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.

Chemical composition of bunya nuts (Araucaria bidwillii) compared to Araucaria angustifolia and Araucaria araucana species.

Three of nineteen Araucaria tree species from around the world produce large edible seeds. While composition is established for edible pinhão and piñones nuts from Brazil and Chile, respectively, the first detailed characterisation for the composition of edible Araucaria bidwillii (bunya nut) from Australia is provided. Almost half of the kernel weight is moisture and the main component in the dried kernel is starch. Whilst low in protein and fat, it contains all essential amino acids and half the fatty acids are polyunsaturated (Omega-3 and 6). Bunya nuts are a source of dietary fibre, folate and minerals (Cu, Mn, Fe, Mg), while the nut husks and inner coating are high in phenolics, mainly catechin. The composition supports the Traditional Knowledge of Aboriginal Australians that the bunya nut is an energy dense and nutrient rich food. Similarities in the composition among the three different edible varieties were found, which should assist in developing sustainable value chain propositions via shared knowledge on processing and utilisation.

Sensory properties of Australian bunya nuts.

Bunya nuts are the seeds of Araucaria bidwillii, a conifer native to South-East Queensland, Australia. They are one of the 19 species of Araucaria family found around the world, with the nuts from South America being the most commonly consumed. They are traditionally eaten boiled or roasted. This study aims to profile the sensory properties of bunya nuts with chestnut as a comparator. Since chestnuts do not come from a conifer tree, it is expected that there will be differences. Different methods of preparation are also expected to change the sensory attributes. Representative samples were collected from a variety of locations in South-East Queensland, prepared and presented to a panel of 14 experienced tasters applying conventional sensory descriptive profiling. During training, the panel developed a lexicon of 23 sensory attributes together with definitions and reference. Profiles of the boiled and roasted bunya nuts revealed higher scores for hardness on the first bite than chestnuts and, when chewed, became more crumbly, dry, and grainy. They had a savory aroma and flavor, and roasted samples exhibited a roasted aroma. Bunya nut samples were less sweet than chestnut samples. Differences in the sensory properties due to method of preparation were also observed. Boiled bunya nuts were softer and moister, with lower scores for crumbly and grainy. This research is foundational in providing technical information on the sensory profile of this important Indigenous Australian nut and provides a strong basis to support novel food sector opportunities for the bunya nut as a reemerging food source not only in Australia, but also South America. PRACTICAL APPLICATION: There is an increase demand for local, sustainable, and natural foods. Bunya nuts are native to Australia and are part of the Araucaria family, which includes 19 species that can be found around the world. To the best of our knowledge there is no study characterizing Araucaria nuts in terms of sensory attributes. This study builds a lexicon for bunya nuts and compares to chestnuts. It also shows how different preparation methods affect its sensory attributes, as well as possible future uses in product development. The outcomes might provide information to support studies on Araucaria nuts in other countries.

Viscoelasticity of non-colloidal hydrogel particle suspensions at the liquid-solid transition.

Suspensions of soft particles transition from a viscous fluid to a soft material upon increases in phase volume. The criteria defining the transition to this jammed state are difficult to define due to the porous and deformable nature of soft particles. Here, we characterise the rheology of aqueous suspensions of industrially relevant non-colloidal, polydisperse, frictional agarose microgels and evaluate shear and viscoelastic behaviour across a range of phase volumes from the dilute regime to the highly concentrated regime. In order to model the viscoelastic response of suspensions without free fitting parameters, the random close packing volume fraction (φrcp) and the particle modulus are determined, respectively, from particle size distribution measurements and direct measurements of reduced elastic modulus of individual particles (Erp) using Atomic Force Microscopy. It is found that at φrcp, previously shown to correspond to divergence of the viscosity, also corresponds to the suspension transition from a viscous to viscoelastic fluid. However, the transition to a jammed solid-like state (φj) occurs at phase volumes exceeding this value (i.e. φj > φrcp). The suspension modulus and its sudden growth at φj are well-predicted by the Evans and Lips model that incorporates the Erp of the hydrogel particles. This rheological behaviour showing a dual transition is reminiscent of two families of systems: (i) colloidal suspensions and (ii) frictional-adhesive non-colloidal suspensions. However, it does not strictly follow either case. We propose that the width of the transition region is dictated by frictional contact, particle size distribution and particle modulus, and plan to further probe this in future work.

The biofilm matrix scaffold of Pseudomonas aeruginosa contains G-quadruplex extracellular DNA structures.

Extracellular DNA, or eDNA, is recognised as a critical biofilm component; however, it is not understood how it forms networked matrix structures. Here, we isolate eDNA from static-culture Pseudomonas aeruginosa biofilms using ionic liquids to preserve its biophysical signatures of fluid viscoelasticity and the temperature dependency of DNA transitions. We describe a loss of eDNA network structure as resulting from a change in nucleic acid conformation, and propose that its ability to form viscoelastic structures is key to its role in building biofilm matrices. Solid-state analysis of isolated eDNA, as a proxy for eDNA structure in biofilms, reveals non-canonical Hoogsteen base pairs, triads or tetrads involving thymine or uracil, and guanine, suggesting that the eDNA forms G-quadruplex structures. These are less abundant in chromosomal DNA and disappear when eDNA undergoes conformation transition. We verify the occurrence of G-quadruplex structures in the extracellular matrix of intact static and flow-cell biofilms of P. aeruginosa, as displayed by the matrix to G-quadruplex-specific antibody binding, and validate the loss of G-quadruplex structures in vivo to occur coincident with the disappearance of eDNA fibres. Given their stability, understanding how extracellular G-quadruplex structures form will elucidate how P. aeruginosa eDNA builds viscoelastic networks, which are a foundational biofilm property.

Frictional behaviour of molten chocolate as a function of fat content.

Soft tribology is used to probe the lubrication behaviour of molten chocolate between soft contacts, analogous to in-mouth interactions between the tongue and palate. Molten chocolate is a concentrated suspension of solid particles (sugar, cocoa and milk solids) in cocoa butter. We hypothesise that the complex frictional behaviour of molten chocolate depends on its particulate nature and thus solid volume fraction (sugar & cocoa solids/fat content). In this work, we assess the properties of molten chocolate as a function of fat content by diluting milk chocolate containing 26, 27 and 29% fat with cocoa butter. The tribological behaviour of molten chocolate deviates notably from the typical Stribeck curve of Newtonian fluids. Additional transitions are observed in mixed and elastohydrodynamic lubrication which are respectively attributed to the effect of shear-thinning rheology (i.e. breakdown of aggregates) and the selective entrainment or exclusion of particles depending on interfacial gap height. These transitions are more pronounced in chocolate of high solid fraction, and correlate with the influence of particle aggregation on rheology. In addition, we assess oral lubrication by preparing model chocolate boluses with aqueous buffer, which produces a ternary system of oil droplets and insoluble cocoa solids dispersed within a continuous aqueous phase. The frictional behaviour of chocolate boluses is determined by the viscosity ratio between cocoa butter and aqueous phase, in agreement with previous findings for oil-in-water emulsions. We provide a conceptual model to interpret how fat content influences the oral lubrication and mouthfeel of chocolate during consumption.

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.

Particle-wall tribology of slippery hydrogel particle suspensions.

Slip is an important phenomenon that occurs during the flow of yield stress fluids like soft materials and pastes. Densely packed suspensions of hydrogel microparticles are used to show that slip is governed by the tribological interactions occurring between the samples and shearing surfaces. Both attractive/repulsive interactions between the dispersed particles and surface, as well as the viscoelasticity of the suspension, are found to play key roles in slip occurring within rheometric flows. We specifically discover that for two completely different sets of microgels, the sliding stress at which slip occurs scales with both the modulus of the particles and the bulk suspension modulus. This suggests that hysteresis losses within the viscoelastic particles contribute to friction forces and thus slip at the particle-surface tribo-contact. It is also found that slip during large amplitude oscillatory shear and steady shear flows share the same generic features.

Viscosity of soft spherical micro-hydrogel suspensions.

The rheology of soft particle suspensions is considered to be a function of particle micromechanics and phase volume. However, soft particles such as microgels present a challenge because they typically contain solvent in their polymeric network structure, and their specific volume can alter in response to mechanical forces and physiochemical effects. We investigate how particle elasticity affects the viscosity of microgel suspensions as a function of effective phase volume (ϕ0) using non-colloidal hydrogel spheres that, unlike many colloidal-scale microgels, are not highly responsive to physiochemical effects. In our unique approach, we compare the viscosity of microgel suspensions to a theoretical hard sphere viscosity model that defines the maximum packing fraction using the geometric random close packing fraction (ϕrcp) obtained from the measured particle size distribution. We discover that our harder microgels follow the hard sphere model up to random close packing, but softer microgels deviate around ϕ0/ϕrcp∼50% which indicates that their specific volume is decreasing with increasing ϕ0. This effect arises because microgels at high phase volumes do not fully re-swell during their preparation. We conclude that particle elasticity does not directly affect the viscosity of soft sphere suspensions up to the random close packing fraction. We highlight a convenient method for analysing the viscosity of microgel suspensions with potential to be applied to a wide variety of soft sphere suspensions.