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.