Development of Taste
OBJECTIVE NUTRITION n°64 (July 2002)
By Natalie RIGAL
University of Paris-X-Nanterre, Paris, France
Is taste, that is to say the preference for certain foods and the refusal of others, innate or the result of the cultural or social environment? While the physiology of taste can partially account for a wide variety of individual differences at birth, studies show that food neophobia is a normal behavior in children. To teach the child of omnivorous parents to overcome the initial refusal of certain foods instead of giving him guilt-inducing advice - in itself an only slightly effective means of prevention of food phobia - it is preferable to familiarize the child by means of sensory training and education.
Taste is a complex constellation of mood-related sensations as reactions to the physical-chemical properties of foods. It is comprised of the perception of tastes and flavors, most of which are odors accounting for approximately 80%. Beyond genetically determined perception, some factors (familial, religious or cultural) guide a child's gustatory preferences.
FOCUS : THE COMPONENTS OF TASTE
Taste consists of a combination of several sensations:
- taste: taste strictly speaking analyzed according to at least four criteria: sweet, salty, acidic, bitter,
- flavor: the sum total of oral sensations, i.e. taste, smell and texture,
- hedonism: "I like it or I don't like it".
AN OVERVIEW ON THE PHYSIOLOGY OF TASTE
Taste buds located on the tongue detect gustative molecules. Specificity of these taste buds is very poor: a receptor can react to several dozen different molecules. The olfactory molecules are perceived by nasal epithelial cells either directly (during nasal inhalation) or indirectly (during mastication). In receptors, the gustatory or olfactory chemical signal is converted into electrical impulses.
Gustative stimuli are transported at the same time as tactile and thermal information to a first nerve relay which provides it with a simplified image. Olfactory stimuli follow their own pathway up to the olfactory bulb.
Simplified gustative and olfactory images are transported to the memory area where they are identified, and in a specific center where they join and combine with all exterior information (at the time of eating) and the internal environment (in particular sensations of hunger and satiety).
Lastly, this multi-sensory collection of information is projected into the cortex where it becomes conscious. This consciousness in turn searches one's memory for prior identifications (during previous ingestions) linked to experiences of pleasure or displeasure. The pleasure-displeasure center (which is composed of only one structure located in the lateral hypothalamus) is particular in that it is connected to all other parts of the brain Thus we can recognize the taste of chocolate and know whether we like it or not. This general process changes very little during childhood. Only the number of taste buds appears higher at birth, but it is not known whether these cells are functional.
IS TASTE AN INNATE CHARACTERISTIC?
The reply to this question is positive if we consider the quote "taste-flavor" equation: starting with the first moments of life or even during intrauterine development (i.e. month 8 of pregnancy), a child accepts a sweet taste in contrast to acidic and bitter ones which are universally rejected. Odors, however, are not subject universal hedonistic responses.
From birth, very wide individual differences are observed in olfactory appreciation. Some babies are attracted more by the odor of excrement than that of vanilla while, others present the reverse pattern of preferences. In fact, each baby has a genetically determined sensitivity profile specific for him/her. Studies (even though they are sometimes contradictory), have demonstrated that the degree of sensitivity of subjects can account for some of their rejection of some items.
Genetic heritage determines the diversity of individual specificities more than it contributes more than it contributes to establishment of preferences and universal rejection of foods. A child must then learn to diversify his/her diet to meet the conditions of an omnivorous diet : Such learning is done in a cultural setting which does not take into account individual differences, but the child's choice in this setting will be guided by his genetic heritage.
Data obtained based on surveys of a large number of children throughout France, demonstrate the existence of a child-like taste beyond the differences of sex, social or geographic origin. (Table 1).
On the side of preferences, there is a common denominator which groups sweet, salty and simple foods, as well as certain meats and dairy products.
With regard to foods that are rejected, we find sweet breads, most vegetables and some products with a very strong flavor.
As they mature, children learn to overcome their sensory dislike for certain tastes. Vegetables and foods with a strong flavor are increasingly accepted as a result of a process of familiarization (see below). On the other hand, cognitive dislikes increase with age in particular to sweet breads which most often are refused because of intellectual criteria (i.e. origin and nature of the product).
Generally, favored foods are nourishing. Their repeated ingestion makes it possible to establish a link between the taste which they arouse and their effects on the body. Pasta, rice, pizza and appetite satisfying products quickly relieve the sensation of hunger which may account for the fact that they are preferred to vegetables. (Insert 1).
REFUSAL TO EAT VEGETABLES, A PRESENTATION WHICH SHOULD BE QUALIFIED
Vegetables are not always massively refused. The percent of subjects who refuse a product is always lower than the percentage of subjects who like it. Thus, spinach is disdained by 25 % of children but accepted by 41%. This high variability between subjects does not exist for fruits. The refusal of vegetables can seem massive in light of the consequences which it produces daily, but it cannot be considered as an absolute phenomenon. It may vary as follows:
• Depending on the cooking method: 51 % of children choose cauliflower when it is prepared "au gratin", 36% if it is steamed, and 15% if it is served in a salad.
• Depending on the subject's own characteristics:
- test for vegetables increases with age especially in girls who , starting in adolescence, fins these products to be pleasant and especially raw, chopped vegetables.
- olfactory and gustatory sensitivity: the case of PROP (6-n-propylthiouracyl) or of PTC(phenylthiocarbamide) illustrate this event. These unpleasant sulfated compounds produce a bimodal distribution of levels of perception in the mouth: in one part of the population, they produce a sensation even at low concentrations ("tasters" subjects who taste different foods); another part of the population perceives them only when they are at very high concentrations (subjects who are non-tasters, i.e. 30% of the European and U.S. population). Cruciferous vegetables (cabbage family) contain such compounds which may account for the fact that subjects who tend to taste different foods refused them, while subjects who do not taste different foods accept them more readily.
Neophobia (fear aroused by new foods) is a common universal event: all omnivorous animals show reticence to ingest an unknown food.
Between the ages of 2 and 10 years, 77% of children spontaneously refuse to taste foods which they do not know; thus neophobia is normal at this age. Rare before 1 ½ to 2 years of age, it is especially intensely manifested between age 4 and 7 years: children only agree to taste something unknown to them if they are strongly encouraged to do so. Starting at age 7, children are more flexible: they agree to taste a product without prejudice against it.
Four hypotheses may account for such neophobia: opposition to one's parents (the "no" phase) the search for a secure sector (while in school, the learning operations multiply) the result of increasing autonomy (the ability to feed one self leads to questions such as "is this food good for me?"), and, perceptual rigidity (at this period of life there are as many new foods as there are ways of presenting a given product).
In light of current knowledge; it is not possible to explain why some children are more neophobic than others. However, it has been shown that children with "hyperguesia" (high gustatory reactivity at birth) are shown to be more selective and difficult at meals at 2 years of age than children with hypoguesia.
Neophobia may be diminished or even overcome by learning. Children prove to be conservative in the area of diet, they need to know what is being introduced into their body. For this reason, all processes of familiarization help them to overcome their initial refusal.
In the short term, this involves developing contact between the child and the food before the food is presented in the child's dish. The child will willingly agree to taste a product which he or she has picked or cooked. By education of the senses, subjects can become accustomed to products which are initially unknown. This education consists mainly of speaking to the child about what he is eating in terms other than hedonistic ones ("I like it") or normative terms (it's good for you").
In the longer term, familiarization, repeated ingestion over time, results in an increased taste for the product. (Figure 1) unless it produces true feeling of displeasure or that its sensory characteristics are very distant from the child's familiar universe. Some conditions strengthen the positive effects of repeated exposure, i.e. a food which is not very satisfying will be more appreciated when it is associated with a feculent (a starch or starchy food). The child who shares his/her meal in a warm emotional context with others who have favorable attitudes to the food served, will gradually develop a lasting preference for them. "Instrumentalization" of the rejected food (such as "eat your spinach or no dessert") will only strengthen the child's desire to reject that food.
In conclusion, it appears preferable to give the child a sensory education with the objective of teaching him/her to appreciate (for example) the taste of vegetables rather than to use nutritional advice which is often difficult to assimilate, which provides feelings of guilt, and which, in the end, is only slightly effective in terms of prevention.
Associate professor of psychology and child development
Université of Paris-X-Nanterre
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