This study indicates that the stage of children's cognitive development plays a role in their preferences for, and perceptions of, fruit and vegetables. As children mature, their cognitions relating to fruit and vegetables increase in number and become more abstract.
Although cognitive development as a viewpoint from which to study fruit and vegetable preferences is new, our findings are not incompatible with previous research. Age related differences in preferences have been found in other studies [8,37]. An interesting finding from our study was that cognitive development is related to the attributes children consider when evaluating products. Young children focus on appearance and texture, whereas older children focus on taste aspects. Rose et al. [38] found similar results with sensory preferences for meat. For 6–7-year-old children, mouth feel characteristics were most important for liking, whereas in 10–11-year-olds taste and smell were most important. The diminished importance of textural attributes is possibly due to children's development of their teeth and jaws [29,39]. Szczesniak [39] stated that texture would be especially important for disliking products, but in our study it was also the most important attribute for liking among the two youngest age groups.
Young children could tell whether they liked or disliked the taste of a product, but could not identify the specific taste. This finding is in line with the study of Liem et al [40] where 4-year-old children could indicate which solution they preferred but failed to distinguish sweetness intensities during discrimination tests. In our study, the 4–5-year-old children knew and could properly use the taste salt, but not the other basic tastes. Older children had an improved understanding of the four basic tastes and, consequently, were more specific about taste when talking about likes and dislikes.
A shift from appearance to more functional attributes was found in children's reasoning with regard to preferences, the appropriate situations to eat fruit and vegetables, and healthiness. In other studies it has been found that pre-operational children focus on the most striking attributes that catch the eye, whereas older children use more functional and underlying attributes [23,41].
One aspect of cognitive development is an increase in the level of abstraction [21,22]. This development was clearly seen in children's improved understanding of health and the basic tastes, the shift in categorization from concrete categories to abstract categories, the expansion of abstract associations, and more abstract arguments concerning healthiness. Furthermore, the arguments for appropriate occasions were very concrete in groups A and B, whereas this was not the case in group C. The reduction of egocentrism [21,22] was reflected in the emergence of social norms and consideration of others' behaviour in the older age group in relation to their argumentation for appropriate eating times.
Roos [42] found that 9–11-year-old children could correctly identify which foods are considered healthy, a finding that is in line with ours. As in the findings of Hart et al. [43] among 7–11-year-old children, food-nutrient and food-health links were most often used in groups B and C as an explanation for the healthiness of a food. In our study, only pre-operational children mentioned preference links as an explanation for healthiness. This is in contrast to the findings of Hart et al. [43], where preference links were especially used by the older boys (10–11 years).
Parents play an important role in the nutrition behaviour of children [44-46]. Hart et al. [43] showed that parents used different rules depending on the age of the child. Food deals were more frequently reported by younger children (7–8 years) compared to older children (10–11 years). Although the age range is somewhat different, these findings are in agreement with our findings, where instrumental rewarding, which is comparable to a food deal, was used in respect of the youngest children but disappeared as children grow up. Probably, parents use instrumental rewarding, because it is a concrete strategy for the child ("If you eat your vegetables, you will get a candy"). The finding that parental effort is used as an argument in the oldest age group appears to fit their cognitive capacities. At this age, children are less egocentric [21,22], they can see another's perspective [22,23], and have a better understanding of value [23]. So children of this age can understand this argument. The strategies that children in group B invent to cope with eating vegetables reveal their emergent idea of conservation [22].
Limitations
Although a qualitative approach is the best method to explore a new topic, the limitations should be acknowledged. A small number of children participated in the conversations. Therefore it is not easy to generalize to a larger and broader population [31]. Further research is needed to confirm our findings.
Qualitative research is sometimes criticized for being low in both reliability and validity because of the subjective interpretation of results [31,32]. Several different actions were taken in this study to ensure reliability and validity. We used accepted systematic procedures for data collection, data handling and data analysis. The fact that children were assured that there were no wrong answers and that we did not finish their responses for them supports validity [47]. As advised by Morrison-Beedy et al. [34], the conversations were discussed immediately afterwards by the moderator and research assistant. Major topics, confusing and conflicting data were discussed. The analyses and interpretations were thoroughly discussed with the co-authors, with other researchers and with experts in research with children and taste. In addition, the comparison of our results with other findings in the literature strengthens evidence [47]. We have been very careful with interpretation and are confident that the findings are an accurate reflection of what the children said.
Another limitation is that we did not measure cognitive development. It is true that children develop at different rates, and this can result in differences within an age group. However, on the basis of cognitive development theories we are convinced that the differences in cognitive development between children of distinct age groups are larger than the differences between children within an age group. Besides, it would have been very impractical to measure the whole concept of cognitive development, as this is enormously time consuming and would have been a heavy burden for the children.
Practical implications and future research
A great advantage of our study method is that we found important practical implications and fruitful directions for future research that would have been missed with a quantitative approach. In our study, vegetables came out as least favourite food in all age groups. This barrier needs to be tackled in order to increase children's vegetable consumption. A promising finding was that fruits (especially apples and strawberries) were liked and that almost all children liked at least one vegetable. It was also positive that children considered fruit and vegetables as food for themselves as well as for adults; it would be a barrier if they perceived it only as adult food. It is often thought that fruits and vegetables are not cool enough for children. However, our study found that this did not play a role in children's consumption.
A reason often mentioned for eating fruits and vegetables at a particular time was "Because I eat it at that time". So if we teach children to have more fruit and vegetable eating times during the day, we could increase their intake. In our study, just a few children in group C associated fruit with the computer or TV. Because children spend many hours in front of the TV or computer nowadays [48], making this activity a fruit or vegetable eating time would be a first step towards improving children's diet.
In group C, it became apparent that the children did not think of cucumber and tomato as vegetables. When the research assistant mentioned that cucumber and tomato are also considered vegetables, then the children suddenly saw more time opportunities for eating vegetables: slices of tomato on bread during lunch or a piece of cucumber during the morning break at school. This finding could be very valuable in promoting vegetable intake by increasing the number of daily vegetable eating times.
In our study, older children were more specific about the preferred preparation of vegetables, and young children valued textural attributes and appearance more, whereas older children valued taste aspects more. Thus by using different preparation methods to match the right attributes to the desires of each specific age group, we might be able to change children's fruit and vegetable preferences and consequently their intake.
A very surprising finding in our study was that the youngest children argued that foods are healthy, because they taste nice. Research has shown that children associate healthy with distaste [49-51]. However, the age of the children in these studies was nine years and older, whereas the children in this group in our study were 4–5 years. It may be that young children associate healthy with tasty through the connecting term "good"; tasty food is good and being healthy is also good. However, at a certain age point, there seems to emerge a differentiation: not all healthy foods taste good. It would be interesting to investigate at what age this negative change in association occurs and how this change comes about.
In concordance with Hart et al. [43], our results indicated that parents do not take many positive actions. The children should eat healthily, are restricted to specific foods and are often persuaded to eat fruit and vegetables because they are healthy. It is not clear whether parents did not apply more positive strategies or whether more positive strategies were just not reported by the children. If parents use many negative strategies for healthy products, then this may be a reason why children develop a negative taste association for these healthy foods [36]. Our study suggests that children's cognitive development influences the strategies that parents use to shape the eating behaviour of children. It would be very interesting to investigate this interaction between parents and children further, together with the effects of this interaction.
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