ORAL PHYSIOLOGY AND TEXTURE PERCEPTION OF SEMISOLIDS

ABSTRACT

In the oral cavity, the food is subjected to several mechanical and chemical processes. It is fractured by the teeth, diluted and broken down by saliva, heated or cooled by the ambient temperature of the mouth, formed into a bolus and finally swallowed. Numerous receptors in the oral cavity and nose respond to the initially ingested food and monitor the changes during processing. This leads to central perceptions of taste, odor, irritation and texture of the food. Most sensations associated with food texture occur only when the food is manipulated, deformed or moved across the oral receptors. In addition, people assessing the same stimulus differ in their ratings of that stimulus and their oral physiological parameters also exhibit inter‐individual variations. This paper is based on the PhD thesis of one of the authors. It gives an overview of this study and includes related work of other authors. The aim of our research was to improve the understanding of oral texture perception, in particular to examine the role of oral physiological processes in oral texture perception of semisolids and to investigate whether individual differences in perception could be attributed to differences in oral physiology among subjects. The results of our study demonstrate that oral physiological parameters such as oral sensitivity, tongue movements, temperature and saliva composition are of importance for texture perception of semisolids. Many parameters of oral physiology correlate with various perceived texture attributes. This implies that intra‐individual differences in texture perception could be attributed to variations in oral physiology. Oral physiology thus plays a role in texture perception of semisolids and should be taken into account in future texture research.

PRACTICAL APPLICATIONS

By understanding the processes in the mouth while eating, a better understanding of the sensations and the perceptions of the food can be gained. Taking for each food the relevant physiological parameter into consideration when performing and designing rheological/instrumental measurements grants more natural conditions and better predictions of sensory perception. This could save time and money on time‐consuming and expensive sensory panels in the earlier steps of product development and renovation. Knowing how physiological processes highlight specific flavor/texture sensations may be useful for product development or quality control where one typically wants to focus on certain sensations and ignores others. In addition, a future application could be to tailor products for personalized nutrition, individual choices or clinical nutrition based on physiological groups.     

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.     

Effect of saliva on physical food properties in fat texture perception

Sensory properties of food drive our food choices and it is generally accepted that lipids greatly contribute to the sensory properties of many foods and consequently to eating pleasure. Many studies have investigated the mechanisms of the fat perception. Unfortunately they used a variety of methods and products, thereby making generalization very difficult. The mechanism of fat perception in oral cavity is combined of several processes. Lipid composition and its properties strongly influence food structure. During consumption food is exposed to a range of in-mouth processing steps. Oral sensation of fat texture changes with time, from a first bite to chewing, while mixing with saliva, up to swallowing and even after swallowing. The present work reviews many aspects of fat texture perception from physical chemistry to physiology. Understanding the underlying mechanisms of in-mouth lipid processing would provide new concepts to produce low-fat food products with full-fat perception.     

不同结晶态脂肪比例对乳液奶油感感知的影响

通过调配无水乳脂肪的脂肪酸组成，构建含不同结晶态比例的乳脂肪模型和与牛奶脂肪含量和粒径匹配的乳液体系，进而模拟口腔加工，测定含不同结晶态脂肪比例乳液的口腔聚合性和摩擦学特性，并评价其奶油感。成功构建了含不同结晶态比例（10%～85%）的乳脂肪模型（F10～F85），所构建乳液（E10～E85）中脂肪球平均粒径为4 μm，在模拟口腔加工后，乳液粒径显著增大，其中E40样品增大程度最大。随着结晶态脂肪比例的增加，乳液口腔摩擦系数（μ20）先减小后增加，其中E40样品μ20最小，为（0.04±0.01）。描述性感官评价结果显示，随着结晶态脂肪比例的增加，乳液样品的整体奶油感先增大后减小，其中E40样品得分最高，为8.06±0.73。主成分分析结果显示，乳液的口腔光滑感和糊口感与整体奶油感相关性最强，其中E40样品与整体奶油感相关性最强。暂时性感官支配评价结果显示，口腔加工中后期，乳液在口腔感知中最显著的优势感官属性为口腔光滑感和糊口感，其中E40样品的口腔光滑感和糊口感优势率最高。本研究明确了结晶态脂肪比例对乳液奶油感感知的影响，为开发低脂但具备全脂口感的乳制品提供理论依据。     

Linking oral processing behavior to bolus properties and dynamic sensory perception of processed cheeses with bell pepper pieces

The addition of food particles to food matrices is a convenient approach that allows to steer oral behavior, sensory perception and satiation. The aim of this study was to determine the influence of physical-chemical properties of heterogenous foods on oral processing behavior, bolus properties and dynamic sensory perception. Bell pepper gel pieces varying in fracture stress and concentration were added to processed cream cheese matrices differing in texture. Addition of bell pepper gel pieces to processed cheeses increased consumption time, decreased eating rate and led to harder and less adhesive bolus with more saliva incorporated. Addition of bell pepper gel pieces to processed cheeses decreased dominance rate and duration of creaminess, smoothness, melting and dairy flavor and increased graininess and bell pepper flavor. Increasing fracture stress of bell pepper gel pieces from 100 to 300 kPa resulted in longer consumption time and lower eating rate. For hard/non-adhesive processed cheese matrices increasing gel pieces fracture stress lead to a boli with larger particles and more saliva. These changes were accompanied by decreased dominance perception of creaminess and bell pepper flavor and increased dominance of graininess. Increasing the concentration of bell pepper gel pieces from 15 to 30% did not affect oral behavior but led to the formation of harder and less adhesive bolus with larger particles and less saliva that were perceived with reduced dominance of creaminess, meltiness and dairy flavor while dominance of graininess and bell pepper flavor increased. Changing the texture of the cheese matrix from soft/adhesive to hard/non-adhesive decreased consumption time, increased eating rate, did not influence bolus properties and decreased dominance rate of creaminess, smoothness and melting sensations. Number of chews and total consumption time were positively correlated with saliva content of the bolus, number of bolus particles, bolus hardness, dominance of firmness, chewiness and graininess. We conclude that the modification of physical-chemical properties of processed cheeses and embedded bell pepper gel pieces can be a strategy to steer oral behavior and bolus properties which consequently determine dynamic sensory perception.     

Food oral processing: Mechanisms and implications of food oral destruction

BackgroundFood oral processing is a simultaneous process of food destruction and sensory perception. How a food breaks down its structure inside the mouth and what mechanisms control this process are hugely important to our eating experience and sensory perception. A proper understanding of this process is urgently needed by the food industry for better design and manufacturing of quality tasty food.Scope and approachThis review article analyses research findings from literature and from author's own laboratory in order to identify main controlling mechanisms of food oral destruction. Appropriate experimental evidences are given wherever available to demonstrate the important implications of different destruction mechanisms to sensory perception.Key findings and conclusionsThree major controlling mechanisms of food oral destruction are identified: the mechanical size reduction, the colloidal destabilisation, and the enzymatic interactions. These mechanisms may be applicable to different food materials either independently or collectively. They could also be applicable through the whole eating process or just at a certain stage of an eating process.     

Relationships between saliva and food bolus properties from model dairy products

During food consumption, complex oral processing occurs to transform the food into a bolus, ready to be swallowed. The objective of this study was to relate food, saliva and bolus properties, by using model dairy products, to better understand the role of saliva in bolus formation. Un-stimulated and stimulated saliva was collected from 5 subjects and biochemical and enzymatic properties were measured. Food bolus was then obtained from 8 different dairy products, varying in composition and ranging from liquid to gelled samples. The rate of saliva incorporation, pH, spreading ability and bolus rheological properties were determined. Some correlations seemed to exist between lysozyme activity and bolus properties. Subject and food product had a significant effect on almost all bolus properties. The rheology of bolus was highly correlated with food product texture. Even though preliminary, this approach could be used to better understand stimulus release and perception during food consumption.     

Partial coalescence as a tool to control sensory perception of emulsions

This study evaluates the role of partial coalescence of whey protein-stabilized emulsions on sensory perception. The selection of fats was restricted to vegetable fats that are essentially melted at oral temperatures. The sensitivity to partial coalescence was controlled by a variation in the fat melting curve and by addition of unsaturated monoglyceride. Most fat-related sensory attributes appear to be well-correlated to an increase in viscosity and coalescence in the mouth due to partial coalescence. Moreover, it was found that in-mouth aeration induces extra coalescence, which increases the perception of fat-related sensory attributes significantly.     

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.     

Liking of food textures and its relationship with oral physiological parameters and mouth-behavior groups

Researchers have categorized people into four “mouth-behavior” groups based on their oral processing preferences, and claimed that members of those mouth-behavior groups differ in their food texture preferences. If people could be classified into groups based on their liking of different textures, food products could be targeted to specific subgroups, potentially enhancing consumer acceptability. In the first part of our study, we grouped people based on their liking ratings of a wide variety of food textures by asking 288 participants to rate their liking of 106 food texture attributes in an online survey. In the second part of our study, we further examined relationships among individuals' food texture liking ratings, mouth-behavior group membership, and measurements of four oral physiological parameters (saliva flow rate, chewing efficiency, biting force, and particle size sensitivity). One-hundred participants completed the online survey on food texture liking, classified themselves into one of four mouth-behavior groups (Chewers, Crunchers, Smooshers, and Suckers), and were measured for four oral physiological parameters. We refuted the idea that large texture-liking subgroups exist. Although our participants self-categorized themselves into the four mouth-behavior groups similarly to previous researchers, our texture liking measurements did not support the presumed preferences of those mouth-behavior groups. Clustering of participants on their oral physiological measurements produced a “low particle-size sensitivity” cluster, a “high biting force” cluster, a “high saliva flow rate” cluster, and a “low saliva flow and low chewing efficiency” cluster. Neither our texture liking nor our oral physiological measurements predicted membership in the four mouth-behavior groups.     

Reduced Fat Food Emulsions: Physicochemical,Sensory, and Biological Aspects

Fat plays multiple important roles in imparting desirable sensory attributes to emulsion-based food products, such as sauces, dressings, soups, beverages, and desserts. However, there is concern that over consumption of fats leads to increased incidences of chronic diseases, such as obesity, coronary heart disease, and diabetes. Consequently, there is a need to develop reduced fat products with desirable sensory profiles that match those of their full-fat counterparts. The successful design of high quality reduced-fat products requires an understanding of the many roles that fat plays in determining the sensory attributes of food emulsions, and of appropriate strategies to replace some or all of these attributes. This paper reviews our current understanding of the influence of fat on the physicochemical and physiological attributes of food emulsions, and highlights some of the main approaches that can be used to create high quality emulsion-based food products with reduced fat contents.     

口腔软摩擦理论与测量技术及其在食品感官感知研究中的应用

食品口腔加工是一个复杂的动态过程，包括食物持续破碎、食团尺寸减小、唾液分泌和食团形成等一系列过程。它既是食品的口腔物理处理过程，又是食品的感官感知过程。在此过程中，食品颗粒之间以及食品颗粒与口腔表面之间普遍存在着相对移动。因此，如何准确描述与该过程控制相关的物理原理，已超出了传统的流变学范畴，而且许多感官特性（如光滑感、脂肪感、油腻感、后余味等）亦不可用简单的流变学测量来表征。近年来兴起的口腔软摩擦研究被认为是研究食品口腔加工与口腔感官的可行技术手段。本文阐述了摩擦学基本原理和口腔软摩擦的基本知识，主要包括口腔软摩擦的生物学基础和测量方法，同时介绍了软摩擦技术在食品研发与感官品质评定中应用的最新进展。     

Textural perception of liquid emulsions: Role of oil content,oil viscosity and emulsion viscosity

This work describes a study on the in-mouth textural perception of thickened liquid oil-in-water emulsions. The variables studied are oil content, oil viscosity, and the concentration of polysaccharide thickener. Gum arabic was chosen as the thickener because of the nearly Newtonian behavior of its solutions and special care was taken to suppress aroma clues. Based on the experimental results and findings from previous studies, this work shows that the emulsion droplets influence textural sensory perception of liquid emulsions by three main mechanisms, each of which relate to changes in specific sensory attributes, and none of which were found to be significantly dependent on the viscosity of the oil: 1) by increasing the viscosity, 2) by becoming incorporated in the mucous oral coating, and 3) by spreading oil at the oral surfaces. Based on these results, the possibility for replacement of emulsified fat by a polysaccharide thickener is evaluated.     

Influence of emulsion composition on lubrication capacity and texture perception

The role of fat content and type of surfactant for oral friction processes and texture perception of oil-in-water emulsions was assessed. Emulsions at 22% and 33% fat content containing either sodium caseinate or a sucrose ester as emulsifier were compared with viscosity-adapted aqueous solutions of dextran using both sensory evaluation and friction coefficient measurements on a tongue-palate contact model. The impact of saliva on the emulsion droplet size distribution was studied using tribology, differential interference contrast microscopy and light diffraction measurements. Emulsions at 33% and the corresponding iso-viscous aqueous solutions were discriminated in the sensory experiment whereas emulsions at 22% were not. Friction was significantly different for emulsions at 33% and the corresponding iso-viscous aqueous solutions. The difference was not significant for emulsions with 22% fat level when compared with the iso-viscous aqueous solutions. Saliva significantly decreased the friction coefficient of emulsions depending on the surfactant and amount of fat used.     

Perception of creaminess in foods

Food texture and mouthfeel play a crucial role in product and consumer acceptability. Creaminess, enjoyed by consumers, is a complex, multimodal sensory perception involving olfactory, gustatory and tactile cues. Oral viscosity and lubrication are the key underlying physical properties that define the mechanism of creaminess perception. Thickness, smoothness, mouth-coating, and dairy flavor can together play roles in the sensation of creaminess. The aim of this review is to present an understanding of the term” creaminess” along with different modalities involved in its perception, and to explore the oral physiological parameters and key physical properties that may be involved in the different oral modalities. An analysis of the previously examined links between food structure and composition and oral physiological parameters is presented. The review also presents a brief summary of previous models describing contributions of taste, aroma, and textural sensations. It emphasizes on the role of oral processing in testing proposed models with experimental evidence supporting those models and the future trends to enhance creaminess.     

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.     

UTILIZATION OF INVERSE WATER-IN-OIL EMULSIONS AS FAT REPLACERS IN FRANKFURTER MODEL SAUSAGES: INFLUENCE OF FAT EMULSION CONTENT ON THE ORGANOLEPTIC AND MECHANICAL PROPERTIES

The effect of fat content on the oral perception and the mechanical properties of frankfurter-type sausages was studied by means of absolute and hedonic organoleptic and mechanical tests. Panelists were able to discriminate changes of 5–10% in fat content. A correlation between fat content and organoleptic attributes was found, supporting a perception that the full-fat product is the de facto preferable product, to whose attributes others must conform.
Substituting fat with water-in-fat globules was found to imitate a full-fat sausage adequately in mechanical tests; however, organoleptic tests assessed the true fat content of the sausages. The tests indicate that it might be the absolute amount of fat, not the surface area of fat-covered globules, that correlates to the perception of fattiness. The above should be attributed to the lubrication and flocculation–coalescence of fat droplets with saliva in the mouth.

PRACTICAL APPLICATIONS

The substitution of fat globules by inverse water-in-fat globules in sausages was found to imitate a full-fat sausage adequately in mechanical tests; however, sensory tests confidently assessed the exact, true fat content of the sausages. This observation points out some limitations concerning the use of inverse emulsions as fat substitutes in relevant products.
Correlations between sensory attributes and fat level in sausages could imply that specific norms relating to fat level exist in the consumers, to which low-fat products may have to conform to.
The testers were able to attribute different scores, thus, discriminate sausages different at times by just 5–10% fat content. This could be of importance in the development of new low-fat sausage products.     

The role of rheological behaviour in flavour perception in model oil/water emulsions

Decreasing the fat content of a food produces changes in a range of physical, chemical and sensory properties that are important in flavour perception. The aim of this paper was to study the role of rheological behaviour in flavour perception in model oil/water emulsions, using a series of samples with different hydroxypropyl methyl cellulose (HPMC) and oil content. The composition of the emulsions was adjusted to deliver iso-release aroma in vivo and to produce samples with the same viscosity in-mouth (measured as the Kokini oral shear stress). The results showed that there was no significant difference in perceived thickness between the three samples providing further evidence that the Kokini oral sheer stress is well correlated with in-mouth thickness. However, there was a significant difference in perceived volatile fruity flavour and sweetness. Samples containing the highest oil content were perceived as significantly less fruity and sweet despite having the same volatile release and in-mouth thickness.