Lubrication of chocolate during oral processing

Abstract

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.

Friction Measurements with Molten Chocolate

“…From the images, we suggest there is a breakdown of the chocolate structure followed by loss of sugar ejected from the contact. In the mouth, sugar is also removed by the action of saliva [8]. Cocoa particles and free lipids released during rubbing preferentially remain in the contact zone.…”

“…It is widely considered that as the film in the tongue-palate interface is sheared and thins, it is the tribological rather than rheology mechanism which dominates [1,[7][8][9]. Chocolate as a tribological material and the effect of oral processing on friction have proved difficult to study, and very few papers have been published [7][8][9]. Typically, molten chocolate is introduced into a sliding contact and friction coefficient measured over a range or speed or load values.…”

“…Although there is a considerable body of published literature on chocolate formulation, rheology and structure, there are relatively few tribological studies [7][8][9]. There are laboratory tests to measure "snap" [11] but not "creaminess" or "smoothness".…”

“…Typically, molten chocolate is introduced into a sliding contact and friction coefficient measured over a range or speed or load values. The contacting specimens vary: polymer (PDMS)/ polymer (PDMS) [8] or polymer/ceramic [7,9] have been used. In these papers [7][8][9], the effects of chocolate composition and preparation method on friction were studied.…”

“…The contacting specimens vary: polymer (PDMS)/ polymer (PDMS) [8] or polymer/ceramic [7,9] have been used. In these papers [7][8][9], the effects of chocolate composition and preparation method on friction were studied. Although differences in the friction values and response to changing speed for different chocolate composition were observed, it was unclear how to relate these trends to mouthfeel texture attributes.…”