A review on children’s oral texture perception and preferences in foods

Abstract

Texture properties of foods are particular drivers for food acceptance and rejection in children. The texture preferences follow the developmental progression of the child and these changes modulate the present and future food habits. This paper reviews the development and factors influencing texture preferences in children and the methods in food texture research with children. The child’s acceptance of more complex food textures is age-dependent. The progression is indorsed by the development of oral processing skills at an early age and bolstered by repeated exposures to foods with varying textures. Children generally reject foods containing pieces or bits (i.e., geometrical textural properties); however, the impact of mechanical textural properties on food acceptance is less clear. Child characteristics such as food neophobia, picky eating, and tactile over-responsivity, negatively affect the acceptance of more diverse food textures. Depending on the child’s age, the prevailing methods of characterizing food texture preferences in children include observational techniques and self-reported questionnaires. Despite knowledge of children’s development of masticatory skills, learning, and cognitive abilities, the relationships of these changes to food texture acceptance and the recommended test methodology for evaluating product texture acceptance in this period of life are still limited.

Keywords:

• Child development
• food texture
• perception
• preference learning

1. Introduction

Texture is a salient sensory attribute of food that plays a key role in food acceptance, more profoundly being a driver for food aversion. It is defined as “the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the senses of vision, hearing, touch and kinesthetic” (Szczesniak 2002). In the early days, consumer research showed that people often took texture for granted and used it as an indicator of food quality. They would not pay much attention to food texture unless it was completely off, inappropriate, or mismatched with their expectations (Szczesniak and Kahn 1971). People like to have control over the food placed in their mouths, thus foods that are sticky, slimy, slippery, or contain unexpected particles may be regarded as aversive and easily rejected (Kauer et al. 2015; Pellegrino and Luckett 2020). These negative responses to the texture properties of food can even elicit disgust reactions, regardless of whether the properties are actual or perceived (Martins and Pliner 2006).

Research in adults shows that texture preferences are influenced by multiple factors, ranging from the physical properties of foods, individual differences in oral processing behaviors to exposure and cultural contexts. For instance, mealiness and grittiness are undesired textural properties of foods that can negatively affect product acceptance (Engelen et al. 2005; Jaeger et al. 1998; Liu et al. 2021). When consumers assess their overall liking of foods, they differ in the extent of attending to various sensory inputs. Those who were more texture-orientated would focus more on texture attributes but less on flavor attributes in foods (Moskowitz and Krieger 1995). Consumers using different approaches for the oral breakdown of food would give rise to individual differences in texture perceptions and preferences (Brown and Braxton 2000). In a series of studies on mouth behaviors, Jeltema, Beckley, and Vahalik (2016; 2015) showed that people could be grouped into “chewers”, “crunchers”, “suckers” or “smooshers”, according to the way they preferred to manipulate food in their mouth. Their food texture choices and preferences were driven by the mouth behavior group they belonged. Recent studies further demonstrated the cultural effect on people’s preferred oral food processing and texture liking. Using a broader categorization of mouth behavior group into “soft processing likers” and “firm processing likers”, Cattaneo et al. (2020) clustered Chinese and Danish consumers into two patterns where the majority of Asian Chinese consumers were classified as likers of foods that required soft oral processing. Caucasian Danish consumers were mostly classified as likers of foods that require firm oral processing, such as biting and chewing. Furthermore, ethnic groups had a significant influence on consumers’ liking of yogurts with different degrees of granularity (Liu et al. 2021).

Children are generally considered more sensitive to food texture than adults; one could expect textures to have a more important role in driving their food preferences. However, relatively little is known about how food texture preferences develop in children, despite a growing interest in these areas (Appiani et al. 2020; Laureati et al. 2020; Lukasewycz and Mennella 2012; Chow et al. 2022). Children from infancy to teenage have different cognitive abilities and attentional spans to comprehend sensory or consumer tests (Guinard 2000). This also presents a challenge to researchers who want to discuss and quantify texture perception and preferences with children.

Szczesniak (1972) suggested that children have a general preference for food textures that are easy to control and manipulate in the mouth. However, children acquire oral processing skills at different developmental stages in childhood, contributing to the acceptance of a wider variety of foods (Demonteil et al. 2019; Cichero 2017; Le Reverend, Edelson, and Loret 2014). These changes would continually shape children’s texture acceptance. The entire process modulated by other factors, such as food exposure and learning, appears to be dynamic. Accordingly, if no unpleasant experiences occur, texture preferences are expected to gradually change from soft, smooth, and unidimensional textures to hard, chewy, lumpy, or other complex textures (Szczesniak 1972). Moreover, rejection of complex textures is associated with food neophobia and picky eating (Coulthard, Harris, and Emmett 2009; Smith et al. 2005; van der Horst et al. 2016). Food neophobia and picky eating often increase from a low baseline at weaning. Their development across childhood is affected by factors such as children’s personalities, parental practices, and social influences (Dovey et al. 2008). Complex textures encountered in dairy products, meat, fish, fruits, and vegetables require learning to accept. Once preferences or rejections are learned, they may be difficult to change and persist across adolescence and adulthood. Therefore, preventing the rejection of complex textures is crucial for children to develop adequate eating habits.

This review aims to relate the developmental aspects in children with the ability to orally process food textures and the development of food texture preferences. The review focuses primarily on healthy children from infancy to late childhood representing four stages of development: infant (birth to 18 months), toddler (18 months to 3 years), preschool (3 to 6 years), and school-aged (6 to 12 years). The work is organized into two parts. The first part identifies factors that affect children’s food texture preferences and the drivers and barriers for children to accept/reject textures. The second part compiles and assesses the methods used to measure food texture preferences and perception in children from early to late childhood.

2. Development in food oral processing

The oral structure and function of a child undergo rapid changes during the first few years of life. Infants’ palatal and mandibular dimensions may develop over the first year as much as between one year and puberty (Le Reverend, Edelson, and Loret 2014). Together with the development of milky teeth and later exchange to permanent teeth, these changes in oral dimensions and masticatory ability allow the acceptance of a wider variety of food textures and affect children’s preferences.

Infants and toddlers accept soft, smooth, and simple textures such as puree more readily than complex textures (e.g., chopped, lumpy, diced, and larger-sized foods), as the latter requires greater effort in oral processing (Birch et al. 1998; Blossfeld et al. 2007; Demonteil et al. 2019; Lundy et al. 1998; Remijn et al. 2019). However, children become more interested in complex textures when they grow up. Lundy et al. (1998) showed that toddlers responded more positively to lumpy and diced textures than infants. Between 6 and 18 months of age, children progressively accepted harder foods and a larger variety of textures (Demonteil et al. 2019). Szczesniak (1972) suggested that the development in oral physiology has a profound influence on texture acceptance in growing children, which shifts from the passive (soft/smooth) to the more forceful (firm/rough) textures. Evidence from dental research indicated that oral development is dependent on specific chewing loads applied to a child at appropriate ages (Le Reverend, Edelson, and Loret 2014). The manipulation of the food in the mouth is partially under the conscious control of the child, however, masticatory processes like chewing, transporting and swallowing occur autonomous and are regulated by auto reflexes. In the following part, the development in these masticatory processes are described as they form an intricate part of food texture acceptance learning.

2.1. Mastication

One of the purposes of mastication is to break down food particles into bolus for safe and comfortable swallowing (Chen 2009). The four major components of the mastication apparatus: bone, muscles, teeth, and soft tissues (i.e., the tongue, lips, and cheeks) develop at different rates with age (Le Reverend, Edelson, and Loret 2014). These developments are associated with the maturation of oral processing skills and affect the degree of texture acceptance in children. During the first few months of life, infants drink liquids and lack masticatory capacity; thus, their oral texture exposure is limited. Liquids require only a transport onto the tongue, a holding in the cavity between the tongue and hard palate, and a subsequence passage during swallowing/sucking movement, where the liquid glides downwards to the pharynx. However, by 4 to 5 months, infants develop certain tongue and lips mobility to move foods to the back of the mouth to swallow (Carruth and Skinner 2002). This allows them to consume semi-liquid foods such as smooth purees.

The infant’s teeth typically begin to erupt between 6 and 9 months, leading to the development of chewing skills and allowing them to consume firmer and lumpier foods (Carruth and Skinner 2002; Szczesniak 1972). For example, 12-month-old infants with more teeth tended to consume more chopped carrots than their toothless peers (Blossfeld et al. 2007). Between 6 months and 2 years, children took the longest time to chew on solid food, followed by viscous food and puree (Gisel 1991). Their efficiency for chewing the same portion of food textures increased with age.

Oral physiological development continues throughout the childhood and teenage years, and children keep refining their oral processing skills. Gisel (1988) reported that children’s chewing efficiency for solid, viscous, and pureed textures improved and matured between 2 and 8 years of age. Children’s bite force increases progressively between 3 and 17 years (Kamegai et al. 2005; Figure 1). However, little is known about how the maturation in masticatory performance affects children’s food texture acceptance. Oral development may support the acceptance of harder foods. Evidence showed that with increasing age, children’s preferences for hard foods become aligned with their adult counterparts (Lukasewycz and Mennella 2012). Contrarily, having a positive attitude to hard foods may improve children’s masticatory performance and support healthy orofacial development. Yamanaka et al. (2009) reported that in Japanese children aged between 7 and 12 years, their preference for hard foods such as cabbage and celery was positively related to bite force and occlusal contact area.

Figure 1. Mean bite force for children aged between 3 and 17 years (n = 2594). Bar charts are created using results from Kamegai et al. (2005).

2.2. Reflexes in the oral, pharyngeal and laryngeal regions

In the oropharyngeal regions, multiple reflexes function as supportive networks of neutrons assisting in the control of complex motor responses, such as chewing, intraoral food transport, and swallowing (Miller 1999). In fact, the upper aerodigestive tract, which acts as a conduit for air and food passages, is the most complex human neuromuscular unit in the body (Delaney and Arvedson 2008). While oral feeding for a newborn (e.g., rooting, suckling, and swallowing) is completely reflexive and involuntary, establishing complex reflexes depend on modifying or building reflex responses to a particular oral sensory input, where learning is involved (Miller 1999; Stevenson and Allaire 1991; Dodrill and Gosa 2015).

2.2.1. Swallowing

There are notable changes in the anatomy of the swallowing apparatus, including the oral cavity, the pharynx, and the larynx, from infancy to childhood (Figure 2). In the first few months of life, the infant’s tongue occupies most of the oral cavity. The larynx is still positioned high, causing the tip of the epiglottis to overlap the soft palate. This arrangement facilitates the suckling feeding on liquid diets and helps protect against aspiration. When the head and neck grow further, the oral cavity enlarges and laryngeal structures descend to a lower position as in adults (Matsuo and Palmer 2008; Stevenson and Allaire 1991). These changes allow increased oral movements and support the oral processing of firmer foods.

Figure 2. The mouth and pharynx anatomy in infants and adults.

When the child starts eating solid foods, the sequence of events during oral processing can be summarized into three phases and described with the Process Model of Feeding (Palmer et al. 1992; Cichero 2006; Figure 3). During the oral phase, food is first transported to the occlusal surface for oral processing. While chewing continues, the triturated food can pass into the oropharynx and accumulate there for bolus formation. Then, the tongue creates a trough, elevates, and projects the bolus backward to the posterior pharyngeal wall, triggering the swallowing reflex. The bolus is propelled through the pharynx and upper esophageal sphincter to the esophagus (pharyngeal phase) and subsequently moved to the stomach (esophageal phase). The pharyngeal and esophageal phases of swallowing are involuntary activities, whereas the oral phase comes under voluntary control with the development of mastication abilities (Dodrill and Gosa 2015). The swallowing mechanism remains fairly stable after this stage of development (Cichero 2006).

Figure 3. The sequence of events during oral processing can be summarized into three phases and described with the Process Model of Feeding (Palmer et al. 1992), illustrated from left to right.

In addition, the normal development of tongue strength and coordination is important for safe and efficient swallowing (Youmans and Stierwalt 2006). A recent study reported developmental changes in children’s tongue strength between 3 and 17 years (Potter et al. 2021). Tongue strength increased rapidly with age from 3 to 6.5 years from where is continued to increase albeit at a slower rate.

2.2.2. Gagging

As a natural defensive mechanism, the gagging reflex shares a similar receptive field as swallowing reflexes (Miller 1999) and may play a role in the development of texture preferences. Gagging is a reflex action evoked by a stimulus (e.g., food, fingers, toys, or utensils) touching the back of the mouth. The posterior oral and pharyngeal muscles constrict and prevent unwanted objects from entering the oropharyngeal regions (Bassi, Humphris, and Longman 2004). Carruth and Skinner (2002) reported that the mean age for infants to eat foods with tiny lumps without gagging was 8.7 months, and the mean age for them to chew and swallow firmer foods without choking was 12.2 months. The gradual disappearance of gagging appears to coincide with the time when infants start learning to take solid foods. It is suggested that the introduction of ‘lumpy’ puree to infants between 6 and 9 months is important for desensitizing the gag reflex (Cichero 2006).

The normal passage of food across the pharyngeal and velar regions does not usually incite gagging. However, it has been reported that children with tactile over-responsiveness or picky eating were more likely to gag on foods (Boquin et al. 2014; Smith et al. 2005). The brain center for gagging is in the medulla oblongata, which is close to the vomiting, salivating, and cardiac centers, which can be potentially triggered during gagging (Bassi, Humphris, and Longman 2004). Excessive salivation, vomiting, sweating, or fainting may occur in a gagging episode, leading to a negative experience with the food stimulus.

3. Learning and accepting food textures

Exposure to tastes and flavors in utero and infancy can increase the acceptance of the same tastes and flavors in foods later in life (Mennella and Beauchamp 1991; Mennella, Johnson, and Beauchamp 1995). Contrarily, texture acceptance in children is subject to postnatal development, depending on both the structure (texture) of the food and the oral structures and functions in children. Birch et al. (1998) demonstrated that only a few exposures were needed to increase infants’ intake of the targeted food (a commercially-prepared banana puree), as well as to generalize this exposure effect to a food with a similar flavor (peaches/pears puree). However, the exposure effect was not effective when the mothers prepared the same-flavored food (a homemade banana puree), which was lumpier than the commercial version of fruit puree. The development of food texture preferences in children can be more complex as it is growth-dependent and, at the same time, influenced by other developmental aspects of a child.

3.1. Sensitive periods for texture introduction

A sensitive period in development refers to the optimal time for learning a particular behavior pattern to take place. After this period, the behavior can still be learned, but an extra effort is needed (Knudsen 2004). Illingworth and Lister (1964) suggested a sensitive period for giving solid foods to children – at the age of 6 to 7 months when they are developmentally ready to chew. A delay in doing so could lead to feeding difficulties such as failing to chew, refusing solid foods, or vomiting. This hypothesis was later confirmed in a prospective study in the UK (the ALSPAC study). Children who were introduced to lumpy solids at 10 months or later were more difficult to feed and more likely to refuse solids by 15 months (Northstone, Emmett, and Nethersole 2001). At 7 years old, these children were also pickier and consumed fewer fruits and vegetables (Coulthard, Harris, and Emmett 2009). Similarly, children who received tube feeding during infancy and were weaned off the tube later experienced difficulties processing and swallowing solids or lumpy texture due to the oral-motor delay (Mason, Harris, and Blissett 2005). Thus, a timely introduction of solid foods to infants has a critical impact on their further widening of texture acceptance later in life.

Although the maturation of oral processing skills follows the orofacial growth in a child, they are learned progression of behaviors that rely upon exposure, i.e., whether the child has the opportunity to process different food textures (Stevenson and Allaire 1991). Infants having an early experience with difficult-to-chew textures develop a preference for complex textures earlier in their later developmental period (Blossfeld et al. 2007; Lundy et al. 1998). Consistent with these studies, a recent large-scale survey study (n = 2999) conducted with mothers of 4- to 36-month-old children confirmed the importance of exposure to diverse food textures (Tournier et al. 2020). Offering more opportunities for children to experience food textures (e.g., soft and small pieces until 18 months, and hard/large pieces and double texture until 29 months) facilitated food texture acceptance in their later ages. Pilot interventions with infants also showed that a larger exposure to food pieces helped improve infants’ chewing abilities (da Costa et al. 2017) and led to greater texture acceptance (Tournier et al. 2021).

3.2. Oral motor skills

During the first year of life, the gradual acceptance of food textures (i.e., from soft and smooth textures to firm and complex textures) has a strong relationship with structural/physiological development in the oral cavity and the pharynx, as well as the development of oral processing skills (see Section 2 for details). This developmental pattern is also concurrent with infants’ global development. For example, improving head and trunk stability provides a gross motor foundation for children to move foods in the mouth and swallow them safely (Carruth and Skinner 2002; Stevenson and Allaire 1991).

Infants and young children have a higher risk of choking and aspiration during their developmental progressions in oral processing skills (Altkorn et al. 2008; Chapin et al. 2013). Foods that are round with hard, elastic, or lubricating textures can be choking hazards for them (Altkorn et al. 2008). Szczesniak (1972; 2002) suggested that children’s general dislike of lumps or particles in foods stems from the fear of choking or gagging. Foods with “things in it” (e.g., yogurt with fruit particles) can give a double sensation between the liquid phase and the solid particles. In a heterogenous food matrix, oral processing becomes more difficult than when the particles or liquids are presented alone. Because the large enough particles trigger the oral reflexes in chewing, whereas the liquid phase may enter the pharynx and trigger the swallowing/gagging reflex. A study on oral behavior in healthy young adults and elderly showed that when adding peach gel particles (15% w/w) into yogurt, the number of chews and consumption time increased more than double compared to the homogeneous yogurt, and decreased the eating rate up to 60% for both consumer groups (Aguayo-Mendoza et al. 2020).

Children learning to consume (semi-)liquid foods with lumps or particles may be difficult since the fundamental process of oral food detection and oral reflexes may be inherently conflicting or confusing in foods with multiple textures, which poses a challenge for a child to learn to swallow these foods safely. Learning to accept particles in a liquid/viscous food requires the tuning of these reflexes during the movement of the food in the oral cavity, whereas negative experiences such as a gagging or choking episode can lead to instantaneously acquired dislikes and long-lasting aversion to that food (Birch 1999; Laureati and Pagliarini 2018). One could anticipate that children will have different abilities to learn these skills and that personality traits like “fear” versus “sensation seeking” could also play a role in this learning process.

Beyond early childhood, ongoing physiological development is expected to continually influence children’s texture preferences (Szczesniak 1972). However, relatively little is known in this area. Several studies suggested that texture is a key driver of food liking among young children, whereas older children tend to focus more on the taste and flavors of foods (Rose et al. 2004a; 2004b; Zeinstra et al. 2007). Parents of young children reported in interviews that food texture particularly influenced their children’s food dislikes (Russell and Worsley 2013). Young children consumed significantly less of a well-liked yogurt when extra fruit pieces were added to that yogurt, but the acceptance was not affected when changing the taste or color of that yogurt (Werthmann et al. 2015). Similarly, Zeinstra et al. (2007) showed that the most important drivers for liking and disliking fruit and vegetables shifted from appearance and texture among younger children (4 to 5 years) to taste attributes among older children (11 to 12 years). It is possible that when children advance in their oral motor skills and coordination of oral processing reflexes, they can handle different kinds of food textures with ease. Texture may become less prominent in driving food acceptance in growing children. For example, school-aged children (10 to 13 years) reported that unpleasant taste or smell was a more important reason than the dislike of texture for food rejection (Sick, Højer, and Olsen 2019).

3.3. Food neophobia and picky eating

Children often reject foods with textures that are difficult to control and manipulate in their mouths. This may explain why children who are high in food neophobia tend to prefer foods with simple and unidimensional textures (Laureati et al. 2020; Lukasewycz and Mennella 2012; Cappellotto and Olsen 2021; Chow et al. 2022). Lukasewycz and Mennella (2012) and Chow et al. (2022) demonstrated that the neophobia trait was related only to rejections of foods containing particles but not their preferences for hardness in foods. Also, texture rejections are common to picky eating (Boquin et al. 2014; Nederkoorn, Jansen, and Havermans 2015; Russell and Worsley 2013). Van der Horst et al. (2016) reported in the US that compared to children who were non-picky eaters, there was a high prevalence of texture refusal in very picky children (34% vs. 3%).

Between 2 and 6 years, food rejection behaviors increase sharply since children become more independent in eating and have to learn what is safe for them to consume (Cashdan 1994; Dovey et al. 2008). Working on the developmental sequence of food rejections in children, Fallon and coworkers concluded that rejection first appeared based on sensory-affective reasons (e.g., distaste). Rejections based on anticipated harm after ingestion (e.g., danger) appeared next, and the ideational type of rejection (e.g., inappropriate or disgust) appeared the latest (Fallon, Rozin, and Pliner 1984). It is possible that when children have immature oral skills and swallowing coordination, they experience more difficulties in handling complex textures. These textures may induce an unpleasant oral tactile experience, require more chewing and represent a higher risk of gagging/choking. For example, a gritty sensation when consuming lumpy foods or sticky foods adhered to the teeth or palate surface can induce oral discomfort (Demonteil et al. 2019; Donadini, Fumi, Vanoni, et al. 2012). Slimy and tough textures that require a long chewing time for oral processing can be unappealing or even unsafe to eat (Russell and Worsley 2013).

Children’s general dislike of food containing particles may also be driven by a feeling of disgust. The unusual presence of particles in foods (e.g., pieces, pips, or chunks) can indicate a sign of contamination (Wardle and Cooke 2008). For example, vegetables with brown coloring, yogurt, or smoothie with food pieces contain visual features that can lead to dislike or rejection of that food at the mere sight. Although it remains inconclusive on whether disgust can be a motivation for food rejection at young ages (Brown and Gillian 2012), studies in adults showed that negative responses to the perceived texture properties of foods (i.e., without actual tasting) could be disgust-eliciting (Martins and Pliner 2006).

3.4. Sensory processing

Children’s differences in their characteristics (e.g., personalities, moods, and behaviors) affect the strength and duration of their food rejection behaviors (Dovey et al. 2008; Harris and Mason 2017). One of such is their degree of sensory processing abilities. Children vary in their sensitivity to stimuli across sensory domains (e.g., visual, taste, touch, and auditory), and their ability to receive, process, and respond to information from their senses impacts their behaviors in daily life (Dunn 1997; 2001). Eating offers a rich multi-sensory experience, and children who have higher levels of sensory over-responsivity may have more difficulties with the taste, texture, temperature, or smell of foods. They are reported to be more neophobic and/or pickier in eating (Coulthard, Harris, and Emmett 2009; Coulthard, Harris, and Fogel 2016; Farrow and Coulthard 2012; Johnson et al. 2015; Nederkoorn, Jansen, and Havermans 2015).

Coulthard, Harris, and Fogel (2016) measured sensory processing in infants and found that infants’ tactile over-responsivity was strongly related to the first acceptance of solid foods in their early complementary feeding period. Infants who were more sensitive to tactile stimuli consumed less carrot than their peers with low or moderate tactile responsivity. Children being tactilely defensive were characterized as having a strong aversion to certain food textures and gagging more often when offered a disliked food (Smith et al. 2005). Schoolchildren with preferences for soft and smooth textures were more sensory responsive across all sensory domains (i.e., tactile, smell and taste, and visual and auditory) than their peers with preferences for hard and particulate textures (Cappellotto and Olsen 2021).

More recently, Ross et al. (2022) proposed using five specific questions to classify children as food texture sensitive. The study identified around 16–22% of the children (4 to 36 months) as texture-sensitive. These children were more likely to reject specific food textures, such as chewy, hard, and lumpy, and display negative behaviors to foods compared to children in the categorization of non-texture sensitive. In a larger sample size of children aged between 5 to 12 years, texture-sensitive children were significantly fussier than their non-texture-sensitive peers (Ross et al. 2022). Nederkoorn, Jansen, and Havermans (2015) studied the relationship between texture selectivity and appreciation of tactile stimuli in 4- to 10-year-old children. Children were asked to touch different tactile stimuli with their hands and taste different foods that varied in textures and temperatures. The study found that children who were more dismissive to tactile stimulation in their hands were also more tactilely selective in foods, especially for younger children.

These results suggest that over-responsiveness in oral tactile processing may be an underlying factor of texture rejections in children. In line with research on adult consumers, tactile responsivity and picky eating were related modalities, and this relationship was mediated by people’s appreciation of the mouthfeel in foods (Nederkoorn, Houben, and Havermans 2019). However, there was no evidence supporting an association between oral tactile processing/food texture sensitivity and the lingual tactile acuity assessed with a psychophysical tool (Ross et al. 2022). The notion of “sensory sensitivity” may not necessarily imply an increased perceptual sensitivity (i.e., oral tactile perception) but heightened responsiveness to stimuli (Prescott, Chheang, and Jaeger 2022). Clearly, further research is needed to understand the role of over-responsiveness in oral tactile processing on texture rejections in children.

3.5. Cognitive development

Children grow in the way they perceive, think, and construct knowledge about their world. While most research about children’s food preferences pays little attention to the possible role of cognitive development, theories derived from this field may provide insight into children’s food choices and their perceptions of foods and eating (Rohlfs Domínguez 2020). According to the cognitive theory of psychologist Jean Piaget, infants are grouped in a “sensorimotor” stage, and children up to 7 years in a ‘pre-operational’ stage. Children at these two stages have a limited ability for information processing. They then progress to “concrete-operational” and “formal operational” stages with an increasing ability for causality thinking and logical reasoning (Flavell 1963). During the pre-operational period, young children’s attention is centered on a limited aspect of a stimulus, and food choices are primarily based on affective appraisals of the sensory attributes, such as appearance and texture (Contento 1981; Rohlfs Domínguez 2020; Frerichs et al. 2016). Children tend to focus on the more salient sensory attributes of foods in their hedonic judgments (Guinard 2000; Oram 1994). They prefer simpler foods and dislike foods being mixed up or food having “things in it” (Szczesniak 1972). As a result, foods containing particles could be rejected at the mere sight. In contrast, concrete-operational children have a greater ability to perceive stimuli multi-dimensionally and understand abstract concepts. This allows them to evaluate foods or eating experiences with multiple sensory attributes or more based on abstract attributes, such as healthiness (Zeinstra et al. 2007; Rohlfs Domínguez 2020).

Oram (1998) studied children’s use of food vocabulary and compared their ability to identify vocabulary of food texture and chemosensory perceptions with adults. Young children (6 to 7 years) recognized very limited words that referred to different texture parameters, but their amount of vocabulary increased cumulatively as they aged. Interestingly, most young children did not recognize any word that referred to geometrical textural properties in foods, e.g., lumpy, chunky, smooth. Although children’s specific understanding of the words was not evaluated, these results reflect the increasing ability of children to describe textures and awareness toward different food textures. The categorization of foods (e.g., based on sensory attributes) can strongly influence children’s preferences and eating behaviors by guiding them to recognize and form expectations to foods, particularly to unfamiliar foods (Aldridge, Dovey, and Halford 2009; Lafraire et al. 2016; Pliner 2008). A review by Rohlfs Domínguez (2020) proposed a theoretical model for children’s neural and cognitive development, particularly on understanding health-related abstract concepts, that encourages vegetable consumption. Thus, further research may examine whether the conceptualization of food texture in a growing child may foster the development of texture preferences.

4. Food texture acceptance and oral tactile perception

Beyond early stages, children have a more diverse experience with foods that can further widen texture acceptance. Since texture is a multi-parameter attribute, a specific parameter can have an important contribution to the overall acceptance of one particular food but not in another food, e.g., crunchiness in vegetables or toughness in meats. This leads to the question of whether there are general food texture preferences among children, which are comparable to their innate preference for sweet taste and rejection of bitter taste (Drewnowski 1997). Recent studies have developed questionnaires to study texture preferences in children (Laureati et al. 2020; Lukasewycz and Mennella 2012; Chow et al. 2022). Consumer studies with children also investigated how textural parameters govern the acceptability of specific foods.

The senses of touch and pressure are considered the most important for food texture detection (Szczesniak 2002; Chen 2009). It would be expected that one’s sensitivity to food texture perception is related to the sensitivity to oral tactile perception. These led to investigations on whether individual differences in oral tactile perception could explain the differences in food texture preferences (Appiani et al. 2020; Lukasewycz and Mennella 2012; Ross et al. 2022). The following sections summarize findings on texture preferences and acceptance/rejection of specific food textures in children, followed by a discussion on their relations with oral tactile perception.

4.1. Textural parameters governing food acceptability

Hardness and particle content in foods are frequently investigated parameters in the research of children’s texture preferences. These parameters align with the two common textural characteristics, i.e., mechanical and geometrical properties in foods (Szczesniak 1963). Lukasewycz and Mennella (2012) showed that children preferred softer foods and those containing fewer particles than their adult counterparts. As they grew up, their preferences for hardness in foods became more adult-liked, but their preferences for particle content in foods remained. Research across European countries suggested a cultural effect on children’s texture preferences. Results showed that a higher proportion of schoolchildren preferred hard and particle-containing foods in Northern Europe than in Southern Europe, which may be due to differences in culinary habits and food selection between countries (Laureati et al. 2020). A preference for oral processing of firm foods among adults in Denmark confirmed such a cultural dimension (Cattaneo et al. 2020).

4.1.1. Mechanical textural properties

Mechanical textural parameters such as hardness, cohesiveness, crispness, and crunchiness have a strong influence on the acceptability of categories of solid and semi-solid foods like vegetables, cereals, confectionary, diary products and meats. Baxter and coworkers showed that textural properties in vegetables were particularly associated with aversion among children in the UK (Baxter, Schröder, and Bower 2000; Baxter, Jack, and Schröder 1998). However, the textural parameters that led to dislike were different between the two studies, which may be because different sets of vegetables were investigated. Children in the first study preferred vegetables with hard and crunchy textures and disliked vegetables with soft and mushy textures (Baxter, Jack, and Schröder 1998), but children in the second study had opposite preferences for textures in vegetables (Baxter, Schröder, and Bower 2000). In a cross-cultural study among children in Chile, China, and the US, softness and juiciness were important sensory attributes for driving vegetable liking (Estay et al. 2019). Research on the effect of preparation methods (i.e., baking, mashing, boiling, deep-frying that can lead to texture changes) on vegetable liking showed mixed results. For example, boiling and steaming were the most preferred preparations for carrots and French beans among children of 4 to 12 years, which could be attributed to the crunchiness of the vegetables (Zeinstra et al. 2010). Donadini, Fumi, and Porretta (2012) studied how preparation methods (i.e., raw, boiling, and oven-baking) would influence preschool children’s liking of vegetables. Tough textures in vegetables lowered acceptance since they might be hard to chew for preschoolers. However, Poelman and Delahunty (2011) reported that textural differences in the vegetables imparted by preparation methods did not influence the acceptance of sweet potato, cauliflower, and beans.

Children’s acceptance of cheese, meat, and fish were affected by the cohesive and chewy textures in products. Preschool children accepted firm and crumbly textures in cheese but rejected deformable and adhesive textures (Donadini, Fumi, Vanoni, et al. 2012). Children also rejected soft, jelly-like textures in fish (Donadini, Fumi, and Porretta 2013). Children liked chewy textures in meat products (Rose et al. 2004a) but disliked these textures when presented in bread products (Jervis et al. 2014).

4.1.2. Geometrical textural properties

Geometrical textural parameters are those related to the geometry (particle size, shape, and orientation) of foods, e.g., gritty, granular, and fibrous; they are perceived when the food is first placed in the mouth (Szczesniak 1963; 2002). Children generally prefer smooth and homogenous textures but reject lumpy and granular textures in foods. Textural contrast in foods (e.g., double or multiple textures) reduces acceptance. This pattern of food acceptance is repeatedly observed in different categories of foods, for example, in vegetables (Blossfeld et al. 2007; Zeinstra et al. 2010), fruit products (Kildegaard et al. 2011; Laureati et al. 2017), yogurts (Kildegaard et al. 2011; Nederkoorn et al. 2018; Werthmann et al. 2015), and cereal products (Jervis et al. 2014; Sandvik et al. 2020). Moreover, children tend to reject foods that fall apart easily in the mouth, as observed in fish dishes (Donadini, Fumi, and Porretta 2013). Preferences for homogeneity in foods are also reflected in children’s liking of food appearance. They liked vegetables with uniform surfaces but disliked those with brown coloring (Donadini, Fumi, and Porretta 2012; Zeinstra et al. 2010). In a qualitative interview by De Moura (2007), children also expressed that the presence of pips and excessive softness in foods were displeasing for them. An exception to this texture acceptance pattern was reported by Demonteil et al. (2019). In their longitudinal study on food texture acceptance in children aged between 6 and 18 months, double textures in purees with soft lumps were highly accepted by all ages. This may be attributed to the small textural contrast presented in the products, which were homogenous enough for simple oral processing. For children between 12 and 18 months, their liking for muesli tended to decrease with age. Muesli had a larger textural contrast between the liquid phase (milk) and the hard pieces, which could present challenges for processing the two different phases.

4.2. Oral tactile perception and their influence on texture acceptance

Mechanoreceptors in oral tissues are responsible for tactile perceptions of objects in the mouth, such as foods, liquids, and oral devices (Haggard and de Boer 2014). Evidence showed that a heightened oral tactile responsivity in children is associated with rejections of more complex textures (Harris and Mason 2017; see Section 3.4 for details). The detection or discrimination of oral tactile stimuli is the biological system underpinning food texture perception. It was hypothesized that individuals who are more sensitive to oral tactile stimuli might better detect the textural properties of foods, which in turn leads to their aversion or rejection of certain textures (Liu et al. 2022). However, when the sensitivity was measured with psychophysical tools (e.g., the letter recognition method, the point pressure test, and the grating orientation test), no relationship could be found with texture preferences in children (Appiani et al. 2020; Lukasewycz and Mennella 2012). These results also suggested that oral tactile perception did not differ between children and adults. Consistent with previous studies with adult subjects (Cattaneo et al. 2020; Liu et al. 2021), no evidence was found to support the association between oral tactile perception and food texture preferences.

Preliminary evidence was found for young children (6 to 7 years) with a higher level of food neophobia to be more sensitive to oral touch as measured by the point pressure test (Appiani et al. 2020). However, in a study using a large sample size (n = 987; 5 to 12 years), oral tactile perception assessed by the grating orientation test was not related to food texture sensitivity in children (Ross et al. 2022). Gisel and Schwob (1988a, 1988b) investigated the relationship between oral stereognosis ability and chewing behaviors in 5- to 8-year-old children. Children’s ability to discriminate size improved significantly between 6 and 7 years old. Also, the size perception error in the oral stereognosis test was significantly correlated with the time required for a child to chew a difficult texture (viscous foods). The author suggested a possible association between oral sensation (oral form perception) and chewing ability.

Overall, there are mixed results on the impact of mechanical textural properties on children’s food acceptance. This may be because different sets of foods were investigated and the insufficient characterization of textural properties. More research is needed to develop a more coherent view on acceptable ranges of mechanical properties of foods in relation to children’s food acceptance. Children often reject geometric textural properties presented as the presence of pieces or bits in foods. The effect of age on the development of food texture preferences beyond early childhood is unclear. Although it is expected that oral tactile perception is linked with texture aversion in children, this association was not confirmed in studies using different psychophysical assessments. It appears that oral tactile perception (as a biological system underpinning food texture perception) and oral tactile responsivity (as the processing of tactile information) are two unrelated modalities. A recent review on oral tactile sensitivity suggested that current measurement techniques, such as a grating orientation task or von Frey filaments, typically represent a single dimension of tactile perception, which can be difficult to link directly with food texture perception/preference (Liu et al. 2022). Alternative methods are needed to study food texture perception in childhood. Such methods should focus less on absolute oral tactile perception and more on the perception of specific food texture attributes.

5. Oral defensiveness in special children groups

Knowledge of severe food texture aversions may provide insight into the normal working of how texture preferences are developed in typically developing children. This session briefly summarizes atypical texture preferences and eating patterns commonly seen in children with developmental disorders, e.g., autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), and Down syndrome. Parents of these children frequently reported their children being picky or highly selective in eating (Cermak, Curtin, and Bandini 2010; Ghanizadeh 2013). For example, children with autism could have very restricted repertoires of food acceptance, and the texture or consistency of foods was a key driver of their food choices (Schmitt, Heiss, and Campbell 2008; Williams, Dalrymple, and Neal 2000). Ross et al. (2022) reported a significantly higher percentage of children with Down syndrome classified as texture-sensitive than among typically developing children.

Researchers have suggested oral defensiveness as a factor underlying texture selectivity in these children. Oral defensiveness may be a component of tactile defensiveness and related to a larger problem of impairment in sensory processing (Cermak, Curtin, and Bandini 2010). They can be a barrier for both typically developing children and children with developmental disorders to accept complex textures in foods. Although ADHD and autism have strict criteria for making the diagnosis, it is unclear if oral defensiveness applies to a broader range of children. At the same time, studies on food neophobia in children (Damsbo-Svendsen, Bom Frøst, and Olsen 2017) are not specific enough on food texture. There is a need to develop better research tools to investigate texture aversions in childhood.

6. Methods in food texture research with children

Food texture investigation with children, such as the hedonic and perceptual aspects of texture, can be approached in different ways. This section reviews the methods that have been used in children to (1) characterize texture preferences and acceptance, (2) measure oral tactile perception, and (3) measure tactile responsivity in relation to their texture preferences/acceptance. Studies included in this section are derived from former sections of the review.

6.1. Texture preferences and acceptance

Measuring food texture preferences or acceptance in children at different ages is important for understanding the texture progression in children’s diets. Results are also useful for exploring the association of texture rejections with child characteristics and other background information, such as dental status, eating behaviors, personality traits, and sociocultural influences. Table 1 summarizes the tools or techniques used in research on children’s food texture preferences or acceptance. An overview of consumer or intervention studies that investigated specific textural parameters governing children’s food acceptance is given in Table 2.

Table 1. Methods for measuring texture preferences and acceptance in children.

Reference	Method(s)	Texture parameters/food investigated	Population	Research aim(s)	Measurement(s)
(Lundy et al. 1998)	Observational techniques	Pureed, lumpy, and diced apple products	Infants aged between 6 and 12 months (n = 12) and toddlers aged between 13 and 22 months (n = 12)	To determine whether food texture preferences differ during infancy and toddlerhood To determine whether experience with textures influenced infants’ texture preferences	Children were videotaped when fed with apple products of different textures. Experimenters coded for children’s texture preferences (facial expression, vocalization, head and body movement), food acceptance, and the number of chewing cycles.
(Demonteil et al. 2019)	Observational techniques	Purees, double textures, cooked pieces, sticky and hard foods	Infants at 6 months (n = 24) and children at 12 months (n = 25) The 6-month-old infants were observed at 6, 8, and 10 months; the 12-month-old children were observed at 12, 15, and 18 months.	To measure the evolution of food texture acceptance and feeding behaviors between 6 and 18 months To investigate the link between acceptance and chewing behavior from a developmental perspective using a longitudinal set-up	Different food textures were offered to children when they were developmentally ready, including purees, double textures, cooked pieces, and sticky and hard foods. Children were videotaped when fed with different textured foods, which were offered in 3 spoons. Experimenters developed a behavioral grid to code for acceptance (ability to process and swallow the foods), feeding behaviors (sucking and chewing), oral discomfort (gagging, coughing/risk of choking, difficulties with the foods), and sensory behaviors shown before intake. Parents assessed children’s food liking on a 10- point scale.
(Tournier et al. 2021)	Observational techniques	Smooth and hard pieces (carrot, chicken, pasta), double texture (carrot pieces in puree), and finger foods of different hardness (raw cucumber, raw apple, and cheese)	Infants at 8 months (n = 60)	To study the effect of providing parents with recommendations on textured food introduction between 8 and 15 months on children’s experience with and acceptance of textured foods	Parent/child dyads were randomly assigned to receive current French recommendations on food texture introduction (control) or receive extended recommendations and monthly counseling (intervention). Children’s food texture acceptance was assessed pre- and post-intervention in laboratory settings, and children were videotaped when fed with different textured foods. Experimenters coded for different acceptance-related behaviors (refuse, spits out, swallow) and other feeding behaviors using a behavioral grid (Demonteil et al. 2019). Parents assessed children’s food liking on a line scale.
(Blossfeld et al. 2007)	Observational techniques; intake measurement	Purred and diced cooked carrots	Infants at 12 months (n = 70)	To determine whether there were differences in food texture acceptance within infants of the same age To study the behavioral, demographical, or experiential factors associated with observed differences in texture acceptance	Infants were videotaped when their mothers fed carrots of different textures. Experimenters coded for the number of spoons accepted or refused and the amount of carrots consumed by the infant. Mothers evaluated food liking on a 9-point hedonic scale after feeding.
(Nederkoorn et al. 2018)	Intake measurement	Three types of strawberry desserts: yogurt with pieces, smooth yogurt, and jelly	Children aged 3–10 years (n = 66)	To test whether tactile exposure through touching a non-food texture by hand could increase the acceptance of food with the same texture	Children were randomly assigned to play with a colorless and odorless jelly with their hands (tactile exposure group) or play a board game (control group). They were then asked to eat the three different strawberry desserts. Food acceptance was measured as the number of spoons of the dessert eaten by a child.
(Tournier et al. 2020)	Parental report	188 food-texture combinations representing three texture levels: (1) purees, (2) soft small pieces, and (3) hard/large pieces and double textures	Children aged 4–36 months (n = 2999) and their mothers	To report the evolution of food texture acceptance with age To study the individual factors associated with acceptance among children of a given age	Mothers completed an online survey listing various food-texture combinations representing three texture levels. They reported on the children’s acceptance of each offered combination, which was then calculated into a global food texture acceptance score (TextAcc), indicating their ability to eat different textured foods.
(Yamanaka et al. 2009)	Self-reported measure	Japanese foods categorized into 10 levels of hardness	Schoolchildren aged 7–12 years (n = 347)	To examine the relationship of texture preferences to bite force and occlusal contact area in Japanese elementary school children	Children completed a questionnaire on texture preferences and had the bite force and occlusal contact area measured during a dental health checkup. Children indicated their preferences for 15 food items in like ⁄dislike form.
(Lukasewycz and Mennella 2012)	Self-reported measure	Pictures of 20 food pairs differed in hardness or particle content	Children aged 7–10 years (n = 52) and their mothers (n = 46)	To explore the relationship of food texture preferences, assessed by a forced-choice questionnaire, to lingual tactile acuity and other factors such as age and food rejection behavioral traits (neophobia/picky eating)	Children completed a forced-choice questionnaire consisting of pictures of 11 food pairs varied in hardness (hard vs. soft) or 9 food pairs varied in particle content (with particles vs. no particles). Children scored 1 for choosing the hard/with-particles food or 0 for choosing the soft/no-particles food in each pair. The hardness and particle preference ratios were calculated for each child by dividing the total scores by the number of choices made in each textural dimension.
(Laureati et al. 2020)	Self-reported measure	Pictures of 17 food pairs differed in hardness or particle content	Schoolchildren aged 9–12 years (n = 610)	To develop and validate a child-friendly tool to explore individual differences in texture preferences in school-aged children in Europe To explore the association between children’s texture-liker status, background variables, and consumption frequency of healthy foods	Children completed the Child Food Texture Preferences Questionnaire (CFTPQ), which consisted of food pairs differing in hardness (hard vs. soft) or particle content (particulate vs. smooth). A CFTPQ index was calculated for each child based on their choices for all valid food pairs. Children were categorized into ‘soft texture-likers’ or ‘hard texture-likers’ based on the total distribution of the index (25^th and 75^th percentile as the cut-off). The test-retest reliability of the CFTPQ was assessed by a subsample of children within two months.
(Chow et al. 2022)	Self-reported measure	Pictographic drawings of 12 food pairs differed in hardness or particle content	Schoolchildren aged 7–10 years (n = 97)	To develop a forced-choice pictographic method to measure texture preferences in children To assess the validity of the method with the provision of actual food stimuli and questionnaire re-testing	Children completed a forced-choice pictographic questionnaire, which consisted of drawings of 6 food pairs differing in hardness and 6 food pairs differing in particle content. Individual response patterns were segmented into preference clusters for each textural dimension. A subgroup of children performed a paired preference test using actual food stimuli corresponding to 6 food pairs, while another group of children was re-tested with the questionnaire. Children’s consistency in food choice between questionnaires and tasting and the questionnaire repeatability was assessed.

Studies are grouped according to the research methods (observational techniques, intake measurement, parental report, and self-reported measures), and studies within each group are arranged chronologically by publication year.

Table 2. Summary of studies investigating the role of texture in governing food acceptability in children.

		Method			Food stimuli
References	Intake	Interview/focus group	Scales/ranking	Tasting	Visual image	Food types tested
Infant and toddler:
(Birch et al. 1998)	×			×		Fruit and vegetables
(Remijn et al. 2019)	×			×		Fruit and cereals
Preschool:
(Poelman and Delahunty 2011)			×	×		Vegetables
(Donadini, Fumi, and Porretta 2012)			×	×		Vegetables
(Donadini, Fumi, Vanoni, et al. 2012)		×	×	×		Dairy
(Donadini, Fumi, and Porretta 2013)			×	×		Fish
(Werthmann et al. 2015)	×			×		Dairy
Preschool and school-aged:
(Zeinstra et al. 2007)		×		×	×	Fruit and vegetables
(Zeinstra et al. 2010)			×	×		Vegetables
(Estay et al. 2019)			×	×		Vegetables
School-aged:
(Baxter et al. 1998)		×	×		×	Vegetables
(Baxter et al. 2000)		×	×		×	Vegetables
(Rose et al. 2004a)			×	×		Meat
(Rose et al. 2004b)			×	×		Meat
(Jervis et al. 2014)		×			×	Cereals
(Kildegaard et al. 2011)			×		×	Dairy and fruits
(Laureati et al. 2017)			×	×		Fruit
(De Moura 2007)		×			×	French cuisine
(Sandvik et al. 2020)			×	×		Cereals

6.1.1. Observational techniques

Observational techniques use trained individuals, such as experimenters or parents, to make qualitative judgments on children’s texture preferences. These methods were used in texture research with infants and toddlers since young children until 18 months of age cannot express their liking or preferences for foods adequately (Guinard 2000). In these studies, children were served different food textures of research interest, e.g., pureed, lumpy, or diced foods; eating sessions were videotaped in the laboratories or their homes. Intake or coding of eating behaviors, such as facial expression, head, and body movement, food acceptance or refusal, and chewing and swallowing behaviors, were used to indicate preferences or acceptance. (Blossfeld et al. 2007; Demonteil et al. 2019; Lundy et al. 1998; Tournier et al. 2021). Parents or experimenters may also assess children’s liking of food textures to supplement the behavioral evaluations.

Observational techniques generate rich and in-depth information for understanding the progression of food texture acceptance during the first few years of life and evaluating the effectiveness of intervention in promoting food texture introduction. However, these methods are labor-intensive and time-consuming, and only a limited number of subjects can be investigated at a time.

6.1.2. Self-report measures

Self-report forced-choice questionnaires were developed as rapid tools to measure texture preferences among school-aged children. By presenting multiple pairs of pictures or drawings of foods that varied in hardness (hard vs. soft) or particle content (with particles vs. no particles), children selected their preferred texture variant in the food pairs (Laureati et al. 2020; Lukasewycz and Mennella 2012; Chow et al. 2022). Children’s texture preferences could be analyzed based on their responses to individual food pairs (Yamanaka et al. 2009; Chow et al. 2022) or calculated into a total score or index for the questionnaire (Laureati et al. 2020; Lukasewycz and Mennella 2012).

Questionnaires can be administrated to a large number of children or applied in a cross-cultural context. They are cost-effective methods to segment children according to their texture preference status. However, except for Chow et al. (2022), the validity of these tools to children’s texture preferences in reality, measured with the provision of actual foods, has not been reported. To accurately capture children’s texture preferences, the reliability and validity of these questionnaires need to be critically evaluated.

6.1.3. General food acceptance methods

In consumer or intervention studies, texture can be investigated as a part of sensory attributes that govern food acceptability (Table 2). Most studies used sensory profiling techniques to identify whether specific texture attributes were drivers for product acceptance, while others compared the role of texture with taste and appearance on food acceptance. Methods used in these studies to assess children’s hedonic responses varied largely because of differences in research purpose and the age range of children being investigated. These studies provide information for the food industry to improve product quality and for health professionals to design interventions to promote healthy eating behaviors in children.

6.2. Oral tactile perception

Several studies have measured oral tactile perception in children to determine the relationship between hedonic and perceptual aspects of food texture. Four psychophysical tools are available for measuring oral tactile perception in children: the letter recognition task, the grating orientation test, the point pressure test, and the oral stereognosis test. These tools provide objective measurements for individuals’ perceptual differences in detecting or discriminating oral tactile stimuli. Studies that applied these tools with children are presented in Table 3.

Table 3. Psychophysical assessments to measure oral tactile perception in children.

Reference	Method(s)	Population	Research aim(s)	Measurement(s)	Specific modifications for children
(Lukasewycz and Mennella 2012)	Spatial acuity test	Children aged 7–10 years (n = 52) and their mothers (n = 46)	To verify the use of a modified letter-identification task to study lingual tactile acuity in children and examine age-related differences in sensitivity. To explore whether lingual tactile acuity and other factors such as age and food rejection behavioral traits relate to food texture preferences.	Participants used their tongues to identify a series of English alphabet letters embossed onto Teflon strips. Letters used in the task (A, I, J, L, O, T, U, W) were manufactured in 7 sizes (2.5, 3, 4, 5, 6, 7, and 8 mm in letter height). Using a staircase approach, participants who correctly identified the letter were presented with another letter one size smaller and vice versa. The task ended when they reached eight performance reversals, and the lingual acuity threshold was calculated as the mean of the letter size of the reversals.	Children were not blindfolded during testing Children were presented with a paper listing all the alphabet letters in the same font as the stimuli to assist the identification task The task was shortened by ending after eight reversals in performances
(J. Lee et al. 2022)	Spatial acuity test	Children aged 5–12 years (n = 1817)	To determine the suitability of the grating orientation task to measure oral tactile acuity in children To examine associations of oral tactile acuity with children’s characteristics	Participants used their tongues to identify the orientation and sureness of oral grating squares. The stimuli varied in groove widths (0.5, 0.75 or 1.25 mm) and square size (1 or 2 cm^2). The squares were presented in random order 8 times in total. Each time participants indicated the orientation of the groove (horizontal/vertical) and the sureness of their responses (sure/unsure). Lingual tactile acuity was calculated as R-index.	Children were familiarized with the task and procedures using a 1.0 mm grate before the actual testing Children were encouraged to use their hands or referred to visual diagrams with vertical and horizontal lines to indicate the orientations
(Appiani et al. 2020)	Spatial acuity test; point pressure test	Children aged 6–13 years (n = 145) and their parents (n = 65), and a separate sample of adult (n = 70)	To verify the use of Von Frey filaments and gratings orientation tests in children and examine age-related differences in sensitivity. To investigate the relationships between lingual tactile sensitivity, food neophobia, preferences, and consumption of foods with different textures	Von Frey filaments test: Participants reported the presence or absence of stimulus touching their tongues. The test used 2 filaments with a 0.008 g and 0.02 g force. The test was conducted 24 times for each filament, 12 times having true touch and 12 times without touching. Each time participants indicated whether they felt the touch (yes/no) and their degree of sureness (sure/unsure). Lingual tactile sensitivity was calculated as R-index. Gratings orientation test (GOT): Participants used their tongues to identify the orientation of square blocks. The stimuli consisted of 6 types of squares varying in groove/bar widths (0.2, 0.25, 0.5, 0.75, 1.00 and 1.25 mm). Each grating was presented 6 times. Each time participants indicated the orientation of the blocks (horizontal/vertical) and their degree of sureness (sure/unsure). Lingual tactile acuity was calculated as R-index.	Children were not blindfolded during testing The tests were created into a game-like situation and split into two rounds to prevent children from losing attention Children were only tested with three different-sized groove widths to reduce fatigue In the GOT, experimenters showed the two possible orientations of the grooves before testing and encouraged children to use their hands to indicate the orientations In the von Frey filaments test, experimenters made a demonstrative touch on children’s hands before testing to prevent children from mistaking the filament for a needle
(Gisel and Schwob 1988a)	3D shape recognition test	Children aged 5–8 years (n = 86)	To investigate the development of oral stereognosis skills in children	Participants identified whether two oral forms were the same or different when placed consecutively into their mouths. The test used 10 plastic forms as stimuli: 2 triangular, 3 circular, 3 rectangular, and 2 biconcave forms (approximately 1.5 × 1.0 × 0.3 cm) and 50 randomly chosen pairs for testing. After feeling the first oral form in the mouth for 5 seconds, participants extruded that form and were given the second form for 5 seconds to compare. Oral stereognosis ability was analyzed as the error in confusing oral forms of different sizes and contours.	Children were instructed to keep their eyes closed during testing

Studies are grouped according to the psychophysical tools used (spatial acuity, point pressure, and 3 D shape recognition).

6.2.1. Spatial acuity tests

Lingual tactile acuity refers to the capacity of the tongue to extract and process spatial information of a stimulus with the sense of touch (Essick et al. 2003). It can be measured by the letter identification task and the grating orientation test. The letter identification task requires subjects to use their tongues to identify a series of English alphabet letters embossed onto Teflon strips (Essick, Chen, and Kelly 1999; Essick et al. 2003). These alphabet letters vary in size, and lingual tactile acuity is determined by the size of letters that can be accurately identified. Subjects who correctly identified a letter would be presented with a letter one size smaller, whereas subjects who incorrectly identified the letter would be presented with a letter one size larger. Lukasewycz and Mennella (2012) modified the test to a child-friendly version to compare the lingual tactile acuity between children (7 to 12 years old) and their counterpart adults. The task was shortened and ended when subjects reached eight reversals in performances (letters correctly or incorrectly identified).

Alternatively, the grating orientation test requires subjects to identify the orientation (horizontal or vertical) of oral grating squares when placed on their tongues (Van Boven and Johnson 1994). These squares are engraved with ridges (gratings) of different sizes of grooves/bars. Lingual tactile acuity is determined by calculating the R-index, which refers to an estimated probability for subjects to correctly identify a target stimulus (the signal) when they are presented pairwise with a second stimulus (the noise) (Lee and Van Hout 2009). Appiani et al. (2020) compared the lingual tactile acuity in children (6 to 13 years old) with adults using three sizes of gratings. A further study by J. Lee et al. (2022) used a large sample of children with over 1800 participants to identify the best discrimination oral grating square (1 cm² grating square in 0.5 mm groove width) to assess lingual tactile acuity.

The spatial acuity tests can be adjusted according to the developmental needs of school-aged children. For example, children may feel uncomfortable with being blindfolded. Both tests could be run without blindfolding, and experimenters ensured that children closed their eyes or did not look at the stimuli. The grating orientation test may be more suitable than the letter identification task for younger children or in a cross-cultural context, as the letter identification task requires subjects to have certain abilities to recognize the alphabets (Cattaneo et al. 2020).

6.2.2. Point pressure test

Von Frey filaments are classical tools for measuring sensitivity to tactile pressure (Levin, Pearsall, and Ruderman 1978). In food texture research, the measurement consists of multiple touch detection tasks where subjects report the presence or absence of the stimulus on their tongues. Using filaments of different sizes (i.e., applied force), lingual tactile sensitivity is determined by calculating the R-index. The von Frey filaments test was successfully used in school-aged children (Appiani et al. 2020). Since children may mistake the filament for a needle, the study suggested making a demonstrative touch on children’s hands to assure them before running the test in their mouths. Also, younger children have a limited attention span. Creating the assessment into a game-like situation or splitting the tasks into sub-sections can help children stay focused during the test (Appiani et al. 2020).

6.2.3. 3D Shape recognition test

Oral stereognosis is defined as the ability to recognize and discriminate oral forms (Jacobs, Bou Serhal, and van Steenberghe 1998). The test is often used for neurophysiological and dental research. Oral stereognosis involves the synthesis of multiple oral sensory inputs in higher brain centers, thus being a more complex process than the simple detection of oral tactile stimuli (Jacobs, Bou Serhal, and van Steenberghe 1998). Gisel and Schwob applied an oral stereognosis test with children (5 to 8 years old) and correlated the results with their chewing abilities (Gisel and Schwob 1988a, 1988b). In the test, multiple pairs of oral forms were placed in the oral cavity; children indicated if the shape and size of the forms were identical or not. The stereognosis ability was determined based on the accuracy of their responses.

6.3. Tactile responsivity

An overview of measurements to access children’s tactile responsivity and its relation with texture preferences/acceptance is given in Table 4. Most studies have used different versions of the Sensory Profile (i.e., the Infant/Toddler Sensory Profile or the 38-item Short Sensory Profile) for measurements (Coulthard, Harris, and Fogel 2016; Smith et al. 2005; Cappellotto and Olsen 2021).

Table 4. Methods of measuring oral tactile processing/texture responsivity in children in relation to their texture preferences/acceptance.

Reference	Method(s)	Population	Research aim(s)	Measurement (s)
(Smith et al. 2005)	Parental report	Children aged 3–10 years (n = 62) and their parents	To explore whether tactile defensive children exhibited picky eating habits	Tactile defensive children were screened with the Short Sensory Profile (Dunn 1999); non-tactile defensive children matching the characteristics of the experimental group were recruited for comparison. Parents completed a questionnaire on their feeding habits, children’s eating behaviors, acceptance of texture, temperature, and smell of foods.
(Coulthard, Harris, and Fogel 2016)	Parental report	Infants aged 5 months (n = 61) and their mother	To test the hypothesis that infants’ early reactions to a vegetable may be associated with sensory processing, particularly tactile over-responsivity To explore whether the relationship between sensory over-responsivity and vegetable consumption was moderated by the age of the infant	Mothers completed the Infant/Toddler Sensory Profile (Dunn and Daniels 2002) to report their children’s sensory responsivity.They fed their children pureed cooked carrots in a controlled eating session, and the amount of carrots consumed by children was assessed.
(Cappellotto and Olsen 2021)	Parental report	Children aged 6–13 years (n = 70) and their parents	To investigate the association between children’s texture preferences, sensory processing, and food neophobia levels To explore whether parent’s texture preferences were correlated with those of their children	Mothers completed the Short Sensory Profile (Dunn 1999) and the Food Neophobia Scale (Pliner 1994). Both children and parents completed the Child Food Texture Preference Questionnaire (Laureati et al. 2020) to indicate their texture preferences.
(Ross et al. 2022)	Parental report	Study 1: Parents of typically developing children aged between 4–36 months (n = 328) Study 2: Typically developing children (n = 635) and children with Down syndrome (n = 149) aged between 11–58 months Study 3: Typically developing children (n = 113) and children with Down syndrome (n = 111) aged between 11–58 months Study 4: Typically developing children aged between 5–12 years (n = 987) and their parents	To develop a short survey completed by parents in nonclinical settings to provide a categorization for food texture sensitivity in children To evaluate the distribution of children as texture sensitive (TS) or non-texture sensitive (non-TS) and the predictive validity of the questionnaire to explain rejections of specific food textures To examine the relationship between TS/non-TS categorization and lingual tactile sensitivity, and food fussiness	Parents answered five questions derived from the Short Sensory Profile (Dunn 1999) and a feeding behavior assessment scale to measure their children’s food texture sensitivity. For each question, 1 point was assigned to texture-sensitive responses; children were classified as texture-sensitive (TS) if they scored 2 points or above. Datasets from a home-use test with textured foods and lingual tactile sensitivity measured by grating orientation task (J. Lee et al. 2022) were used to evaluate the validity of the 5-question survey.
(Nederkoorn, Jansen, and Havermans 2015)	Behavioral measure	Children aged 4–10 years (n = 44) and their parents	To test the hypothesis that children who were more sensitive to touch and disliked the feel of various tactile stimuli would be more dismissive of tactile stimulation in their mouth, therefore being more selective in eating	Children’s touch responsivity was measured by offering 10 tactile stimuli to touch with their hands, and their selective eating was measured by offering 10 food items to taste. The tactile stimulus and food items varied in textures or temperatures, and children indicated their liking of each stimulus on a 5-point scale. The score for tactile appreciation ranged from 5 to 50, and the score for selective eating ranged from 0 to 50 (i.e., children scored 0 when they refused to taste the foods).

Studies are grouped according to research methods (parental reports or behavioral measures), and studies within each group are arranged chronologically by publication year.

The Sensory Profile measures children’s sensory processing abilities in different domains, such as tactile, taste and smell, and visual and auditory (Dunn 1999). The tools have been validated in both typically and atypically developing children (Ermer and Dunn 1998; Tomchek and Dunn 2007). Parents responded to each question on a 5-point scale anchoring from always (1) to never (5). Results on the tactile domain (e.g., avoids going barefoot, especially in sand or grass; reacts emotionally or aggressively to touch) are often investigated with texture rejections in children. A high score in the domain suggests a normal level of sensory processing, whereas a low score indicates tactile over-responsivity.

More recently, a study used five specific questions derived from the Short Sensory Profile and feeding behavior assessment scales to measure food texture sensitivity in children (Ross et al. 2022). The predictive validity of the tool was evaluated with a home-use test, which compared texture-sensitive and non-texture-sensitive children on their reactions to various food textures.

Apart from parental reports, Nederkoorn, Jansen, and Havermans (2015) used a behavioral test to measure children’s appreciation of tactile stimuli. Children were offered 10 tactile stimuli that varied in textures and temperatures to touch with their hands, e.g., sandpaper, jelly, velvet cloth, ice cubes, etc. They rated their liking of each stimulus on a 5-point smiley scale, giving a total score of tactile responsivity ranging from 5 to 50. The results were analyzed with children’s acceptance of foods differing in textures and temperatures.

Overall, different methods or tools are available for characterizing texture preferences and acceptance in children. Depending on the child’s age, the prevailing methods of characterizing food texture preferences in children include observational techniques and self-report questionnaires. However, only a few studies were conducted using each of these methods. Additional research with preschool children is needed to understand how texture preferences develop across the infancy/toddlerhood and school-aged periods. This would also help identify when the acceptance of a particular texture parameter (e.g., lumpiness) occur.

Oral tactile perception has been assessed among school-aged children by adapting psychophysical tools into child-friendly versions. However, many studies did not find evidence to support the association between food texture preferences and oral tactile sensitivity (see Section 4.2 for details). Oral tactile perception may be more relevant to dimensions other than food texture preferences, such as masticatory performance (Engelen, Van Der Bilt, and Bosman 2004; Gisel and Schwob 1988b; Hirano, Hirano, and Hayakawa 2004; Shupe, Wilson, and Luckett 2019). Thus, it is questionable to continually explore the use of these tools in children to relate their oral tactile perception with food preferences. Alternatively, assessments on children’s tactile responsivity suggested a potential link with texture aversion (see Section 3.4 for details). While most of these measures rely on parental reports, further studies may develop equivalent tools for children to investigate tactile responsivity and texture aversions in childhood directly.

7. Conclusion and prospects

Texture of food plays a profound role in children’s food preferences and acquiring healthy eating habits. Food texture acceptance is learned from an early age as a development matching between children’s ability of food oral processing (i.e., mastication and swallowing) and the complexity of the texture (i.e., ease of manipulation). For young children, timely exposures to foods with varying textures (e.g., hard, lumpy, and gritty) facilitate the acceptance/preference development of those textures. However, individual differences in oral motor skills, food rejection behaviors, sensory processing abilities, and cognitive abilities can affect how food textures are perceived, thus contributing to the development of texture preferences. In particular, food neophobia, picky eating, and tactile over-responsivity are identified barriers for children to accept more diverse food textures. A link between oral tactile perception and texture aversion has not been established.

There are mixed results on the impact of mechanical textural properties on children’s food acceptance. In contrast, children often reject geometric textural properties presented as the presence of pieces or bits in foods. It is suggested that such texture aversions may be linked to the child’s autonomous reflexes dealing with the control of chewing and swallowing, which would require repeated and/or conscious training.

The methodology for studying texture preferences in children mainly concerns intake measures and observational techniques in infants and self-administrated questionnaires in older children. Children’s oral tactile perception has been assessed by adapting psychophysical tools into child-friendly versions. There is still scope for developing validated and relevant test tools for texture acceptance studies in children. Furthermore, there is a need for a more integrated approach to fully understand inter-individual differences and factors underpinning food texture acceptance and rejection.

Disclosure statement

The authors report that there are no competing interests to declare.

Additional information

Funding

This work was supported by Arla Foods amba as part of the industrial PhD program of the Innovation Fund Denmark (grant number: 0153-00158B).