EFFECTS OF ORAL CHEMICAL IRRITATION ON TASTES AND FLAVORS IN FREQUENT AND INFREQUENT USERS OF CHILI

Pergamon

0031-9384(95)02052-W

Physiology & Behavior, Vol. 58, No. 6, 1117-1127, 1995 Copyright © 1995 Elsevier Science Inc. Printed in the USA. All rights reserved

0031-9384/95 $9.50 + .00

Effects of Oral Chemical Irritation on Tastes and Flavors in Frequent and Infrequent Users

of Chili

JOHN P R E S C O T r I AND RICHARD J. STEVENSON

Sensory Research Centre, CSIRO Division of Food Science & Technology, P.O. Box 52, North Ryde NSW 2113, Australia

Received 19 January 1995

PRESCO'VI', J. AND R. J. STEVENSON. Effects of oral chemical irritation on tastes and flavors in frequent and infrequent user's of chili. PHYSIOL BEHAV 58(6) 1117-1127, 1995.--The studies reported here addressed the question of whether the pungent principle in chilies, capsaicin, suppresses taste and flavor intensity. Over a period of several minutes, groups of frequent and infrequent eaters of chili repeatedly rated the taste and flavor intensities of sweet and sour solutions that also contained either orange or vanilla flavor, and capsaicin at 0, 2, 4, or 16 ppm. As well as the intensity of the qualities while in the mouth, measures of the number of rating periods for the intensity to dissipate to zero, and the summed total intensity were also derived. Infrequent chili users rated the capsaicin burn as more intense than did the frequent users. With few exceptions, and for both groups, sweetness was suppressed by the presence of capsaicin. By contrast, sourness was unaffected by capsaicin. Flavor intensities also showed suppression by capsaicin. High correlations between ratings of sweetness and flavor were found, suggesting that perceptual confusion between the two qualities may have been responsible for the flavor suppression. A second experiment examined the effects of capsaicin on ratings of strawberry flavor alone. This study produced little evidence of flavor suppression by capsaicin. These results are discussed in terms of an attentional model of capsaicin's effects.

Capsaicin Sweetness Sourness Flavor Time-intensity Chili

A HIGH percentage of the world's population consumes foods every day that are characterised by the presence of pungent or irritant ingredients. Such ingredients not only add intensity to foods but will also often form part of the "flavor principle" of that cuisine (20). All such pungent ingredients (e.g., ginger, chili, mustard, horseradish, pepper, and CO 2) produce a range of common sensations such as burning, tingling, and warmth (2) that are a result of their stimulation of the trigeminal nerve in the mouth or nose.

There has been increasing interest in recent years in the properties and functions of the trigeminal system, stemming perhaps from a realisation that our understanding of this sense has lagged behind that of smell and taste. The result has been a considerable increase in our understanding of the psychophysics of this sense in the nose and mouth and an understanding of the differences in the mode of action of the trigeminal system from that of taste or smell [see, e.g., (10)].

Possibly due to the increasing use of pungent compounds in Western diets, a key question that has received attention is how

the trigeminal sense interacts with taste and smell. Anecdotal, plus limited research evidence (21), indicates that many people believe tastes and flavors to be suppressed in the presence of strongly pungent food ingredients. Supporting this, some of the first research on olfactory/trigeminal interactions in humans indicated that, at least in the case of CO2, nasal irritation can suppress the intensity of odors (1).

The effects of CO 2 in the mouth have also been examined. Yau and McDaniel (24) found no suppression of tastes in CO2/taste mixtures. Commetto-Muniz et al. (3) found concentration-dependent reductions in bitterness by CO 2 but no reductions in sweetness, sourness, and saltiness. These results suggest that taste suppression is not a necessary outcome of oral irritation per se.

The vast majority of research on oral irritation has used capsaicin, the pungent principle in chili, not least because chili is the most commonly consumed oral irritant (16). Research on the effects of capsaicin has shown that both odors and tastes are judged as less intense following prestimulation with capsaicin,

1 To whom requests for reprints should be addressed.

1117

1118 PRESCOTT AND STEVENSON

even at relatively low levels of burn, whereas flavor identification, on the other hand, appeared to be unaffected (13,14).

Because of the contiguity of the sensory qualities, investigating tastes/irritant interactions in mixtures provides a much closer analog of such interactions in foods than does prestimulation with capsaicin. Moreover, perceived reductions in taste intensity following prestimulation with capsaicin may not be an effect of capsaicin per se, but may be due to context effects resulting from the contrast between an intense stimulus, capsaicin, and the subsequent weaker tastes [see, e.g., (23)].

The first study of interactions in capsaicin/taste mixtures by Cowart (4) found no taste suppression using capsaicin at relatively low burn levels (2 ppm). In contrast, Prescott et al. (18) found that capsaicin, particularly at higher levels of burn (4 and 8 ppm), reduced sweetness in both solution and a food (soup), although similar reductions were not observed for saltiness.

The mechanisms behind taste suppression by capsaicin are unclear. One mechanism that has been proposed (13,18) is that capsaicin dominates the mixture to such an extent that attention is diverted from the taste, thus leading to a perceived reduction in taste intensity. The failure to find saltiness suppression may be the result of a perceptual confusion (6) between capsaicin and NaC1, due to the latter's irritant properties (9).

If an explanation of taste suppression as a result of attentional factors is correct, then capsaicin could also be expected to dominate flavors because the perceived location of flavors in the mouth can be seen as a purely cognitive phenomenon (19). However, to date, there has been little research on the effects of capsaicin in more complex systems of tastes and flavors, such as are typically found in foods. Green (8) has recently demonstrated that the detection thresholds of both sucrose and citral were increased by the presence of capsaicin. However, this research did not demonstrate suppression of the intensity of a suerose/citral mixture by capsaicin. Green's (8) experimental procedure used mixtures of capsaicin (2 and 5 ppm) and flavor, although the effective burn can be considered to have been higher than is suggested by these concentrations because the mixture was preceded by three warm-up rinses of capsaicin.

The present research was carried out to examine capsaicin's effects in complex taste/flavor mixtures, using generally higher levels of burn (up to 16 ppm) than have been previously used. It was envisaged that use of mixtures containing flavors as well as tastes would provide insight into the viability of an attentional (cognitive) explanation for capsaicin's effects.

The flavors used in this study were chosen to examine effects in both familiar and unfamiliar taste/flavor combinations. In an orange/sweet/sour mixture, as used here, subjects may be cued to the relative intensities of each component by the strengths of the other components due to the familiarity of this combination. For this reason, we also chose to use an unfamiliar combination, namely vanilla/sweet/sour, in which it presumably would be difficult for the subjects to derive accurate information regarding the intensity of one component (e.g., vanilla) from the intensities of the sweet and sour components.

This research also sought to determine if the effects of capsaicin on tastes and flavors were different for frequent and infrequent users of chili. Within the framework of an attentional view of capsaicin's effects, it might be expected that frequent users of chili would be less influenced by capsaicin if it was acting as a distracter than would infrequent users. Previous research (14) found that frequent and infrequent chili eaters, though differing in their intensity ratings of the burn of capsaicin, did not differ in the extent to which prestimulation with capsaicin produced taste suppression. This may have been due to the relatively low level of capsaicin burn used (2 ppm).

Previous research has examined the interactions of capsaicin and tastes at a single point in time. However, the time course of capsaicin's burning effects (2) and those of tastes and flavors differ. In addition, measures of intensity may not necessarily be the only indicators of capsaicin's effects, which may, for example, be reflected in flavor or taste duration. For these reasons, this research also examined the effects of capsaicin over time and derived a number of measures that would encompass different taste and flavor parameters.

EXPERIMENT 1

M E T H O D

Subjects

Staff from CSIRO Division of Food Science & Technology were screened using a questionnaire on frequency of chili use and liking for the burn of chili (14). Those who either ate chili very infrequently (less than once a month) or very frequently (at least three to four times a week) were assigned to respectively to group Infrequent (n = 16; 10 males, 6 females) or group Fre- quent (n = 16; 5 males, 11 females). Table 1 illustrates group characteristics and differences.

Stimuli

All test solutions contained 0.29 M sucrose, 0.04 M citric acid (Univar), and either 0.40% vanilla flavor or 0.36% orange flavor (Quest). These concentrations produced approximately equal flavor intensities. Solutions also contained either capsaicin (Sigma) at concentrations of 1, 4, or 16 ppm and vehicle (0.20% ethanol in distilled water), or vehicle alone. Solutions were refrigerated until use, then warmed to 37°C in a water bath prior to serving. Subjects received 15 ml of each test solution.

Procedure

All test sessions were separated by at least 48 h and subjects were requested to refrain from eating chili in the 24 h preceding each session. Subjects received only one test solution per session. On the first test day, subjects were given a practice trial to familiarise them with the rating scales, using tomato juice as the test solution. Following this, subjects rinsed their mouths with mineral water and proceeded to evaluate the first experimental solution. All trials were controlled by computer and took place alone in a sound-attenuated room.

On test trials, the computer presented instructions to the subject outlining the basic plan of the trial and the rating scales to be used. Subjects were instructed to pour the test solution into

TABLE 1 G R O U P C H A R A C T E R I S T I C S F O R E X P E R I M E N T 1

Frequent Users Infiequent Users (n ~ 16) (n = 16)

Mean/ S E / Mean/ SE/ Median Range Median Range Difference

A g e 33.2 10.1 34.0 10.8 t(30) ffi 0.22 Questionnaire score 28.8 3.5 15.8 7.4 t(21) = 6.33* Burn l iking 7 .1 t 1.2 3 .9 t 1.9 t(25) = 5.64* Frequency of use 5:~ (None) 2~. 1-2~: z (U) = 5.11"

* Signif icant ly different at p < 0.001. t Nine-point hedonic scale (1 ffi dislike extremely). :~ 1 = Once a year or less, 2 = less than once a month, 5 = three to

four t imes a w e e k .

ORAL IRRITATION, TASTES, AND FLAVORS 1119

their mouths and to gently swill it around while making their ratings. The computer tlaen serially presented four visual ana- logne rating scales in random order on the monitor. Responses were made by positioning a bar at the point along the scale that subjects felt reflected their sensations at the moment the scale appeared. The following attributes were rated: • burn intensity (How strong is the burning sensation now?

Anchors: No~ce, Extremely strong); • sweetness (How sweet is the taste now? Anchors: Not sweet,

Extremely sweet); • sourness (How sour is the taste now? Anchors: Not sour,

Extremely sour); and • flavor intensity (How strong is the flavor now? Anchors: No

flavor, Extremely strong). Subjects were specifically instructed not to include ratings of burn intensity in their ratings cf flavor intensity.

After completion of these ratings ( ~ 20 s), the computer instructed subjects to expectorate, and to then gently move their tongues around their mouths while making the rest of the ratings. Subjects were then presented with blocks of the same four rating scales in randomised serial order, nine times. If the four scales were completed in under 20 s, the computer waited until this time period was up before stal~ing the next block of four scales. This 20-s timer was reset every time a new block of scales were started. By completion of the practice trial, subjects had little difficulty in completing t?ae scales within the 20-s period.

In total, subjects completed eight randomised test trials, com- prised of orange flavor (plus sucrose and citric acid) and vanilla flavor (plus sucrose and citric acid) at 0, 1, 4, and 16 ppm capsaicin. Four subjects had to repeat one trial each, either as a result of incorrectly following instructions or because they had initially failed to recall eating chili in the 24 h preceding the experiment.

Following the last experimental trial, subjects completed a questionnaire that asked them to recall any miscellaneous information about the test solutions that they had observed, their tobacco usage, whether tl~e burn intensity in food they consumed was generally hotter, the same, or cooler than the hottest sample in the study, and finally, how much they had liked or disliked the hottest sample, using a nine-point category rating scale.

Analysis

Burn intensity data were analysed using ANOVA. Vehicle- only trials were excluded from the ANOVA as subjects did not

generally make any response. Analysis of sweetness, sourness, and flavor intensity data utllised all trials, but did not use one overall ANOVA because of the large variation in the times for the intensity of these qualities to dissipate to zero, meaning that a large number of zero values occurred, particularly in the last five ratings periods. Instead a number of different measures, including those representing effects over time, were derived and analysed. These were the effects of capsaicin on: 1) ratings while the solutions were in the mouth and just after expectoration (RAT 1 and RAT2, respectively); 2) the number of trials taken for the sensation to dissipate to zero (T... o); and 3) the total sum score of all ratings on a particular trial (TOTs,=).

In all ANOVAs, subjects were included as a random effect and all reported main effects and interactions were significant at least at the 5% level, unless otherwise stated. Following significant ANOVA main effects and interactions, linear, quadratic, and cubic contrasts were undertaken to examine the effects of capsaicin doses, and Fisher's Least Significant Differences were calculated to compare differences between means. All reported means are out of 75, the maximum score on the rating scale.

R E S U L T S

Subjects

The two groups significantly differed on all the Lawless et al. (14) questionnaire variables (see Table 1). On the final questionnaire, infrequent users reported on average the use of a less intense burn than the one experienced during the 16 ppm trials. This level of burn was the norm for frequent users and groups differed significantly in this respect, z(U) = 2.96. Frequent users also liked the 16 ppm burn significantly more than infrequent users, who disliked it (t26 = 4.38). When asked to provide "any miscellaneous information about the test solutions that they had observed," approximately two-thirds of subjects explicitly stated that they were unable to recognise one or both of the flavors used in the experiment. Indeed, many subjects were not even aware that two different flavors had been used.

Burn Intensity

A four-way repeated-measures ANOVA was used with frequent and infrequent users (group) as a between-subjects factor, and orange and vanilla (flavor), capsaicin concentration (dose), and time of rating after oral presentation (time) as within-subject

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FIG. 1. Mean ratings of frequent and infrequent chili users for burn intensity over the 10 measurement periods at each capsaicin concentration. The bar represents the 5% least significant difference (LSD).


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R A T I N G P E R I O D

FIG. 2. Mean ratings of frequent and infrequent chili users for sweetness over the 10 measurement periods at each capsaicin concentration. The bar represents the 5% least significant difference (LSD).

factors. Infrequent chili users rated the burn as significantly more intense than frequent users [main effect of Group, F(1, 30)= 9.99]. All subjects found that the burn intensity increased as the

capsaicin concentration was increased [main effect of Dose, F(2, 60) = 170.69] and generally decreased over the rating periods [main effect of time, F(9, 270) = 75.15], with weaker capsaicin

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FIG. 3. Mean ratings of frequent and infrequent chili users for sourness over the 10 measurement periods at each capsaicin concentration. The bar represents the 5% least significant difference (LSD).


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FIG. 4. Mean ratings of frequent and infrequent chili users for orange flavor over the 10 measurement periods at each capsaicin concentration. The bar represents the 5% least significant difference (LSD).

concentrations decaying more rapidly than more intense concentrations [dose × time interaction, F(18, 540)= 14.67] (see Fig. 1).

Sweetness, Sourness, and Flavor Intensity

Figures 2 -5 show the mean ratings of frequent and infrequent chili users for sweetness, sourness, and orange and vanilla flavor

intensity over the 10 measurement periods at each capsaicin concentration. The mean values of RAT 1 plus the TI measures (To 0, TOTsum) for each taste and flavor quality across the levels of capsaicin are given in Table 2.

RAT 1 and RAT 2 Ratings of sweetness, sourness, and flavor intensity made

while the solutions were in the subject's mouth (RAT 1) and just

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1122 PRESCOTt AND STEVENSON

following expectoration (RAT a ) were analysed by four-way repeated-measures ANOVAs, with flavor (orange and vanilla), dose (0, 1, 4, 16 ppm capsaicin), and time (before and after expectoration) as within-subject factors and group (frequent and infrequent users) as a between-subject factor.

Sweetness. The groups did not significantly differ in their overall perception of sweetness, which decreased following expectoration, F(1, 30)--28.22. For both groups sweetness was suppressed in the presence of capsaicin [main effect of dose, F(3, 90) = 10.92]. The magnitude of this effect was approximately a 16% reduction at 16 ppm compared to sucrose alone, which is comparable to previous findings, even with lower levels of burn (18). The relationship between the degree of suppression and capsaicin dose was explored by trend analysis, which revealed significant linear, F(1, 30)= 22.68, and quadratic, F(1, 30)= 5.77, effects. This suggests that the suppression of sweetness was a curvilinear function of the capsaicin doses used. Although sweetness ratings decreased as capsaicin dose increased, for frequent users' responses on the orange-flavored samples, suppression only appeared to occur with the 16 ppm capsaicin dose [flavor × dose X group interaction, F(3, 90) = 2.93].

Sourness. The groups did not differ significantly in their overall perception of sourness; however, frequent users found the vanilla-flavored solutions more sour than the infrequent users [flavor × group interaction, F(1, 30) = 4.22]. Sourness decreased uniformly in both groups following expectoration [main effect of time, F(1, 30)-- 15.38], but capsaicin had no significant effects on sourness perception.

Flavor. The groups did not significantly differ in their overall ratings of flavor intensity, which decreased following expectoration [main effect of time, F(1, 30)= 21.84]. Capsaicin suppressed ratings of flavor [main effect of dose, F(3, 90) = 2.72], although this was only apparent at 16 ppm. This relatively small effect was reflected in a marginally significant ( p = 0 . 0 5 5 ) quadratic contrast.

T- , 0

The number of time periods taken for sweetness, sourness, and flavor intensity to dissipate to a zero rating (T_, 0) was analysed using three-way, repeated-measures ANOVAs, with dose and flavor as within-subject factors and group as a between-subject factor.

Sweetness. Sweetness ratings decreased to zero faster when capsaicin was present [main effect of dose, F(3, 90) = 4.22] than when it was absent. Moreover, as the capsaicin dose increased, sweetness dissipated more quickly [linear trend, F(1, 30) = 9.16].

Sourness. There was no effect of capsaicin on the time taken for the sour taste to dissipate. No other effects or interactions were observed.

Flavor. There was no effect of capsaicin on the time taken for the flavors to dissipate. No other effects or interactions were observed.

TOT~um

Total ratings for sweetness, sourness, and flavor intensity across all measurement periods (TOTs, m ) were analysed using three-way, repeated-measures ANOVAs, with dose and flavor as within-subject factors and group as a between-subject factor.

Sweetness. Overall, subjects perceived the orange-flavored stimuli to be sweeter than the vanilla [main effect of flavor, F(1, 30) = 4.34]. As before, the presence of capsaicin suppressed sweetness [main effect of dose, F(3, 90)= 6.69]. The total sweetness rating decreased as a function of increases in capsaicin dose [linear trend, F(1, 30) = 15.24]. Dose × group, F(3, 90) --- 4.29, and dose × flavor X group, F(3, 90) = 3.08, interactions reflected a general trend for decreased sweetness on higher-dose capsaicin trials, but show large fluctuations in responses between groups, especially on orange-flavored trials.

Sourness. There was no effect of capsaicin on the total sourness rating. No other effects or interactions were observed.

Flavor. There was a main effect of dose, F(3, 90) = 3.10, on the total flavor rating. The pattern of results closely resembled those occurring pre- and postexpectoration, in that ratings only decreased on the strongest capsaicin trial, as shown by a significant quadratic contrast for dose, F(1, 30) = 7.31.

Relationship Between Variables

In an attempt to establish whether ratings for flavor, sourness, sweetness, and burn intensity were independent, correlations between ratings for all pairs of qualities were computed (Pearson's r). Comparisons were made on data calculated by summing the rating scores across trials during the period in which the stimuli were in the subject's mouth. That is, data for each subject on each rating type were collapsed across flavor and all capsaicin levels, excluding vehicle-only trials where burn ratings were consistently at zero. The rationale for this approach was twofold. First, the results of an analysis on data generated while the stimuli were just in subjects' mouths closely resembled the general pattern of findings reported above, thus justifying this time point as representative. Second, collapsing the data across trials allowed us to examine consistent variations in rating be-

T A B L E 2

MEANS 4-SE FOR RAT 1, T~ 0, AND TOTsu m DERIVED FROM RATINGS OF SWEETNESS, SOURNESS, ORANGE AND VANILLA BY FREQUENT AND INFREQUENT USERS OF CHILI IN EXP. 1

FLAVOR

Sweetness Sourness Orange Flavor Vanilla Flavor Capsaicin (ppm) Freq. In freq. Freq. Infreq. Freq. Infreq. Freq. lafreq.

RAT× 0 51 4- 4 46.94- 4.2 3 5 . 6 5 : 4 . 9 22.8+ 4.2 49 .9+ 5.2 39 .9+ 5.7 4 8 . 1 5 : 4 . 5 37.6 1 46.7 4. 3.9 47.3 4. 4.4 36.2 + 5.3 29.4 4- 4.3 46.1 4- 5.7 41.9 4- 3.4 44 .1+ 4.3 42.5 4 49 .1+ 3.5 38.2 4. 4.5 28.6 4. 5.1 28.3 4- 4.8 44.2 4- 4.3 42.8 + 5 44.8 4- 5 41.4

16 3 9 . 3 5 : 4 . 1 36 4. 4.3 36.6+ 5.3 25.1 4- 4.5 42.1 4- 4.8 3 2 . 8 5 : 4 . 8 40 .2+ 4.4 38.4 T ~ 0 0 7 .7+ 0.7 7.94. 0.6 6 .3+ 0.8 5.84. 1 7 .6+ 0.8 7 .7+ 0.7 7.44. 0.7 7.6

1 7.3 4- 0.7 7.9 4. 0.6 6.24- 0.7 7 .2+ 0.9 7.3 + 0.9 8.9 4. 0.4 7.2 + 0.7 8.3 4 6.84. 0.7 7.54- 0.7 6.1 4- 0.7 6.34- 0.9 7 .3+ 0.7 8.34- 0.7 7 . 2 5 : 0 . 6 8.8

16 6.54. 0.7 6.84- 0.8 6 4- 0.7 5.95: 1 6 . 7 5 : 0 . 8 7 .3+ 0.8 6 .9+ 0.7 7.7 TOTsu m 0 217.6 4- 32.7 206.3 4. 28.6 137.7 4. 23.5 84.4 + 17.3 210.6 + 37.1 166 4- 28.5 187.2 + 32.1 152.8

1 175.6 + 27.3 230.4 4. 39 149.8 + 33.7 130.8 + 23.8 177.5 4. 36.2 209.2 + 32.6 180.2 4. 32.6 206.8 4 214.5 4. 29.3 168.7 4. 29.1 134.4 4. 30.7 115.5 + 24.4 198 4. 32.1 182.8 4. 29.2 151.7 + 19.4 182.9

16 163.8 + 29.8 136 4- 25.4 130.4 4- 29.4 112.1 4- 26.4 170.1 + 35.8 144.4 4- 27.5 146.4 4- 18.5 150.8

4. 5.4 4- 3.9 4- 4.7 + 3.6 5 : 0 . 7 5 : 0 . 6 4- 0.5 4- 0.8 4- 28 + 37.1 5:30 + 26.5


haviour regardless of the medium on which these ratings were made. Finally, these correlations were examined by group, to reveal any variation in rating behaviour between them.

The only significant correlation for infrequent users was between flavor and sweetness ( r = 0.91). This was also significant in frequent users (1" = 0.84), as was the correlation between flavor and sourness ratings ( r = 0.68). However, there was no significant difference between groups when the correlations for sourness and flavor were compared (z = 1.23).

The fact that sweetness and flavor were highly correlated for both frequent and infrequent chili users suggests the possibility that flavor suppression in this study was merely an artifact of a perceptual confusion that existed between these two qualities, so that when sweetness was suppressed, flavor also appeared to be suppressed. Frank et al. (6) have demonstrated that tastes and flavors show a high degree of perceptual confusion to the extent that taste qualities (e.g., sweetness) may be attributed to odors. Under these conditions, odors have been shown to enhance the sweetness of sucrose solutions.

To examine the possibility that the flavor suppression found was merely the result of perceptual confusion with sweetness, another study was conducted in which flavor was mixed with capsaicin without the concurrent presence of tastes. The presence or absence of flavor suppression under these circumstances would provide evidence of whether the flavor suppression found in the present study was an artifact.

EXPERIMENT 2

METHOD

Subjects

Sixteen subjects, eight frequent and eight infrequent chili users, who had compleled the first experiment, were asked to take part in another five experimental trials. Subjects who had participated in Experiment 1 were recruited both to ensure com- patibility with the results of that experiment and to take advan- tage of the training they had received. The majority of subjects had at least a 1-month break before starting the second experiment (the minimum intermission was 1 week in one subject). The frequency of chili use had not changed for any subject since

TABLE 3. GROUP CHARACTERISTICS FOR EXPERIMENT 2

Frequent Users Infrequent Users (n = 8) (n = 8)

M e a n / S E / M e a n / S E / Median Range Median Range Difference

Age 32.1 8.9 36.7 10.7 t(14) = 0.94 Questionnaire score 27.6 3.6 15.0 9.0 t(9) = 3.67* Burn liking 7.3? 1.1 3.6? 1.8 fill) = 4.84* Frequency of use 5:~ (None) 2~: 1-2:~ z(U) = 3.56*

* Significantly different at p < 0.005. 1" Nine-point hedonic scale (1 = dislike extremely). :~ 1 = Once a year or less, 2 = less than once a month, 5 = three to

four times a week.

finishing the first experiment. Subject characteristics are detailed in Table 3.

Stimuli

All test solutions contained 0.40% strawberry flavor (Quest). Solutions also contained either capsaicin (Sigma) at concentrations of 1, 4, or 16 ppm and vehicle (0.20% ethanol in distilled water), or vehicle alone. Solutions were presented in the same manner as in Experiment 1.

Procedure

The same procedure was used as in Experiment 1, with the following exceptions. On the first trial, subjects received strawberry flavor (0.40%) and distilled water (rather than the ethanol vehicle) and were told "This sample principally contains a flavor". Subjects then used the same computer program as described in Experiment 1, except this time only two ratings were made, burn intensity and flavor intensity, and the interval between blocks was reduced to 10 s. Following completion of this first training trial and a water rinse, subjects were given a second sample containing 4 ppm capsaicin and vehicle but no strawberry flavor, and were told, "This sample principally contains a bum." The same computer program was then used to collect burn and flavor intensity ratings. On the four subsequent trials, each at

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FIG. 6. Mean ratings of frequent and infrequent chili users for burn intensity over the 10 measurement periods at each capsaicin concentration. The bar represents the 5% least significant difference (LSD).

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FIG. 7. Mean ratings of frequent and infrequent chili users for strawberry flavor over the 10 measurement periods at each capsaicin concentration. The bar represents the 5% least significant difference (LSD).

least 48 h apart, subjects received strawberry flavor and capsaicin (1, 4, 16 ppm) or vehicle, and used the computer program described in Experiment 1 to rate just flavor and burn intensity.

Analysis

The same analysis strategy was applied to burn intensity and flavor ratings as described for Experiment 1.

RESULTS

Subjects

Groups significantly differed on the Lawless et al. (14) questionnaire score (see Table 3), on their normal level of preferred burn, z(U) = 2.34, and on their liking for the strongest capsaicin

trial ( t 9 = 2.77). All comparison data were collected during the preceding experiment.

Burn Intensity

A three-way repeated-measures ANOVA was conducted with frequent and infrequent users as a between-subject factor (group) and capsaicin concentration (dose) and time of rating after oral presentation (time) as within-subject factors. As can be seen in Fig. 6, burn intensity increased in all subjects as a function of increasing capsaicin dose [main effect of dose, F(2, 28) = 77.96] and ratings decreased following expectoration [main effect of time, F(9, 126) = 16.46]. The rate of this decrease also depended upon the capsaicin dose [dose × time interaction, F(18, 2 5 2 ) = 9.21].

TABLE 4 MEANS 5: SE FOR RAT 1, T~ o, AND TOTs. m DERIVED FROM RATINGS OF STRAWBERRY FLAVOR

BY FREQUENT AND INFREQUENT USERS OF CHILl IN EXP. 2

Capsacin RAT1 T ~ o TOTsam

(ppm) Freq. Infreq. Freq. In freq. Freq. [nfreq.

0 20.3 + 8.4 29.8 + 5.4 7.9 + 1.1 6.5 + 1.2 120.4 + 45.1 136.8 + 54.1 1 23.1 + 10.7 29.5 + 6.7 7.1 5:0.9 7.8 5:1.2 132.6 5:45.6 158.1 5:43.4 4 24.5 5:10.4 32.9 5:4.6 6.4 5:1.3 7.6 5:1.2 115.9 + 49.2 206.6 5:53.1

16 23.6 + 10.1 32.5 -I- 6.3 5.4 + 0.9 6.9 5:1.6 105.3 + 47.4 186.3 + 55.2

THIS SECTION COMES AFTER TOTAL RATINGS (EXP 1) AND BEFORE RELATIONSHIP BETWEEN VARIABLES


Differences were again apparent between the groups' burn intensity ratings. Figure 6 shows that infrequent users rated the burn of 1 ppm as more Jintense than frequent users, but the burn of 4 and 16 ppm the same as frequent users [group × dose interaction, F(2, 28)= 8.58]. The pattern of response over the rating periods also diffeled between the groups [group X dose X time interaction, F(18, 252)= 2.03]. Frequent users tended to show rapid increases then decreases in rated intensity postexpectoration, whereas infrequent users' responses were more static.

Flavor Intensity

The mean ratings of frequent and infrequent chili users for strawberry flavor intensity over the 10 measurement periods at each capsaicin concentr~Ltion are shown in Fig. 7. The analyses conducted were identical to those used for each measure in Experiment 1. The mean values of the TI measures (RAT1, T . 0, TOTsm) for flavor intensity across the levels of capsaicin are given in Table 4. There were no significant difference between groups nor any effect of capsaicin on flavor intensity ratings for RAT1, RAT2, T_. 0, and TOTs, m.

DISCUSSION

In agreement with previous findings (14,22), in Experiment 1, infrequent chili users rated the burn of capsaicin as more intense than frequent users. Moreover, this difference persisted over time and occurred across the range of burn intensities examined in Experiment 1. The data from Experiment 2 also show differences between frequent and ir~equent chili users in ratings of burn intensity, although the effect is confined to 1 ppm capsaicin. It may be for this smaller sample of subjects, a ceiling effect on burn was operating for the higher capsaicin concentrations. An- other possible factor is that participation in the first experiment reduced the differences between the two groups by providing a common context in which to judge the capsaicin burn.

Although some form of physiological adaptation and/or chronic desensitisation in frequent users cannot be ruled out as an explanation for the difference between frequent and infrequent chili users, recent studies have suggested that an individual's range and frequency of burn experiences in everyday life may provide a context for burn judgements in the laboratory (23). This would cause frequent users of chili to generally judge burn to be less intense than infrequent users because of their experience with burns that are generally more intense. Data from Experiment 1 on subjects' comparisons of the burn of 16 ppm and their everyday experiences with hot food supports this interpretation. Infrequent users not only had fewer experiences with chili than the frequent users but these experiences were also less intense, being on average less than the 16 ppm capsaicin, which, by contrast, was the norm for the frequent users.

In Experiment 1, sweetness was suppressed by capsaicin in a concentration-dependent :manner, an effect that was apparent not only in the first two ratings (RAT 1, RAT 2) but was also evident in those indices that represented the effects over time, namely T_. o, and TOTs, m. This constitutes the first evidence that capsaicin burn can alter the time course of a taste. Together with previous studies showing sweetness suppression in both solutions and a food context (18), the present results indicate that sweetness suppression by cai~saicin in mixtures is a reliable phenomenon, if only at mode.rate to high levels of burn.

One explanation advanced to account for the sweetness suppression observed by Prescott et al. (18) was that the dominant nature of capsaicin burn captured the attention of subjects to the detriment of sweetness perception. In other words, capsaicin

acted as the dominant component within the mixture, acting to suppress weaker components. One implication of this is that the intensity of any oral sensation in a mixture with capsaicin that was weaker than the capsaicin bum should also be suppressed. Investigating the effects of capsaicin bum on flavor intensity was undertaken to test the hypothesis that sweetness suppression was essentially an attentional (or cognitive) phenomenon. The data from these experiments shed light on this notion in a number of ways.

The flavor suppression found in Experiment 1 appeared to add weight to the argument for capsaicin's effects being due to diversion of attention. However, the results from Experiment 2 cast considerable doubt on this interpretation. As could be expected if the flavor suppression in Experiment 1 was the result of perceptual confusion between sweetness and flavor, Experiment 2 failed to show any suppression of flavor by capsaicin, when the flavor was presented alone.

There is thus the strong suggestion that capsaicin does not suppress the intensity of flavors. In Experiment 1, flavor and sweetness were rated as similar in intensity, whereas flavor intensities in Experiment 2 were lower than those of sweetness. Because subjects will perceive flavor to be located in the mouth, these data do not support the view that capsaicin's suppressive effects act primarily by demanding attention or by dominating a mixture due to higher stimulus intensity.

There are other aspects of the present results that are inconsis- tent with a simple attentional model in which capsaicin's effects dominate mixtures. First, despite differences in the experience of burn intensity, in agreement with previous findings (14), sweetness suppression was generally equivalent for both frequent and infrequent chili users. An attentional account would predict that there would be differences between groups. With their greater exposure to burn experiences and their familiarity with the bum of 16 ppm, frequent chili uses could be expected to be less susceptible to the distracting effects of capsaicin's burn than infrequent users.

Second, in contrast to capsaicin's effects on sweetness, there was no suppression of sourness by capsaicin. This is despite the fact that sourness was a less prominent component of the mixture than was sweetness in that its ratings were lower than those of sweetness. This again suggests that capsaicin's suppression of sweetness was not merely the result of the general suppression of weaker mixture components by a stronger.

The question then arises as to why capsaicin would selectively suppress one taste (sweetness) while leaving another (sourness) unaffected. One possible reason, consistent with the present results, why sourness was not suppressed is confusion between the qualities of sourness and burning. Like capsaicin, very sour solutions are associated with sensations of irritation (7,12). Frank et al. (6) have shown that taste intensities in mixtures can be either enhanced or suppressed by other tastes or odors, depending on the rating categories provided and also the extent to which the components are perceptually similar to one another. Mixture components that are perceptually or conceptually similar may show enhancement. This phenomenon of perceptual confusion was demonstrated in the high correlations between sweetness and flavor in Experiment 1. Another example was the demonstration in a previous study that sodium chloride alone produced no sensation of bum, yet in mixtures with capsaicin, it added to the burn intensity (18).

Similar differential effects on tastes by another irritant, namely CO 2, have also been demonstrated (3). Subjects rated pungency and taste intensity in mixtures of CO 2 with sweet, salty, sour, and bitter solutions. Although there was essentially no interaction between sweetness and CO 2, sourness and oral CO 2 pungency


showed mutual enhancement (although this may have been due to the additional sourness of carbonic acid). However, saltiness also enhanced oral pungency, and CO 2 enhanced saltiness at low NaC1 concentrations.

By contrast, those mixture components that are dissimilar may show suppression. Because of the irritation frequently associated with sourness and the lack of irritation of sweetness, perceptual confusion may have occurred in the mixtures used in the present study. This is also consistent with the sweetness, but not saltiness, suppression by capsaicin found previously (18).

However, the failure to find flavor suppression casts doubt on the mechanism of perceptual similarity/dissimilarity as a general rule when considering the actions of capsaicin in taste/flavor mixtures. The flavor used in Experiment 2 should have been vulnerable to suppression in that it shares no common features with capsaicin (unlike, for example, sourness). Moreover, strawberry flavor has been shown to be closely identified with sweetness (6).

This leaves open the possibility that capsaicin's interaction with tastes occurs at the receptor level. In other words, capsaicin suppresses sweetness through its interaction with sweetness receptors. By contrast, capsaicin's lack of impact on sourness and saltiness may be a function of similarities in transduction mechanisms. Thus, the irritation that is characteristic of sourness and saltiness at high concentration is mediated via the lingual nerve as is capsaicin's irritation (12), and there is increasing evidence that the transduction of sourness and saltiness involves mechanisms in common with capsaicin. Gilmore and Green (7) have demonstrated that capsaicin desensitisation produced decrements in both the irritation and taste intensity of NaCI and citric acid. This raises the possibility that not only the irritation produced by saltiness and sourness, but also a proportion of what we perceive as their taste intensity, is mediated by capsaicin-sensitive fibres.

Further evidence for capsaicin and sour/salty stimuli sharing common mechanism comes from electrophysiological responses to tastants recorded from chorda tympani of rats following subep- ithelial injection of substance P (5), the primary neurotransmitter involved in capsaicin transduction (17). Responses to NaC1 and citric acid increased as a function of SP concentration, whereas there were no changes in responses to sucrose and quinine. More

recently, Liu and Simon (15) have identified a capsaicin-activated current in rat neurons that is also activated by acidic stimuli, again pointing to common mechanisms of action.

Further research is needed to examine the specificity of capsaicin's interactions with sweetness. To determine whether or not sweetness in general was affected, as opposed to the sweetness of sucrose, it would be useful to examine the impact of capsaicin on other sweet compounds that have been proposed to have different receptor mechanisms to sucrose (11). In addition, it would be informative to examine the impact of capsaicin on another nonirritating taste [e.g., umami (MSG taste)]. Suppression of another nonirritating taste such as umami, in addition to sweetness suppression, might suggest that the failure to find flavor suppression was due to capsaicin's action simply being confined to interactions with taste receptors (rather than being a specific effect on sweetness).

If, as the evidence suggests, neither tastes such as sourness and saltiness nor flavors are suppressed by capsaicin, the question remains of what lies behind individuals' complaints that food with too much chili reduces the taste/flavor. Data from subject questionnaires reveal this to be a real complaint. For example, in the present studies, 38% of subjects reported this phenomenon [see also (14)].

The answer to this question may lie in the difference between the laboratory setting and "real l ife." Subjects in a psychophysics experiment are typically encouraged to adopt an analytical frame of mind in which individual qualities can be judged independently, even in mixtures. By contrast, a real meal may be dominated by initial, most dominant, or most strongly positive or negative sensory experiences. Following consumption of highly spiced food, the individual may be left with a strong impression that tastes/flavors were reduced whereas what really occurred was that one component (the burn) was simply a more prominent sensory experience. This impression may be reinforced by the persistence of the burning sensations long after the tastes and flavors have disappeared. In other words, it may be the memory of the experience that dictates beliefs about the interaction of irritants, tastes, and flavors. It is unclear whether or not such an impression would remain if attention was drawn to the intensity of the flavor components during food consumption.

R E F E R E N C E S

1. Cain, W. S.; Murphy, C. L. Interaction between chemoreceptive modalities of odor and irritation. Nature 284:255-257; 1980.

2. Cliff, M.; Heymann, H. Descriptive analysis of oral pungency. J. Sens. Stud. 7:279-290; 1992.

3. Cometto-Muniz, J. E.; Garcia-Medina, M. R.; Calvino, A. M.; Noriega, G. Interactions between CO 2 oral pungency and taste. Perception 16:629-640; 1987.

4. Cowart, B. J. Oral chemical irritation: Does it reduce perceived taste intensity? Chem. Senses 12:467-479; 1987.

5. Esakov, A. I.; Serova, O. N. Influence of substance P on taste receptor organ and salt solution intake in rats. Neurosciences 14:321-327; 1988.

6. Frank, R. A.; van der Klaauw, N. J.; Schifferstein, H. N. J. Both perceptual and conceptual factors influence taste-odor and taste-taste interactions. Percept. Psychophys. 54:343-354; 1993.

7. Gilmore, M. M.; Green, B. G. Sensory irritation and taste produced by NaC1 and citric acid: Effects of capsalcin desensitization. Chem. Senses 18:257-272; 1993.

8. Green, B. G. The effect of oral capsaicin on the perceived intensity and detectability of taste and retronasal odor. Presented at the Association for Chemoreception Sciences conference (AChemS XVI), Sarasota, FL, April 13-17, 1994.

9. Green, B. G.; Gelhard, B. Salt as an oral irritant. Chem. Senses 14:259-271; 1989.

10. Green, B. G.; Lawless, H. T. The psychophysics of somatosensory chemoreception in the nose and mouth. In: Getchell, T. V.; Bar- toshuk, L. M.; Doty, R. L.; Snow, J. B., eds. Smell and taste in health and disease. New York: Raven Press; 1991:235-253.

11. Jakinovich, W.; Sugarman, D. Sugar taste reception in mammals. Chem. Senses 13(1):13-31; 1988.

12. Kawamura, Y.; Okamoto, J.; Funakoshi, M. A role of oral afferents in aversion to taste solutions. Physiol. Behav. 3:537-542; 1968.

13. Lawless, H. T.; Stevens, D. A. Effects of oral chemical irritation on taste. Physiol. Behav. 32:995-998; 1984.

14. Lawless, H. T.; Rozin, P.; Shenker, J. Effects of oral capsaicin on gustatory, olfactory and irritant sensations and flavor identification in humans who regularly or rarely consume chili pepper. Chem. Senses 10:579-589; 1985.

15. Liu, L.; Simon, S. A. A rapid capsaicin-activated current in rat trigeminal ganglion neurons. Proc. Natl. Acad. Sci. USA 91:738-741; 1994.

16. Moore, F. W. Food habits in nonindustrial societies. In: Dupont, J., ed. Dimensions of nutrition. Boulder, CO: Assoc. Uni. Press; 1970:121-181.


17. Nagy, J. I.; Goedert, M.; Hunt, S. P.; Bond, A. The nature of the substance P-containing nerve fibres in taste papillae of the rat tongue. Neuroscience 7:3137-31151; 1982.

18. Prescott, J.; Allen, S.; Stephens, L. Interactions between oral chemical irritation, taste and temperature. Chem. Senses 18:389-404; 1993.

19. Rozin, P. "Taste-smell confusions" and the duality of the olfactory sense. Percept. Psychopl~ys. 31:397-401; 1982.

20. Rozin, P. Getting to like the burn of chili pepper. In: Mason, R.; Green, B. G.; Kare, M., eds. Irritation. New York: Marcel Dekker; 1990:231-269.

21. Stevenson, R. J. The nature of and psychological mechanisms under- lying the development of liking for the chili bum. Unpublished D.Phil. dissertation, University of Sussex; 1993.

22. Stevenson, R. J.; Yeomans, M. Differences in ratings of intensity and pleasantness for the capsaicin bum between chili likers and nonlikers; Implications for liking development. Chem. Senses 18:471-482; 1993.

23. Stevenson, R. J.; Prescott, J. The effects of prior experience with capsaicin on ratings of its bum. Chem. Senses 19:651-656; 1994.

24. Yau, N. J. N.; McDaniel, M. R. Carbonation interactions with sweetness and sourness. J. Food Sci. 57:1412-1416; 1992.

EFFECTS OF ORAL CHEMICAL IRRITATION ON TASTES AND FLAVORS IN FREQUENT AND INFREQUENT USERS OF CHILI

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