Bolus Formation and Disintegration during Digestion of Food Carbohydrates

Abstract: The first step in the digestion process is mastication, or chewing, when food is broken down, lubricated with saliva, and formed into a cohesive mass known as the food bolus. Upon swallowing, the bolus moves to the stomach and undergoes further breakdown during gastric digestion. The subject of this review is the formation of the food bolus and its subsequent breakdown in the stomach. Bolus formation has been widely studied, especially in terms of food particle size and lubrication. However, information about bolus disintegration is limited, and this review focuses on the breakdown of bread and starch‐based foods. Bolus formation and disintegration are key steps in the overall digestion process, as they control the rate at which ingested food components and nutrients are absorbed and released into the body. Information on the rate kinetics of bolus disintegration is necessary in developing a quantitative understanding of the food digestion process.     

Particle size distribution in the food bolus after mastication of natural foods

The main goal of mastication is to prepare a food bolus suitable for deglutition. The bolus preparation consists in food breakdown and processing during which oral sensations are generated. This study was performed to examine the particle size distribution in the bolus formed by chewing 10 natural foods. Ten young subjects with normal dentition were asked to chew the food and to expectorate the bolus just before swallowing, while masticatory parameters were recorded. The particle size distribution of each bolus was evaluated by wet sieving. The number of cycles, sequence duration and masticatory frequency varied among subjects and foods. The particle size distributions differed among foods but were similar among subjects. The median particle size d₅₀ gave a range from 0.82 to 3.04 mm allowing a food classification based on the state of the bolus. The d₅₀ value reflected the fracturability and may be useful to describe food behaviour in the mouth during bolus preparation.     

In-mouth mechanisms leading to flavor release and perception

During eating, foods are submitted to two main oral processes-chewing, including biting and crushing with teeth, and progressive impregnation by saliva resulting in the formation of a cohesive bolus and swallowing of the bolus. Texture influences the chewing behavior, including mastication and salivation, and in turn, these parameters influence texture perception and bolus formation. During this complex mouth process, flavor compounds are progressively released from the food matrix. This phenomenon is mainly dependent on the food texture, the composition and in-mouth breakdown, and on saliva impregnation and activity, but an individual's anatomical and physiological aspects characteristics should also be taken into account. This article reviews the knowledge and progresses on in-mouth processes leading to food breakdown and flavor release and affecting perception. Relationships between food texture and composition, food breakdown, oral physiology, and flavor release are developed and discussed. This review includes not only the mechanical aspects of oral physiology but also the biological aspects such as the influence of saliva composition, activity, and regulation on flavor perception. In vitro and in silico approaches are also described.     

Texture changes in bolus to the “point of swallow” - fracture toughness and back extrusion to test start and end points

The change in texture of a food bolus during chewing, from first bite to swallow, is dramatic for solid foods and a variety of analytical techniques are required to quantify the texture at any given point in the chewing cycle. The objective of the work presented in this paper is to develop mechanical and rheological tests relevant to a model food, allowing the texture of the bolus to be quantified at first bite, and when masticated to the point of swallowing. This paper presents one aspect of the “Food Structure Platform” programme, a multi-disciplinary New Zealand programme investigating the influence of structure on the textural attributes of solid foods. The programme team is developing model foods and novel techniques to test their mechanical and rheological properties.The first model food developed by the Food Structure Platform is a biscuit with a well defined range of hardness within one basic recipe. This was tested in 3-point bending to determine fracture stress and relate that to texture perceived on first bite. The biscuit samples were also masticated to the point immediately prior to where the subject would have normally swallowed then expectorated for rheological testing. Modified TPA and back extrusion, based on a cup and piston test piece, were used to test the rheological properties of the bolus from each of the biscuit models. Good correlations were found with fracture stress of the biscuit and sensory hardness for first bite. At the point of swallow the bolus had a consistent cohesiveness and saliva content irrespective of starting texture, whilst the hardness and adhesiveness was affected by starting texture/recipe.     

Interrelations among physical characteristics,sensory perception and oral processing of protein-based soft-solid structures

Oral processing is essential in breaking down the physicochemical structure of the food and thus important to the sensory perception of food in the mouth. To have an understanding of protein-based, soft-solid texture perception, a multidisciplinary approach was applied that combined studies of food microstructure with mechanical properties, sensory evaluation, and oral physiology. Model foods were developed by combining ion-induced micro-phase separation and protein-polysaccharide phase separation and inversion. Activities of masseter, anterior temporalis and anterior digastric muscles during oral processing were recorded by electromyography (EMG), while jaw movement amplitudes, durations, and velocities were simultaneously collected by a three-dimensional jaw tracking system (JT-3D). Changes in the microstructure of mixed gels significantly altered the characteristics of the chewing sequence, including the muscle activities, number of chews, chewing duration and chewing frequency. Mechanical attributes related to structural breakdown and sensory perception of firmness were highly correlated with the amount of muscle activity required to transform the initial structure into a bolus ready for swallowing. Chewing frequency was linked to mechanical properties such as recoverable energy, fracture strain and water holding capacity of the gels. Increased adhesiveness and moisture release also resulted in slower chewing frequency. Evaluation of oral processing parameters at various stages (i.e., first cycle, first 5 cycles, and last 3 cycles) was found to be a useful method to investigate the dynamic nature of sensory perception at first bite, during chewing and after swallowing. The study showed that muscle activity and jaw movement can be used to understand the links between physical properties of foods and sensory texture.     

Oral processing behavior and dynamic sensory perception of composite foods: Toppings assist saliva in bolus formation

Composite foods consist of combinations of single foods, such as bread with toppings. Single foods can differ considerably in their mechanical and sensory properties. This study aimed to investigate the effect of toppings on oral processing behavior and dynamic sensory perception of carrier foods when consumed as composite foods. Two carriers (bread, crackers) and three toppings (firm cheese, cheese spread, mayonnaise) were selected and six carrier-topping combinations were prepared. Mastication behavior, bolus properties (33, 66 and 100% of total mastication time) and dynamic sensory perception were determined for single carriers and all carrier-topping combinations. Both carriers with cheese spread and mayonnaise were chewed shorter and with fewer chews than single bread and crackers, although twice the mass of food was consumed. These toppings contributed to a faster bolus formation by providing moisture, so that less saliva was incorporated into the bolus during mastication. As a result of the moisture incorporation, carrier boli with toppings were softened and perceived less firm and less dry than carrier boli alone. The largest effects of toppings on oral processing behavior and perception were found for liquid-like mayonnaise, and these effects were more pronounced in dry crackers than in moist bread. We conclude that toppings assist saliva in bolus formation of carriers. Carriers drive oral processing behavior and texture perception whereas toppings drive overall flavor perception. This knowledge contributes to food design tailored for specific consumer segments and future personalized nutrition.     

Next target for food hydrocolloid studies: Texture design of foods using hydrocolloid technology

Texture is important in terms of both food palatability and the safety of eating. Recently, the importance of texture has been emphasized for the development of nursing-care foods, including dysphagia foods, in recent aged society, where the number of patients with mastication and swallowing difficulties is increasing. Texture design of these food products is now one of the most important tasks in the food industry in Japan. Texture of these food products should be optimized by modulating viscoelasticity using hydrocolloids so that they can easily transform to ‘ready-to-swallow’ bolus during oral processing. This article reviews the importance of texture as an essential attribute of foods and also the usefulness of hydrocolloids as an ingredient to modify and control food texture. The article also covers recent trials by the author’s research team on bolus rheology and in vivo acoustic analysis. The trials are to find some objective parameters describing the mastication and swallowing eases as an alternative to conventional bulk rheology and subjective sensory analysis.     

Food oral processing—A review

Food oral processing is an essential procedure not only for the consumption and digestion of foods but also for the appreciation and pleasure of food texture and food flavour. The consumption of a food inside mouth involves various oral operations, including first bite, chewing and mastication, transportation, bolus formation, swallowing, etc. Exact mechanisms and governing principles of these oral operations are still not fully understood, despite of continuous efforts made by scientists from food, psychology, physiology, dental and clinical studies, and other disciplines. This article reviews recent progresses and literature findings about food processing and transformation in mouth, with particular attention on the physiology and rheology aspects of oral operations. The physiological behaviour of human's oral device is discussed in terms of biting capability, tongue movement, saliva production and incorporation, and swallowing. The complexity of oral processing is analysed in relation to the rheology and mechanical properties of foods. The swallowing and oral clearing process is also examined for its criteria, triggering mechanism, bolus deformation, and the rheology of swallowing.     

The role of friction in perceived oral texture

Instrumentally measured in vitro friction in semi-solid foods was related to oral texture sensations. Increased fat content resulted in lower sensations of roughness, higher sensations of creaminess, and lower friction, suggesting that lubrication is the mechanism by which fat affects oral texture in low fat foods. Starch breakdown by salivary amylase in low fat foods resulted in reduced friction, possibly through the release of fat from the starch food matrix, and the migration of fat to the surface of the bolus where it becomes available for lubrication. No evidence was found that salivary mucins or salivary viscosity play a role in lubrication. Astringent sensations may be related to reduced lubrication and increased friction caused by particles, either resulting from precipitation of salivary protein rich proteins or from flocculation of dead cells.     

Physical Aging of Amorphous Starches (A Review)

In recent years, physical behaviors of glassy foods are greatly focused as many processed foods are consumed in their glassy states. Physical aging is one of the important phenomena for glassy foods, which is responsible for quality changes vs. retention of freshness during storage. The aging phenomenon can be predicted on the basis of a process of thermodynamic relaxation, induced by structural rearrangements in amorphous matrices. Thus, it can be evaluated by using calorimetric, volumetric, and mechanical analyses. Starch is one of the principal components in most cereal‐based glassy foods, and its aging is thus relevant to the overall changes in quality and freshness of various cereal‐based food products. The polymeric theory for the physical aging of glassy starch is reviewed on the basis of the existing literature, and the aging kinetics based on the changes in various physical and thermal properties are discussed.     

Modelling the breakdown mechanics of solid foods during gastric digestion

Solid food disintegration within the stomach has a major role on the rate and final bioavailability of nutrients within the body. Understanding the link between food material properties and their behaviour during gastric digestion is key to the design of novel structures with enhanced functionalities. However, despite extensive research, the establishment of proper relationships has proved difficult. This work builds on the hypothesis that to bridge this knowledge gap a better understanding of the underlying mechanisms of food disintegration during digestion is needed. The purpose of this study is to propose a new protocol that, by uncoupling the physicochemical processes occurring during gastric digestion, allows for a more rigorous understanding of these mechanisms. Using steamed potatoes as a product model, this study aims to develop a viable methodology to characterize the role of gastric juice and compressive forces on the breakdown mechanics of solid foods during digestion. From a general viewpoint, this work not only reveals the importance of the parameter used to describe the size distribution of food particles on the interpretation of their breakdown behaviour, but also provides a new framework to characterize the mechanisms involved. Results also illustrate that food breakdown during gastric digestion might well not follow a unimodal behaviour, highlighting the need to characterize their performance based on parameters describing broad aspects of their particle size distribution rather than single point values. Arguably simplistic on its approach, this study illustrates how an improved understanding of the role of chemical and physical processes on the breakdown mechanics of solid foods can facilitate valid inferences with respect to their in-vivo performance during digestion. In particular, it shows that while the contraction forces occurring in the stomach can easily disintegrate the potato matrix at the molecular level, the continuous exposure to gastric juices will promote their disintegration into progressively smaller debris. A discussion on the challenges and future directions for the implementation of a more general and standardized protocol is provided. Not intended to reproduce the breakdown behaviour of foods during gastric digestion, but rather to characterize the mechanisms involved, the proposed protocol would open new opportunities to identify the material properties governing the performance of different foods upon ingestion.     

FOOD TEXTURE AND ITS EFFECT ON INGESTION, MASTICATION AND SWALLOWING

The oral processing of most semisolid and solid foods can be summarized in terms of two opposing mechanical influences: forces that fracture food particles versus those that make them adhere to each other. During either the ingestion of food into the mouth or the early stages of mastication, the aim of processing solid food particles is usually to fracture them. However, later on towards swallowing, adhesion is desirable in order to try to form a sticky food bolus that could clear the mouth of isolated food fragments. Neither of these tendencies is actually a function of any particular force (or stress) or displacement (or strain) on food particles, but is instead controlled by energy. Food particles can adhere not only to each other but to the mouth's surfaces. This produces friction. While this is essential for the tongue to grip food particles and move them around the mouth, it also adds to the work that mouthparts must do during processing and may affect sensory perception of food quality. Successful processing of foods in the mouth requires a considerable amount of neural feedback from sensory receptors. We focus here on recent evidence about these sensory receptors with an attempt to reinterpret their role in terms of textural perception.     

A review of acoustic research for studying the sensory perception of crisp,crunchy and crackly textures

The relationship that acoustic sensations have with the perception of texture has been studied for crisp, crunchy and crackly products. This has involved evaluating the contribution of chewing sounds to the perception of these textures or recording noises produced during mastication and evaluating various acoustic parameters from the resulting amplitude–time curves. Combining the analysis of acoustic recordings with mechanical testing results has been successful for predicting crispness and crunchiness of snack foods. The next stage for acoustic research should be to relate the structure of the products to the sounds produced during mechanical breakdown of the products in order to fully understand the textural properties of the food.     

Towards modelling of fluid flow and food breakage by the teeth in the oral cavity using smoothed particle hydrodynamics (SPH)

The mechanical functions of the oral cavity, such as chewing and swallowing, present many modelling challenges. Plastic strain and fracturing occur in food due to interactions with hard and soft tissues, whilst the food moves, collides and mixes with fluid. Smoothed particle hydrodynamics (SPH) is a meshless numerical method that uses particles instead of meshes to discretise material. The Lagrangian nature of SPH means that it is well suited to modelling complexities such as fluid-free surfaces or solid fracture, interactions with complicated deforming boundaries, and temperature and chemical dynamics. We propose a combined SPH–biomechanical model of the oral cavity and present five model applications that address a broad range of material behaviours observed during eating. Interactions between the gums, teeth and the moving tongue with fluids in the anterior oral cavity; biting and chewing of elastoplastic foods; and biting and crushing of brittle foods are each simulated. In each case, the proposed meshless SPH–biomechanical model was found to be well suited to modelling the complex motions, boundary interactions and material responses. The modelling framework shows promise as a tool for simulation of food breakdown and taste release for foods of different material behaviours.     

Human digestion – a processing perspective

The human digestive system is reviewed in the context of a process with four major unit operations: oral processing to reduce particle size and produce a bolus; gastric processing to initiate chemical and enzymatic breakdown; small intestinal processing to break down macromolecules and absorb nutrients; and fermentation and water removal in the colon. Topics are highlighted about which we need to know more, including effects of aging and dentition on particle size in the bolus, effects of different patterns of food and beverage intake on nutrition, changes in saliva production and composition, mechanical effects of gastric processing, distribution of pH in the stomach, physicochemical and enzymatic effects on nutrient availability and uptake in the small intestine, and the composition, effects of and changes in the microbiota of the colon. Current topics of interest including food synergy, gut–brain interactions, nutritional phenotype and digestion in the elderly are considered. Finally, opportunities for food design based on an understanding of digestive processing are discussed. © 2015 Society of Chemical Industry     

Particle Size Distribution of Brown and White Rice during Gastric Digestion Measured by Image Analysis

The particle size distribution of foods during gastric digestion indicates the amount of physical breakdown that occurred due to the peristaltic movement of the stomach walls in addition to the breakdown that initially occurred during oral processing. The objective of this study was to present an image analysis technique that was rapid, simple, and could distinguish between food components (that is, rice kernel and bran layer in brown rice). The technique was used to quantify particle breakdown of brown and white rice during gastric digestion in growing pigs (used as a model for an adult human) over 480 min of digestion. The particle area distributions were fit to a Rosin–Rammler distribution function. Brown and white rice exhibited considerable breakdown as the number of particles per image decreased over time. The median particle area (x₅₀) increased during digestion, suggesting a gastric sieving phenomenon, where small particles were emptied and larger particles were retained for additional breakdown. Brown rice breakdown was further quantified by an examination of the bran layer fragments and rice grain pieces. The percentage of total particle area composed of bran layer fragments was greater in the distal stomach than the proximal stomach in the first 120 min of digestion. The results of this study showed that image analysis may be used to quantify particle breakdown of a soft food product during gastric digestion, discriminate between different food components, and help to clarify the role of food structure and processing in food breakdown during gastric digestion.     

Texture and texture assessment of thickened fluids and texture‐modified food for dysphagia management

Thickened fluids and texture‐modified foods are commonly used in the medical management of individuals who suffer from swallowing difficulty (known as dysphagia). However, how to reliably assess texture properties of such food systems is still a big challenge both to industry and to academic researchers. This article aims to identify key physical parameters that are important for objective assessment of such properties by reviewing the significance of rheological or textural properties of thickened fluids and texture‐modified foods for swallowing. Literature reviews have identified that dominating textural properties in relation to swallowing could be very different for thickened fluids and for texture‐modified foods. Important parameters of thickened fluids are generally related with the flow of the bolus in the pharyngeal stage, while important parameters of texture‐modified foods are generally related with the bolus preparation in the oral stage as well as the bolus flow in the pharyngeal stage. This review helps to identify key textural parameters of thickened fluids and texture‐modified foods in relation to eating and swallowing and to develop objective measuring techniques for quality control of thickened fluids and texture‐modified foods for dysphagia management.     

Swallowing profiles of food polysaccharide gels in relation to bolus rheology

Swallowing profiles of food polysaccharide gels were investigated in relation to bolus rheology. Polysaccharide gel from either gellan gum or a mixture of gellan gum and psyllium seed gum was used as a model food. Acoustic analysis and sensory evaluation were carried out to investigate the swallowing profiles using the same human subjects. Model bolus was prepared through instrumental mastication using a mechanical simulator to mimic the action of the human jaw in the presence or absence of artificial saliva and was subjected to dynamic viscoelasticity measurements to investigate the rheological properties. Bolus from the binary gel was shorter in time required to transfer through the pharyngeal phase due to mass flow and was scored higher in sensory perceived cohesiveness (bolus forming) than that from gellan gum gel. Model bolus from the binary gel showed a rheologically weak gel (or structured fluid) behavior and was higher in structural homogeneity than that from gellan gum gel. Also, dynamic viscoelasticity parameters of the binary gel were less dependent on the addition level of saliva. Results indicate that the viscoelasticity balance is a key for texture design of dysphagia foods in relation to the saliva miscibility.     

Respective impact of bread structure and oral processing on dynamic texture perceptions through statistical multiblock analysis

Texture perception is a multidimensional and dynamic phenomenon resulting from both the initial structure of food and its breakdown during oral processing. The aim of this study is to identify the respective contribution of food and bolus properties to temporal changes in texture perceptions during bread consumption. For this purpose, the perception dynamics of three French baguettes with dif\ferent structures were assessed through Temporal Dominance of Sensations and Progressive Profiling. Samples of crumb with and without crust were tasted by trained panelists. The intensities of nine texture attributes were evaluated at three key stages of oral processing (10%, 40% and 100% of individual swallowing time) using the Progressive Profiling method. Six of them were related with a Multiblock Partial Least Squares (MB-PLS) regression to the initial bread properties and to some bolus properties measured at these three stages. The evolution during oral processing of some attributes such as “soft”, “dry”, “doughy” and “sticky” was more influenced by modifications of bolus properties than by the initial characteristics of the breads. Among bolus properties, the MB-PLS highlighted that the hydration and texture properties of the bolus had a greater impact on texture perceptions than bolus structure. The “aerated” perception was more affected by the crumb structure, while the “heterogeneousness” and the “crispiness” were more affected by the presence of crust. This study thus contributes to improving our understanding of dynamic texture perceptions through a statistical model that takes the physical properties of bread and bolus during oral processing into account.     

The determining role of bolus rheology in triggering a swallowing

Swallowing is the final stage of an eating process. Even though individuals know exactly when and how to swallow, the controlling mechanisms and the determining criteria of bolus swallowing are still not yet clear. One hypothesis is that bolus rheology, i.e. its flow-ability and stretch-ability, determines the triggering of a swallowing and the main aim of this work was to test this hypothesis. A wide range of fluid foods, including 18 commercial products and 10 lab-constituted foods, were examined for easiness of swallowing by a panel of 19 subjects. Oral residence time, defined as the time from the ingestion till the completion of swallowing, was determined for each eating process. It was observed that the oral residence time had a linear relationship with the sensed difficulty of swallowing. That is a food sensed difficult-to-swallow tends to stay longer in the oral cavity. Rheological properties (both shear and stretching flow) of these foods and their simulated boluses (mixture of food and simulated saliva) have also been determined at body temperature. The apparent shear viscosity showed a positive correlation with the sensed difficult of swallowing. However, the stretching behaviour of a fluid food, characterised by the maximum stretching force and the work of stretching showed much improved correlation to the sensory perceived easiness of swallowing. It was concluded that bolus rheology, in particular its extensional stretch-ability, had the most important influence on the ease of swallowing.