Tribology of o/w Emulsions Under Mouth-like Conditions: Determinants of Friction

Abstract

Fat-perception is thought to be related to a complex interplay between fat-associated flavor release and mouth-feel. Friction sensed between the tongue and the palate seems to play a prominent role: in previous work, we have shown that emulsions that are more sensitive toward coalescence give rise to a lowering of the orally perceived and experimentally measured friction and, probably as a consequence, to an enhanced fat-perception. In this paper, we study in detail the factors determining friction of protein-stabilized emulsions using a novel mouth-mimicking tribometer and model surfaces consisting of PDMS modified in various ways (hydrophobicity, deformability, roughness). We show that unlike in many technological applications where lubrication is essentially hydrodynamic, for physiologically relevant loads, the modified PDMS is boundary and/or mixed lubricated, which is like in-mouth lubrication. We find that an increased sensitivity of the emulsions toward coalescence results in a lower friction, confirming previous results obtained with pig's tongue. Surface-induced coalescence (or spreading of emulsion droplets) seems to be very important in this, surface hydrophobicity being the dominant trigger. Viscosity of the dispersed phase does not have such a strong influence on both the measured friction and the oral perceived friction. We do find a strong influence of the presence of bulk proteins and saliva on friction. Finally, hardly any dependence of measured friction on fat content of the emulsion was observed, indicating that only a small amount of fat is needed to alter the friction.

Rheology of food materials

“…Tribology, lubrication, triborheometry, and micro-gap rheometry has recently received considerable attention to study the effect of shear, squeeze flow and the role of confinement at length scales that are approaching the length scale of the food microstructure [87][88][89][90]. For example, the lubricating properties of human saliva is influenced by health and diet conditions but also depends on the food material it interacts with [91][92][93][94].…”

Relating Food Emulsion Structure and Composition to the Way It Is Processed in the Gastrointestinal Tract and Physiological Responses: What Are the Opportunities?

“…Incorporation and controlled release in the mouth of these aroma compounds in reduced fat products in a similar way as in the full-fat product is difficult to achieve. Furthermore, the lubricating ability of liquid fats 209,210 and the possible existence of specific fatty acid receptors [13][14][15] have been suggested.…”

Colloidal aspects of texture perception

“…In literature different devices have been described to measure the friction coefficient/behaviour of food products [103,[106][107][108][109]. Lubrication properties of food varied with their ingredients (e.g.…”

“…For instance emulsified fat increased not only the viscosity of products, but in addition it has also an important separate effect by improving lubrication [2,107]. On the other hand thickeners (e.g.…”

“…Under normal conditions in the mouth, the tongue surface behaves strongly hydrophilic due to the presence of a mucous layer. In vitro measurements showed that an increased sensitivity to coalescence leads to a decrease in friction of rubber surfaces [107] and to an increased oil spreading on glass [130]. The same factor leads to an increased retention of emulsion droplets and oil at the tongue surface.…”