Interaction of human whole saliva and astringent dietary compounds investigated by interfacial shear rheology

Human whole saliva (HWS) is a complex physiological secretion that performs a number of essential functions such as protection of oral health, lubrication of mouth tissues, as well as predigestion of food. In addition, saliva forms the perireceptor environment of the oral cavity and, by its interactions with food and beverages, influences the transduction of aroma, taste and tactile (mouthfeel) perceptions.We investigate and report the impact of addition of compounds commonly found in oral health and beverage products (sodium dodecyl sulphate (SDS), citric acid and tea polyphenols: epigallocatechin gallate (EGCG), epicatechin (EC), rutin) on the interfacial shear elasticity of HWS obtained from a single subject. The aim of this work is to probe the relationship between surface shear elasticity of saliva and the astringent (puckering-like) mouthfeel arising from the consumption of products containing the investigated ingredients.Saliva proteins are extremely surface active and adsorb to the air–liquid interface to form a high-elastic “solid-like” surface film. Our findings show that the interfacial shear elasticity G′ of saliva is significantly reduced when mixed with citric acid, SDS or a tea polyphenol containing a galloyl ring in the molecule (EGCG). These results support the hypothesis that astringency arises from aggregation of salivary proteins, thereby reducing lubrication of oral tissues. Minor or no effect is observed from the interaction of saliva with EC or rutin.Furthermore, we investigate the possibility of using mucins as a simple model for saliva. Two selected mucins, tested at different concentrations and ionic strength, failed to mimic the elastic interfacial behaviour shown by saliva.