Capturing the onset of oral processing: Merging of a model food emulsion drop with saliva

The events occurring before and during the merging of a model liquid food emulsion with saliva have been captured ex vivo using confocal microscopy. In the order of a few seconds, millimeter-sized drops of liquid food and saliva touch and are deformed; the two surfaces eventually collapse, resulting in the merging of the two phases, in a process reminiscent of emulsion droplets coalescing. The model droplets then surge into saliva. Based on this, two distinct stages can be distinguished for the insertion of a liquid food into the oral cavity: A first phase where two intact phases co-exist, and the individual viscosities and saliva-liquid food tribology should be important to texture perception; and a second stage, dominated by the rheological properties of the liquid food-saliva mixture. The importance of the surface properties of saliva and liquid food are highlighted, as they may influence the merging of the two phases.