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.

Solid food undergoes drastic structural changes during its transformation into a cohesive bolus, which play a key role in the perception of different textures during mastication. This article describes the structural changes in biscuits during mastication through particle agglomeration. Apparent particle size and bolus moisture content were measured at different chewing stages by image and gravimetric analyses. The microstructure of the ready-to-swallow bolus was also examined with light microscopy and cryo-scanning electron microscopy (cryo-SEM). The results reveal that moisture content increased at a parabolic rate during mastication, and the absorbed saliva increased apparent particle size and decreased particle number through agglomeration. Furthermore, both cryo-SEM and light microscopy indicate that a majority of the solid particles in the ready-to-swallow bolus were individual starch granules. The results from cryo-SEM also suggest that saliva composition varied locally within the final bolus, thereby implying that laminar mixing governs bolus formation during mastication. PRACTICAL APPLICATIONSIn materials science it is well-known that the properties of a material are related to its structure. This concept can also be applied to the food bolus: the structure of a bolus determines its mechanical and rheological properties. These properties are perceived by senses within our mouths during mastication and translate to the perception of food texture. Hence, texture perception cannot be fully understood without analyzing bolus structure. This article also suggests that bolus structure is related to the type of mixing that operates within the mouth. Identifying the mechanisms involved in mastication will contribute towards a better understanding of texture perception, and can be applied in computational models of food breakdown and flavor release. This knowledge will allow solid food structures to be designed to enhance sensory appeal or follow a particular breakdown path in health food applications. 1988;Knecht 1990;Prinz and Lucas 1997;Bornhorst and Singh 2012). Throughout this process, food texture and bs_bs_banner A journal to advance the fundamental understanding of food texture and sensory perception Journal of Texture Studies ISSN 1745-4603 89 Journal of Texture Studies 45 (2014) 89-96

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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...

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