Oral Tribology Research Articles

Investigation on the structure characteristics, stability evaluation, and oral tribology of natural oleanolic acid-based water-in-oil high internal phase and multiple Pickering emulsions as realistic fat analogues

Investigation on the structure characteristics, stability evaluation, and oral tribology of natural oleanolic acid-based water-in-oil high internal phase and multiple Pickering emulsions as realistic fat analogues

Thirst and the influence of ionic concentration; an investigation into the effect on salivary lubrication and the role of MUC5B

Saliva plays a crucial role in lubricating the oral cavity and maintaining a normal hydrated mouthfeel, with the main macromolecule in saliva, MUC5B, playing a significant role in its lubricating properties. Mucins form a hydrated biogel that covers the oral epithelium and forms part of the mucosal pellicle, maintaining proper hydration. This study investigates the relationship between ionic concentration, saliva lubrication, and the perception of oral dryness. At baseline, sodium was found to be ten times more concentrated in the mucosal pellicle compared to bulk saliva, in contrast to potassium which was only 1.5 times more concentrated in the pellicle. Increasing concentrations of mouthwashes containing sodium or potassium indicated a preferential sequestering of sodium in the mucosal pellicle with sodium chloride rinses being associated with an increased thirst perception. We therefore propose that sodium binds to MUC5B, reducing the electrostatic repulsions between its negatively charged moieties. This interaction subsequently leads to a compaction of the mucin structure, which decreases the hydration of MUC5B and reduces its lubricating properties. We determined that increased sodium concentrations compact the mucin structure as indicated by transmission electron microscopy. However, we found that increased sodium concentrations were associated with improved the lubrication of salivary and MUC5B films in an oral tribology mimic, decreasing the coefficient of friction from 0.044 to 0.018, and 0.416 to 0.100 with the addition of 200 mM sodium chloride for saliva and MUC5B, respectively. MUC5B was also demonstrated to become more hydrated with increasing ionic concentrations, as indicated by Quartz Crystal Microbalance with Dissipation. We therefore propose that increased hydration shells surrounding the sodium counterions are responsible for this improved lubrication via the hydration lubrication mechanism. Lastly, it was observed that the spinnbarkeit of saliva decreased with increasing ionic concentrations, indicating a reduced stretchiness in its rheological properties. However, the concentration of MUC5B alone did not appear to affect spinnbarkeit, highlighting that many factors influence the biophysical properties of saliva.

Investigating the role of water in rheological and friction properties of pureed foods: A deep dive into avocado puree

We examined how water content and distribution shape the network structure and oral processing characteristics (i.e., rheology and friction properties) of pureed foods. Our findings suggested that a decrease in water content resulted in increased hardness, tighter binding between water and puree components, and a denser gel formation. When the ratio of avocado puree to water surpassed 1:1.5, the sample assumed a gel-like texture. Oral tribology tests revealed that in puree by water, lower viscosity yielded better lubrication. Nevertheless, as the proportion of slurry increased, the lubricating efficacy of lipid droplets and pectin became more pronounced, causing the friction coefficient to decline with increasing viscosity. Our study sheds light on the effects of water content on the rheology and friction of pureed foods, providing a straightforward solution for families dealing with different levels of swallowing challenges seeking safe food choices.

Correlation of rheology and oral tribology with sensory perception of commercial hazelnut and cocoa-based spreads.

This study examined the effects of spread formulation and the structural/lubricant properties of six different commercial hazelnut and cocoa spreads on sensory perception. Rheology, tribology, and quantitative descriptive analysis (QDA) was assessed by also evaluating the correlation coefficients between the quality descriptor and the rheological and textural parameters. The viscosity was evaluated at different temperatures to better simulate conditions before and after ingestion. Tribological analysis was executed at 37°C to mimic the human oral cavity. The effect of saliva presence and the number of runs on tribological behaviors was investigated. Moreover, textural, calorimetric, and particle size distribution measurements were performed to reinforce the correlation between structural/thermal parameters (e.g., firmness, stickiness, sugar melting point) and sensory aspects. "Visual viscosity," defined as a sensory attribute evaluated prior to consumption, negatively correlated with apparent viscosity measured at 20°C and 10 s-1, whereas "body," defined during oral processing and related to creaminess, positively correlated with apparent viscosity measured at 37°C and 50 s-1. These attributes were mainly influenced by particulate microstructure and solid volume fraction within the formulation. Textural stickiness positively correlated with sensory "adhesiveness" and was related to fat composition and milk powder addition, while "sweetness" was related to sucrose content and sugar melting enthalpy. Tribological data provided meaningful information related to particle-derived attributes, as well as after-coating perception (fattiness/oiliness), thus better predicting food evolution during oral consumption.

Influence of dispersion media on the rheology and oral tribology of the konjac glucomannan/xanthan gum thickener

Influence of dispersion media on the rheology and oral tribology of the konjac glucomannan/xanthan gum thickener

New insight in characterization of red wine astringency using soft tribology method

AbstractThe variation of friction coefficient (μ) of model wine and model saliva mixtures with entrainment speed (VR) on simulated oral surfaces was evaluated by oral tribology. Combined with techniques of dynamic light scattering (DLS), atomic force microscopy (AFM), rheometer, and quartz crystal microbalance with dissipation monitoring (QCM‐D), the correlation between characteristics of the model wine and model saliva mixtures (particle size and morphology, suspension viscosity, adsorption film thickness, and viscoelasticity) and the oral lubrication loss was established. The results showed that the higher the concentration of tannin in the model wine, the larger the size of the complexes with the model saliva, and the higher the thickness, viscoelasticity, and roughness of the adsorption film formed, which resulted in the increase of friction coefficient in the boundary lubrication regime. Different from previous results, it is found that the maximum value of the friction coefficient (μmax) in the boundary regime has the best positive correlation with the astringency perception intensity of the model wine accordingly.

Oral tribology of dairy protein-rich emulsions and emulsion-filled gels affected by colloidal processing and composition

Designing nutritious food for the elderly population often requires significant quantities of leucine-rich whey proteins to combat malnutrition, yet high-protein formulations can cause mouth dryness and increased oral friction. This study investigated how various colloidal processing methods and compositions impact the in vitro oral tribological properties of protein-rich emulsions and emulsion-filled gels. Oil-in-water emulsions with oil fractions from 1 wt% to 20 wt% were prepared, alongside emulsion-filled gels containing whey protein isolate (WPI), hydrolysed whey protein (HWP), or a blend of both (10 wt% protein content). Two processing approaches were employed: creating emulsions with an initial 10 w% protein content (M1) and initially forming emulsions with 0.1 wt% protein content, then enriching to a final 10 wt% concentration (M2). The hypothesis was that formulations with HWP or method 2 (M2) would offer lubrication benefits by inducing droplet coalescence, aiding in the formation of a lubricating boundary tribofilm. Surprisingly, the tribological behavior of high-protein emulsions showed minimal dependence on oil droplet volume fraction. However, both HWP-based emulsions and those processed with M2 for WPI exhibited significant friction reduction, which may be attributed to the presence of coalesced oil droplets, supporting our hypothesis. Substituting 50 wt% of WPI with HWP in emulsion-filled gel boli resulted in very low friction coefficients in the boundary lubrication regime, suggesting oil droplet release from the gel matrix. These findings provide insights into designing high-protein foods with improved mouthfeel for the elderly population, necessitating further validation through sensory studies.

Friction study of honey-water in a mimicked soft oral environment

Food oral tribology plays an important role in studying the oral processing behavior of foods. This article explores the lubricity of different mass fractions of honey-water solutions (HWS) combining experimental and simulation study. Friction experiments are performed in simulated soft oral environment, and the complete Stribeck curve is successfully constructed. It is found that as the mass fractions of honey increase, the friction coefficients decrease and the curves move towards higher Vη (speed × viscosity) values. Higher viscosity and wettability enhance the film-forming and entrainment ability of the fluid, while reducing the friction coefficients in the boundary and mixed regimes. However, changes in wettability have little impact on the elastohydrodynamic lubrication regime of pure honey. In addition, a lubrication model is established using finite element method to examine the fluid movement in the lubrication area. The distribution of lubrication film exhibits a horseshoe shape. Influences of the sliding speed, fluid viscosity, and solid viscoelasticity on the distribution of fluid pressure and film thickness are revealed. This study provides guidance for exploring the oral processing behavior of fluid foods and improving the pleasure of eating.

Tailoring the oral sensation and digestive behavior of konjac glucomannan-gelatin binary hydrogel based bigel: Effects of composition and ratio

In the food industry, there is a growing demand for bigels that offer both adaptable oral sensations and versatile delivery properties. Herein, we developed bigels using a binary hydrogel of konjac glucomannan (KGM) and gelatin (G) combined with a stearic acid oleogel. We closely examined how the oleogel/hydrogel volume ratio (φ) and the KGM/G mass ratio (γ) influenced various characteristics of the bigels, including their microstructure, texture, rheological properties, thermal-sensitivity, oral tribology, digestive stability, and nutraceutical delivery efficiency. A noteworthy observation was the structural evolution of the bigels with increasing φ values: transitioning from oleogel-in-hydrogel to a bicontinuous structure, and eventually to hydrogel-in-oleogel. Lower γ values yielded a softer, thermally-responsive bigel, whereas higher γ values imparted enhanced viscosity, stickiness, and spreadability to the bigel. Oral tribology assessments demonstrated that φ primarily influenced the friction sensations at lower chewing intensities. In contrast, γ played a significant role in augmenting oral friction perceptions during more intense chewing. Additionally, φ dictated the controlled release and bioaccessibility of curcumin, while γ determined digestive stability. This study provides valuable insights, emphasizing that through meticulous selection and adjustment of the hydrogel matrix composition, bigels can be custom-fabricated to achieve specific oral sensations and regulated digestive behaviors.

Effect of variation in basic emulsion structure and polysaccharide content on the physicochemical properties and structure of composite-based emulsion gels as cube fat mimetics

This study aimed to reveal the effect of different basic emulsion structures (W/O/W and O/W) and polysaccharide additions on protein-polysaccharide composite-based emulsion gels utilizing soybean protein isolate, palm oil and konjac glucomannan. The results of texture profile, rheological tests, microstructure observations, and oral tribology showed that basic emulsion structures and konjac glucomannan addition had significant effect on the emulsion gels' properties, while the impact of konjac glucomannan addition was stronger. W/O/W double emulsion gels (DEG) exhibited lower oral friction coefficients and obtained higher scores for oiliness and juiciness during the sensory evaluation. However, O/W single emulsion gels (SEG) displayed a firmer texture and higher chewiness, a 29.62% and 49.57% increase compared to the DEG at 7% konjac glucomannan addition on the hardness and chewiness respectively. It has demonstrated the emulsion gels' potential as cube fat mimetics and feasibility of adjusting their properties by changing the basic emulsion structure.

Effect of cooking methods and polysaccharide addition on the cooking performance of cubic fat substitutes

This study examined the effect of two different cooking methods (grilling and frying) and konjac glucomannan (KGM) addition on the cooking yield, water distribution, color, texture, oral tribology, sensory characteristics, and thiobarbituric acid (TBA) content of cubic fat substitutes (CFS). Grilled samples displayed higher yellowness and lightness values and cooking yield. Low-field nuclear magnetic resonance analysis showed that the fixed water relaxation time gradually decreased at a higher KGM level, indicating that KGM changed the distribution and migration of internal water. The relaxation time and amplitude of the grilled samples were slightly higher, indicating a slightly lower fixed water level. The textural and sensory evaluation showed that the overall sensory score of the fried cubic fat substitutes and pork backfat exceeded that of the grilled samples. A higher KGM content improved the hardness, springiness, and chewiness of the cubic fat substitutes to improve the texture and taste. The thiobarbituric acid reactive substance (TBARS) value of all the cubic fat substitutes was significantly lower than in the pork backfat, while these values were slightly higher when fried than when grilled. Scanning electron microscopy (SEM) revealed that KGM addition rendered the network structure more compact, denser, and similar to that of pork backfat. This study provided a theoretical basis for cooking cubic fat substitutes and research involving properties such as the cooking yield and water distribution after cooking.

Insights into the Multiscale Lubrication Mechanism of Edible Phase Change Materials.

Investigation of a lubrication behavior of phase change materials (PCM) can be challenging in applications involving relative motion, e.g., sport (ice skating), food (chocolates), energy (thermal storage), apparel (textiles with PCM), etc. In oral tribology, a phase change often occurs in a sequence of dynamic interactions between the ingested PCM and oral surfaces from a licking stage to a saliva-mixed stage at contact scales spanning micro- (cellular), meso- (papillae), and macroscales. Often the lubrication performance and correlations across length scales and different stages remain poorly understood due to the lack of testing setups mimicking real human tissues. Herein, we bring new insights into lubrication mechanisms of PCM using dark chocolate as an exemplar at a single-papilla (meso)-scale and a full-tongue (macro) scale covering the solid, molten, and saliva-mixed states, uniting highly sophisticated biomimetic oral surfaces with in situ tribomicroscopy for the first time. Unprecedented results from this study supported by transcending lubrication theories reveal how the tribological mechanism in licking shifted from solid fat-dominated lubrication (saliva-poor regime) to aqueous lubrication (saliva-dominant regime), the latter resulted in increasing the coefficient of friction by at least threefold. At the mesoscale, the governing mechanisms were bridging of cocoa butter in between confined cocoa particles and fat coalescence of emulsion droplets for the molten and saliva-mixed states, respectively. At the macroscale, a distinctive hydrodynamic viscous film formed at the interface governing the speed-dependent lubrication behavior indicates the striking importance of multiscale analyses. New tribological insights across different stages and scales of phase transition from this study will inspire rational design of the next generation of PCM and solid particle-containing materials.

In vitro oral simulation based on soft contact: The importance of viscoelastic response of the upper jaw substitutes.

Real oral processing is the squeezing and shearing between two soft surfaces. The importance of soft palate surface cannot be ignored while focusing on tongue substitutes. Thus the effects of viscoelasticity, roughness of upper jaw substitutes, and fluid rheological properties on lubrication properties were explored by in vitro oral tribology experiments. Different palate substitutes significantly changed the friction curves of pure water, milk, and yogurt. The boundary friction coefficients of pure water and milk are higher under softer or smooth palate substitutes due to stronger viscoelastic responses of friction pairs. Their boundary friction coefficients are lowest at rigid upper jaw substitutes owing to smaller contact angles and deformation. However, the boundary friction coefficient of yogurt is lower owing to its high viscosity, low loss factor, and large particle size under soft friction pairs. In addition, it is highest at rigid palate friction pair because a smaller contact area reduces the entrainment of yogurt, resulting in poor lubricating performance.

Create Fat Substitute From Soybean Protein Isolate/Konjac Glucomannan: The Impact of the Protein and Polysaccharide Concentrations Formulations.

In this study, soybean protein isolate (SPI) and coconut oil were emulsified and konjac flour was added to prepare the protein/polysaccharide composite emulsion gel. The SPI/polysaccharide compound fat substitute was obtained by vacuuming. The effects of protein and konjac flour addition on the gel system of the mixed emulsion were explored. Sensory evaluation experiments showed that the overall acceptability of fat substitutes added with 1% SPI was higher. With the increase of protein and konjac content, the juiciness of the samples decreased gradually. The increase of konjac content reduced the brightness of compound fat substitutes, and the yellowness of compound fat substitute increases significantly with the increase of protein content. The rheological results showed that the G′ and loss modulus (G″) increased with the increase of protein and konjac content, forming a rigid elastic gel matrix, which provided a basis for the preparation of fat substitutes. Texture profile analysis (TPA) results showed that the springiness of all samples was similar to the natural fat after 20 min of heating. With the increase of protein and konjac content, the hardness of the samples increased gradually. The results of oral tribology showed that the friction coefficients of all samples were very small. The friction behavior of the samples with SPI content of 1% was similar to that of natural fat, which could better simulate the swallowing feeling and lubricity of natural fat. To sum up, the appearance of solid fat substitutes prepared with SPI and konjac flour is similar to pork fat. They show ideal functional characteristics in mechanical properties and oral tribology. Among them, the fat substitute with the protein content of 1% and konjac content of 4% is the most popular among consumers.

Construction of 3D printed reduced-fat meat analogue by emulsion gels. Part II: Printing performance, thermal, tribological, and dynamic sensory characterization of printed objects

The manufacture of a high-quality meat analogue needs the utilization of appropriate ingredients through the application of cutting-edge technology to mimic the functional properties of traditional meat without negatively affecting the product features. Here, the reduced-fat soy-based emulsion gels, prepared by biosurfactant variants, were printed via an extrusion-based printer to manufacture a well-defined 3D structure. The printing performance revealed the 3D printed meat analogues formulated by dodecenyl succinylated inulin and ethyl (hydroxyethyl) cellulose presented a finer resolution compared to acetylated and octenyl succinic anhydride modified starches. The microstructure images provided evidence for the formation of an interconnected network with a highly porous structure in the 3D printed samples. Thermal and crystalline behaviors showed that the biosurfactants affected the crystalline structure, offering an increase in the hardness. The oral tribology measurements lead to the conclusion that the presence of biosurfactants reduced the friction coefficients. The presence of a large quantity of aggregated proteins was related to the possible formation of hydrogen bonds between the protein molecules. Moreover, disulfide bonds became progressively important in the samples, including modified inulin and modified cellulose, initiating the development of the fibrous structure. Temporal dominance of sensations also indicated that the biosurfactants induced a fibrous sensation in the printed meat analogues. The results of this study demonstrated how the application of emulsion gels in 3D printing process can further produce reduced-fat meat analogues with desired 3D structure and modified textures for enhanced eating experiences.

Development of a simulated tongue substrate for in vitro soft “oral” tribology study

The tribological principle of the relative movement amongst food particles as well as between food particles and oral surface is considered as an important physical mechanism governing food oral processing and sensory perception. Various in vitro techniques which mimic oral lubrication have been reported in literature, but most of such approaches were not representative of oral features, in particular the substrate. Therefore, it is necessary to reveal the topographic characteristics of human tongue and the morphological features of fungiform and filiform papillae. With this background, this work applied imaging technique to investigate distributions of the fungiform and filiform papillae among young Chinese subjects. The distribution and average size of the two types of papillae were analyzed. A significant negative correlation was observed between the density of filiform papillae and average tongue roughness (r = −0.47, p < 0.01). A substrate simulating topographic features as well as mechanical strength of human tongue was fabricated using a moulding technique. The reliability of the substrate was verified by lubrication testing over a wide range of sliding speed and surface load, using water and syrup solutions as model systems. Results show that the current substrate provides a feasible application to soft tribometers for in vitro oral tribology study.

Application of Pickering emulsions in 3D printing of personalized nutrition. Part II: Functional properties of reduced-fat 3D printed cheese analogues

The current interest in personalized food through the application of surface-active biopolymers and additive manufacturing has identified a requirement to tailor a 3D printed healthy diet with well-defined geometries. In this study, a reduced-fat casein-based Pickering emulsion was stabilized via different ratios of acetylated microcrystalline cellulose, and then it was printed through an extrusion-based printer. The micro-biosurfactant contents led to differences in the printing performance and resolution of printed architectures. The original compact and uneven structures of 3D printed objects were reduced in presence of micro-biosurfactant, providing a uniform and porous matrix. Thermal investigation indicated that oil and aqueous phases in the reduced-fat 3D printed systems were less readily frozen. Proton molecular mobility became less mobile and moved to the populations with a shorter relaxation time upon micro-biosurfactant addition. Oral tribology was shown to depend on micro-biosurfactant content, where less deformable (firmer) 3D printed objects decreased surface-surface contact and reduced friction coefficients, improving lubrication property. The dynamic sensory evaluation by temporal dominance of sensations showed promising results regarding creaminess and mouth-coating, which was correlated well with instrumental readings. This work established an integrated attitude, from fundamental and practical viewpoints, supporting the application of Pickering emulsions in the 3D printing process to develop functional foods, broadening the micro-biosurfactant utilization in food printing.

Oral tribology, adsorption and rheology of alternative food proteins

Mechanistic knowledge using tribology and adsorption may help to screen various proteins with better lubrication; aiding the fast tracking of new ingredient formulations for use in low-fat/high protein food development. The aim of this study was to compare the lubrication, adsorption and physicochemical properties of alternative proteins (pea, potato, lupin and insect proteins) with whey protein isolate (WPI) as the control. Soluble fractions (1–10 wt%) of pea protein concentrate (PPCsol), insect protein concentrate (IPCsol), potato protein isolate (PoPIsol) and lupin protein isolate (LPIsol) were chosen as the alternative proteins. All proteins were negatively-charged at neutral pH and showed various degrees of aggregation (hydrodynamic diameters ranging from 25 nm for PoPIsol to 244 nm for PPCsol). The boundary friction coefficient (μ) at 5 wt% protein followed the trend as PPCsol > LPIsol > IPCsol > PoPIsol > WPIsol, highlighting excellent lubrication performances of PoPIsol, IPCsol and WPIsol. At higher protein concentrations (10 wt%), μ significantly increased for LPIsol, PoPIsol and IPCsol, while decreasing for WPIsol. Quartz crystal microbalance with dissipation monitoring (QCM-D) results revealed formation of rigid elastic films on hydrophobic surfaces by PoPIsol and WPIsol giving rise to low μ while more viscous films by PPCsol led to high μ. PPCsol had the highest hydrated mass (11.0 mg m−2) as compared to WPIsol (8.0 mg m−2) with lower values reported for other proteins (5.0–5.4 mg m−2). Strong correlations existed between μ scaled to viscosity, size and hydrated mass and viscoelasticity of films in alternative proteins, validating the surface-linked phenomena in frictional response.

A new biomimetic set-up to understand the role of the kinematic, mechanical, and surface characteristics of the tongue in food oral tribological studies

A biomimetic tribological set-up was developed to better integrate physiological characteristics in oral friction studies. Polyvinyl alcohol was used to design tongue-mimicking samples (TMSs) with controlled bulk rigidity and surface roughness. The set-up allowed normal stresses and sequences of shearing motion to be applied between a TMS and a rectangular aluminum plate mimicking the hard palate. Cottage cheese with and without suspended microcrystalline cellulose particles of two sizes (small: 15 μm and large: 250 μm) was used as the model food in the experiments, making it possible to introduce heterogeneities well above the perception threshold, which often get excluded in conventional tribology studies.The feasibility of the set-up was tested by measuring how friction coefficient values were affected by food properties (viscosity, particle presence), TMS properties (surface roughness, bulk rigidity), and operational parameters (normal stress, shearing velocity). The measurements with the set-up were found to be repeatable, confirming its reliability. Particle presence led to increases in friction coefficient values, while food viscosity showed less of an influence, comparatively. Increases in the surface roughness and bulk rigidity of the TMSs led to a pronounced augmentation of the friction coefficient values. The friction coefficient values showed dependence on normal stress, increase in normal stress led to significant decrease in friction. Moreover, friction notably increased between the lowest and highest shearing velocities. In conclusion, this set-up proved to be efficient and reliable in measuring friction coefficient values and variation, paving the way for a better understanding of oral tribology and its dynamic nature.

Tribology and rheology of bead-layered hydrogels: Influence of bead size on sensory perception

The aim of this study was to understand the influence of the size of soft beads embedded in layered hydrogels on mechanical performance as well as sensory discrimination and perception. Layered hydrogels were designed using a monolayer of calcium alginate (CaA) beads of small, medium and large size (diameter of 805, 1413 or 1725 μm, respectively) sandwiched in between layers of kappa-carrageenan (κC) gel matrix, with controls created using pure κC hydrogels and κC + sodium alginate (NaA) mixed gels. Large deformation rheology of the hydrogels followed by apparent viscosity as well as tribological properties of the hydrogel boli (after homogenising with simulated saliva) were analysed. Sensory discrimination tests (n = 113) and intensity ratings (n = 60) were conducted with untrained panellists. Bead size did not have an influence on the rheological properties of the layered hydrogels and hydrogel boli, respectively (p > 0.05). However, the lubrication behaviour of the layered hydrogel boli was influenced by bead size, with gels containing large-sized beads showing highest lubrication in both boundary and mixed regimes (p < 0.05). Although panellists were able to discriminate non-layered gels from bead-layered counterparts based on textural attributes, such as “hard”, “chewy” and “pasty”, they could not distinguish between small and large-sized bead-layered gels in contrast to the oral tribology results. The low modulus of the beads appeared to be the limiting factor to detect differences based on bead sizes in this study. Findings on instrumental characterization and consumer perception of bead-layered hydrogels can have important implications for development of novel food texture.