solution. The concentrations
of PROP increased in half-log steps and were determined as
described in previous studies (Drewnowski et al., 1997a).
The PROP concentrations were 3.2 × 10–6, 10.0 × 10–5, 3.2 ×
10–5, 10.0 × 10–4, 3.2 × 10–4, 10.0 × 10–3 and 3.2 × 10–3 mol/l.
These correspond to solutions 3, 5, 7, 9, 11, 13 and 15 in the
regular PROP series (Drewnowski et al., 1997a). The solutions were prepared at least 1 day before testing and were
stored at 4°C.
Sucrose solutions
The concentrations of five sucrose solutions in distilled
water were 2, 4, 8, 16 and 32% (w/v) (Drewnowski et al.,
1997a,b).
Caffeine solutions
Seven solutions of caffeine (Sigma Chemical Co.) were
prepared using distilled water. Caffeine concentrations,
increasing in quarter-log steps, were 1.8 × 10–3, 3.2 × 10–3,
5.6 × 10–3, 10.0 × 10–2, 1.8 × 10–2, 3.2 × 10–2 and 5.6 ×
10–2 mol/l.
NHDC solution
The solution of NHDC (Zoster SA, Barcelona, Spain)
in distilled water had a concentration of 3 × 10–4 mol/l.
Sweetness intensity of this concentration of NHDC was
equivalent to that of an 11% sucrose solution (Borrego et
al., 1991). All solutions were prepared at least 1 day prior to
tasting and were stored at 4°C.
Food stimuli
White chocolate (Guittard, Burlingame, CA), milk chocolate
(Felchlin, Schwyz, Switzerland) and bittersweet chocolate
(Felchlin, Schwyz, Switzerland) were purchased and stored
at room temperature for 2 weeks before testing. The percentages of fat (cocoa butter) for the white, milk and dark
chocolate were 36, 35.1 and 43.6%, respectively.
Scaling procedures
Respondents placed a PROP filter paper on their tongue,
allowed it to moisten and then rated bitterness intensity
using a 9-point category scale, anchored at each end, where
1 is not at all bitter and 9 is extremely bitter. Sensory
response to PROP solutions was assessed using whole
mouth tasting and the standard sip-and-spit technique
(Drewnowski et al., 1997a,b,c; Kaminski et al., 2000).
Respondents were asked to take a sip of each solution, hold
it in the mouth for ~3 s and then expectorate into the cup
provided. The solutions, mean volume 10 ml, were presented in 30 ml medicine cups. Respondents were required
to rinse with water between tasting each sample and each
successive tasting was separated by a minimum 60 s interval.
Taste intensity was assessed using a 9-point category scale
where 1 was not at all sweet (or bitter) and 9 was extremely
sweet (or bitter). Hedonic preferences were also assessed
using a 9-point category scale that ranged from 1 (dislike extremely) to 9 (like extremely) (Drewnowski et al., 1997a,b).
Bitterness intensity, sweetness intensity and preference for
the three types of chocolate were also assessed using 9-point
category scales.
Food preference questionnaire
Respondents rated preferences for three types of chocolate
(white, milk and dark) and for coffee beverages. They were
asked how often they consumed these foods [several times a
week, once per week, several times per month, rarely (less
PROP Tasting and Response to Caffeine 43
than once per month) or never] and what type of coffee beverages they usually consumed (drip or espresso beverages).
Those who drank drip coffee were asked if they consumed it
black, with milk/cream or with sweetener only, or with milk/
cream and sweetener. Those who drank espresso beverages
were asked what type they usually selected (espresso shots,
Americanos or lattes) and if they drank it sweetened or not.
Study design and procedures
Respondents rated the bitterness intensity of PROP filter
papers during the screening session. Those who rated the
papers 1 or 2 on a 9-point scale were classified as potential
non-tasters, whereas those who gave ratings between 3 and 9
were classified as potential tasters.
During the first tasting session respondents tasted and
rated five sucrose solutions, followed by one solution of
NHDC, seven solutions of caffeine and seven solutions of
PROP. All solutions were presented in a random order of
concentrations. PROP solutions were always presented last.
All respondents but one returned no more than 1 week later
for the second tasting session, during which they tasted and
rated seven caffeine and seven PROP solutions sweetened
with NHDC and tasted and rated three types of chocolate.
Sweetness, bitterness and the hedonic response were the only
qualities rated. Respondents also completed food preference
questionnaires. Respondents tasted three PROP filter papers
on three separate occasions to assess the reliability of PROP
filter paper rating.
Figure 1 Distribution of summed bitter intensity ratings for seven PROP
solutions (n = 54).
Statistical analyses
Statistical tests were conducted using SPSS for Windows 9.0
(SPSS Inc., Chicago, IL). Taste data were analyzed using
repeated measures ANOVA where taster status was the
between subject variable and stimulus concentration or
stimulus type (PROP or caffeine solutions with and without
NHDC) was the within subject variable. Questionnaire
data were analyzed using cross-tab analyses followed by χ2
statistics. Significance of correlations among variables was
tested using Pearson correlation coefficients.
Results
PROP tasters and non-tasters
Summed bitterness intensity ratings for the three PROP
filter papers were initially used to assess the perception
of PROP (Drewnowski et al., 1997a; Kaminski et al., 2000).
Reliability tests showed high correlations between successive
bitterness scores (r = 0.84 for filter papers 1 and 2; r = 0.89
for filter papers 1 and 3; r = 0.89 for filter papers 2 and 3).
The sum of bitterness intensity ratings for the three PROP
filter papers showed a bimodal distribution.
As shown in Figure 1, the sum of bitter intensity ratings
for seven PROP solutions also showed the expected bimodal
distribution. There was a high degree of correlation (r =
0.79, P < 0.01) between bitterness intensity ratings for the
Figure 2 Summed bitterness ratings for seven PROP solutions plotted
against bitterness ratings for three PROP filter papers (r = 0.79, P < 0.01).
seven solutions and the three filter papers, as shown in
Figure 2 (Drewnowski et al., 1997a,b,c; Kaminski et al.,
2000). Since the use of solutions appeared to be less subject
to ceiling effects, the sum of bitter ratings for the seven
PROP solutions served as the main method for assigning
PROP taster status (Lawless, 1980). Respondents whose
summed responses were 21 or less were classified as nontasters, whereas those with summed ratings in excess of 21
were classified as tasters. The subject sample was thus
divided into 33 tasters and 21 non-tasters of PROP. As
shown in Table 1, PROP tasters and non-tasters did not
differ from each other in their age, height, weight or BMI.
Taste responses to PROP and caffeine
Taste response profiles for PROP solutions supported the
taster–non-taster distinction (see Figure 3, top). As expected, non-tasters gave very low bitterness ratings to all
PROP solutions, except for solution 15. In contrast, tasters
rated all solutions as more bitter and had lower hedonic
ratings than did non-tasters. As in past studies, PROP
44
A. Ly and A. Drewnowski
tasters disliked PROP solutions more than did non-tasters.
The correlation between bitterness and hedonic ratings was
significant (r = –0.77, P < 0.01).
Tasters found caffeine solutions to be more bitter than did
non-tasters [F(1,52) = 4.9, P < 0.05] and disliked them more
than did non-tasters [F(1,52) = 5.2, P < 0.05]. These data are
shown in the bottom panels of Figure 3. As was the case
with the PROP solutions, respondents who found caffeine
solutions to be more bitter also liked them less (r = –0.71,
P < 0.01).
The relationship between summed bitterness intensity
ratings for PROP and those for caffeine is shown in Figure
4. Whereas some PROP non-tasters gave high bitterness
ratings to caffeine solutions, others did not. In contrast,
PROP tasters also tended to give higher ratings to caffeine.
These data are consistent with the notion that PROP tasting
Table 1
Respondent characteristics (means ± SEM)
Age (years)
Height (cm)
Weight (kg)
BMI (kg/m2)
Non-tasters
(n = 21)
Tasters
(n = 33)
All subjects
(n = 54)
23.4
165.8
58.4
21.4
24.1 ± 0.7
165.5 ± 0.4
61.7 ± 1.6
22.5 ± 0.5
23.8
165.2
60.4
22.1
± 0.7
± 0.6
± 1.8
± 0.5
± 0.5
± 0.4
± 1.2
± 0.4
may involve both a specific sensitivity to PROP and a more
general bitter taste responsiveness (Olson et al., 1989).
PROP tasting and response to sweet taste
PROP tasting had no effect on intensity or on hedonic
ratings for sucrose solutions. Only a main effect of sucrose
concentration, but no effect of taster status and no interactions, was observed. These data are consistent with
previous studies, based on a larger sample of young women
(Drewnowski et al., 1997a). Tasters and non-tasters did not
differ in their sensory responses to a single NHDC solution.
Mean sweetness intensity ratings for NHDC were 7.12 ± 1.8
for tasters and 6.81 ± 2.38 for non-tasters.
Response to sweetened PROP and caffeine solutions
Adding NHDC to PROP solutions reduced bitterness
intensity and increased hedonic ratings. As shown in Figure
5, the suppression of bitterness was stronger for PROP
tasters than for non-tasters, whose ratings were already at
baseline. Analysis of variance of bitterness ratings showed a
significant stimulus type by taster status interaction [F(1,51)
= 8.6, P < 0.01] and no main effect of stimulus. Analysis of
variance of hedonic ratings showed a significant main effect
of stimulus type [F(1,51) = 17.1, P < 0.001].
The addition of NHDC to caffeine solutions reduced
perceived bitterness and increased hedonic ratings, as shown
in Figure 6. PROP tasters still perceived sweetened caffeine
solutions as more bitter than did non-tasters [F(1,50) = 5.5,
Figure 3 (Top) Mean bitterness intensity and hedonic ratings for PROP solutions by PROP taster status. (Bottom) Mean bitterness intensity and hedonic
ratings for caffeine solutions, by PROP taster status.
PROP Tasting and Response to Caffeine 45
P < 0.05]. However, tasters and non-tasters no longer differed in their hedonic response to sweetened caffeine. Only
the main effect of caffeine concentration was significant
[F(6,46) = 53.8, P < 0.001].
PROP tasting and coffee and chocolate consumption
With this small sample size there were no significant differences by PROP taster status in the reported consumption of
chocolate or coffee beverages. However, out of respondents
who consumed drip coffee (n = 22), tasters were more likely
(77%) to drink coffee with milk/cream and sweetener than
were non-tasters (44%). Conversely, a greater percentage of
non-tasters (22%) preferred their coffee black as opposed to
tasters (7%), although these results also were not significant.
There were no differences between taster groups in their
sensory evaluation of white and dark chocolate and in their
bitter intensity ratings for milk chocolate.
Taste factors, food preferences and food consumption
Figure 4 Summed bitterness ratings for PROP solutions plotted against
summed bitterness ratings for caffeine, by PROP taster status.
Taste and food preferences and self-reported frequencies of
food consumption are associated variables. The correlations
between taste preferences, food preferences and reported
frequencies of consumption for coffee and chocolate are
shown in Table 2. All correlations were significant, suggesting that hedonic ratings obtained in the laboratory are linked
with self-reported food preferences and with self-reported
frequencies of food use. Since food frequency question-
Figure 5 Intensity (left) and hedonic ratings (right) for PROP solutions before and after the addition of NHDC, by PROP taster status.
Figure 6 Intensity (left) and hedonic ratings (right) for caffeine solutions after the addition of NHDC, by PROP taster status.
46
A. Ly and A. Drewnowski
Table 2
Pearson correlations among taste preferences, food preferences and frequency of food consumption (n = 53)
Food item
Taste preference × food
preference
Food preference × food
frequency
Taste preference × food frequency
White chocolate
Milk chocolate
Bittersweet chocolate
Coffee/coffee beverages
0.87a
0.53a
0.67a
0.50a
0.50a
0.75a
0.66a
0.47a
0.22
0.56a
a
The correlation is significant at the 0.01 level (two-tailed test).
naires are the mainstay of nutritional epidemiology, showing that food preferences and food frequencies are in fact
linked suggests that chemical senses are an important
predictor of dietary behavior (Kaminski et al., 2000).
Discussion
The finding that PROP tasters found caffeine to be more
bitter than did non-tasters confirms some previous studies
(Hall et al., 1975; Neely and Borg, 1999). PROP tasters also
disliked caffeine solutions more than did non-tasters. The
difference between PROP tasters and non-tasters was
observed across a wide range of caffeine concentrations,
including those found in coffee and in soft drinks
(0.003–0.004 mol/l) (Hall et al., 1975). Bartoshuk suggested
at one point that PROP tasters’ sensitivity to caffeine or
saccharin might lead them to avoid coffee or diet soft drinks
(Bartoshuk, 1979). However, it now appears that moderate
sweetening of caffeine solutions minimizes the impact of
genetic taste differences. Whereas the sweetening of PROP
solutions with NHDC reduced, but did not eliminate, differences in taste response between tasters and non-tasters,
sweetening of caffeine solutions eliminated differences in
hedonic response altogether.
These findings may explain why past studies have
found only a weak effect of PROP taster status on reported
preferences for bitter foods (Drewnowski and Rock, 1995;
Drewnowski et al., 1997c, 1998; Kaminski et al., 2000).
Consumers often mask the taste of bitter foods through
the addition of sweeteners or dressings (Roy, 1990). While
significant differences in bitter taste perception have been
observed under laboratory conditions (Hall et al., 1975;
Bartoshuk 1979; Gent and Bartoshuk 1983), their impact on
actual food choices and eating habits may be limited (Akella
et al., 1997; Drewnowski et al., 1997c; Kaminski et al.,
2000).
For example, there were no significant differences between
PROP tasters and non-tasters in their sensory evaluation
of three types of chocolate. While chocolate contains
both caffeine and bitter polyphenols, its high sugar and
fat content (Rozin et al., 1991) may suppress the differential
sensation of bitterness. We did not find differences in
reported preferences for coffee beverages by PROP taster
status. Sweetening caffeine solutions with NHDC may have
the same effect as adding sugar to coffee. Given that
sweetening caffeine solutions eliminated taster–non-taster
differences in hedonic preference, it may be difficult to
demonstrate the impact of genetic taste factors on food
preferences and eating habits. Although taste and food preferences broadly predict eating habits and food consumption,
people prepare and season foods in different ways. Rather
than avoid bitter foods, PROP tasters may choose to suppress bitterness through the addition of fat, sugar or salt.
Debittering of plant foods, in particular, is a major concern
of the food industry.
NHDC has proved to be an effective debittering agent.
Currently, the most common commercial method for masking bitter taste is the addition of a sweetening agent (Roy,
1990; Schiffman et al., 1994a,b). Schiffman and co-workers
found that the addition of sucrose and sorbitol to caffeine
solutions significantly decreased the perception of bitterness
(Schiffman et al., 1994a,b). Commercially, it is the citrus
juice industry that relies the most on debittering to mask
or remove the bitterness of citrus juices (Roy, 1990). While
neohesperidin, a flavonoid extracted from bitter oranges,
is intensely bitter, NHDC is intensely sweet. NHDC has
been used as a debittering agent when added to bitter foods,
antibiotics, vitamins or bitter pharmaceuticals (Borego et
al., 1991). PROP tasters, who often reject bitter tasting
foods (Drewnowski 1990; Drewnowski and Rock, 1995;
Drewnowski et al., 1998), may be more likely to use caloric
and non-caloric sweeteners than PROP non-tasters.
The relationship between PROP tasting and bitterness
ratings for caffeine was of particular interest. Among PROP
non-tasters were some who rated caffeine as very bitter and
some who did not. In contrast, PROP tasting was associated
with higher bitterness ratings for caffeine. This may explain
why attempts to link PROP taster status with enhanced perception of other bitter compounds have produced inconsistent results (Leach and Noble, 1986; Mela, 1989; Smagghe
and Louis-Sylvestre, 1998). Subject selection procedures or
methodological differences in assigning PROP taster status
may also have played a part. These observations are also
consistent with past suggestions that PROP tasting may
carry with it a more general ability to taste bitter compounds (Olson et al., 1989).
As in some past studies conducted with college age
PROP Tasting and Response to Caffeine 47
females, there was no relationship between PROP sensitivity
and taste responsiveness to sucrose solutions (Drewnowski
et al., 1997a,b). There was no difference between PROP
tasters and non-tasters in their sensory responses to NHDC
at a concentration of 0.3 mmol/l. Gent and Bartoshuk
observed no differences in the sweetness intensity ratings
of NHDC at a concentration of 0.32 mmol/l although
differences by PROP taster status were observed at lower
concentrations (Gent and Bartoshuk, 1983).
Finally, greater perceived bitterness was associated with
lower preference ratings. This inverse relationship held when
PROP and caffeine solutions were sweetened with NHDC.
Given that bitterness and hedonic functions are mirror
images of each other, hedonic ratings may provide an alternative measure of the genetic ability to taste PROP (Akella
et al., 1997; Drewnowski et al., 1997a,c; Kaminski et al.,
2000).
Acknowledgements
This work was supported in part by NIH grant 1RO1 CA61680.
We thank Starbucks Co. (Seattle, WA) for providing caffeine and
Zoster SA (Barcelona, Spain) for supplying neohesperidin DC.
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Accepted July 26, 2000