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.

Carrageenan-induced conjugated oat protein isolate microgel particles as structure modulators in fat analogues and their digestion behaviors

HYPOTHESIS

Engineering plant-based microgel particles (MPs) at a molecular scale is meaningful to prepare functional fat analogues. We hypothesize that oat protein isolate (OPI) and κ-carrageenan (CA) have synergy in MPs formation, using MPs with controllable structure, and further to fabricate fat analogues with adjustable characteristics is feasible. Their digestion fate will also be possibly modulated by interfacial coatings.

EXPERIMENTS

OPI-based conjugated MPs with tunable rigidities by changing crosslinking densities were designed. The relationship between microgel structures, and emulsion gel properties was explored through spectroscopy, microstructure, rheology and tribology. The delivery to lycopene, as well as inhibiting digestion behaviors of fat analogues was evaluated in a simulated gastro-intestinal tract.

FINDINGS

The rigidity of conjugated MPs could be tailored to optimize the performance of fat analogues. OPI-1 %CA MPs could stabilize emulsions up to 95 % oil fraction with fine texture. Tribological behaviors had a dependence on microgel elasticity and interfacial coatings, medium hard MP-stabilized emulsion was less disrupted without coalescence after oral processing. Digestion was delayed by denser and harder MPs by softening the interfacial particle layer or limiting lipase accessibility. Softer conjugated MPs possessed better flexibility and were broken down more easily leading to a higher rate of lipid digestion.

Structure Response of Preadsorbed Saliva Pellicle to the Interaction between Dairy and Saliva Protein

Using the surface characterization techniques of quartz crystal microbalance with dissipation, atomic force microscopy, and scanning electron microscopy, the structure of the salivary pellicle was investigated before and after it was exposed to dairy proteins, including micellar casein, skim milk, whey protein isolate (WPI), and a mixture of skim milk and WPI. We have shown that the hydration, viscoelasticity, and adsorbed proteinaceous mass of a preadsorbed salivary pellicle on a PDMS surface are greatly affected by the type of dairy protein. After interaction with whey protein, the preadsorbed saliva pellicle becomes softer. However, exposure of the saliva pellicle to micellar casein causes the pellicle to partially collapse, which results in a thinner and more rigid surface layer. This structure change correlates with the measured lubrication behavior when the saliva pellicle is exposed to dairy proteins. While previous studies suggest that whey protein is the main component in milk to interact with salivary proteins, our study indicates interactions with casein are more important. The knowledge gained here provides insights into the mechanisms by which different components of dairy foods and beverages contribute to mouthfeel and texture perception, as well as influence oral hygiene.

Effect of Solid Fat Content in Fat Droplets on Creamy Mouthfeel of Acid Milk Gels

Previous studies have shown that emulsions with higher solid fat content (SFC) are related to a higher in-mouth coalescence level and fat-related perception. However, the effect of SFC in fat droplets on the fat-related attributes of emulsion-filled gels has not been fully elucidated. In this study, the effect of SFC on the creamy mouthfeel of acid milk gel was investigated. Five kinds of blended milk fats with SFC values ranging from 10.61% to 85.87% were prepared. All crystals in the blended milk fats were needle-like, but the onset melting temperature varied widely. Blended milk fats were then mixed with skim milk to prepare acid milk gels (EG10−EG85, fat content 3.0%). After simulated oral processing, the particle size distribution and confocal images of the gel bolus showed that the degree of droplet coalescence in descending order was EG40 > EG20 > EG60 > EG10 ≥ EG85. There was no significant difference in apparent viscosity measured at a shear rate of 50/s between bolus gels, but the friction coefficients measured at 20 mm/s by a tribological method were negatively correlated with the coalescence result. Furthermore, quantitative descriptive analysis and temporal dominance of sensations analysis showed that SFC significantly affected the ratings of melting, mouth coating, smoothness and overall creaminess, as well as the perceived sequence and the duration of melting, smoothness and mouth coating of acid milk gels. Overall, our study highlights the role of intermediate SFC in fat droplets on the creamy mouthfeel of acid milk gels, which may contribute to the development of low-fat foods with desirable sensory perception.

The Effect of Corn Dextrin on the Rheological, Tribological, and Aroma Release Properties of a Reduced-Fat Model of Processed Cheese Spread

Low-calorie and low-fat foods have been introduced to the market to fight the increasing incidence of overweightness and obesity. New approaches and high-quality fat replacers may overcome the poor organoleptic properties of such products. A model of processed cheese spread (PCS) was produced as a full-fat version and with three levels of fat reduction (30%, 50%, and 70%). Fat was replaced by water or by corn dextrin (CD), a dietary fiber. Additionally, in the 50% reduced-fat spreads, fat was replaced by various ratios of CD and lactose (100:0, 75:25, 50:50, 25:75, and 0:100). The effect of each formulation was determined by measuring the textural (firmness, stickiness, and spreadability), rheological (flow behavior and oscillating rheology), tribological, and microstructural (cryo-SEM) properties of the samples, as well as the dynamic aroma release of six aroma compounds typically found in cheese. Winter’s critical gel theory was a good approach to characterizing PCS with less instrumental effort and costs: the gel strength and interaction factors correlated very well with the spreadability and lubrication properties of the spreads. CD and fat exhibited similar interaction capacities with the aroma compounds, resulting in a similar release pattern. Overall, the properties of the sample with 50% fat replaced by CD were most similar to those of the full-fat sample. Thus, CD is a promising fat replacer in PCS and, most likely, in other dairy-based emulsions.

Characterization of Reduced-Fat Mayonnaise and Comparison of Sensory Perception, Rheological, Tribological, and Textural Analyses

Reduced-fat products can help to fight obesity and its associated health risks. To develop appealing products, both product-specific fat replacers and suitable analytical methods for the characterization of fat-associated properties are important. The rheology, tribology, texture, and spreadability of a reduced-fat mayonnaise with different concentrations of corn dextrin were analyzed to determine properties such as flow behavior, viscosity, lubricity, firmness, and stickiness. Additionally, a sensory panel analyzed the samples for their mouthfeel (creaminess, firmness, and stickiness). Correlations between the results of the instrumental methods suggested that the analytical effort for the future development of appealing reduced-fat food products can be reduced. In addition, several correlations were identified between the instrumental and the sensory data. Results from tribological measurements correlated with the sensory attribute of stickiness, suggesting that tribometry can complement or constitute an alternative to complex and expensive human sensory tests. Additionally, the use of Stevens' power law showed a high correlation between the Kokini oral shear stress and the sensory attribute of creaminess. The instrumental texture properties (firmness, stickiness) also correlated with the sensory sensation. The identified correlations obtained from comparing different methods may help to estimate the possible applications of new fat replacers and facilitate innovative product development.

Application of texture analysis methods for the characterization of cultured meat

Mechanical characterization supposes a key step in the development of cultured meat to help mimicking the sensorial properties of already existing commercial products based on traditional meat. This work presents two well stablished methods that can help studying cultured meat mechanical characteristics: texture profile analysis (double compression test) and rheology. These techniques provide data about the elastic and viscous behaviour of the samples but also values about other texture characteristics such as springiness, cohesiveness, chewiness and resilience. In this work, we present a comparison of cultured meat-based samples with commercial of the shelf common meat products (sausage, turkey and chicken breast). Results show that both Young's and Shear modulus in the cultured meat samples can be compared to commercial products in order to understand its properties. The texture characteristics for the cultured meat studied, show values within the range of commercial products. These results demonstrate the applicability of this methodology for the adjustment of mechanical properties of cultured meat products.

Roles of viscosity, applied load and surface wettability on the lubrication behaviour of model liquid/semi-solid foods: Measurements with a bespoke tribo-cell fixture and rotational rheometer

Thin film sliding and friction phenomena of food bolus confined between tongue-palate surfaces during oral processing can be explored using tribological measurements. However, these measurements are still limited within the food industry due to the requirement of expensive commercial instruments which are not commonly used in the food industry. This work has designed and manufactured a modular "tribological cell" (tribo-cell) that can simulate lubricated soft-hard contact interfaces and can be mounted on a rotational rheometer to perform tribological measurements. The tribo-cell was validated by performing tribological measurements using a range of corn syrup solutions as model liquid foods. It was shown that the Stribeck curve describing the change in friction behaviour with entrainment speed or with the product of entrainment speed and liquid viscosity could be obtained. Since tribology deals with surface property, the cell was then used in the further studies to demonstrate the effects of applied normal load and surface wetting on the tribological response of lubricated hard-soft contact of the designed fixture. These parameters were shown to have a marked influence on in the boundary and mixed-lubrication regimes. The designed tribo-cell was also used to illustrate the impact of fat content on the lubrication properties of commercial liquid and semi-solid foods with different fat contents, thus, pointing out to the importance of tribology as a vital tool for product formulation designs in food and beverage industry.

Rheological behaviour of attractive emulsions differing in droplet-droplet interaction strength

HYPOTHESIS

We hypothesise that interaction strength between oil droplets determine the rheological properties of oil-in-water (O/W) emulsions by simultaneous formation and break-up of bonds between droplets. Using small (SAOS) and large (LAOS) amplitude oscillatory shear measurements, we aim to distinguish different classes of emulsions based on the specific microstructural evolution of the emulsions.

EXPERIMENTS

Concentrated O/W emulsions differing in droplet-droplet interaction strength were obtained. Different interaction strength was obtained using different types of interactions; (a) electrostatic attraction, (b) salt bridging, or (c) crosslinking.

FINDINGS

In line with our hypothesis, different rheological events in emulsions depend on the droplet-droplet interaction strength. Strong interactions lead to monotonous yielding, and droplets undergo jamming or densification to provide strain hardening and gel-like behaviour. Emulsions with weak interactions exhibit two-step yielding (SAOS) and continuous yielding in LAOS; indicating a soft-glassy material. In emulsions above maximum packing, and with weak interactions the rheology is controlled by cluster/cage breaking, and transient formation of new clusters. For medium-strength interactions, two-step yielding was reduced, and apparent stain-hardening occurred. The probability of two distinct time scales of yielding is hindered by stronger interactions and jamming. Overall, in concentrated emulsions, yielding is determined by network rupture and reformation, cluster rearrangement and -breaking, which in turn is influenced by interaction type and strength. We present a more differentiated categorisation of emulsions based on interaction strength.

Food polysaccharides and roles of rheology and tribology in rational design of thickened liquids for oropharyngeal dysphagia: A review

In today's market environment, an aging society is recognized as one of the megatrends in the world. The demographic change in the world population age structure has driven a huge demand in healthcare products as well as services that include the technological innovation for the health and wellness of the elderly. Dysphagia or swallowing difficulty is a common problem in the elderly as many changes in swallowing function come with aging. The presence of a strong relationship between swallowing ability, nutritional status, and health outcomes in the elderly leads to the importance of dysphagia management in the population group. Modification of solid food and/or liquid is a mainstay of compensatory intervention for dysphagia patients. In this regard, texture-modified foods are generally provided to reduce risks associated with choking, while thickened liquids are recommended for mitigating risks associated with aspiration. In this review, we discuss thickened liquids and other issues including the importance of their rheological and tribological properties for oropharyngeal dysphagia management in the elderly. The review focuses on both commercial thickeners that are either based on modified starch or xanthan gum and other potential polysaccharide alternatives, which have been documented in the literature in order to help researchers develop or improve the characteristic properties of thickened liquids required for safe swallowing. Furthermore, some research gaps and future perspectives, particularly from the nutrition aspect related to the interaction between thickeners and other food ingredients, are suggested as such interaction may considerably control the rate of nutrient absorption and release within our body.

Sensorial Perception of Astringency: Oral Mechanisms and Current Analysis Methods

Understanding consumers' food choices and the psychological processes involved in their preferences is crucial to promote more mindful eating regulation and guide food design. Fortifying foods minimizing the oral dryness, rough, and puckering associated with many functional ingredients has been attracting interest in understanding oral astringency over the years. A variety of studies have explored the sensorial mechanisms and the food properties determining astringency perception. The present review provides a deeper understanding of astringency, a general view of the oral mechanisms involved, and the exciting variety of the latest methods used to direct and indirectly quantify and simulate the astringency perception and the specific mechanisms involved.

Interfacial polymerization of a thin film on contact lenses for improving lubricity

HYPOTHESIS

A large number of contact lens wearers drop out each year due to end of day discomfort, which could possibly be reduced by designing lenses with highly lubricious surfaces. We hypothesize that polymerizing a thin film of dimethyl acrylamide (DMA) on the surface of the lenses will improve lubricity.

EXPERIMENTS

The thin film is polymerized by loading a commercial contact lens (1-DAY ACUVUE® TruEye®) with N,N,N',N'-Tetramethylethane-1,2-diamine (TEMED) and soaking it in a solution of DMA and ammonium per sulfate (APS). The two components of the redox couple (APS and TEMED) mix near the surface of the lens due to diffusion and react rapidly to form free radicals. The free radicals lead the polymerization of the DMA monomer near the surface resulting in the formation of the thin hydrogel layer that is attached to the lens matrix through activation of unreacted vinyl groups or possibly through formation of entanglements with the lens polymer.

FINDINGS

The thickness of the layer is controlled by the polymerization time which is limited to 30 s to create a layer of DMA only at the surface. The presence of the DMA layer is confirmed through measurements of Fourier-transform infrared spectroscopy (FTIR) spectra in total internal reflection mode. The layer is determined to be about 3-5 µm thick with a water content of about 285%. The presence of the layer significantly improves lubricity as is evident through the qualitative rubbing test and quantitative measurement of the friction coefficient. A preliminary one-week safety study in rabbits show that lens wear does not cause any toxicity.

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.

Human saliva and model saliva at bulk to adsorbed phases - similarities and differences

Human saliva, a seemingly simple aqueous fluid, is, in fact, an extraordinarily complex biocolloid that is not fully understood, despite many decades of study. Salivary lubrication is widely believed to be a signature of good oral health and is also crucial for speech, food oral processing and swallowing. However, saliva has been often neglected in food colloid research, primarily due to its high intra- to inter-individual variability and altering material properties upon collection and storage, when used as an ex vivo research material. In the last few decades, colloid scientists have attempted designing model (i.e. 'saliva mimicking fluid') salivary formulations to understand saliva-food colloid interactions in an in vitro set up and its contribution on microstructural aspects, lubrication properties and sensory perception. In this Review, we critically examine the current state of knowledge on bulk and interfacial properties of model saliva in comparison to real human saliva and highlight how far such model salivary formulations can match the properties of real human saliva. Many, if not most, of these model saliva formulations share similarities with real human saliva in terms of biochemical compositions, including electrolytes, pH and concentrations of salivary proteins, such as α-amylase and highly glycosylated mucins. This, together with similarities between model and real saliva in terms of surface charge, has led to significant advancement in decoding various colloidal interactions (bridging, depletion) of charged emulsion droplets and associated sensory perception in the oral phase. However, model saliva represents significant dissimilarity to real saliva in terms of lubricating properties. Based on in-depth examination of properties of mucins derived from animal sources (e.g. pig gastric mucins (PGM) or bovine submaxillary mucin (BSM)), we can recommend that BSM is currently the most optimal commercially available mucin source when attempting to replicate saliva based on surface adsorption and lubrication properties. Even though purification via dialysis or chromatographic techniques may influence various physicochemical properties of BSM, such as structure and surface adsorption, the lubricating properties of model saliva formulations based on BSM are generally superior and more reliable than the PGM counterpart at orally relevant pH. Comparison of mucin-containing model saliva with ex vivo human salivary conditioning films suggests that mucin alone cannot replicate the lubricity of real human salivary pellicle. Mucin-based multi-layers containing mucin and oppositely charged polyelectrolytes may offer promising avenues in the future for engineering biomimetic salivary pellicle, however, this has not been explored in oral tribology experiments to date. Hence, there is a strong need for systematic studies with employment of model saliva formulations containing mucins with and without polycationic additives before a consensus on a standardized model salivary formulation can be achieved. Overall, this review provides the first comprehensive framework on simulating saliva for a particular bulk or surface property when doing food oral processing experiments.

Tribology and its growing use toward the study of food oral processing and sensory perception

Here we provide a comprehensive review of the knowledge base of soft tribology, the study of friction, lubrication, and wear on deformable surfaces, with consideration for its application toward oral tribology and food lubrication. Studies on "soft-tribology" have emerged to provide knowledge and tools to predict oral behavior and assess the performance of foods and beverages. We have shown that there is a comprehensive set of fundamental literature, mainly based on soft contacts in the Mini-traction machine with rolling ball on disk configuration, which provides a baseline for interpreting tribological data from complex food systems. Tribology-sensory relationships do currently exist. However, they are restricted to the specific formulations and tribological configuration utilized, and cannot usually be applied more broadly. With a careful and rigorous formulation/experimental design, we envisage tribological tools to provide insights into the sensory perception of foods in combination with other in vitro technique such as rheology, particle sizing or characterization of surface interactions. This can only occur with the use of well characterized tribopairs and equipment; a careful characterization of simpler model foods before considering complex food products; the incorporation of saliva in tribological studies; the removal of confounding factors from the sensory study and a global approach that considers all regimes of lubrication.

Finite size effects of ionic species sensitively determine load bearing capacities of lubricated systems under combined influence of electrokinetics and surface compliance

The behaviour and health of lubricated systems in various natural and artificial settings are often characterized by their load bearing capacity. This capacity stemming from the lift force associated with confined fluid flow can be significantly altered due to surface compliance and electrokinetic effects. Here, we highlight the influence of finite size of the ionic species participating in electrokinetic transport with substrate compliance in determining the electromechanical characteristics of lubricated systems. With these new considerations, anomalous trends previously observed for the load bearing capacity corresponding to high values of zeta potential are corrected. Simultaneously, trends associated with the finite ionic size are also found to be reversed, but fall in line with the consistent theory. Importantly, despite an intricate interplay among the various influences - electrokinetic, hydrodynamic, geometric, and elastic - previously established trends due to geometric (non-parallel slider geometry) and elastic effects are found to persist. Specifically, in the presence of electrokinetic effects, an increase in the obliqueness of the slider geometry results in lower values of load bearing capacity while an increase in the stiffness leads to higher values. These results point to a certain robustness in the overall theory and it is hoped that they can contribute to better practical designs of slider bearings and an improved understanding of lubricated sliding surfaces in biological settings.

Impact of oral processing on texture attributes and taste perception

Mastication is the first step of food digestion, where foods are broken down and simultaneously impregnated by saliva resulting in the formation of semi-fluids known as food boluses. This review focuses on the impact of oral processing on texture attributes and taste perception. The article describes the oral actions in which texture characteristic are measured for the critical conditions that trigger swallowing. Taste perception also plays a key role in oral processing and oral sensations. There are still challenges in terms of determining different oral physiological characteristics. These include individual chewing behavior regardless of the temporal aspects of dominant processes of comminution, insalivation, bolus formation and swallowing. A comprehensive approach is essential to process favorable foods with respect to the food properties of texture and taste.

A comparison of the lubrication behavior of whey protein model foods using tribology in linear and elliptical movement

Lubrication is an important factor in the sensory evaluation of food products. Tribology provides a theoretical framework and instrumental methods for evaluating frictional properties between two moving surfaces and the lubrication behavior of products between these surfaces. Relating frictional measurements to sensory properties detected during oral processing requires careful and pertinent choices in surface materials and testing conditions. The aims of this study were to investigate: (a) differences in lubrication behavior of a range of food textures and (b) the differences between linear and elliptical movement and added saliva to understand the contribution of food structure to friction. Six whey protein model food samples, ranging in texture from fluid to semisolid to soft solid, were analyzed using a pin on disk tribometer to determine the coefficient of friction (COF) across a range of sliding speeds. The samples were analyzed in their initial form and post-oral processing (n = 4) in both linear and elliptical movements. Elliptical movement slightly decreased coefficients of friction and extended the shape of the friction curve. Increases in test food viscosity decreased the COF but differences in viscosity were not apparent when test foods were mixed with saliva. Data correction for viscosity shifted the friction curves horizontally, indicating that lubrication had a greater impact upon friction than viscosity. This study provides initial insights for further comparison of linear and elliptical movement with a variety of sample compositions.

PRACTICAL APPLICATIONS

Sensory perception of smoothness and creaminess are often major contributors to overall hedonic food liking and are a major reason why products high in fat and sugar are more highly preferred over other foods. These parameters are influenced by friction and lubrication between the tongue, palate, teeth, food products, and saliva during oral processing. Tribology provides an instrumental method to evaluate friction between moving surfaces that mimic oral surfaces and the lubrication behavior of foods. Trends in frictional measurements can be correlated with sensory ratings of the same foods to better understand why preferences exist for certain foods or food compositions and how to effectively improve the acceptability and enjoyment of healthier foods.

Influence of fluid viscosity and wetting on multiscale viscoelastic lubrication in soft tribological contacts

Friction (and lubrication) between soft contacts is prevalent in many natural and engineered systems and plays a crucial role in determining their functionality. The contribution of viscoelastic hysteresis losses to friction in these systems has been well-established and defined for dry contacts; however, the influence of fluid viscosity and wetting on these components of friction has largely been overlooked. We provide systematic experimental evidence of the influence of lubricant viscosity and wetting on lubrication across multiple regimes within a viscoelastic contact. These effects are investigated for comparatively smooth and rough elastomeric contacts (PTFE-PDMS and PDMS-PDMS) lubricated by a series of Newtonian fluids with systematically controlled viscosity and static wetting properties, using a ball-on-disc tribometer. The distinct tribological behaviour, characterised generally by a decrease in the friction coefficient with increasing fluid viscosity and wettability, is explained in terms of lubricant dewetting and squeeze-out dynamics and their impact on multi-scale viscoelastic dissipation mechanisms at the bulk-, asperity-, sub-asperity- and molecular-scale. It is proposed that lubrication within the (non-molecularly) smooth contact is governed by localised fluid entrapment and molecular-scale (interfacial) viscoelastic effects, while additional rubber hysteresis stimulated by fluid-asperity interactions, combined with rapid fluid drainage at low speeds within the rough contact, alter the general shape of the Stribeck curve. This fluid viscosity effect is in some agreement with theoretical predictions. Conventional methods for analysing and interpreting tribological data, which typically involve scaling sliding velocity with lubricant viscosity, need to be revised for viscoelastic contacts with consideration of these indirect viscosity effects.

Lubrication of chocolate during oral processing

The structure of chocolate is drastically transformed during oral processing from a composite solid to an oil/water fluid emulsion. Using two commercial dark chocolates varying in cocoa solids content, this study develops a method to identify the factors that govern lubrication in molten chocolate and saliva's contribution to lubrication following oral processing. In addition to chocolate and its individual components, simulated boluses (molten chocolate and phosphate buffered saline), in vitro boluses (molten chocolate and whole human saliva) and ex vivo boluses (chocolate expectorated after chewing till the point of swallow) were tested. The results reveal that the lubrication of molten chocolate is strongly influenced by the presence of solid sugar particles and cocoa solids. The entrainment of particles into the contact zone between the interacting surfaces reduces friction such that the maximum friction coefficient measured for chocolate boluses is much lower than those for single-phase Newtonian fluids. The addition of whole human saliva or a substitute aqueous phase (PBS) to molten chocolate dissolves sugar and decreases the viscosity of molten chocolate so that thinner films are achieved. However, saliva is more lubricating than PBS, which results in lower friction coefficients for chocolate-saliva mixtures when compared to chocolate-PBS mixtures. A comparison of ex vivo and in vitro boluses also suggests that the quantity of saliva added and uniformity of mixing during oral processing affect bolus structure, which leads to differences in measured friction. It is hypothesized that inhomogeneous mixing in the mouth introduces large air bubbles and regions of non-emulsified fat into the ex vivo boluses, which enhance wetting and lubrication.