Relating the effect of saliva-induced emulsion flocculation on rheological properties and retention on the tongue surface with sensory perception

Perception of food emulsions can often not be directly related to the structure of the products before consumption. Taking into account the changing product structure upon oral processing might increase understanding of the relation between perception and product properties. This study aims to gain insight in the effect of saliva-induced flocculation on perception of emulsions at neutral pH. Whey protein (WPI)-stabilized emulsions flocculating in a reversible manner with saliva were compared with lysozyme-stabilized emulsions that irreversible flocculate with saliva. The main emulsion variables, besides the emulsifying protein, were oil content (2.5% oil vs 10% oil), and the effect of emulsion thickening with guar gum (at 10% oil). To relate perception to processes occurring in the oral cavity, the emulsions were characterized before and after oral processing with respect to morphology and rheological properties (viscosity, storage and loss moduli). In addition, insight in retention of emulsion droplets on the tongue surface was obtained by measuring emulsifier and oil content in tongue swabs. Saliva-induced emulsion flocculation clearly shows a large effect on perception of the here studied emulsions. WPI-stabilized emulsions showed little retention on the tongue surface and perception was characterized by creaminess, fattiness and thickness. Guar gum thickening further increased perception of these attributes. On the other hand, for lysozyme-stabilized emulsions perception was largely related to attributes like dryness, roughness and astringency. In addition, a large viscosity increase upon oral processing and clear retention of emulsion droplets on the tongue surface was observed. Guar gum thickening decreased the effects of irreversible flocculation, likely because of its lubricating properties and increased viscosity. Although the amount of mucins recovered from the tongue surface was unaffected by orally processing of lysozyme-stabilized emulsions, the sensory characteristics of these emulsions reminds one of astringency perception of e.g. tannins that precipitate salivary proteins.