Chapter 1: The Human Oral Cavity and Oral Processing of Foods

The oral processing of foods is a multistage dynamic process occurring in the oral cavity that simultaneously encompasses sensory perception, mechanical destruction, biochemical changes, and colloidal destabilization of consumed food. The brain and human neural system are efficient at coordinating the sensory perception of food.^1,2  Ingested food experiences various processes, such as mechanical deformation and flow, decrease in size, saliva lubrication/wetting, temperature changes leading to phase modification, changes in consistency and rugosity, food particle clustering to form a bolus that can be swallowed readily, and residue formation in and coating of the mouth.^3,4  The oral processing of foods is a concurrent process of sensory perception and food breakdown.^5–7  The nervous system and the brain regulate the entire process.⁸  Oral processing of foods can be categorized into three classes: oral physiology, oral physics, and oral psychology.¹ 

Oral physiology examines the different responses and alterations that occur in the oral cavity while chewing and swallowing food, including jaw movement, forces generated by the orofacial muscles, salivary secretion, tongue sliding, tongue pressure against the hard palate, and stimulation of receptors such as the mechanoreceptors and thermoreceptors present in the mouth.^9,10  Oral physics examines how applied physical and mechanical forces cause food to deform, fracture, and undergo microstructural changes, and also how those changes affect the oro-sensory perception of the texture of food.^11,12  With the aid of the neurological system’s feedback processes, sensory psychology focuses on the perception and measurement of sensory aspects, including the sensation of mouthfeel properties.^13,14  Figure 1.1 depicts various categories of food oral processing.

Recent research in the field of sensory psychology has been focused on neurological facets to comprehend how sensory stimuli and brain signals interact.¹⁵  Some studies provide a deeper understanding of the brain’s “reward pathways” which are activated during mastication and texture perception and are captured with the help of functional magnetic resonance imaging (fMRI).^16–18  Nevertheless, some previous investigations were focused additionally on the motor-control signals required to fracture solid foods that are soft and hard in texture.^19,20  This chapter attempts to provide an overall and comprehensive picture of the anatomy and physiology of the oral cavity, various stages and dynamic changes that occur during the oral processing of food, and different sensorial, analytical, and instrumental techniques used to understand the complex and intricate mechanisms of oral processing of foods and sensory perceptions. The chapter gives a brief overview of anatomical and physiological features of the oral cavity and oral processing of different food matrices, and explains the significance of oral processing studies and oro-sensory perception in detail.

This is the starting point of the gastrointestinal tract (digestive tract). Major functions of the oral cavity include mastication, swallowing, gustation (taste perception), olfaction (flavor perception), and texture perception, apart from other activities such as breathing and social and behavioral functions (speaking, smiling, yawning).²¹  The oral cavity consists of the upper and lower lips anteriorly, the cheeks covering both sides laterally, the top covered by a soft and hard palate, and the tongue at the base. Further, teeth are embedded in the dentition supported by bones, the maxilla (upper jaw) and mandible (lower jaw), which are located adjacent to each other in the oral cavity.²²  The oral cavity’s end section is a pharyngeal arch structure made up of the hyoid bone, muscles, and larynx cartilage.²³  The hard and soft palate is covered by a thin layer of mucosa, lips that aid in the ingestion of food, and the pharynx that links the oral and nasal cavities to the food pipe (esophagus), which transports the food bolus to the stomach. Figure 1.2 illustrates the various parts of the human oral cavity.

Humans have diphyodonty (two generations of teeth) such as the primary deciduous dentition and the secondary permanent dentition. There are around 28–32 teeth in normal adults, which can be classified into four types: incisors, canines, premolars, and molars.²²  Incisors (cutting teeth) are thin with blade-like crown structures, which are mainly used for cutting the ingested food. Canines have a single cone-shaped cusp on their crowns; they are known as piercing/tearing teeth. Premolars are bicuspid teeth that are unique to permanent dentition. Molars consist of more than one cusp, usually four or five, on a broader and flatter biting surface. The molariform teeth, which include premolars and molars, are referred to as grinding teeth.²²  The tooth structure can be divided into the upper crown and lower root; the latter is embedded in the jaw bone and held firmly by the gums. The upper crown is the visible region of teeth that is covered with enamel, which is the toughest tissue in the human body and comes in direct contact with food during mastication.²⁴  Teeth are the major component used for the size reduction of the ingested food (chewing). The mastication efficiency of an individual is greatly dependent on their teeth. Thus a significant decline in mastication performance is noted in individuals with removable dentures, implant-supported prostheses, and the loss of post-canine teeth.^25,26  Further, studies have found that individuals do not chew their food properly after losing posterior teeth and end up swallowing larger food particles due to poor mastication function.²⁷ 

The tongue is the muscular organ that is attached to the floor of the mouth and occupies the major space within the oral cavity. The tongue gets its support from the hyoid bone from the bottom and it is attached to the inner surface of the mandible near the midline.²²  The predominant work of the tongue is to deliver the food/food fragments appropriately between the teeth for mastication, aid in lubrication with saliva, and assist the transportation of food to the back of the oral cavity to facilitate swallowing. Further, the tongue plays a crucial part in the ingestion of food.²¹  Movement of the tongue during mastication aids in the migration of food from the front of the mouth to the back of the throat, which supports swallowing and the formation of a bolus. The superficial surface of the tongue consists of many taste buds that can detect five major tastes: salty, sweet, bitter, sour, and umami. Further, chemoreceptors and mechanoreceptors on the surface of the tongue sense the chemical nature and mechanical properties of food, respectively.²⁸ 

The dorsum of the tongue is convex and it can be subdivided into two parts: the palatal part (anterior two-thirds) and the pharyngeal part (posterior one-third). The tongue is divided symmetrically into two halves by a median sulcus and it terminates in a depression called the foramen cecum. The sulcus terminalis is a shallow V-shaped groove that is the junction where the palatal and pharyngeal parts meet. Massive amounts of papillae cover the palatal portion of the tongue, whereas the pharyngeal portion is smoother and has more lymph follicles and muciparous glands (lingual tonsil).²⁹  Circumvallate papillae, foliate papillae, fungiform papillae, and filiform papillae are the four different kinds of papillae. Taste buds are present in the first three papillae, but not in the filiform papillae. However, it is responsible for transducing touch, temperature, and nociception.²⁴ 

The circumvallate papillae are situated ahead of the sulcus terminalis and foramen cecum, creating an array on either side in the shape of an inverted “V”. Foliate papillate are slit-like folds of mucosa containing over 100 taste buds, which are positioned on both the sides of the tongue at the lateral border. The palatal region of the tongue (anterior two-thirds part) is covered with copious amounts of filiform papillae, which look like dwarf-whitish, hair-like conical elevations. The fungiform papillae are isolated deep reddish mushroom-shaped structures that are interspersed between the filiform papillae.²²  The fungiform papillae are most numerous at the tip of the tongue; in addition they are also present on both sides and also scattered unevenly and frugally over the dorsum. Filiform papillae are keratinized whereas fungiform papillae are non-keratinized. Additionally, taste buds, which are essential components of the gustatory sense, are dispersed unevenly throughout the mucous membranes of the mouth and tongue.^21,30 

Saliva is the lubricant found in the oral cavity and it comprises around 98% water. The remaining 2% consists of other components such as enzymes (α-amylase and lingual lipase), electrolytes (calcium, bicarbonate, magnesium, sodium, chloride, potassium, phosphate), proteins (enzymes, albumin), glycoproteins, immunoglobulins, mucins, and other peptides. It also contains minor amounts of urea, glucose, and ammonia.^31,32  The major role of saliva is to provide lubrication and buffering, maintain tooth integrity, protect against bacteria, assist taste perception, and aid in digestion.^33,34  Figure 1.3. details various significant functions of saliva. The pH of saliva can range between 5.3 and 7.8. In general, humans will produce around 600 mL of saliva per day. The secretion of saliva is higher during mastication and less pronounced during sleep.³⁵  There are three major salivary glands in the oral cavity, namely parotid, submandibular, and sublingual glands, which are responsible for more than 90% of the saliva secreted, the remainder of the saliva being secreted by minor glands.²²  The details of various salient features of salivary glands are provided in Table 1.1.

Many muscles present in the head and neck region are responsible for oral processing functions. However, muscles such as the masseter, temporalis, and medial and lateral pterygoid are jointly known as “muscles of mastication”.²²  The lateral and medial pterygoid muscles are positioned deeper within the infratemporal fossa, while the masseter and temporalis muscles are situated superficially on the face. The muscles of mastication work jointly to move the mandible in various directions at the temporomandibular joint during the mastication process. The mandibular division of the trigeminal nerve supplies all the muscles involved in mastication with their innervation. The muscles present in the suprahyoid region include digastric, mylohyoid, and geniohyoid muscles, which are positioned on the floor of the oral cavity. These muscles run from the hyoid bone to the mandible in the form of sheets of parallel fibrous tissues. Further, the digastric muscle is closely associated with the functions of mastication muscles as it is the principal muscle involved in jaw movement. The muscles present in the suprahyoid region function as jaw intruders and jaw elevators (mouth opening and closing muscles).²⁹ 

The muscles of the tongue are classified into two types, intrinsic and extrinsic muscles. The three fiber groups of the intrinsic muscles, namely transverse, longitudinal, and vertical, are confined to the substance of the tongue and alter its shape.²¹  The extrinsic muscles are divided into four groups, the genioglossus, hyoglossus, styloglossus, and palatoglossus. The extrinsic muscles originate from areas outside the tongue such as the skull and hyoid bone, and then spread into the body of the tongue. The extrinsic muscles are for the bodily movement of the tongue.²²  The soft palate is held in place with the help of fibrous palatine aponeurosis. There are pairs of four muscles, the levator veli palatini, the palatoglossus, the tensor veli palatini, and the palatopharyngeus uvulus muscles. All four of these muscles join at the aponeurosis and they are responsible for alterations in the shape and position of the soft palate. Further, all four muscles play a crucial part in opening and shutting of the airway during swallowing, with the levator veli palatine serving as the soft palate’s chief elevator.²³ 

The oral cavity is packed with many nerve endings which are responsible for the significant sensitivity towards various stimuli such as tactile, taste, and olfaction. Through the use of very specific structures that serve as receptors for appropriate stimuli, the body has several groupings of sensory cells that are extremely sensitive to particular stimuli. The main function of receptors is to monitor and collect vital information regarding various stimuli (temperature, chemical composition) of a given environment and convey it to the central nervous system (CNS). Further, the complex coded data of stimulus in the CNS are then processed in the brain into a perception that is then utilized to produce suitable actions. The receptors must be both selective and sensitive to the corresponding stimulus. The receptors in the human body can be broadly classified into four, based on their sensitivity to physical energy, namely electromagnetic, thermal, chemical, and mechanical. The oral cavity contains all types, except for photoreceptors sensitive to electromagnetic radiation.³⁶  The receptors of the oral cavity aid in the sensing and perception of food that is consumed. The oral cavity consists of taste, olfactory, and tactile receptors (mechanoreceptors, proprioceptors, thermoreceptors, and nociceptors) to sense the various attributes of food consumed. Table 1.2 summarizes various significant features of oral receptors for the sensory stimuli taste, tactile and olfaction.

The three cranial nerves, namely trigeminal, glossopharyngeal, and facial nerves, innervate the oral region and transmit sensory information to the brain. The chorda tympani branch of the glossopharyngeal nerve and facial nerve innervates the taste buds in the posterior one-third and anterior two-thirds of the tongue, respectively. Thus, both of them are responsible for the sensation of taste.³⁶  Further, tactile sensations (proprioception, temperature, and nociception) are detected by the trigeminal nerve (three branches), which innervate maximum areas of the orofacial region. The receptors of the oral cavity convey tactile information to the CNS with the aid of the trigeminal somatic sensory system.³⁷ 

In addition, the pons, pontomedullary, and rostral medulla are the levels at which the trigeminal, chorda tympani, and glossopharyngeal nerves, respectively, enter the brainstem. The spinal nucleus and the principal nucleus are the two significant parts of the trigeminal brainstem complex and they are responsible for processing heat and painful stimuli, and mechanosensory stimuli, respectively.²⁴  The sensory signals (taste, vision, olfaction, and touch) from various sensory regions in the human body are conveyed to the complex multimodal regions of the brain, namely the orbitofrontal cortex.^38,39  Further, their sensory signals are processed and combined to form the comprehensive result which is perception.⁴⁰ 

The oral processing of food comprises six stages, namely (A) first bite, (B) size reduction, (C) granulation, (D) bolus formation, (E) swallowing, and (F) residue or mouth coating.⁴¹  During the first bite, the bulk of the food is deformed in the mouth and textural characteristics (mechanical or rheological), including fracturability, hardness, and viscosity, are recognized. Size reduction is characterized by tribological and rheological principles and it occurs as a result of prolonged chewing or grinding. The tribological effect is brought on by the meal’s relative rubbing on the oral surfaces, which leads to the release of oil or moisture from the food, the secretion of saliva from the salivary glands, and dryness. The size reduction stage allows the perception of mechanical features such as gumminess, chewiness, and adhesiveness, and also geometric tribology dominant traits including roughness, grittiness, and powderiness. In granulation due to capillary bridging, saliva causes particles to cluster together. During the bolus formation stage, the granular particles tend to clump together because of the tongue’s shearing and squeezing action against the hard palate, and also due to the inclusion of saliva, which results in the creation of a soft, cohesive bolus that can be swallowed. Different mouthfeel characteristics, including slipperiness, smoothness, and creaminess, can be felt as the bolus forms. The key factors that affect the swallowing process are the bolus’s moisture content, particle size, and cohesiveness. The term “residue” refers to mouth coatings that are still present after swallowing the bolus and they are accountable for mouthfeel qualities such as astringency. Significant features of the oral processing of food are presented in Figure 1.4.

Food is mostly destroyed in the mouth by three complicated mechanisms that occur simultaneously: mechanical breakdown, biochemical and enzymatic interaction, and colloidal destabilization.²  Additionally, they are also the primary reason for the perception of texture in the oral cavity.

Solid foods undergo mechanical breakdown in the oral cavity, which results in a reduction in size and, along with continual salivation, a swallowable bolus consistency is reached. Semisolid and viscous fluid meals are often modified in numerous ways during oral processing, including temperature modification resulting in melting, chemical breakdown, shearing, and reduction in viscosity. Semisolids and high-viscosity liquids, on the other hand, undergo very little or no mastication since they are already in a state that can be swallowed.⁴² 

Mechanical deformation makes it easier to perceive a variety of sensory phenomena, including textural characteristics, bursts of flavor and aroma, and the taste of consumed meals.^43,44  Food is mechanically destroyed as a result of several motions and pressures released by various areas of the mouth cavity. These include the relative sliding speeds and pressures produced by the tongue against the hard palate, and the chewing and biting action of teeth that are embedded in the jaws (incisors assist in cutting, premolars and molars help in fine grinding, and canines support ripping). All solid meals, including both hard and soft foods, are subjected to mastication (biting and chewing), which reduces the size of the food’s particles and turns it into a bolus that is soft and easy to swallow. A non-flowable solid meal is therefore transformed into a flowable bolus by mechanical breakdown with the aid of saliva, which supports the transition of the ingested food from the mouth to the gastric chamber through the esophagus for digestion. This is crucial because texture perception during oral processing is dynamic. Food undergoes bulk deformation and fracture at the earliest stage of oral processing, and texture perception can be described as a rheological regime. The impression of texture perception during the later phases of oral processing is mostly a tribology-dominated regime that is substantially influenced by the surface characteristics of the food and oral surfaces, wear and friction at the tongue–food, tongue–food–palate, and palate–tongue interfaces, and the lubricating nature of saliva.⁴⁵ 

Salivary glands in the human oral cavity create a complex heterogeneous, colloidal, clear fluid known as saliva. Saliva is normally composed of 98% water and 2% additional inorganic and organic chemicals including mucin, glycoproteins, amylase, lingual lipase, electrolytes, and antibacterial agents.⁴⁶  There are four distinct structural phases for saliva: components such as salivary micelles and less water-soluble proteins in the salivary network, a mucin–protein continuous scaffold-like network, bacterial, lipid, and epithelial cells, and an electrolyte–water continuous phase.⁴⁷  The colloidal nature of human saliva has been validated using microscopic images of freeze-dried saliva.⁴⁸  In addition to lubricating the mouth and reducing dry mouth and discomfort, saliva also protects oral surfaces such as the teeth and other surfaces from microbial attack. Additionally, saliva also acts as a buffering agent, lubricates solid and semi-solid foods, aids in making a swallowable cohesive bolus, and facilitates the removal of oral residues. Saliva as a lubricant also maintains oral health by protecting the teeth and other oral surfaces from bacterial attack, preventing dry mouth and irritation.^49,50 

Saliva comes immediately in contact with food after ingestion, making it an essential element of oral processing and texture perception. Additionally, the interaction between food and saliva leads to colloidal destabilization, which has a substantial impact on several mouthfeel qualities.⁵⁰  The oro-sensory perception of various mouthfeel qualities such as creaminess,⁴⁹  astringency,⁵¹  and fatty perceptions such as oiliness and greasiness^49,52  is greatly impacted by the colloidal destabilization of emulsions.

Four fundamental mechanisms have been described in relation to the oral destabilization of food emulsions, namely: (1) bridging flocculation, (2) depletion flocculation, (3) salt-induced aggregation, and (4) coalescence that results from mouth shear and saliva incorporation.^50,53  The concepts of bridging and depletion flocculation are the most widely accepted among these mechanisms. Mucin rich in proline is the vital constituent for bridging and depletion flocculation mechanisms to occur in the mouth. Additionally, mucin has a gel-like consistency and is primarily responsible for saliva’s lubricating properties.⁵⁴ 

A study showed how the charge of the surfactant employed to stabilize food emulsions affects the oral stability of emulsions.⁵⁵  In the oral environment, strongly negatively charged emulsions remain completely stable because of strong electrostatic repulsion effects and only experience dilution effects from saliva inclusion. Weakly negatively charged and neutral emulsions undergo reversible depletion flocculation as a result of osmotic pressure generated from their interaction with salivary protein (mucin). In contrast, as a result of bridging flocculation, positively charged emulsions experience rapid phase separation and permanent destabilization in the oral cavity, which is brought on by the attachment of mucins and other large salivary proteins to the emulsion surfaces.⁵⁵  Therefore, the mechanism of colloidal destabilization plays a significant role in the perception of food texture, particularly for emulsion-based foods and for liquid foods such as drinks.^2,56 

Generally, a stable emulsion that has been well distributed will have a creamy or smooth mouthfeel, whereas an emulsion that has been destabilized will generate an altogether different texture impression, demonstrating roughness/dryness.⁴⁸  Emulsions can occasionally have an oily or greasy mouthfeel as a result of the coalescence of oil droplets brought on by severe flocculation.

The decrease in lubrication by oral colloidal destabilization is caused by the accumulation of salivary protein with tannins and polyphenols. Astringency is felt as a result of reduced surface lubrication and increased friction caused by the disappearance of the mucin (lubricating protein).⁵⁷ 

Lingual lipase and α-amylase are the two key enzymes found in saliva that play a vital role in the digestion of lipids and starches, respectively, and also in oral sensory perception. The amylose and amylopectin’s 1-4-linkages of starch molecules are hydrolyzed by the α-amylase to generate maltose sugar. As a result, two important effects are observed when starch-rich foods are broken down in the mouth and come into contact with saliva: oral thinning of the food, which occurs when the food is viscous, and the perception of a sweet taste as a result of sugar synthesis. The oral thinning phenomenon of starchy and sugary foods has been demonstrated in several studies.⁵⁸  The viscosity of custard was reduced by a factor of 10 upon exposure to saliva for less than 10 s.⁵⁹  Additionally in that study, under two in vitro settings, saliva and water, the impact of oral deterioration on two distinct gel types consisting of tapioca starch and whey protein isolate was investigated. A discernible reduction in viscosity was seen in samples with saliva during the course of a normal eating process. Therefore, caution must be exercised when selecting emulsifying agents that are starch-based in the manufacture of food emulsions since they may flocculate upon exposure to the oral environment, which may be induced by the enzymatic breakdown mediated by α-amylase.² 

Lingual lipase is a further significant enzyme found in human saliva. Because of its hydrophobic nature, it can enter fat globules. Triglyceride molecules are hydrolyzed, yielding glycerol and the appropriate fatty acids. According to some research, the presence of free fatty acids is what causes oro-sensory sensations, including the sense of creaminess and fat.⁶⁰  Furthermore, it has been demonstrated that fatty meals are associated with tactile or physical sensations based on data from neural imaging.⁶¹  However, some disagree with this viewpoint, arguing that the level of salivary lipase is low and the quantity of free fatty acids produced to provide a sense of fattiness is very inadequate.² 

The food matrix/structure plays a significant part in the oral processing fate of ingested food. The structure of food is a systematic organization of various constituents such as molecules to form a food product along with multidimensional spatial scales such as nano-, micro-, and macroscopic levels. The structure or matrix of food is immensely intricate, ranging from self-oriented natural foods of plant and animal origin to those that are formulated, predesigned, and produced by food processors. There are various food matrices, such as solid, liquid, gel, colloidal, emulsion, cellular, dense, viscoelastic, porous, and fibrous. The term matrix is commonly used when one component is embedded in another.⁶²  Further, foods can be classified into various categories based on how they are perceived in the human oral cavity (texture). Based on the texture, foods are commonly categorized into hard solids, soft solids, semi-solids, and liquids (thin and viscous).^63,64  Table 1.3 categorizes the food products based on their structure in relation to oral processing, sensory and mechanical properties.

Oral processing is significantly influenced by food structure and type of matrix.⁶⁵  The kind of matrix, filler, and individuals will determine the particle size distribution, average particle size, and breadth of the size distribution.⁶⁶  When food is processed orally, the food matrix breaks down, allowing released chemicals to interact with salivary enzymes.⁶⁷  For example, astringency is produced by the interaction of polyphenols with proline-rich proteins and α-amylase hydrolyzes starch into glucose and dextrin. Additionally, the matrix and microstructure of the food impact the level of interactions. Additionally, during oral processing, the disintegration of the food matrix and hydration assist in releasing taste, flavor, and aroma components that will diffuse into the saliva.^68–70 

Various compounds in the food matrix influence the degree of release and interaction, e.g. the presence of polysaccharides and carbohydrates will reduce the aroma release due to the increase in viscosity of the matrix or interaction with flavor compounds directly.⁷¹  The presence of protein will lead to various molecular interactions such as ionic bonding, hydrogen bonding, and hydrophobic bonding. The presence of lipids will separate the oil- and aqueous-based aroma compounds.⁷²  Further, the increased hardness and chewiness (mechanical properties) of the matrix will result in a prolonged mastication time, increased lubrication, and extended flavor perception with less intensity.⁷³  Recent research on the oral processing of various food matrices is summarized in Table 1.4.

Texture perception is influenced by material characteristics, including the food’s physical state, mechanical and surface properties, microstructure, and composition, and also system characteristics, including saliva’s lubricating properties, the force that the tongue and teeth exert on the food, and the friction and lubrication that occur between them. Oral processing of foods involves various functions such as jaw movements, muscle activities, and tongue movements, all of which contribute to the conversion of food to a bolus that is safe for swallowing. Concurrently, oral receptors aid in the perception of texture, taste, and flavor while the food structure transforms into a bolus. However, the perception of food texture during oral processing is highly complex and dynamic as the structure of foods changes continually owing to mechanical and biochemical breakdown along with the lubricating effect of saliva.

It is generally known that texture perception in the mouth depends on mechanical characteristics, which are a mix of rheology and tribology, rather than just rheology. Oro-sensory perceptions (texture) and their dominant regimes are summarized in Table 1.5. Furthermore, rheology dominates the early stages of oral processing when there is bulk deformation, but tribology dominates during the latter stages when there are surface contacts, lubrication, and friction between the tongue, food, palate, and bolus. Additionally, there is a period in between during which both rheology and tribology have a vital role in texture perception.⁷⁴  Figure 1.5 illustrates the inter-relationship between food oral processing, oro-sensory perception and its dominant regimes.

Rheology is the study of the deformation and flow of matter, which involve forces, deformation, and time. The fundamental rheological tests aid in understanding various intrinsic food properties such as strength, deformability, modulus, toughness, and elasticity, which can be sensed during the processing of food in the mouth. This indicates that classical rheological and fracture mechanics properties will provide us with information on texture perception whereas oral processing and also molecular mechanisms explain structure–function relationships. Based on the state and structure of materials/foods, their rheological properties are governed by various factors and laws. The rheological properties of solids (ideal elastic) and fluids (ideal viscous) are governed by the relationship between stress and strain (time–rate independent) and stress and strain rate (time dependent), respectively.⁷⁵  Solids obey Hooke’s law, which states that the relationship between stress (applied force per unit area) and strain (deformation per unit length) is linear, whereas fluids obey Newton’s law, which states that the relationship between stress and strain rate is linear. However, most foods behave as both solid-like and fluid-like, which is known as viscoelastic behavior. The Deborah number (De) is often used in rheology to characterize the nature of food (solid/fluid) with respect to time. For solid foods, De is expected to be large (De ≫ 1).⁷⁶  Figure 1.6 depicts the classification of foods based on their rheological behavior.

Oral rheology focuses on how food deforms and flows under the influence of various factors such as bite force, degree of chewing, addition of saliva, and bolus consistency (optimum to swallow) within the oral cavity.²⁴  Further, the oral rheology of foods has a significant impact on the rate of transfer of aroma and taste components to the retronasal cavity and chemoreceptors on the tongue, respectively,⁷⁷  and the tactile sensation (mechanical) originating from the resistance to shear enforced by the teeth, tongue, and hard palate.³³  Further, the consistency of the food is continuously altered during oral processing from first bite to swallow owing to mastication and mixing of saliva. Hence the knowledge of the rheological properties of a bolus (food–saliva mixture) throughout oral processing is essential to understand its influence on the sensory perception of foods. However, the understanding of the oral rheological behavior of foods is very tedious and also it is difficult to predict how and to what extent it will impact the sensory perception of food. Table 1.6 provides a brief overview of various techniques and instruments used to investigate the rheological properties (food and bolus) and associated sensory perception.

Rheological investigations at a shear rate of 50 s⁻¹ on liquid and semi-solid meals when assessed showed a link between the viscosity and perceived thickness of different fluid foods.⁷⁸  Additionally, extremely thin fluids and highly viscous fluids demonstrated greater association at shear rates of 10 and 1000 s⁻¹, respectively.⁷⁸  Rheological tests such as storage modulus and apparent yield stress measurements showed a favorable correlation with texture perceptions such as firmness for soft foods, e.g. mayonnaise and yogurt.⁷⁹  Using instrumental methods and other physiological tests as objective techniques, it was found that viscosity measurement at a shear rate of 50 s⁻¹ did not have any correlation with complex sensory perceptions including fattiness, stickiness, smoothness, and creaminess for soft foods (such as custard).^59,80  In reality, the contact of the bolus with the oral surfaces, its surface characteristics, and its rheological characteristics all play a role in complicated sensory experiences.⁸¹ 

“Tribology”, which means rubbing or sliding, is described as “the science of friction, wear, and lubrication between interacting surfaces that are in relative motion, with or without the presence of a medium separating them”. It is a very ancient discipline that has been widely utilized in the automotive industry,⁸²  engineering,⁸³  polymer technology,⁸⁴  nanotechnology,⁸⁵  medicine,⁸⁶  and cosmetics.⁸⁷  Only lately has its importance in food applications been recognized. Several fundamental, empirical, and imitative tests may be used to predict the texture that will be experienced during the early stages of oral processing. This explains why tribological studies are necessary; by combining the strengths of existing methods, an ideal instrumental method of texture measurement must be developed that can simulate oral processing and quantify textural properties as perceived in the oral cavity. Substantial efforts have been made in soft tribology in recent years to comprehend and quantify the physics of oral tribological phenomena. However, more widespread studies and research must be conducted.

During the oral processing stage, food directly engages with the body and is deformed as the texture is perceived. Over time, surface characteristics become more important than bulk features. Further, in texture perception, both oral tribology and lubrication have a vital role. The major factors involved in oral tribology are illustrated in Figure 1.7. During the oral processing of food, the oral cavity’s interacting surfaces, such as the teeth, hard palate–tongue, teeth–food, and food–tongue–palate interfaces, tongue–teeth, food–tongue, and lips are all in motion.⁴⁵  Additionally, the tongue lacks the hard palate’s smoothness, and its roughness is caused by the existence of papillae which are 100 µm in height and diameter.⁸⁸  A thin layer of saliva coats every component of the oral cavity, ranging in thickness from 5 µm on the hard palate to roughly 25 µm on the tongue.^34,89  This offers a defense against microbial assaults and also friction and wear, such as surface irritation or dryness.³² 

The lubricating role of saliva in the mouth is frequently overlooked during tribological examination, which results in significant variations in the recorded friction values. In the mouth cavity, saliva works very well as a lubricant, interacting with different elements of the food system and preventing wear by producing a coating of salivary proteins that is resistant to wear.⁹⁰  Additionally, saliva-lubricated smooth surfaces have a friction coefficient of about 0.02.⁹¹  The tribological characteristics of saliva are significantly influenced by several factors, including surface roughness, the presence of surfactants, ionic strength, and applied force.⁹¹  It is recommended to coat or absorb a small layer of saliva on the device’s surface before the addition of a food sample to mimic the lubricating properties of saliva in the mouth.⁹²  Additionally, during the oral processing stage, the tongue glides across the palate and squeezes it with a force ranging from 0.01 to 90 N and a sliding speed of up to 200 mm s⁻¹.²⁸  However, the tongue’s exerted pressures and sliding rates change depending on the food type, taking into account the impacts of structure and hardness, and also the change in time and location on the tongue. Therefore, it is strongly advised to evaluate the tribological characteristics of food under a variety of loads and sliding speeds to gain further insights into oral lubrication and processing.⁹³  These are significant factors that are not taken into account in traditional textural measuring techniques. To understand better how textures are perceived, it is important to understand how foods behave under various conditions of friction and lubrication since these properties change over time, based on the type of food and also depending on how the tongue moves.⁹³ 

According to recent research on the oral processing of food, oro-sensory perceptions of texture and frictional measures are connected; nonetheless, it is extremely difficult to measure and identify their empirical links. Furthermore, as tribology is a “system” property, great care should be taken to optimize additional elements such as properties of the food sample, inclusion of saliva, sliding speed, and selection of the tribo-pair as these components are crucial in estimating the correlations amongst friction coefficients and texture. Thus, establishing a link between friction coefficients and oro-texture perception is perhaps the most evident problem. The main cause of this problem is that the friction coefficient depends on several factors in addition to the food matrix, including the instrumentation, lubrication, testing procedure, sliding speed, applied load, tribo-pair, and surface characteristics. Numerous elements of food tribology have advanced significantly over the past 10 years, including the creation of diverse tribological systems with various tribo-pairs and a wide range of loads and sliding speeds. However, in measuring the complexity of oral processing, the optimization of “system” parameters remains a significant difficulty. In addition to the disadvantages already described, it is also difficult to comprehend the proper entrainment rates at which the correlation between texture-related sensory qualities and friction coefficient could be elucidated. To acquire a comprehensive knowledge of the link between oral processing and sensory perception, an integrative approach incorporating several additional characterization techniques such as rheology, microstructure, and texture analysis along with tribology is essential. Table 1.7 summarizes recent tribological studies of foods with a special focus on food texture.

Taste (gustation) is also a significant sensation that is perceived during the oral processing of foods. The sensation of taste is perceived when the active chemical compounds in the food are released due to mastication and hydration with saliva and interact with the taste receptors present in the taste buds.²⁴  Taste buds are specialized onion-shaped structures that consist of 50–150 taste receptors.⁹⁴  The taste buds are mainly located on the upper surface of the tongue (dorsum) and other parts of the oropharynx, such as the larynx, pharynx, and epiglottis.⁹⁵  There are five basic well-recognized and approved taste modalities, namely sweetness, sourness, saltiness, bitterness, and umami (savory).⁴⁴  Various regions of the tongue are responsible for the sensation of different gustatory preferences such as back (bitter), posterior lateral edges (sour and bitter), anterior lateral edges and tip (salt), and front (sweet) due to the presence of specific taste receptors in the taste buds that are distributed among different papillae in the tongue epithelium.⁹⁵ 

Further, recent research suggests that there are other taste modalities, namely oleogustus (perception of fat) and starchy (perception of oligosaccharides), which have significant potential to become the sixth taste sense. Further, there are numerous theories related to the taste perception of fat, such as free fatty acids (high-fat foods) as a chemosensory signal for dietary fat,⁹⁶  lipid-binding protein CD36 (located in the tongue epithelial cells) that could function as both a scavenger and receptor of lipid,⁹⁷  and the role of lingual lipase in the enzymatic hydrolysis of triglycerides.⁶⁰  A study observed that the lingual lipase is active during the oral processing of high-fat foods, which requires a complex oral processing effort.⁹⁶  However, it has also been reported that lingual lipase does not contribute to the detection of oral fats. Further, a recent study showed that oligosaccharides can be detected without using sweet receptors.⁹⁸ 

The taste sensation is very unique and complex because its perception and intensities vary between individuals based on the age, sex, genetic variants of receptors, and taste preferences. A study was conducted to analyze the impact of age, sex, and genetics on the taste acuity of relatively healthy, European populations between the ages of 18 and 80 years. A decrease in the perception of all taste stimuli, especially bitter and sour tastes, was observed with increasing age, females sensed all the tastes more than men (except for umami), and TAS2R38-rs713598 polymorphism was strongly linked with bitter taste perception and also altered the sex differences for bitterness based on the genotype.⁹⁹ 

Further, the composition, texture, and structure of foods, the temperature of foods, the degree of mastication, and hydration by saliva also play a crucial role in the gustatory perception and intensity of various taste and flavor (odor–taste–texture) modalities.^39,100–102  In addition, individuals also vary in their sensitivity to certain compounds; for instance, individuals specifically sensitive to phenylthiocarbamide (a bitter-tasting compound) are acknowledged as supertasters. Further, it has been found that approximately 25% of the population are “supertasters”, a further 25% are “non-tasters”, and the remaining 50% are medium tasters.¹⁰³  Also, the supertasters are more sensitive to other taste stimuli.¹⁰⁴  An oral processing study was conducted using bread as a model food, with three different varieties of bread in terms of structure and/or composition but with the same salt content. The denser bread was perceived as less salty and exhibited a simple texture perception sequence compared with the others. The study acknowledged the influence of structure and texture on the perception of salty taste.¹⁰⁵ 

Flavor perception is a complex combination of multiple sensations such as gustation (taste), olfaction (retronasal and orthonasal), and oral–somatosensation (tactile, trigeminal, temperature, and pain).¹⁰⁶  In addition, visual (food color) and auditory (food-eating sound) signals also play a significant role in the flavor perception of foods.¹⁰⁷  The olfactory sensory system, especially both the retronasal system (which involves posterior nares to detect aromas emanating from the foods while we consume them) and the orthonasal system (a conventional system that is associated with the inhalation of external odors) play a crucial role in the detection of food flavors.¹⁰⁸  Further, various oral processing parameters (mastication, salivation, and swallowing), breathing, and, most importantly, individual differences (the way in which they orally process the foods and retronasal release of aroma compounds) have a significant influence on the perception of flavor and its intensity.¹⁰³ 

The sense of taste and smell plays a vital part in multisensory flavor perception. Indeed, several studies reported that olfactory cues are more critical than taste signals, as they contribute around 80–90% towards human flavor perception.¹⁰⁹  A few studies have provided evidence that there is a significant enhancement of the perception of olfactory stimuli (both retronasal and orthonasal) when the gustatory signal is maintained at either a sub- or supra-threshold level.^110–113  However, this kind of multisensory integration was observed to be dependent on certain combinations of surfactants and tastants used (i.e. stimulus congruency – the degree to which pairing of two stimuli in a food product is acceptable),^114,115  and on the cultural background of the participants studied.^103,116 

Furthermore, the oral-somatosensory system provides us with tactile cues (temperature and texture of food) that appear to play an important role in multisensory flavor perception.^107,117  A study highlighted the key role played by texture (mouthfeel) in driving multisensory flavor perception.¹¹⁸  The study found that regardless of whether the odor was provided orthonasally or retronasally, as the viscosity of the solution (milk) increased the perceived flavor intensity (creamy) decreased.¹¹⁸  Further, a few studies have elegantly demonstrated the cross-modal interactions between temperature and taste sensations.¹¹⁹  Around 33–50% of the population experiences “thermal taste” illusions such as variation of the temperature of the tongue (increase or decrease) can provoke basic taste senses (sweetness, sourness, saltiness, and bitterness).¹⁰²  Further, thermal tasters (people who perceive the illusion of thermal flavor) tend to perceive all oral stimuli (sweet, sour, salty, bitter, and astringent) as more intense compared with non-thermal tasters.¹²⁰ 

The auditory cues experienced during the consumption of food play a vital role in the multisensory perception of flavors and also texture.¹²¹  Most of the published research has made an effort to establish a correlation between subjective scores and objective/mechanical measurements for various foods (potato chips), especially for sensorial attributes such as crispiness/crunchiness and/or freshness. Further, it was found that the auditory cues heard during the oral processing of potato chips contribute around 15% to the perception of their crispness and/or freshness.¹²²  In a study conducted with potato chips, recorded sounds of the auditory feedback of the oral processing of chips (biting) were played in headphones. When the overall sound level was amplified and/or when just the high-frequency sounds were intensified, the panelists reported that chips were both significantly crisper and fresher. Similarly, when the overall intensity of the sound was reduced and/or when the high-frequency sounds were attenuated, the panelists scored the crisps as significantly staler and softer.¹²²  Subsequently, another study was conducted to demonstrate that modifying the loudness of popping sounds of carbonated fizzy beverages could significantly impact participants’ ratings.¹²³  Further, it is very important to note that external environmental noise and background music also have a significant impact on our eating and drinking behaviors.^124,125 

Over the years, several studies have reported that altering the color of foods to either appropriate, inappropriate, or absent has a major role in influencing people’s decisions regarding the taste, aroma, and flavor of the foods being evaluated. However, other factors such as opacity and varied intensities of color (colored beverages) have been less researched and also gave mixed results. A study demonstrated the effect of color on the taste perception of consumers.¹²⁶  The impact of red-, yellow-, and green-colored solutions on the detection thresholds and taste sensitivity for sweet, sour, bitter, and salty tastes (the four basic tastes) was investigated. The study revealed that the participants could only identify the taste of colored solutions at higher concentrations of tastants compared with uncolored solutions. Colorants did not affect the taste detection thresholds of the salt solutions, probably because salty food comes in a range of colors. However, in the case of a sweet solution, the taste sensitivity of participants varied based on the colors such as yellow (decreased), green (increased), and red coloring (no effect). In the case of bitter taste, yellow and green coloring of the solution had no effect whereas a substantial decrease in participants’ sensitivity was observed for a red-colored solution. For sour taste, both yellow- and green-colored solutions decreased the sensitivity of participants and red color had no significant effect.¹²⁶ 

Further, various studies investigated the impact of visual cues on the discrimination of olfactory stimuli (if presented in their appropriate color, inappropriate color, and uncolored),¹²⁷  identification of flavored beverages (both fruit and non-fruit flavors),¹²⁸  and also on the odor intensity.¹²⁹  Moreover, a study was conducted to explore the impact of color on how people perceive the aroma of wine (red and white wine) and validated the domination of visual signals over orthonasal olfaction.¹³⁰  In addition, several studies were conducted to identify the impact of visual contribution directly on flavor perception by subjecting participants to various combinations (pairs) of flavor and color signals,¹³¹  for instance, appropriate pairing (a cherry-flavored drink colored red), inappropriate pairing (a lime-flavored drink colored red), and incomplete pairing (either colorless with flavor or flavorless with color).¹³² 

The olfactory and gustatory stimuli are captured by the sensory receptors and transmitted to the piriform cortex and primary taste cortex, respectively; and both of the sensory signals are projected to the orbitofrontal cortex.^133,134  Currently, available evidence indicates that the orbitofrontal cortex plays a crucial role in modulating the multisensory interactions in flavor perception.¹³⁵  On the other hand, the oral–somatosensory signals such as proprioception, chemical irritation, texture, nociception, and temperature are sensed by the oral receptors during the oral processing of food. The tactical signals are transmitted to the brain through the trigeminal nerve, which projects directly to the primary somatosensory cortex. Hence the oral texture sensation is also represented in the orbitofrontal cortex.¹³³  Some researchers used functional magnetic resonance imaging (fMRI) to observe the neural interactions in the orbitofrontal cortex for congruent and incongruent pairings of various olfactory and gustatory stimuli.^136,137  They observed enhanced neural activity in the orbitofrontal cortex and increased pleasantness on evaluating congruent pairs of the olfactory–gustatory stimulus compared with incongruent combinations. Further, several studies applied various cognitive neuroscience techniques such as neurophysiology, neuroimaging, psychophysics, and computational modeling, in addition to conventional sensory techniques, to understand the reasons behind the complex sensory perception of foods.^121,138 

Sensory evaluation is the scientific discipline that is used to “evoke, measure, analyze, and interpret reactions to those characteristics of food as they are perceived by the senses of sight, smell, taste, touch, and hearing”. The sensory evaluation/profiling of foods is mandatory to determine consumer acceptability, product quality control, storage and shelf-life, processing changes, product development, and product reformulation.¹³⁹  There are two different approaches to ascertain the sensory characteristics of foods, namely instrumental techniques (objective methods) and sensory evaluation by human panelists (subjective methods).¹⁴⁰  Further, when both sensory evaluation techniques (objective and subjective) are combined in the characterization of food products, we obtain a great deal of information and a holistic picture regarding the product. However, the proper selection of instrumental and sensory methods is crucial to obtain a valid result about the sensory features of a product. Table 1.8 summarizes sensory profiling methods and instrumental and analytical techniques used in investigating different sensory perceptions.

Sensory profiling techniques can be broadly classified into two categories based on the period of evaluation, namely static and dynamic sensory evaluation techniques. Various conventional sensory evaluation techniques, such as descriptive and difference tests, consider sensory attributes as a static phenomenon (sensation).¹⁴¹  However, we know that the oro-sensory perception of texture, flavor, and taste is complex and dynamic in nature. In the case of static sensory profiling, the sensory perception over the entire period of oral processing (first bite–mastication–bolus formation–swallow–aftertaste) is expressed as a single sensation even though the sensation was dynamic over time.¹³⁹  In dynamic sensory evaluation techniques such as temporal dominance of sensation (TDS),¹⁴²  time–intensity (TI) analysis, progressive profiling (PP), and temporal check-all-that-apply (TCATA), the sensory attributes perceived throughout the various stages of the oral processing of food are measured over time. Also, the sensory information acquired from food oral processing helps us comprehend better the connection between sensory perception (texture, taste, and flavor) and the structure and composition of food. The dynamic evaluation techniques allow sensory panelists to record a series of sensory changes that occur during oral processing due to the dynamic structural changes (mastication and mixing of saliva) and release of flavor, taste, and aroma compounds.¹⁴³ 

The progressive profiling method is a relatively low-resolution dynamic method compared with TDS, TI, and TCATA.^139,144  TI analysis aids in measuring the intensity perception of specific traits over the food consumption period. However, at one time only the perceived intensity of mostly one or a maximum of two sensory attributes [dual-attribute time–intensity (DA-TI)] can be determined effectively.¹⁴⁵  The TI technique is quantitative in nature so it requires extensively trained panel members.⁴  The TCATA technique is a new dynamic method for investigating the multidimensional sensory properties of products as they evolve. TCATA is the dynamic extension of the check-all-that-apply (CATA) method.¹⁴⁶  In this method, the selection and deselection of sensory attributes are tracked continuously over time, allowing the assessors to demonstrate the evolution and changes in sensory attributes of products over time. Multiple attributes can be selected simultaneously in TCATA, which describes the sensations that arise either sequentially or concurrently.¹⁴⁷ 

The TDS method is a relatively new dynamic methodology that is frequently used to assess how sensory perception changes dynamically during the whole oral processing event, from the initial bite to the swallowing point.¹⁴⁸  TDS helps to illustrate the order of change in about 10 sensory attributes during food consumption at one time. Owing to its novelty, ease of use, and affordability, this approach has attracted a lot of research interest. TDS is the most preferred method compared with the TI method because of its ability to reveal the product perception pattern over time.¹⁴⁹  Further, the TDS technique, apart from providing the sequence of change in qualitative sensory attributes, also provides approximate quantitative data regarding the intensities (maximum or minimum) of dominant sensory attributes.¹⁵⁰  In contrast to TI, the sensory panelist needs less intensive training, and inexperienced panelists can also be used.⁴ 

In the recent past, there have been many studies on food oral processing to determine the link between food structure and the oro-sensory perception of texture, flavor, and taste. Consumers are extremely aware of health and well-being owing to the prevalence of numerous non-communicable lifestyle disorders such as diabetes, hypertension, cardiovascular disease, and obesity. Both government and food processors are forced to implement changes in food formulations to reduce sugar, salt, and fat content to produce healthy foods without altering their sensory properties. A lot of research is being carried out to develop new products (food structuring) with improved health benefits by choosing healthy ingredients and processing methods to preserve and retain their fullest nutritional and sensory properties. This chapter has summarized how food is processed in the mouth and its governing mechanisms and factors, and also how sensory attributes are perceived and factors that influence the sensory perception of food in the mouth. Additionally, a brief compilation of recent research work related to food oral processing, sensory perception, and various techniques employed in their investigation has been presented and discussed. Nevertheless, there is still a huge need at present and in the future to expand our existing knowledge on the oral processing of food to develop new food products with superior nutritional quality, health benefits, and modified sensory attributes so as to satisfy the needs of various sections of the population.