Interactions of Lemon, Sucrose and Citric Acid in Enhancing Citrus, Sweet and Sour Flavors (2024)

Subjects

The 12 subjects were 7 women and 5 men, aged 18–50 years (M = 29.1, SD = 8.2), all of whom participated in all 3 experiments. Subjects included students and employees at Yale University, as well as other members of the New Haven community. All subjects reported being nonsmokers, and none reported impaired flavor perception before or during the experimental sessions. The research complied with the Declaration of Helsinki for Medical Research involving Human Subjects. All subjects gave informed consent, in accordance with protocols approved by the Human Subjects Committee at Yale University. Subjects received $10 per hour to participate.

Materials

The stimuli in all 3 experiments were created by combining an olfactory flavorant, natural lemon extract (Spice Barn: 7% oil of lemon, dissolved in 74% ethyl alcohol and 19% water), with 1 or both of 2 gustatory flavorants, sucrose (Sigma-Aldrich, CAS #57-50-1, C₁₂H₂₂O₁₁) and citric acid monohydrate (J.T. Baker, CAS #5949-29-1, HOC(COOH)(CH₂COOH)₂·H2O). All flavorants (single flavorants and mixtures) were dissolved in deionized water to desired concentrations (i.e., by combining flavorants before dilution, not by mixing solutions).

Experiment 1 (lemon-sucrose) presented 9 stimuli, created by factorially combining each of 3 possible concentrations of lemon (0.0, 17.6, and 35.3 mL/L) with each of 3 possible concentrations of sucrose (0.0, 0.05, and 0.10 M). Experiment 2 (lemon-citric acid) presented 9 stimuli created by combining each of the same 3 concentrations of lemon extract with each of 3 possible concentrations of citric acid (0, 0.0006, and 0.0012 M). Experiment 3 (lemon-sucrose-citric acid) presented 8 stimuli, representing all combinations of each of the 3 flavorants at a concentration of either zero or the higher of the 2 nonzero concentrations used in Experiments 1 and 2. Thus, these 8 stimuli were: (i) deionized water (every component at zero concentration); (ii) lemon at 35.3 mL/L, (iii) sucrose at 0.10 M, and (iv) citric acid at 0.0012 M; (v) lemon-sucrose, (vi) lemon-citric acid, and (vii) sucrose-citric acid; and (viii) lemon-sucrose-citric acid. Solutions were mixed fresh every week and refrigerated until use. All of the stimuli, as well as the water rinses, were presented at room temperature, approximately 21 °C.

Method

Subjects were instructed to avoid eating or drinking anything except water for 1 hour before each session. Subjects completed each experiment in a single session of approximately 1 h on a separate day, with the order of the 3 experiments counterbalanced over subjects. The procedure was the same in all 3 experiments. On each trial, the subject sipped the stimulus solution (5 mL of solution in a transparent 30 mL disposable plastic cup), held it in the mouth for about 3 s while breathing naturally through the nose, spit it out, and then rinsed with deionized water before sampling the next stimulus, about 30 s later.

After sipping and expectorating each stimulus, but before rinsing, the subject rated the perceived intensity of each of 3 flavor qualities: “sweet,” “sour,” and “citrus,” on a Labeled Magnitude Scale (Green et al. 1996) presented on a computer screen 3 times, separately for each rating, in the fixed order “sweet,” “sour,” and “citrus”. Each LMS displayed descriptive adjectives from “no sensation” to “strongest imaginable,” spaced quasi-logarithmically along a straight vertical line. Each intensity rating was coded by computer software as a number between 0 (“no sensation”) and 100 (“strongest imaginable”). A brief training session before each condition familiarized subjects with the LMS.

Experiments 1 and 2 presented the subjects a total of 72 trials in each, comprising 8 replicates of the 9 stimuli, given in unique, pseudo-randomized (without replacement) sequences of the 72 trials that varied from subject to subject. Experiment 3 presented a total of 64 trials, comprising 8 replicates of the 8 different stimuli, also presented to the subjects in unique, pseudo-randomized (without replacement) sequences.

Data analysis

Ratings obtained in each experiment were first averaged across replicates separately for each subject, rating scale and stimulus. To assess how the 3 flavorants, separately and together, affected the intensity ratings of each flavor quality, the mean responses in each experiment were both plotted graphically (Figures 1–3) and analyzed statistically using multivariate analysis of variance (MANOVA). Using the data of Experiments 1 and 2, for each experiment, we performed a repeated-measures MANOVA with each of the 2 flavorants (lemon and sucrose in Experiment 1; lemon and citric acid in Experiment 2) serving as a within-subject variable having 3 levels (concentrations), replicate serving as a third within-subject variable (8 levels) and the ratings of sweet, sour, and citrus serving as 3 separate dependent variables. Using the data of Experiment 3, we performed a repeated-measures MANOVA with each of the 3 flavorants (lemon, sucrose, and citric acid) serving as a within-subject variable having 2 levels (concentrations, defined as the presence or absence of each flavorant), replicate serving as a third within-subject variable (8 levels), and the ratings of sweet, sour, and citrus again serving as 3 separate dependent variables. Results of the MANOVAs provided measures of both univariate and multivariate main and interaction effects; in the interest of clarity, we report only the (more useful) univariate results.

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Figure 1.

Mean ratings of sweet intensity (left panels), sour intensity (center panels), and citrus intensity (right panels) in Experiment 1, plotted against both the concentration of sucrose (upper panels) and the concentration of lemon (lower panels). Error bars depict standard errors of the means.

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Figure 2.

Mean ratings of sweet intensity (left panels), sour intensity (center panels), and citrus intensity (right panels) in Experiment 2, plotted against both the concentration of citric acid (upper panels) and the concentration of lemon (lower panels). Error bars depict standard errors of the means.

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Figure 3.

Mean ratings of sweet intensity (left), sour intensity (center), and citrus intensity (right) given to the 8 stimuli in Experiment 3: water, W; lemon, L; sucrose, Su; citric acid, CA; and mixtures of lemon and sucrose, L-Su; lemon and citric acid, L-CA; sucrose and citric acid, Su-CA; and lemon, sucrose, and citric acid, L-Su-CA. Error bars depict standard errors of the means; for clarity, only the negative parts of symmetrical error bars are shown. Significant results of post hoc t-tests between stimuli are indicated by brackets and an asterisk.

To compare stimuli common to multiple experiments, we ran 6 separate repeated-measures MANOVAS, in all of which, again, ratings of sweet, sour, and citrus served as 3 separate dependent variables: (i) the first MANOVA tested the ratings given to water in all 3 experiments, with experiment serving as a within-subject variable and (ii) its complement did the same using ratings given to lemon. (iii) A third MANOVA tested rating given to sucrose in Experiments 1 and 3 with the experiment serving as a within-subject variable, and (iv) its complement did the same testing ratings given to the lemon–sucrose mixtures. (v) Another MANOVA tested ratings given to citric acid in Experiments 2 and 3 with the experiment as a within-subject variable, and (vi) its complement did the same for lemon-citric acid mixtures.

All statistical analyses were performed using SPSS, version 24. Where sphericity was violated, we report values of P after Greenhouse-Geisser correction, but present the original (uncorrected) values of df. We used a value of alpha of 0.05 to assess significance. In order to improve the clarity of the presentation, we report statistics on only the significant results.

Experiment 1. Lemon and sucrose: Separate and joint effects

Figure 1 shows the mean ratings from Experiment 1, with the upper and lower pairs of panels on the left, middle, and right showing, respectively, the ratings of sweet intensity (1A, 1D), sour intensity (1B, IE), and citrus intensity (1C, 1F). For ease of evaluation, the upper panels (1A–1C) plot the ratings against the concentration of the sucrose component, and the lower panels (1D–1F) plot the same ratings against the concentration of the lemon.

Sweet intensity increased significantly with increasing concentration of sucrose (Figure 1A), F(2,22) = 21.495, P < 0.001), and not with increasing lemon (Figure 1D). Sweet intensity was not significantly affected by replicate (repeated presentation over the course of the sessions), and the interaction of sucrose and lemon on sweet was also not significant. There were no first-order or second-order interactions with replicate.

Sour intensity was not affected by sucrose (Figure 1B). On the other hand, sour intensity did increase with increasing concentration of lemon, although the magnitude of this increase declined as the concentration of sucrose decreased (Figure 1E). The main effect of lemon on sour taste was significant, F(2,22) = 7.251, P = 0.015, as was the interaction between sucrose and lemon, F(4,44) = 4.512, P = 0.017—as though lemon itself produced a slight sour taste that was suppressed (masked) by increasing sweetness.

Sour intensity increased over the first 3–4 presentations, then tended to level off or even decline slightly (Supplementary Figure 1A), the main effect of replicate being significant, F(7,77) = 3.133, P = 0.035; interactions involving replicate, however, were not. For completeness, we illustrate the effect of replicate on sour intensity in Supplementary Figure 1B.

Finally, citrus intensity did not increase with increasing concentration of sucrose when lemon was absent (Figure 1C). Of course, citrus intensity did increase markedly and significantly with increasing concentration of lemon (Figure 1F), F(2,22) = 24.917, P < 0.001. Replicate did not affect citrus intensity. There was, however, a significant interaction of lemon concentration and replicate on ratings of citrus intensity, F(14,154) = 2.389, P = 0.005, which seems to result from citrus intensity being greater when lemon was present than when it was not (Supplementary Figure 1C). The other interactions involving replicate were not significant.

To summarize: in Experiment 1, we presented mixtures of the olfactory flavorant lemon extract and the gustatory flavorant sucrose. Increasing the concentration of the gustatory sucrose, as expected, “appropriately” (i.e., reflecting its main quality) increased ratings of sweetness, but not ratings of sourness or citrus. Increasing the concentration of the olfactory lemon extract increased—again “appropriately”—ratings of citrus and did not reliably increase those of sweetness. Contrary to our expectations, however, the gustatory sucrose did not increase (enhance) the citrus quality, nor did the olfactory lemon extract enhance sweet. Increasing the concentration of lemon extract, however, did lead to greater sourness ratings. Interestingly, the sour gustatory note evoked by this olfactory flavorant was itself suppressed by the presence of the sweet gustatory flavorant, sucrose, suggesting cross-stimulus or cross-sensory quality interaction. This unexpected effect on a taste note that is associated with a flavorant (citric acid) that is not present in the set of stimuli, but has probably been experienced in daily life in close association with the olfactory flavorant, lemon, points to a possible role of experience. Experiment 2 investigated gustatory–olfactory flavor interactions between citric acid and lemon: two flavorants that are often experienced together and that produce flavor qualities, sour and citrus, that are perceived as congruent.

Experiment 2. Lemon and citric acid: Separate and joint effects

Figure 2 shows the ratings of sweet, sour, and citrus intensity obtained in Experiment 2 with lemon, citric acid, and their mixtures. Sweet intensity did not increase with increasing concentration of citric acid (Figure 2A) or concentration of lemon (Figure 2D). The interaction of citric and lemon too was not significant.

By way of contrast, sour intensity increased markedly and significantly with increasing concentration of both citric acid (Figure 2B), F(2,22) = 8.557, P = 0.012, and lemon (Figure 2E), F(2,22) = 5.811, P = 0.024, although the interaction was not significant. The effect of lemon on citrus intensity was, not surprisingly, robust and significant, F(2,22) = 13.007, P = 0.003. There was no interaction of lemon and citric acid.

There were no significant univariate or multivariate main effects of replicate, nor any significant interactions involving replicate on ratings of sweet, sour, or citrus intensity. For completeness, we illustrate the effect of replicate on sweet, sour, and citrus intensity in Supplementary Figure 2.

To summarize: in Experiment 2, we observed greater ratings of sourness with increasing citric acid concentration, as well as greater ratings of citrus with increasing lemon concentration. Ratings of sweetness were not affected by citric acid or by lemon extract. The only prominent multistimulus/multiquality effect was an increase in sourness ratings with an increase in the concentration of lemon extract. Thus, contrary to our expectations, the gustatory flavorant did not significantly enhance the intensity of the olfactory quality, whereas the olfactory flavorant did significantly enhance the gustatory quality, suggesting an asymmetry in the cross-modal interactions that is opposite in direction to that of recent reports (Green et al. 2012; Fujimaru and Lim 2013; Isogai and Wise 2016; Linscott and Lim 2016). Lemon extract enhanced sour taste even when the stimulus contained no citric acid, and in this sense, the lemon presumably evoked the sour quality. In Experiment 1, lemon presumably also at least partly evoked sour taste. This evocation of sour taste might signify confusion or lack of separability of the sour and citrus flavor qualities. Experiment 3 investigated the interactions observed in both Experiments 1 and 2 by testing all 3 flavorants.

Experiment 3. Lemon, sucrose and citric acid: Separate and joint effects

Comparison of responses to stimuli common to multiple experiments

Because different experiments used overlapping subsets of the flavor stimuli, we can assess which effects were consistent and which varied across experiments, possibly reflecting differential effects of stimulus context, i.e., of the presence of the other stimuli, unique to particular experiments. To this end, Figure 4 shows the mean ratings obtained from the 6 stimuli that were tested in either 2 or all 3 of the experiments: water (W); the highest concentrations each of lemon (L), sucrose (Su), and citric acid (CA); and the mixtures of the high concentrations of lemon and sucrose (L-Su) and lemon and citric acid (L-CA).

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Figure 4.

Mean ratings of sweet intensity (A: top) sour intensity (B: middle) and citrus intensity (C: bottom) given to the 6 stimuli: water, W; lemon, L; high concentration of sucrose, Su; high concentration of citric acid, CA; and mixtures of lemon and sucrose, L-Su, and lemon and citric acid, L-CA, that were common to Experiments 1 and 2, 1 and 3, or 1, 2, and 3. Error bars depict standard errors of the means; for clarity, only the negative parts of symmetrical error bars are shown. Significant results of post hoc t-tests between experiments are indicated by brackets and an asterisk.

Water, presented in all 3 experiments, did not differ significantly in its sweet, sour, or citrus ratings across the experiments. Nor did lemon, also presented in all 3 experiments, in its sweet, sour, or citrus ratings.

Sucrose, presented in Experiments 1 and 3, did not differ significantly in its sweet, sour, or citrus ratings. The mixture of sucrose and lemon, also presented in both experiments, received significantly lower sweet ratings and higher sour ratings in Experiment 3, F(1,11) = 8.267, P = 0.015, and F(1,11) = 15.873, P = 0.002, respectively, whereas citrus ratings did not differ significantly.

Citric acid, presented in Experiments 2 and 3, did not differ significantly in its sweet, sour or citrus ratings in the 2 experiments. The mixture of citric acid and lemon was rated as significantly sweeter in Experiment 3 than Experiment 2, F(1,11) = 6.767, P = 0.025, but not significantly different in sour or citrus flavor.

To summarize: in several instances, the mean ratings given to the same flavor stimulus were remarkably similar across experiments—for example, the ratings of citrus intensity given to the lemon–sucrose mixture in Experiments 1 and 3. But in other instances, the ratings diverged considerably across experiments, for example, the ratings of sweet and sour given to lemon–sucrose in the same 2 experiments.

Asymmetry in cross-modal enhancement

The olfactory evocation/enhancement of gustatory flavor was much more robust than the gustatory enhancement of olfactory flavor. This asymmetry conflicts with recent reports (Green et al. 2012; Fujimaru and Lim 2013; Linscott and Lim 2016) that gustatory enhancement of olfactory responses may be more robust (up to 400%) and reliable than olfactory enhancement of gustatory responses (up to 30%: Isogai and Wise 2016). Note, however, that earlier studies did report clear evidence that olfactory flavorants can enhance gustatory responses (Schifferstein and Verlegh 1996; Djordjevic et al. 2004). The source of the variations in results is not immediately evident, although there are many differences across the studies in details of the procedures. One source of differences may be related to the choice of olfactory flavorant. In many of our earlier experiments with gustatory–olfactory mixtures (e.g., Marks et al. 2012), we, like Green, Lim and colleagues, used citral, a single molecule, as the olfactory flavorant. Subjects often commented, however, on citral’s somewhat unfamiliar, unpleasant, and unnatural flavor. Consequently, we later switched to a different olfactory (citrus) flavorant, natural lemon extract, which is a complex mixture of potentially thousands of chemical components. Subjects generally find the flavor of lemon extract more natural and pleasant than that of citral. Note too that the olfactory enhancement of gustatory responses occurred here only for the gustatory quality sour, which is consistently experienced together with lemon extract and perhaps consequently has a high ecological predictability. It is possible that the use of lemon extract as the olfactory flavorant rather than citral increases the ecological predictability of sour taste and thus led to the robust enhancement here of sour taste by lemon extract. Lastly, as Fujimaru and Lim (2013) showed, olfactory concentration is an important factor in the enhancement of taste. The same might be true of gustatory concentration. Although this hypothesis was tested by Fujimaru and Lim, they exclusively used sucrose as the gustatory flavorant, but did not test the possible interaction of their olfactory flavorant, citral, with citric acid.

The interactions between citric acid and lemon, which can be substantial, may reflect a direct evocation of multisensory qualities—as suggested for the evocation of gustatory qualities by olfactory stimuli (Stevenson et al. 1998, 1999; Gautham and Verhagen 2010)—but may instead, or may also, reflect confusion between qualities or conflation of verbal labels (e.g., Bonnans and Noble 1993). Subjects in the present experiments may have tended to conflate the citrus and sour qualities of the flavors. We do not assume that perceivers can always analyze their flavor perceptions in a simple manner: the components of complex flavors may not be perceptually or decisionally separable, possibilities that future studies might address more directly, perhaps through studies of identification or classification (see Ashby and Townsend 1986).

Regarding the possible contextual effect of variations in the stimuli across experiments, in several instances, the mean ratings given in different experiments to the same flavor stimulus were remarkably similar; but in other instances, the ratings given in different experiments diverged considerably. To be sure, some variability in the mean ratings is expected, given the variability in the responses across individuals. But it is plausible that, in a few cases, the variations in mean ratings across experiments could reflect perceptual or decisional effects of varying the set of stimuli presented within each experiment; that is, variations in ratings to the same stimulus in different experiments might represent what is broadly termed effects of stimulus context (e.g., Stevens 1958; Anderson 1975; see Marks et al. 2012). For example, Experiment 3 excluded those lower concentrations of sucrose, lemon, and their mixtures that were included in Experiment 1 and/or Experiment 2. Consequently, the average concentrations of sucrose, citric acid, lemon, and their mixtures were greater in Experiment 3 than in the other 2 experiments—conducive, for example, to the presence of greater adaptation-like effects of stimulus context in Experiment 3, especially for the gustatory flavorants (see Marks et al. 2012). Moreover, Experiment 3 included citric acid and mixtures of citric acid with sucrose, lemon, and sucrose + lemon, these 4 stimuli all being absent from Experiment 1. And Experiment 3 included sucrose and mixtures of sucrose with citric acid, lemon, and lemon + citric acid, 4 stimuli that were all analogously absent from Experiment 2. These differences in context too may have influenced the ratings (see also Hallowell et al. 2016). We suspect that the effects of stimulus context can be more substantial than, for example, the effects of the ethanol present in the lemon extract, and perhaps more substantial than any effects of the order with which the subjects rated the 3 flavor qualities; nevertheless, the effects of all of these variables could be examined in experiments devoted explicitly to evaluating them.

Possible effects of ethanol

Potentially complicating the findings are the possible effects of ethanol, which served as a diluent for the lemon extract. Because the lemon extract contained 74% ethyl alcohol, the 2 nonzero concentrations of lemon used in these experiments, 17.6 and 35.3 mL/L, were accompanied by concentrations of ethanol equal to 1.3% and 2.6%, respectively, similar to their concentrations in an earlier study (Hallowell et al. 2016). Although ethanol can have a taste, as described below, it appears unlikely that the taste of ethanol materially affected the flavor responses here.

Ethanol at its absolute threshold is often described as sweet (Wilson et al. 1973), although sometimes also as bitter (Mattes and DiMeglio 2001). The lower concentration of ethanol in the present study, 1.3%, lies below the absolute taste threshold of 4.2% reported by Wilson et al. (1973)—who also reported trigeminal (“burning”) responses appearing at 21%—although in the region of the taste threshold values of 1.2% (women) and 1.7% (men) reported by Mattes and DiMeglio (2001). Our lower concentration of ethanol is unlikely to affect taste materially. Our higher concentration of ethanol, however, lies above the just-mentioned taste thresholds, and might conceivably affect flavor. Any bitter taste of the higher-concentration ethanol, however, is likely to be masked by the sucrose in mixtures containing both lemon and sucrose (see Bartoshuk 1975; Keast and Breslin 2002). If not masked, residual ethanol-induced bitterness is likely to be assimilated to ratings of sour intensity, given the common confusion between sour and bitter (O’Mahony et al. 1979). The higher-concentration ethanol might, however, impart a sweet taste. Even so, the effects of the ethanol on taste, even at the higher concentration, should be small: Martin and Pangborn (1970) found only tiny effects on taste intensity of adding 4% ethanol to solutions of sucrose, NaCl, citric acid, and quinine.

Possible effects of fixed order of rating scales

Subjects were asked to provide ratings of the stimuli in the fixed order of “sweet,” “sour,” and “citrus”—similar to the method used by others (e.g., Green et al. 2012; Fujimaru and Lim 2013; Linscott and Lim 2016; Isogai and Wise 2016). Riskey et al. (1979) found that the order of report did not affect judgments of sweetness and of pleasantness—although we cannot exclude the possibility that the particular order of presentation of scales influenced the results. More specifically, given that ours and all studies mentioned above all ask for ratings of taste qualities before asking for ratings of odor qualities, it is possible that odor qualities may be “dumped” more on taste qualities than vice versa. This is possibly consistent with the asymmetry in enhancement observed in previous studies, but seems inconsistent with our pattern of results. In any case, the differences observed between ours and others results—though scale order was similar—seems to indicate that if there is an effect, it is small compared to the multisensory enhancement effects.

Separability of flavor qualities

Do the present multiquality interactions indicate that flavors are phenomenally synthetic? Is the flavor of a lemon analogous to the color orange, a mélange of pure sour and citrus flavors akin to the mélange of pure yellow and red colors? Perhaps. It is less self-evident, however, that flavors are emergent qualities, endowed by their neural creators with unalienably unique properties. To phenomenal experience, a lemon’s flavor seems to contain citrus and sour, entangled though these qualities may be. In our view, flavors may be better described (though perhaps not perfectly described) as fusions (e.g., Skramlik 1926) or sensory blends (James 1890), a notion resurrected 3 decades ago by McBurney (1986). The notion that flavors constitute fusions or blends falls between analysis and synthesis.

Instead of focusing on the phenomenology of flavor experience, however, it may be more fruitful instead to focus on the ways that the flavor system, like other perceptual systems, operates within the evolutionary-cultural service of functional goals, providing adaptively important information, such as: What is this that I’ve taken in my mouth? Is it likely to be helpful or harmful? As pointed out in the Introduction, flavor perception is multisensory, but it is likely that the processes that underlie multisensory flavor perception are shaped at least in part by our experiences with foods and beverages. One implication is that human flavor perception will also reflect whatever contributes to experience, including variations in culture: both variations in the foods and ingredients commonly available within different cultures and variations in the words used to describe food flavors (e.g., Johns and Keen 1985; Enfield 2011). A cross-cultural perspective may prove invaluable in clarifying the contributions of experience to flavor perception (see Howes 1991; Spence et al. 2015).

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