Oral Processing: Implications for Consumer Choice and Preferences Lina Engelen The University of Sydney, Sydney, NSW, Australia
Chapter Outline 1. Introduction 2. Mastication 2.1 Oral Function 2.2 Tongue 2.3 Oral Cavity/Bite Size 2.4 Food Characteristics 3. Saliva 3.1 Volume of Saliva 3.2 Composition of Saliva
401 403 404 405 406 407 408 408 410
3.2.1 α-Amylase410 3.2.2 Lingual Lipase 412 4. Receptors and Perception 412 4.1 Texture 413 4.2 Eating Styles 414 5. Swallowing 415 6. Conclusion 416 References 416
1. INTRODUCTION Eating is an essential part of our daily life. It is a routine process of obtaining the energy and nutrients required for living and well-being, but one that includes sensory appreciation and enjoyment of food. Not only what we eat, but how we eat it is important. Even though this process is perceived to be routine, its governing principles are highly complicated and are still not fully understood. Eating is no longer seen as a simple process of food breakdown, but is recognized as a highly sophisticated process of human responses (physiological, psychological, and neurological) to the changing physicochemical properties of the food. Although food remains for only few seconds in the oral cavity, a great deal of structural and physicochemical transformations occur during this short period (Chen, 2015). Oral processing is the first interaction of our body with food in the digestive system and is a dynamic process Methods in Consumer Research, Volume 1. https://doi.org/10.1016/B978-0-08-102089-0.00016-9 Copyright © 2018 Elsevier Ltd. All rights reserved.
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that involves a sequence of highly coordinated oral actions, including muscle activities, jaw and tongue movements, breakdown, and mixing with saliva, that contribute to preparing food for safe swallowing. Simultaneously, chemicals move to olfactory and taste receptors, food particles interact with oral surfaces, and the net result is an evaluation of taste, aroma, and texture. If we like what we sense, we take another bite; if not, we expectorate before swallowing. Oral processing is hence a very complex physiological process in which decision-making, often unconscious, and preparation of a safe bolus occur simultaneous with information flow to and from the central nervous system (Koç, Vinyard, Essick, & Foegeding, 2013). Oral processing involves many factors: the physiological characteristics of the individual performing the chewing action, such as facial anatomy, gender, age, personality type, time of day, or dentition status, as well as the properties of the food being chewed, such as hardness, moisture content, fat content, food portion size, and food structure (Bornhorst & Singh, 2012). A number of excellent reviews have comprehensively summarized oral processing, the mechanics and interactions between the food and the oral tissues, food perception, and social aspects of eating (Chen, 2009; Koç et al., 2013; Pereira & van der Bilt, 2016). During the past decades, sophisticated methods have been used to characterize, model, and mimic oral processes and integration of perceptions (Chen & Engelen, 2012), but as oral processing is a set of complex processes, it is hard to mimic the real process in vivo (Chen & Lolivret, 2011). The characteristics of the individual, the eater, will affect the way food is manipulated orally and, consequently, the way food is perceived. Although it seems like everyone with a mouth, teeth, and a tongue will interact in a similar way with the food consumed, this is only partly the case. Large differences in oral physiology parameters exist, such as masticatory performance, swallowing threshold, amount and composition of saliva, oral flavor, tactile sensation, and pain sensation as well as the mechanical and chemical breakdown and binding of food particles in the mouth. These differences could be explained by things like status of the dentition, age, gender, etc., which have effects on interactions between the food and the receptors/sensors in the mouth. As a consequence, oral processes as well as food perception may vary largely among individuals. There are many types of consumers, in terms of both previous experiences, expectations, and reasons for eating and how they interact with and break the food down, hence their oral processing. Consumers can be divided into segments owing to characteristics that could have an influence on various aspects of their oral processing, such as children, the elderly, denture wearers, and people with swallowing dysfunction (see Fig. 16.1), which in turn is likely to influence their perception of food. By understanding the processes that take place in the mouth while eating, we can gain a better understanding of the sensations and perceptions of the food and how these relate to individual differences and consumer segments.
Oral Processing: Implications for Consumer Choice and Preferences Chapter | 16 403 Masticatory function
Swallowing Bite force
Oral receptors / sensitivity
Type of manipulation
Saliva amount and composition Age Dentition Disease Drugs Culture Gender FIGURE 16.1 The various food oral factors and how they are related to selected consumer segments that could influence food oral processing.
2. MASTICATION When food is first taken into the mouth, it is placed on the tongue and compressed against the palate; then solid foods are transferred to the postcanine teeth, so that a size reduction can be made to the food. During this process, food particle size, textural features, lubrication, flavor, and taste will be sensed. If the food is perceived as noxious, it will be expectorated at this stage. Once food particles are reduced to a size comfortable to swallow, they will be moved selectively to the back of the oral cavity to form a bolus. Chewing is an important process of digestion and is meant to prepare the food for swallowing and further processing in the digestive system. In the mouth, the food is subjected to several mechanical and chemical processes. Taste and texture of the food are perceived and have their influence on the chewing process. The water in the saliva moistens the food particles, whereas the salivary mucins bind masticated food into a coherent and slippery bolus that can
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be easily swallowed. For more detailed information on the oral management of food, please refer to van der Bilt (2012). A range of muscles are involved in oral processing such as the labial (lip) and buccinator (cheek) muscles, the suprahyoid muscles of the oral floor, and the muscles of the soft palate. The muscles involved in mastication are the masseter, temporalis, and pterygoid muscles, which are all innervated by the trigeminal nerve and move the mandible (lower jaw) in different directions (Pereira, 2012). Masticatory muscle activation and coordination determine the direction of jaw movement and occlusal control (Herring, 2007). The thickness of the masticatory muscles affects bite force (Castelo, Pereira, Bonjardim, & Gavião, 2010). Numerous studies have been performed to quantify various oral factors to get an impression of the functioning of the mouth. For a detailed description of mastication and swallowing, see reviews by Mosca and Chen (2016) and Pereira and van der Bilt (2016).
2.1 Oral Function The teeth are an important part of the masticatory system, mechanically breaking down food into smaller particles during chewing. Masticatory function is an objective measure of how well a person can chew, break down, and mix and knead food (Speksnijder, Abbink, van der Glas, Janssen, & van der Bilt, 2009), while masticatory ability is a subjective measure of a person’s oral function (van der Bilt, 2012). People with high masticatory function are able to break food down effectively during chewing. They also tend to swallow food particles of smaller sizes. The rate of the breakdown of food depends on many anatomical and physiological variables. Many people have decreased oral function due to loss of teeth, periodontal disease, or malocclusion. A clear relationship has been found between dental state and masticatory performance, where about 50% of the variance in masticatory performance could be explained by the number of occlusal units. Individuals with natural dentition have better masticatory function than those who wear dentures or implant-supported prostheses (van Kampen, van der Bilt, Cune, Fontijn-Tekamp, & Bosman, 2004). Denture wearers often have a loss in masticatory function and need more chewing cycles to break the food down (Fontijn-Tekamp et al., 2000; van Kampen, van der Bilt, Cune, & Bosman, 2002). However, often, these individuals do not increase their number of chewing cycles accordingly, but rather swallow larger pieces; hence, declining masticatory function due to compromised dentition is responsible for swallowing poorly chewed food (Fontijn-Tekamp, Vanderbilt, Abbink, & Bosman, 2004). Chewing requires muscle activity to exert the forces needed for mechanical breakdown of the food. The force exerted by the teeth, which is a parameter that characterizes the chewing behavior, varies greatly among individuals. Maximum bite force is often used to evaluate oral function, because the bite force has a large influence on masticatory function (Hatch, Shinkai, Sakai,
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Rugh, & Paunovich, 2001). Gender and age are predictors of bite force, in that male individuals have on average a higher bite force than women, and bite force tends to decrease with higher age in adults (van der Bilt, Tekamp, van der Glas, & Abbink, 2008). Paphangkorakit and Osborn (1997) reported values of biting force of the central incisors ranging between 110 and 370 N. These variations are due to several factors such as gender, age, ethnicity, health status, and dental state. The force exerted by molars is a lot higher (500–800 N). Full denture wearers, on the other hand, have an average maximum bite force of 77–135 N (Van Der Bilt, 2011). The number of chewing cycles needed to prepare a bite of food for swallowing is called the swallowing threshold. This threshold varies largely among people, as illustrated by the number of chewing cycles varying from 17 to 110 in a group of 87 healthy adults chewing 9 cm3 of peanuts (Engelen, FontijnTekamp, & Van der Bilt, 2005). Although there is a large variation among individuals for the same food, the swallowing threshold seems to be quite constant within a person for the same food. Intraindividual thresholds have been seen to vary depending on the characteristics of the food, but they are correlated. Fontijn-Tekamp et al. (2004) suggested that there are “slow and fast swallowers.” Hence, some individuals consistently use a high number of chewing cycles, while others swallow the food after only a few chewing cycles. The swallowing threshold is, however, not associated with the individual’s chewing performance, in that a person with a high masticatory function is not necessarily a “fast swallower” (Fontijn-Tekamp et al., 2004). The swallowing threshold rather seems to depend partly on physiology, but also on contextual parameters and personal characteristics of the individual. Females have been shown to chew more times and spend more time eating meals than males (Isabel et al., 2015; Park & Shin, 2015). Keeping the food for a shorter amount of time in the mouth will have implications on the breakdown, mixing, transport, and, hence, perception of the food. A person with a compromised dentition and low bite force will have difficulty fragmenting the food and as a consequence may have a different perception compared to a person without these issues (Pereira & van der Bilt, 2016). The number of teeth tends to decrease with age, leading to impaired masticatory function.
2.2 Tongue The tongue plays a crucial role throughout the whole process of food oral management. In addition to having a role in sensory perception and speech, it works as a mechanical device for in-mouth food manipulation and transportation (Mosca & Chen, 2016). The dynamic pattern of tongue movements includes displacement in the horizontal and vertical directions, rolling, and folding. With these movements, the tongue aligns the food with the teeth during mastication, collects and aggregates food particles, and transports the bolus for swallowing.
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The movements of the tongue are coordinated with jaw opening and closing (Okada, Honma, Nomura, & Yamada, 2007). It also works as a major sensory organ to sense the temperature, taste the flavor, and perceive the texture of the food. By sensing particle size and bolus lubrication, the tongue sorts the food fragments that are ready for transport, propelling and squeezing them into the oropharynx. Large fragments are returned back for further comminution/size reduction. The sensitivity of the tongue and palate is extraordinary in comparison with most other parts of the body. The mouth is very accurate and sensitive to objects, especially when these objects are between occluding teeth. In a study (Engelen, Prinz, & Bosman, 2002), we found that the perceived size of spheres in the mouth was dependent on the actual size of the sphere and not on the weight. This suggests that the size of the object is derived from the number and spatial arrangement of stimulated receptors, and not from the magnitude of the pressure exerted by the object. There is, however, evidence for an oral size illusion, but the direction of the illusion depended on the size of the object (Crutchfield, Mahoney, Pazdernik, & Rivera, 2016). The illusion has been shown to be reduced when a plastic palate is used to minimize palatal input, hence demonstrating the importance of receptors in the palate and tongue to sense size (Engelen et al., 2002; Engelen, Van der Bilt, & Bosman, 2004). Oral sensorimotor function performs an important role in mastication, in that palatal coverage was associated with reduced masticatory efficiency (Kumamoto, Kaiba, Imamura, & Minakuchi, 2010), but an opening in the palatal portion of a denture seemed to improve oral perception (Koike, Ishizaki, Ogami, Ueda, & Sakurai, 2011). Local anesthesia of the preferred chewing side illustrated that intraoral sensory input can affect both food breakdown and mixing capacities (Yoshida, Fueki, & Wakabayashi, 2015). De Wijk et al. (2008) added polystyrene particles to vanilla custard desserts and found an increase in temporalis and throat muscle activity with increasing particle size for 10 subjects. These results are expected, given that particle size reduction is a requirement for bolus formation and swallowing. Particles sizes as small as 2 μm can be detected when rubbing the tongue against the palate (Engelen, Van der Bilt, Schipper, & Bosman, 2005) if they are hard and irregular. Particles of this size were also found to affect the sensation of roughness and smoothness (Engelen, de Wijk, et al., 2005). Particles that are softer and rounder, however, need to be larger to be detected.
2.3 Oral Cavity/Bite Size The size of the oral cavity varies significantly from person to person. It has been shown that, for adult males, a normal mouthful has the volume of around 30.5 ± 10.1 g water, while for adult females, it is around 25.2 ± 8.1 g water (Medicis & Hiiemae, 1998). Although not an exact measure, this an estimation of the volume size of the oral cavity; however, when eating, we do not tend to
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fill our mouths completely. The volume of each bite size is dependent on the characteristics of the food and the eater (gender). It appears that the amount of food for each mouthful decreases from liquid foods to soft solids and further to hard solids. This is illustrated by males taking an average bite size of 18 ± 4.9 g of banana, while females take 13.1 ± 4.0 g. The bite size becomes smaller for peanuts, 5.5 ± 2.3 g for males and 3.6 ± 1.4 g for females (Medicis & Hiiemae, 1998). The reason for this decrease is probably the increased difficulty of oral breakdown and oral manipulation for solid foods. It has been found that the swallowing threshold linearly increased as a function of the volume of the food in the mouth, hence a larger number of chewing cycles was needed before the food was ready for swallowing (Fontijn-Tekamp et al., 2004).
2.4 Food Characteristics The influence of food characteristics on the oral process has been extensively studied. Hwang et al. (2012) showed that the particle size distribution of the ready-to-swallow bolus depended essentially on food type and the mechanical properties of the food, such as hardness, cohesiveness, and adhesiveness when banana, tofu, cooked rice, and biscuits were eaten by healthy subjects. Immediate muscle response is necessary to maintain a constant chewing rhythm, while food resistance varies from cycle to cycle. More jaw muscle activity and longer burst durations have been observed when harder and tougher foods are chewed (Agrawal, Lucas, Prinz, & Bruce, 1997). The average muscle activity needed to chew firm foods like carrot and peanuts was twice as high as the muscle activity needed for cake or cheese (van der Bilt, Engelen, Abbink, & Pereira, 2007). Characteristics of the food also have a large influence on the number of chewing cycles needed to prepare the food for swallowing. In a group of 87 healthy adults, the number of chewing cycles varied from an average of 17 cycles for a portion of cake to 61 for an equal-sized portion of carrots (Engelen, Fontijn-Tekamp, et al., 2005). With increasing portion size, the number of chewing cycles and time in the mouth increased (Gaviao, Engelen, & Van der Bilt, 2004). Dry and hard products needed more chewing cycles before swallowing than moist and soft products, since a dry food needs more time in the mouth to allow for secretion and mixing with sufficient saliva. Confirming this, the addition of small volumes of water (5 or 10 mL) resulted in a lower number of chewing cycles and less total muscular work (Van der Bilt et al., 2007; Pereira, de Wijk, Gavião, & Van der Bilt, 2006). In addition, participants with a lower salivary flow rate seemed to benefit more strongly from the addition of water than individuals with a higher flow rate. Similarly, spreading butter on dry products just prior to ingestion reduced the number of chewing cycles needed (Engelen, Fontijn-Tekamp, et al., 2005), confirming the need to form a cohesive bolus prior to swallowing.
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The period between the time at which food enters the mouth and when it is swallowed (oral residence time) is very short for liquids that require no manipulation prior to swallowing; longer for semisolids (up to 8 s) (Chen & Lolivret, 2011), which might require some manipulation or mixing with saliva; and longest for solids that require mechanical breakdown, wetting, mixing, and processing into a bolus that is safe to swallow. Owing to the longer residence time for solids, we have more time to sense the texture and the changes in texture as the structure is altered. Much of the input we receive about solid food’s texture is derived from the jaw forces and sounds created while chewing. Important processes taking place during the processing of solid foods are fracturing during the biting by incisors, grinding and chewing by the molars, water uptake and the rate thereof, lubrication, bolus formation, swallowing, and clearing (van Vliet et al., 2009).
3. SALIVA Saliva is paramount to the eating process and to oral health. It would appear that saliva is involved at every step of oral processing, not only as an agent of breakdown, but also as a wetting agent and lubricant, providing surface smoothness and weak interparticle adhesive forces (Lillford, 2016). During the main parts of the day when we are not eating, saliva, as a seromucous coating, lubricates, protects, and maintains oral tissues, acting as a barrier against irritants as well as aiding in the prevention of harmful microbial invasion and growth. During eating, when food has been placed in the mouth, saliva takes on a number of other responsibilities, such as the initial breakdown of food, transport to make taste compounds and odorants available to the respective receptors, buffering of acid compounds to protect the teeth and mucosa, and facilitating manipulation of food in the oral cavity and the mixing and binding of food for safe swallowing, as well as clearance of food residues after swallowing (Carpenter, 2012; Humphrey & Williamson, 2001; Ranc et al., 2006). Saliva is hence expected to be of importance for the perception of food stimuli in the mouth. There is some evidence that both the volume and the composition of saliva present in the mouth while eating are of importance.
3.1 Volume of Saliva The saliva flow rate varies largely among individuals, but also within an individual, depending on the state of the person (rest, activity, eating) as well as the type of stimulation. We studied a total of 266 healthy adult subjects (Engelen, Fontijn-Tekamp, et al., 2005) and found flow rates ranging from 0.16 to 3.8 mL/ min, with a mean rate of 0.45 ± 0.25 mL/min for unstimulated saliva flow and 1.25 ± 0.67 mL/min for stimulated saliva flow. In contrast, in patients suffering from dry mouth syndrome the flow rate at rest was 0.61–0.90 mL/15 min, and the gum-stimulated flow rate was 6.34–15.35 mL/15 min (Hayashida et al.,
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2015). This low level of saliva flow negatively affects teeth and gum health and also swallowing and taste perception. Saliva secretions are reflexively controlled by the autonomic nervous system (Proctor & Carpenter, 2007), particularly by chemoreceptors in the taste buds and mechanoreceptors in the periodontal ligament. Salivation can also be induced by the smell or thought of food. Keesman, Aarts, Vermeent, Häfner, & Papies (2016) showed increased salivary responses to cues from food that was either looked at or simulated to eat. As predicted, foods increased salivation compared to a nonfood control object, especially when they were attractive or sour, and simulation of eating increased flow rates even further. A number of factors are associated with large interindividual differences in saliva flow rate, such as gender, age, and health status. Greater salivary flow rate is observed in males compared to females, often attributed to the larger salivary glands generally found in males (Inoue et al., 2006). The aging process has been related to the sensation of a dry mouth. Resting whole salivary flow rates and resting and stimulated submandibular and sublingual saliva have been found to be significantly lower in older adults, regardless of medication usage (Affoo, Foley, Garrick, Siqueira, & Martin, 2015; Smith et al., 2013). Oral dryness is one of the most frequent drug-induced oral side effects (Scully & Bagan, 2004). Antihypertensive drugs are known as potential causes of reduced salivary flow (de Matos et al., 2010). It has also been shown that diabetic and hypertensive subjects had reduced submandibular and sublingual saliva flow (Dodds, Yeh, & Johnson, 2000). Cancer patients often undergo chemotherapy or radiation, which is frequently coupled with side effects such as dry mouth, tooth loss, and chewing difficulty, resulting in a negative influence on the masticatory process (Caputo et al., 2012). The presence of saliva is crucial during eating. Liedberg and Owall (1991) found that the number of chewing cycles until swallowing, and hence time in the mouth, increased after inducing oral dryness, illustrating the important role of the amount of saliva in chewing and swallowing. However, in a study from our lab (Engelen, Fontijn-Tekamp, et al., 2005), it was shown that the moment of swallowing was only weakly correlated with the individual’s salivary flow rate. Hence a person with a high salivary flow rate does not necessarily swallow food after fewer chewing cycles than a person with a low flow rate. As a consequence, individuals with a relatively high flow rate are used to swallowing better moistened food than individuals with less saliva. This is likely to influence the perception of food. In another study (Engelen, de Wijk, Prinz, Janssen, et al., 2003), we found that the addition of a fluid (water, α-amylase, or saliva) affected rated texture attributes of semisolids. Increased saliva volume has also been shown to facilitate chewing of dry foods, but not fatty and wet products (Pereira, Gavião, Engelen, & Van der Bilt, 2007). Pereira et al. (2007) found that addition of fluid significantly decreased muscle activity and swallowing threshold for dry products, such as melba, cake, and peanuts, but it did not influence the chewing of fatty (cheese) and wet products (carrot). In that study,
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participants with lower salivary flow rate seemed to benefit more strongly from the addition of water than individuals with a higher flow rate. Sensory texture perception of a range of attributes was analyzed relative to salivary flow rate (Engelen, de Wijk, Prinz, Van der Bilt, et al., 2003). No association between subjects’ normal flow rates and their sensory perceptions was found. A subject with a higher flow rate did not rate the food any differently from one with a lower flow rate. This finding suggests that we are used to the amount of saliva present in our mouths while eating. When this amount was changed by adding extra fluids the resulting perception changed for semisolids as well as solid foods (Engelen, de Wijk, Prinz, Janssen, et al., 2003; Pereira et al., 2006).
3.2 Composition of Saliva Saliva contains 99% water and a number of other components, such as minerals; enzymes, e.g., α-amylase; a large number of proteins, such as proline-rich proteins, and mucins; and glycoproteins with a range of functions, e.g., lubrication and antibacterial actions. A number of these constituents of saliva, including water, affect the structure and perhaps also the perception of food. Collected saliva was analyzed for composition, and the salivary components, protein concentration, buffer capacity, mucin level, and α-amylase activity varied considerably among subjects, but also within subjects, as a result of different means of stimulation (Engelen et al., 2007). There are a number of chemicals and enzymes present in saliva that may have some function in oral processing and the resulting perception of the food masticated and manipulated in the mouth. It has been shown that saliva gives emulsions an enhanced sensory feeling (Silletti, Vingerhoeds, Norde, & van Aken, 2007). Variations in salivary components have been found to correlate with sensory perception of a number of flavor, mouthfeel, and after-feel attributes in semisolids (Engelen et al., 2007). For example, wine and tea are often described as astringent. The most commonly described mechanism for astringency of compounds such as tannins and polyphenols is their precipitation of proteins, such as proline-rich proteins; however, alternative explanations for the sensation of astringency or “puckering” have been suggested, including a disruption of the salivary protein layer and loss of salivary lubrication (Gibbins & Carpenter, 2013).
3.2.1 α-Amylase Salivary α-amylase is very common in saliva and contributes to food digestion through the hydrolysis of starch to glucose and maltose (Zakowski & Bruns, 1985). α-Amylase is produced and secreted by glands in two different locations of the body/digestive tract: the parotid glands in proximity to the jaws, secreting into the oral cavity, and the pancreas, secreting into the small intestine. Although the pancreatic secretion of α-amylase into the small intestine is considered the
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most important in terms of in vivo starch breakdown, the parotid secretion into the oral cavity has been shown to be of importance to both starch and structural breakdown (Prinz, Janssen, & de Wijk, 2007) as well as the oral perception of texture of starch-containing food (Engelen et al., 2007; Engelen & Van der Bilt, 2008). Some claim that the oral transit time of food is too short for amylase to have any significant effect on breakdown; however, Woolnough, Bird, Monro, and Brennan (2010) analyzed spat-out boluses of starchy solids and found that about 13% of bread starch, 20% of potato starch, and 27% of wheat starch were hydrolyzed and transformed into smaller carbohydrate molecules during oral processing. It has also been shown that the interaction of α-amylase with starch ingredients produces an almost immediate effect on hydrolysis, and it was found that, in less than 10 s of mixing with the saliva, custard showed a close to 10-fold decrease in its viscosity (Janssen, Terpstra, de Wijk, & Prinz, 2007; Prinz et al., 2007). That this change was also perceived by participants was evident in a number of studies (Engelen, de Wijk, Prinz, Janssen, et al., 2003; de Wijk, Prinz, Engelen, & Weenen, 2004) that asked trained sensory panelists to rate foods eaten on a range of textural attributes. The perceived thickness was affected by the concentration of α-amylase available to the starch during eating. When additional amylase was added at the same time as ingestion the perceived thickness was decreased, and conversely, when an amylase inhibitor was added to the sample, the rated thickness was higher (de Wijk et al., 2004). For starch-based foods, the addition of an α-amylase solution resulted in changed sensations of a range of attributes, such as melting, thickness, and creaminess (de Wijk et al., 2004). This effect was attenuated when an α-amylase inhibitor was added and not present when carboxymethyl cellulose, which is not broken down by α-amylase, replaced starch as a thickener in the food. These results challenge the assumption that semisolids are kept too short a time in the mouth for α-amylase to play a role in food breakdown and perception. There is also a large variation in α-amylase concentration among people, in which the measured concentration in parotid-stimulated saliva varies by a factor of about 10 (Arhakis, Karagiannis, & Kalfas, 2013). This individual variation is likely to have an effect on the way food texture such as thickness is perceived by consumers during eating. α-Amylase has also been suggested to prevent bacterial attachment to oral surfaces and to enable bacterial clearance from the mouth (Bosch, de Geus, Veerman, Hoogstraten, & Nieuw Amerongen, 2003). Mandel and Breslin (2012) found that individuals with high endogenous salivary amylase concentration had more AMY1 gene copies (coding for α-amylase) and lower postprandial blood sucrose levels after ingesting a starch solution than individuals with a low amylase concentration. They suggested that these individuals might be better adapted to ingest starches, whereas lowamylase individuals may be at greater risk for insulin resistance and diabetes if chronically ingesting starch-rich diets.
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3.2.2 Lingual Lipase Lingual lipase is another enzyme present in the mouth. It is an enzyme secreted into the oral cavity by Von Ebner’s serous glands located on the tongue in proximity to foliate and circumvallate papillae (Hamosh & Burns, 1977; Kulkarni & Mattes, 2014). Lipases have the function of breaking down dietary fats, by hydrolyzing ester bonds between the fatty acid and the glycerol moieties of dietary triacylglycerol to produce mono- and diacylglycerols and free fatty acids. Rats have a continuous release of lingual lipase from the circumvallate papilla and a fast generation of free fatty acids (Kawai & Fushiki, 2003) and express a reduced preference for triacylglycerols but not for free fatty acids in the presence of a lipase inhibitor. This suggests that lipolysis leading to the release of fatty acids in the oral cavity contributes to oral detection of fats in rodents. Although the presence of functional lipase in human saliva has not yet been clearly confirmed, studies suggest evidence of lipase activity during oral processing. In a study conducted by Neyraud, Palicki, Schwartz, Nicklaus, and Feron (2012), lipase activity, along with lipocalin and salivary flow, was shown to correlate with fat perception and liking. It is, however, not completely known if lingual lipase actually affects the oral perception or detection of fatty acids in humans. In a study by Kulkarni and Mattes (2014), participants were asked to masticate and sensory rate five high-fat foods varying in physical state and fatty acid composition (almond, almond butter, olive oil, walnut, and coconut) at the rate of one chew per second with and without lipase inhibitor orlistat. The study suggests that lingual lipase is active during oral processing of some high-fat foods, possibly those that require higher oral processing effort. Although these researchers did not find that lingual lipase contributed to oral fat detection, in another study by Pepino, Love-Gregory, Klein, and Abumrad (2012) the presence of lipase appeared to have an influence on fatty acid thresholds, as these thresholds increased with the addition of lipase inhibitor during testing.
4. RECEPTORS AND PERCEPTION The oral receptors are the first step in perceiving food and manipulating food safely and effectively in the mouth. The importance of the mouth as a receiver for tactile and chemical stimuli is very strong in infancy, but it also remains strong in adulthood. Three classes of receptors are present in the mouth, each sensitive to one modality of physical energy: chemical (taste), mechanical (tactile sensations), and thermal (hot/cold). For a detailed summary of the oral receptors and their functions, see Engelen (2012). The mechanoreceptors mediate sensations of touch and proprioception; the thermoreceptors sense the temperature of the body and the objects that we come in contact with, nociceptors signal pain, and chemical receptors respond to taste and smell. All these types of receptors contribute to the total perception of the food that we ingest. To control oral motor behaviors such as biting, chewing, speech, and oral manipulation, the brain relies on sensory information from the receptors in and around the mouth (Trulsson & Johansson, 1996). These receptors serve two
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major functions during eating: transmitting information about the texture of the food in the mouth and providing sensory feedback. This is important in the control of oral functions, such as the position of the tongue, manipulating the position of food in the mouth and guiding the food bolus out to the right positions for chewing and other manipulation; preventing biting of the tongue and cheeks; and swallowing without choking. Proprioceptors sense the static position and movement of the various body parts. Awareness of the position of the tongue, needed to pick up food from one part of the mouth and bring it to another, as well as the muscle movements when seamlessly switching chewing from one side of the mouth to the other relies on the input from proprioceptors. The ability to feed-forward information about the perceived properties of the food from one bite to the next is crucial in conveying appropriate motor commands to the jaw muscles about timing and bite force needed before the teeth make contact with the food. This information is gathered by proprioceptors in conjunction with periodontal receptors. Periodontal receptors are mechanoreceptors located in the periodontal ligament, by which teeth are attached to the alveolar bone, that respond to loading of the teeth and provide information about the direction in which the force is applied (Trulsson, 1993). The sensation of pain (nociception) serves an important protective function; it warns of injury that should be avoided and responds to high-energy noxious stimuli, such as some chemicals, tissue damage, and temperatures outside of the physiological range. Individuals with pain hyposensitivity often burn and chew their tongue and lips and, as a result of undetected damage, lose the tips of their fingers. Some chemicals that in high concentrations could potentially cause tissue damage, such as capsaicin (chili) and eugenol (cloves), produce sensations, when eaten in low concentrations, that are considered pleasant, or even necessary for the flavor of some foods. This sensation is often referred to as trigeminal sensation, as it is transduced by free nerve endings of afferents from the trigeminal nerve. Carbonation in sparkling drinks is another example of a trigeminal sensation. There is a great variation in sensitivity among people for these sensations (Green & Lawless, 1991), and conversely, some individuals show a high preference, while others generally avoid high concentrations in their food and drinks. There seems to be a strong cultural aspect to the absence or presence of this preference, in that some cuisines are famously “hot.” Although there is likely to be a genetic aspect to the sensitivity and preference, it is probable that it is also based on exposure, as low-frequency users of chili rate the burn as more intense than frequent users (Prescott & Stevenson, 1995).
4.1 Texture Texture is defined as the attribute of a substance resulting from a combination of physical properties and perceived by the senses of touch, sight, and hearing (Jowitt, 1974). It is thus the perceived multimodal representation of a food’s structure. Texture is most affected by and related to oral processing of food.
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Please refer to our previous work (Engelen & Van der Bilt, 2008; Engelen & de Wijk, 2012) for more details about oral processing and texture perception. Texture is a strong driving force for the aversion of food (Scott & Downey, 2007). The awareness of food texture is often subconscious, in that we mainly notice it when it is different from expectation. When our expectations are met, we focus more on the taste and aroma of the food. However, if the expectations are breached, then the texture becomes a reason to reject or dislike the food, such as a tough steak, stale bread, or wilted lettuce. The preferred textures are strongly dependent on the specific type of food, but in general, crunchy and juicy are preferred over stringy and slimy (Szczesniak, 2002). Texture also has strong cultural ties, in that crispness seems to be universally positive (perhaps because it is a sign of freshness for vegetables and fruit), whereas attributes such as sticky and slimy are preferable in some cultures, while the same attributes are reasons for food rejection in others. Texture is, hence, greatly important for the appreciation of food, but also for recognition. This has been tested in several studies in which food structure has been altered by blending the food (Schiffman, 1977; Engelen, unpublished). Only 14%–50% of foods, such as grapes, apples, and lettuce, were correctly identified based on taste and aroma only. The receptors involved in the sensations of various texture attributes can be found all over the oral cavity and include tactile, thermal, and receptors for irritant stimuli (for more information, see Engelen, 2012). Given the complex nature of texture, it is likely that the perception of texture arises from a combination of sensory input from several types of receptors. As the concentration of these receptors and the sensitivity to these stimuli vary among people, combined with a range of ways to orally process food and expectations, it is evident that there will be large variations in how texture is perceived by different individuals.
4.2 Eating Styles There are large individual differences in how food is processed orally. These differences can be measured by various means, such as instrumentally, observational, or introspectively. Most characterizations have been done for solids, involving chewing. In an introspective study, participants were asked to describe chronologically how the food was manipulated after they placed it in their mouth (Engelen & van Doorn, 2000). Four basic eating styles for semisolids were identified and designated as simple (50%), in which the food was placed on the tongue and swallowed straight away; taster (20%), in which a series of sucking movements against the palate was made after having moved the food backward along the tongue; manipulator (17%), which describes a wide variety of behaviors including the use of the tongue, palate, teeth, and cheeks; and tongue (13%), in which the tongue was engaged in a series of backward and sideways movements against the palate. This experiment shows that there is
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a wide variety of strategies developed by people, but that it also is possible to segment them into groups of similar processing behaviors. Subsequent experiments showed that individuals seem to have adopted the style of eating that provides them with a maximized sensation, and the sensation of different attributes requires different movements and manipulations (de Wijk, Engelen, & Prinz, 2003). An individual’s normal mastication behavior, which exhibited the largest diversity and complexity of movements, typically resulted in the most intense sensations of flavor and mouthfeel. This suggests that consumers aim to maximize their food sensations. As a result thereof, foods that are not liked should result in mastication behaviors aimed to minimize food sensations. By observing and surveying more than 500 people, Jeltema, Beckley, and Vahalik (2015) found that most people fit predominantly into one of four types of mouth behaviors (crunchers, chewers, suckers, and smooshers) and will choose foods that satisfy that type of mouth behavior. These preferred foods differ particularly in texture and how they can be manipulated in the mouth. For example, a cruncher is more likely to choose or enjoy foods such as raw vegetables and chocolate with nuts, while a smoosher prefers semisolids such as custard and chocolate that melts in the mouth. The authors posit that, hence, it is the mouth behavior that is the primary driver of food choice and product acceptance and rejection (Jeltema, Beckley, & Vahalik, 2016).
5. SWALLOWING After the food structure is sufficiently transformed by the oral actions, the last stage of oral processing, swallowing, takes place. Swallowing is characterized by the transport of the food bolus from the oral cavity into the esophagus and stomach. The cohesiveness of the food bolus has been suggested to identify the moment of swallowing (Prinz & Lucas, 1997). Others have shown that the swallowing threshold is not defined solely by particle size and hardness. The physical parameters springiness, adhesiveness, and cohesiveness and the sensory attribute stickiness were suggested to be important factors in triggering swallowing of a bolus (Peyron et al., 2011). In addition, researchers have also included the capacity of easily deforming, stretching, and flowing to be required for a comfortable swallow with minimal oral effort (Chen & Lolivret, 2011). It seems that the moment to initiate swallowing is decided by an integrative process that combines the perception of several bolus properties. For healthy individuals, swallowing occurs as a natural step during the eating process. However, for a disadvantaged group comprising elderly individuals and patients recovering from operations or suffering from dysphagia, swallowing can provoke choking or suffocation. Each person thus seeks to obtain a critical, optimal particle size before swallowing to avoid adverse effects such as breathing in food particles; alternatives are to avoid difficult foods or to swallow insufficiently prepared foods.
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6. CONCLUSION The oral processing of food is important not only for breaking food down and binding it for safe swallowing and the first step in the metabolism of food, but also for creating a dynamic environment crucial for sensing and perceiving the characteristics of food so important for the enjoyment of food. Oral processing consists of a number of highly specialized steps and interactions between several oral structures, such as the tongue, the teeth, and a range of receptors. It is evident that there can be large variations among people, depending on factors such as age, gender, dentition, drugs, and personality, in their oral processing, as well as the resulting perception. There is evidence to suggest that consumers can be more or less firmly segmented into groups depending on the way they process and manipulate food in their mouth, in that oral processing is likely to be important for food preference and choice. It is not implied that people would choose to eat only foods that easily allow a certain mouth behavior; rather, it is more an indication of foods that are more often chosen or are enjoyed more by consumers of that particular oral processing or eating style. This strengthens the idea that although flavor is an important driver of food choice, the way the product can be manipulated by the consumer can have great implications on how the product is liked and chosen.
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