Taste is of primary influence on food selection particularly in children [1
]. It is one of the key factors determining food palatability and liking, which contribute to food intake [4
]. Taste is also reported to affect food choices in children aged 12–13 and significantly determines food acceptance in 7–11-year-old children [5
Children are individually different in perceiving tastes [7
]. Taste sensitivity is defined as the individual ability in responding to taste stimuli [8
] which could be measured by different methods such as detection threshold (DT), recognition threshold (RT), taste responsiveness, and fungiform papillae (FP) density [9
]. In addition, sensitivity to PROP (6-n-propylthiouracil) has been associated with other taste responses, which makes this test commonly used in taste sensitivity studies [10
]. The DT focuses on low concentrations of taste stimuli and is obtained at the point where the concentration of the taste stimulus can be discriminated from water [9
]. As the concentration is further increased, RT is obtained as the point where the taste is perceived and identified [11
]. Subjects can be separated into taste-sensitive and nonsensitive groups according to their DT or RT as previously proposed in adults [12
] or in children aged 7–14 years [7
]. Taste responsiveness is the subject’s rating of perceived intensity to taste stimuli above the threshold level, also known as the suprathreshold intensity [13
]. The 6-n-propylthiouracil, also known as PROP, is a chemical compound commonly used to measure subjects’ responsiveness to bitterness [14
]. However, responsiveness to PROP may not reflect responsiveness to all bitter compounds; this measure is specifically related to the genetic variants of taste receptor TAS2R38 for bitter perception [10
]. Indeed, PROP is chemically similar to phenylthiocarbamide (PTC) and sensitivity to these compounds was reported to be associated with the TAS2R38 receptor [16
]. Sensitivity testing to PROP can be conducted using water solutions [20
] or impregnated paper [22
]. People can be classified into supertasters, medium-tasters, and nontasters according to their responsiveness to PROP, i.e., their PROP phenotype. This classification is based on intensity ratings perception of the stimulus on a Labeled Magnitude Scale (LMS), and the application of established cut-off points that define the taster categories [23
]. Subjects categorized as tasters perceive a higher bitter sensation of PROP compared to a nontaster [14
]. The quantification of fungiform papillae (FP) has also been used to infer taste sensitivity. However, there is a concern related to using this method, as recent studies involving large population samples concluded that FP density does not directly correlate to taste sensitivity [24
There are five basic tastes—sweet, sour, salty, bitter, and umami—that, respectively, relate to different receptors and mechanisms of responses [27
]. In addition, the taste of fat (also called oleogustus) has been suggested as the sixth taste modality [28
], with a gustatory pathway devoted to the perception of lipids [29
]. The association between the different basic tastes and their taste receptors has been widely investigated, with the results suggesting that genetic variants may contribute to individual taste sensitivity [15
]. Differences in taste sensitivity contribute to a variety of eating practices and food choices [31
]. Taste sensitivity is shown to influence the willingness of 9–11-year-old children to consume fruits and vegetables [32
] and significantly affects their acceptance of new foods [33
]. Moreover, sensitivity to PROP is reported to affect the acceptance of sweet and fatty foods [34
], and children aged 4–5 years with lower bitter taste sensitivity are reported to have a higher vegetable acceptance [35
Regarding sweet taste, previous research has reported that sweetness sensitivity correlates with an increase in the liking of sweet food. This was investigated using sweetened apple juice in 6–9-year-old children [36
]. The sweet sensitivity of 4–6-year-old children has also positively been correlated with their preference for sweetened beverages [37
]. On the opposite end, a bitter taste has been associated with food rejection in children [38
]. Moreover, 4–5-year-old children who were sensitive to PROP have been shown to have a lower acceptance for broccoli and cheese compared to nonsensitive children [21
]. In a review on children’s perception of saltiness, Liem [40
] concluded that saltiness plays an important role in children’s liking for a selection of salty foods, but that saltiness sensitivity does not influence children’s real consumption of salty food products. To our current knowledge, there are few studies about sourness sensitivity in children. Unlike the other basic tastes that consistently have been reported to influence food palatability, the literature indicates that sour taste does not significantly affect liking and preference in nine-year-old children, even after repeated exposure of this taste [41
]. Moreover, a recent taste sensitivity study involving large samples of children aged 6–9 in Europe did not include sourness in the evaluations [42
]. Further, umami has been reported to enhance palatability and acceptance of foods [43
]. Sensitivity to umami has been reported to be significantly different in 13–16-year-old children according to their weight [44
]. The study of the recognition threshold for this taste requires a training session [45
] due to unfamiliarity and confusion between umami and saltiness [45
]. In addition to the basic tastes, fattiness sensitivity has been highlighted to affect the liking and consumption of fatty foods [47
]. However, the correlation between fat sensitivity and food liking in children seems to be inconsistent [49
], particularly when weight status is involved. Previous investigations of fat sensitivity in children often used dairy samples such as milk, cheese, or pudding varying in fat content [50
Although the matter of taste sensitivity and food liking has widely been investigated over the decades, it is still uncertain how different taste sensitivity measures relate to each other and to food liking. Moreover, different methods for measuring taste sensitivity may lead to different results, preventing easy results comparison between studies. A review by Cox et al. [49
] suggested the need to measure the relationship between sensory sensitivity, fattiness, and liking. Earlier research results on the relationship between taste sensitivity and food liking are inconsistent [49
] and studies involving preadolescent subjects are still limited [52
]. The objective of this study is to investigate the relationships of basic tastes and fattiness sensitivity with food liking in 11-year-old children. By understanding the role of basic tastes and fattiness sensitivity in food liking, we may provide insights on how to encourage preadolescents to choose healthier food options, since this group has been reported to have selective eating [53
]. To our knowledge, this is the first study on taste sensitivity investigating the relationship between five basic tastes as well as fattiness and food liking conducted in preadolescents. Moreover, different methods were applied including DT, RT, taste responsiveness, and PROP responsiveness testing to measure taste sensitivity. In addition, water solutions of both caffeine and quinine were utilized in this study to characterize sensitivity to bitterness, since subjects may have different sensitivity thresholds for different bitter compounds [54
2. Materials and Methods
A total of 118, sixth-grade children were invited from two primary schools in Ski county, Norway. Both the children and their parents were provided with short information regarding the study activities in the form of a flyer. Signed written consent from the children and their parents was required to participate in the study. In addition, the children’s verbal consent was enquired at the beginning of the test. A total of 107 children returned the consent form and participated in the sensory testing. One of the subjects did not finish the test, resulting in 106 children involved in the data analyses. The schools received rewards for the benefit of the children for their participation, however, each child’s participation was voluntary. The ethical approval of this study has been granted by The Norwegian Center for Research Data (NSD) No. 747124 and refers to the Declaration of Helsinki, while data protection has followed the General Data Protection Regulation (GDPR).
2.2. Test Procedure: The Taste Detective Game
When conducting sensory testing with children, it is important to implement a test procedure that is appropriate for the children’s age and psychosocial and cognitive ability [56
]. In addition, it needs to be fun and engaging for them [52
]. Therefore, a gamification concept was inserted into the testing procedure and introduced to the children as a game called the “taste detective”. In this sensory game, a short story was narrated, and the children were asked to conduct different tasks as taste detectives. The first task aimed to measure the children’s responsiveness and liking to basic tastes, the second task measured children’s basic tastes DT and RT, the third task measured fat sensitivity, and the last task measured children’s responsiveness to PROP. All the measurements will be explained in the following sections. To evaluate the gamification concept, the children rated how fun and how difficult the sensory game was at the end of the test. This was recorded on a seven-point pictorial scale labeled with “not fun at all!” to “very fun!” and “very difficult!” to “very easy!”.
The children’s age, gender, and self-reported hunger levels were also recorded. The children’s hunger levels were measured using a seven-point pictorial scale anchored from “not hungry at all” to “very hungry”. This practice was applied because previous research has shown that hunger may influence taste sensitivity [57
] and the test was conducted at different times (10:30–11:15 and 11:30–12:15 for School A, and 12:30–13:15 and 13:00–13:45 for School B). Testing was conducted across 6 sessions with around 20 children for each testing time.
Most of the instructions were arranged online and the children’s responses were recorded with the aid of tablets. At the beginning of the test, we explained the rules of the game (i.e., performing the test quietly, not talking to one another, rinsing the mouth with water in-between samples, etc.) and what each task involved. It took the children around 30–45 min to finish all the tasks. All the tests were conducted in the children’s respective schools and classrooms. There were four adults present during the testing time: one person explaining the game and rules, two research assistants helping with the samples for the children, and one teacher.
The subjects’ basic tastes sensitivities to sweet (saccharose), sour (citric acid), salty (sodium chloride), umami (monosodium glutamate), and bitter (caffeine and quinine) were evaluated based upon five concentration levels each (Table 1
). All the taste compounds are food grade and were purchased from Merck Kga, Germany. The samples were prepared by dissolving the tastant in tap water in the sensory laboratory at Nofima (Ås, Norway) a maximum of two days before the evaluation. Around 10 mL of the sample solutions were served to the children at room temperature.
The concentration levels for sweet, salty, umami, and caffeine-bitter followed the study from Knof et al. [58
]. These concentration levels had been used in a large population study in Europe to measure taste sensitivity in 6–9-year-old children [42
]. The sour taste concentrations followed the study from Myhrer et al. [59
] while the bitterness level of quinine was adapted from Vennerød et al. [60
]. All the levels of the sample solutions were first pretested by colleagues at the sensory department at Nofima, adjusted, then piloted with 42 children aged 11–12-years. The results showed that the selected sample solutions covered a suitable concentration range for measuring both DT and RT, and matched one another in concentration level intensity across the basic tastes (results not reported here).
2.4. Taste Responsiveness and Taste Liking
The children’s taste responsiveness was measured at the beginning of the test and using the strongest level (i.e., Level 5, see Table 1
) of each taste compound in 10 mL servings. This level was expected to be clearly perceived by the majority of the children. The children evaluated all the basic tastes including two bitter compounds of caffeine and quinine in a randomized balanced order. Their responses were recorded in an unstructured line scale labeled with “weak” and “strong” and was then scaled into 0–100 for data analysis purposes. For this task, the cups were labeled with the names of the basic tastes. The liking of basic tastes in water solution at the same concentration level was also recorded in a seven-point pictorial hedonic scale. The children were provided a short explanation on how to use the line scale (i.e., by placing a mark on the line according to the strength of their perception after tasting the sample) and the pictorial scale (i.e., by choosing a happy or grim face according to the degree of their liking). This first session also aimed to familiarize the children with the basic tastes’ names and sensations. This was aimed to reduce the children’s confusion between the basic tastes in the following recognition task [61
], particularly for salty-umami and sour-bitter [13
]. Such confusions have been reported to often occur in children aged 7–11 years [45
2.5. Detection and Recognition Test
The children’s taste sensitivity was also measured using detection and recognition thresholds, DT and RT. In this evaluation, they were told to solve six taste mysteries presented with different symbols. One symbol represented one basic taste and consisted of a series of five cups labeled from 1 to 5, corresponding to the increasing concentration levels of the taste (Table 1
). All samples were given to the children at the same time in 10 mL servings, in addition to an identified cup of plain water for reference. The children were instructed to perform the tasting in a staircase order for one series (i.e., from Cup 1 to Cup 5) and could repeat their tasting for each cup. They had to identify the taste of each cup and would record their answers by dragging each corresponding cup on their tablet screen into the right taste box. Seven taste box options were offered: “sweet”, “sour”, “salty”, “bitter”, “umami”, “water”, and “I don’t know”. Note that all seven answer options were available for each cup at any time during the whole test. For each specific concentration level, we assumed that children who answered “water” could not detect any taste (tastant under detection threshold). DT was obtained when the subject could start to differentiate the sample from water, while RT was obtained when the child correctly identified the taste. Last, we assumed that children who either answered “I don’t know” or wrongly identified the taste quality, could detect the tastant; this level was therefore recorded as their DT. On their tablets, the children could freely place each cup in any taste box according to their own perception, without any limitation regarding the number of cups that could be placed in each box. Moreover, the children were not told that each series of five cups all carried the same taste, so they could freely attribute different tastes to cups of a given series. Once a taste series of five cups was completely evaluated, a break was provided with a few items from the food liking questionnaire (see below). Then, the on-screen instructions indicated to the child which symbol they should categorize next. It was not possible for the child to reconsider cups from the previous symbols.
In this test, we informed the children that there were no right and wrong answers, as this depended on their own perception. This point was strongly reminded and inserted as one of the game rules. Moreover, the children had to compare the samples with water and to rinse their mouths between tasting. They could spit out the samples in spitting cups to avoid being bloated during the evaluation. The taste series were tested in a randomized balanced order across children.
2.6. PROP Responsiveness
PROP responsiveness was measured by a paper strip (Precision Laboratories, Inc., Northampton, United Kingdom). The use of this paper strip was adapted from a method by Pickering et al. and Oftedal and Tepper [23
]. The children were asked to place the strip in their mouth and hold it for 30 s before rating the bitterness intensity using the LMS [63
]. Prior to this task, the children were provided with the instructions on how to use the LMS by using examples of foods that have extreme and mild sensations such as syrups and mineral water, salted potato chips and a spoon of salt, a spoon of wasabi, etc. [64
]. The children were classified based on their LMS rating into nontasters (if they rated the bitterness ≤ 13 mm on the LMS), medium-tasters (14–67 mm), and supertasters (>67 mm) [23
]. The test was allocated at the end of the whole testing session to refrain supertasters from being demotivated for further participation. The children received water and fresh fruits (grapes) after this task to clear their mouth from any unpleasant lingering sensation.
2.7. Fattiness Sensitivity Test
A paired comparison method adapted from Alexy et al. [50
] was used to measure fattiness sensitivity. Four milk samples were tested in pairs with 0.5% (low), 1% (medium), and 1.5% (high) fat content differences for each pair (Table 2
). All the milk samples were purchased from a local supermarket. There was no modification to the fat content for each milk sample except for the 2% fat milk, which was obtained from mixing the 3.5% and 0.5% samples in a 1:1 ratio. The milk pairs were presented in disposable cups and labeled with a geometric symbol, followed by a unique three-digit random number for each milk sample. The children’s task was to identify the fattiest milk sample in each pair, in addition to the sample they liked the most. To explain the fatty taste, the children cited examples of fatty foods (i.e., cream, butter, etc.) prior to the evaluation. All the pairs were served at the same time. Both the pairs and the milk samples within pairs were tested by the children in a randomized order. The children were told to rinse their mouth with water between testing the milk samples. Those who reported having a milk allergy and/or lactose intolerance or who declined to taste the milk samples were excluded from the milk evaluation (19 excluded, leading to n
= 87 subjects who completed the task).
2.8. Food Liking Questionnaire
The children completed a food liking questionnaire which consisted of 30 different food items representing five different basic tastes, and fattiness (the list of the food items is presented in Supplementary Material Table S1
). The selected food items and their basic tastes and fattiness profiles were based on a study by Martin et al. [65
], and were relevant within the Norwegian diet as they were listed in the Norwegian dietary survey [66
]. The children were asked about the familiarity of five random food items by either stating “I have tasted it” or “I have never tasted it”. If they had tasted the item before, they then scored their liking using a seven-point pictorial hedonic scale. These practices were conducted six times between the basic taste sensitivity measurements, aimed to provide a short break from the tasting task as well as reducing boredom to cover the list of 30 food items. The food items were evaluated in a randomized balanced order.
2.9. Data Analysis
Multiple Factor Analysis (MFA) was applied to explore the relationship between taste sensitivities measured by different methods. The liking of the basic tastes in water solutions (Level 5 concentration) was included in the MFA as supplementary variables.
The overall DT, RT, and taste responsiveness scores were computed by averaging DT levels, RT levels, and taste responsiveness scores, respectively, across the six compounds tested in water solutions. This was aimed to observe the relationship between each measurement of taste sensitivity. Pearson correlations were computed between the different sensitivity measurements and between taste compounds. The different taste sensitivities (DT, RT, and taste responsiveness), as well as the liking of basic tastes in water solution, were modeled using linear mixed models. This analysis was aimed to explore the effect of taste quality, hunger level, and gender (fixed effects) on taste sensitivity or liking of basic taste in the water solution sample. In these models, a child nested within gender was included as a random effect. In addition, PROP responsiveness was included as a continuous variable. The children’s hunger levels across schools and testing times were also compared and analyzed using a Student’s t-test.
Linear mixed models were also applied to test the effect of taste sensitivity on food liking. Taste sensitivity (as measured by DT or taste responsiveness), taste quality, gender, and hunger level were included as fixed factors, whereas child nested within gender was included as a random effect. In addition, PROP was added as a continuous variable. Note that RT was not investigated in such a model as it was not shown to be influenced by any of the explanatory variables from the previous mixed model analysis. Further, Principal component analysis (PCA) was applied to map the children’s food liking with the children’s liking scores as columns and food items as rows. The food liking scores were double-centered prior to the analysis as this enables us to better observe individual differences [67
]. The taste profiles of the foods were coded as binary variables for each of the basic tastes and fattiness (+1 if present, 0 if not present) and included as supplementary variables. The children were grouped into three liking groups based on PC1 and PC2 loadings, and a two-way ANOVA was then conducted to analyze the group effect and gender effect on taste sensitivity measured by DT for each taste.
Based on their response in identifying the correct milk pair, the children were categorized into sensitive and nonsensitive groups with respect to fattiness. The children who correctly answered the pairs from low- to high-fat levels in a staircase order (as seen in Table 2
) were allocated to the fat-sensitive group (this includes those who correctly identified all the pairs; those who correctly identified both medium and high pairs; and those who correctly identified the high-fat pair only). The remaining children (those who answered all pairs incorrectly, or those who answered other than the above-mentioned pattern) were categorized as the non-fat-sensitive group. This practice was carefully applied to eliminate the chance of guessing and inconsistent answers from the children, as our data showed an inconsistency from several children who correctly classified the low-fat pair (0.5%) but were not able to identify the high-fat pair (1.5%). The effect of fattiness sensitivity on the liking of fatty foods between the groups was analyzed using Analysis of Covariance (ANCOVA), with PROP involved as a continuous explanatory variable.
Pairwise comparisons were conducted using Tukey’s post hoc test with a significance level set to α = 0.05. All statistical analyses were computed using XLSTAT Sensory version 2020.1.2 (Addinsoft, France).