1. Introduction
The sensory properties of grapefruit (
Citrus X paradisi) are distinctive characterizing components and play a key role in reasons why consumers choose or not choose to consume the fruit and its products, e.g., juice. Grapefruit is a rich source of vitamin C, and health-promoting citrus flavonoids and limonoids and has beneficial antioxidant and anti-inflammatory properties [
1,
2]. Its appearance, aroma, flavor, and mouthfeel properties contribute to the sensory perception of the fruit.
Consumers differ widely in opinions on liking or disliking grapefruit and part of this individual preference is attributed to liking or disliking of the bitter taste that characterizes the fruit. Excessive bitterness of the juice was considered to be an important problem in commercial grapefruit juice production [
3]. Naringin and limonin are mainly responsible for the bitter taste commonly associated with grapefruit [
4]. The consumption of fresh grapefruit, and grapefruit products has been declining [
5] and plant breeders are working on ways to select for desirable sensory traits. A better understanding of the impact of the different sensory modalities contributing to the sensory perception of grapefruit products (e.g., juice) might assist product developers to optimize formulations and improve uptake of the products among consumers, thereby, maintaining or enhancing profitability for the role-players, along the grapefruit value chain.
Flavor perception is complex, due to the simultaneous stimulation of a number of senses. It is the result of processes that respond to sensory signals, from the activation of multiple sensory modalities, including smell (retronasal olfaction), mouthfeel (somatosensation), as well as taste (gustation), and to some extent also sight. When different senses are stimulated, concurrently, and perceptually interact with each other, the perceived flavor is the result of the cross-modal sensory interaction [
6]. Cross-modal interactions can change the intensity and perceived character of individual tastes and aromas, and even the overall flavor [
7].
The present study aimed broadly, to determine the relations between the stimulus components of a model beverage (formulated to be similar to grapefruit juice) and their effects on the perceived sensory properties and hedonic responses. We factorially combined, in the same acidified neutral base, each of three possible levels of bitter naringin (low, medium, and high) with each of three levels of sweet sucrose (low, medium, and high), two levels of grapefruit aroma (low and high) and two color variants (red and yellow). We hypothesized that perceived bitterness of the model grapefruit-like beverage will drive consumers dislike for the beverage but that bitterness perception will be a function of cross-modal color–taste, aroma–taste, and sweet–bitter taste interactions.
The color of the natural juice extracted from the grapefruit depends on the variety used and ranges from greenish-yellow to pale yellow, pink, and light red [
8]. We hypothesized that a rose red grapefruit-like beverage would be perceived as sweeter than a pale yellow option. Previously, it was reported [
9] that a red color decreased the perception of bitter taste intensity of a caffeine + water solution, with the yellow and green color having had no effect. The color of food and drinks impacts subsequent perception of taste, flavor, and overall sensory perception. It has been reported in several studies that the color of a solution greatly impacts the ability to identify its flavor and also affects the liking responses [
7].
We hypothesized that a beverage with high, compared to a low grapefruit aroma, would suppress the bitter taste perception and enhance the taste of sweetness [
10]. A new study [
10] reported that lemon extract, sucrose, and citric acid, when presented separately and also together, affected the perception of sweet, sour, and citrus flavors. The aroma of the products can influence the perception of basic tastes and vice versa [
11,
12,
13,
14,
15].
It is well-known that sucrose and other sweet tasting compounds can suppress bitterness. This is practically applied when bitter tasting coffee or tea is sweetened with sugar. Here the expectation was that sweetness would suppress bitterness but an enhancement effect on volatile aroma and flavor compounds was also expected. When sucrose was added to the fruit juices, not only were the perceived level of bitterness and sourness reduced and the sweet taste intensity increased, but the sweet aroma intensity rating also changed [
16].
2. Materials and Methods
2.1. Preparation of the Grapefruit-Like Beverages
Thirty-six grapefruit-like beverages (
Table 1) were manufactured, following a factorial design with deflavored, clarified, deionized, and acidified apple juice, as base, with an addition of naringin (three bitter levels), sucrose (three sweet levels), a grapefruit aroma compound mixture (two intensity levels) consisting of caryophyllene, citral, nootkatone, aldehyde C8 (octanal), aldehyde C9 (nonanal, aldehyde C10 (decanal), and two colorants (red or yellow). The addition of naringin was intended to reflect a low-level, in-between, and a high-level, based on the typical content in the grapefruit juice (218–340 mg/kg) [
17]. The low level of sweetness was based on the industry minimum requirement for export purposes, with incrementally higher levels added to reflect medium and high sweetness. These aroma compound mixture and levels used were selected in consultation with a flavorant supplier. The typical grapefruit juice color was copied using artificial colorants. The red color was a 0.001% solution blend of 30% sunset yellow and 70% ponceau red. The yellow color consisted of 0.0125% quinoline yellow. Standard preparation and mixing procedures were used for all added stimuli to ensure uniformity. The grapefruit-like beverages were filled in 250 mL plastic bottles, with lids, for easy handling and uniformity, and were kept frozen at −18 °C, until use. The beverages were defrosted overnight at an ambient temperature and kept at 14 °C, until served. A summary of the physico-chemical characterization of the 36 grapefruit-like beverages is presented in the
Supplementary Material (Table S1).
2.2. Descriptive Sensory Analysis
The sensory profiles of the beverages were described by a sixteen-member trained sensory panel with one to two years of descriptive sensory analysis experience. The specific training for attribute and methodology development for the evaluation of the beverages consisted of two sessions of 2 h each, using the generic descriptive analysis method [
18]. A total of 21 attributes were generated to characterize the aroma, flavor, and aftertaste of the grapefruit-like beverages (
Table 2). Beverage samples (±30 mL) were served at ±14 °C, in 125 ml polystyrene cups with plastic lids, and marked with randomly selected three-digit numbers. Samples were evaluated in duplicates, 12 beverages per 2 h session per day and a total of six sessions. The presentation order of samples per day for the different panelists followed a Williams Latin square design. Reference standards were available during training and evaluation sessions.
Panel performance was monitored to test reproducibility and consistency of the panel ratings using PanelCheck 1.3.2 (
www.panelcheck.com; Nofima, Ås, Norway).
The attributes were evaluated on a structured horizontal line scale (10 cm) with descriptors at the scale ends ranging from ‘not intense’ (at the left end of the scale, 0 cm) to ‘very intense’ (at the right end of the scale, 10 cm). Data was captured using Compusense® five release 4.6 software (Compusense Inc., Guelph, ON, Canada).
2.3. Consumer Evaluation
Ninety six young South African female consumers aged 18–24 years were recruited by trained fieldworkers. Each consumer completed an online screening survey and were invited to participate if in a self-reported good state of health, and if not limited by any food intolerance(s) and/or allergies. Participants were briefed and gave written consent before evaluating the beverages. Participants were requested not to eat, drink (except for water) or smoke for at least 1 h prior to the session.
The consumers (
n = 90) evaluated liking of the color, aroma and flavor of the 12 most diverse beverages (selected on the basis of composition) (
Table 1) using the Simplified Labeled Affective Magnitude (SLAM) scale [
19], a 10 cm line scale labelled with descriptors ‘greatest imaginable dislike’ (at 0 cm), and ‘greatest imaginable like’ (at 10 cm). Sample preparation and presentation was the same as for the trained panel. The 12 samples were evaluated in one session and the order of presentation to different consumers followed a Williams design.
Data was captured using Compusense® five release 4.6 software (Compusense Inc., Guelph, ON, Canada).
Ethical approval for this study was obtained from the Faculty of Natural and Agricultural Sciences Ethics Committee at the University of Pretoria (EC 130827-088).
2.4. Statistical Analysis
An analysis of variance (ANOVA) model fitted using PROC GLM in SAS v9.4 (SAS Institute Inc., Cary, NC, USA) was used to determine the main effects of the panelists, the bitter level, the sweet level, the aroma level, and the color type, together with the respective two-way interactions on the sensory attributes of the beverages. Tukey’s HSD test (p = 0.05) was used to compare beverages that differed in an attribute. Principal component analysis (PCA) using XLSTAT 2014 (Addinsoft, Paris, France) was applied to the correlation matrix of the sensory panel mean ratings, for all attributes of all grapefruit-like beverages.
Consumer liking of the color, aroma, and flavor of the 12 most diverse beverages was analyzed by a three-way ANOVA model, including the effects of color, aroma level, and tastants (bitter and sweet levels in three combination). Means were compared using Fisher’s least significant difference test at p < 0.05. Data were analyzed using GenStat® (VSN International Ltd., Hertfordshire, UK). Consumer liking ratings (y) for color, aroma, and flavor of beverages were modeled as a function of the descriptive sensory attributes (x), using three separate partial least squares (PLS) regression models. Preliminary models were run with all sensory attributes and their squared terms. Variable importance (VIP), which measures how important a variable is in terms of modeling the liking attributes, was used to select a smaller number of linear and squared terms for the final model. The VIP values summarize the overall contribution of each X-variable to the PLS model, summed over all components, and weighted according to the Y variation, accounted for by each component. Only those linear terms with a VIP greater than 0.8, as well as the five squared terms with the highest contribution, were retained. The PLS models were used to determine the positive and negative drivers of color, aroma, and flavor liking, and also to predict consumer liking of the 24 samples that were profiled by the descriptive sensory panel, but not evaluated by the consumers. The SIMCA-P package (Umetrics, Umea, Sweden) was used for the PLS modeling.
4. Discussion
The research studied the effect of varying the bitterness, sweetness, color, and aroma intensity of grapefruit-like beverages on the cross-modal perception of sensory properties and its effects on consumer liking. A model grapefruit-like beverage standard formulation was created and a sensory lexicon with a total of 21 attributes and definitions were generated to characterize the aroma, the flavor, and the aftertaste of the grapefruit-like model beverage with variations in color, aroma, and gustatory flavorant levels.
Color hue of the grapefruit-like beverage affected the perception and description of the aroma and flavor sensory properties, as evaluated by the trained human panelists. Color of the beverages, and, in particular, the sample with the rose-red hue had a significant enhancing effect on perception of overall aroma intensity and grapefruit, deteriorated/rotten, muddy/moldy, fruity, and sweet aroma intensities. It also corresponded to the consumer liking—the red beverages were liked more than the yellow ones. The cross modal effect of the beverage color on aroma and flavor of the beverages, however, did not lead to significant differences in the liking of aroma or a liking of the flavor of the red and yellow beverages. The difference in methodology followed and the cognitive tasks employed by the two groups of panels might be the reason. When the group of consumers evaluated the liking of the color of the beverages, solely based on appearance, a slight but significant preference for the red-colored beverages was noted. This preference was solely driven by visual cues, since the consumers did not yet smell or taste the beverages. After smelling and tasting the beverages, it is likely that the opinion and preference might have changed, based on the cross-modal, color-aroma/flavor sensory interaction, as demonstrated by the results for the trained panel in this study. Considering that the consumers first evaluated the liking of the color, then the aroma (retronasally), and lastly the flavor (after consumption) of each sample, sequentially, it cannot be excluded that some form of learning, anticipation, and association might have occurred over the evaluation of the sequence of twelve samples, of which 50% were red and 50% were yellow.
A study [
20] reported that the red color decreased the perception of the bitter taste sensitivity of a water solution. Coloring a clear bitter solution red, decreased the perception of the bitter taste, while the addition of yellow and green coloring had no such effect [
9]. Other researchers [
21] suggested that color-induced olfactory enhancement observed when odorous solutions are smelled orthonasally, might be the result of a conditioned olfactory percept caused by the color. Conditioned expectations predict that certain colors would be strongly associated with particular flavors, e.g., red with cherry, orange with orange, and green with lime [
22]; yellow with lemon, blue with spearmint, and red with strawberry, raspberry, and cherry [
23]. In South Africa, the location for the study, both yellow and red/pink grapefruit are marketed. The Star Ruby variety with a red color is the most planted (84%) grapefruit variety in South Africa, followed by the white variety Marsh (16%) (the juice of this type of grapes is pale yellow) [
24]. In another study [
25] it was found that the relationship of green and yellow colors in the lemon and lime-flavored sucrose solutions was altered; such color changes were found to have an impact on the perceived sweetness ratings. In another study, results showed that color–odor solution pairings were rated as having more intense odors with color cues than without, regardless of the color–odor pairing appropriateness [
21]. This cross-modal effect presumably results from the color-cue setting up an expectation concerning the likely identity and intensity of a food or drink’s taste or flavor [
20]. No significant sensory attribute drivers for liking of the color of the grapefruit-like beverages was identified, since the trained sensory panel did not evaluate the appearance attributes.
Aroma level added to the model beverage had a significant enhancing effect, on the majority of the aroma and flavor sensory attributes. The enhancement of overall aroma and characteristic aroma qualities, including citrus flavor, as a function of the level of aroma added, was expected and confirmed. When consumers evaluated liking of the aroma of the beverages, solely based on orthonasal inspection, surprisingly the beverages with low aroma were slightly preferred over those with high aroma. It is possible that the higher aroma level was more distinctive and clearly reminiscent of grapefruit and possibly evoked a stronger cue for those disliking grapefruit. An interesting and unexpected finding was the apparent suppression of bitter and astringent gustatory sensations, due to a higher load of olfactory stimuli (high aroma level). Previous studies have found that aroma–taste interactions can result in complicated changes in the perceived flavor. The addition of an aroma can, e.g., elevate the bitter-detection threshold [
26,
27]. The perceived intensity of tastes in solutions was increased by volatile compounds, especially when there was a logical association between them, such as between sweetness and fruitiness [
28]. Apple and strawberry aromas evoked both sweetness and sourness. A study found that tasteless aromas, namely green tea and coffee, predominantly evoked bitterness, while the vanilla aroma predominantly evoked sweetness [
29]. The grapefruit aroma consisted of a blend of caryophyllene, citral, nootkatone, and various aldehydes; octanal, nonanal, and decanal. No study could be found that specifically indicated that any of these compounds evoked bitterness. Nootkatone at the above threshold concentrations was reported as tasting bitter [
30]. Consumption of a beverage results in the simultaneous perception of aroma and taste, coupled with tactile sensations, all of which contribute to an overall impression of flavor. Compounds that stimulate taste perception (e.g., naringin contributing a bitter taste) can increase the apparent intensity of aromas. In this study, the grapefruit flavor was enhanced by the naringin addition. The aroma compound (containing a citral component) of the grapefruit-like beverages had an enhancing effect on the citrus aroma intensity. An additive effect of the sweet components with citral or limonene volatiles having a ‘citrus’-like aroma was reported by [
31] but was not observed in this study. The suppression of bitterness in the high aroma beverages, however, did not affect the liking of the flavor, since there was no difference found in the liking of the flavor of beverages with low or high aroma levels. Positive drivers for liking of the aroma of the grapefruit-like beverages were fruity aroma
2, citrus flavor, and sweet flavor, while negative drivers were sweet aroma
2, sweet flavor
2, and pungent aroma.
The low bitter/high sweet beverages were preferred over the high bitter/low sweet samples. A study [
32] reported that with an increase in the ratio of °Brix/acidity of reconstituted grapefruit juice, the consumer perception of sweetness increased and bitterness and aroma intensity decreased. Some bitterness in processed grapefruit products is acceptable for consumers, but excessive bitterness is one of the major consumer objections to such products [
28,
31]; this was confirmed in this study. The variation in sensitivity of the individual consumers to bitter compounds in grapefruit beverages could be explored further to identify whether subgroups might have different preferences. As expected, the contribution of varying concentrations of naringin affecting the bitterness of the grapefruit-like beverages did not have a significant effect on any of the aroma attributes. Similarly, [
32] reported that consumers did not find any difference in aroma with increased levels of naringin in processed grapefruit juice. However, the concentration of bitterness of the grapefruit-like beverages had a significant effect on the flavor attributes (astringent, sweet, sour, bitter and grapefruit flavor, and the bitter aftertaste). A study [
32] has also reported that an increase of limonin (also a bitter compound) in processed grapefruit juice, increased the perceived bitterness and tartness, while decreasing the sweetness.
In a previous study, an increase in the °Brix with sucrose, enhanced the taste of sweetness, and had a decreasing effect on the sour, bitter, astringent, and grapefruit flavors, and the bitter aftertaste. When sucrose was added to fruit juices, not only were the perceived levels of bitterness and sourness reduced (as was also found in this research) but the sweet aroma intensity rating also changed [
16] (although this was not found here). Sucrose was also reported to mask the bitter taste of sinigrin, goitrin, and quinine [
33]. In the complex beverage model, increasing sucrose did not have the often reported enhanced effect on the perceived fruity aroma. Increasing the sugar concentration of blueberry and cranberry fruit juices, increased their fruitiness (evaluated by sipping), even though no difference in aroma was perceived by sniffing alone [
16]. Sucrose in the mouth significantly enhanced the “citrus” ratings, compared to when citral was inhaled alone [
12]. Similarly, increases in the intensity of different ‘fruity’ aromas were perceived in a multichannel flavor delivery system [
34], model dairy desserts [
35], and custard desserts [
36], when increasing the sweetness with sucrose. Sweet level also affected the soapy aroma of the grapefruit-like beverages. The reason for the effect on soapy aroma is unclear. It is possible that the aroma blend contributed a slight soapy aroma.
The effect of aroma level and color on the perceived sensory attributes, as observed in this study, are evidence of cross-modal sensory interactions. It was anticipated that the intensity and character of the aroma level of a grapefruit juice would increase the perception of the citrus flavor, a positive driver of grapefruit flavor liking and reduce the negative attributes, the bitter and astringent flavor, as well as the bitter aftertaste. Positive drivers for liking of the flavor of grapefruit-like beverages were the sweet taste, the chemical aroma, and the citrus flavor intensities, while negative drivers were intensity of soapy aroma, bitter aftertaste, and sour taste.