Astringency of Tea Catechins: More than an Oral Lubrication Tactile Percept
Oral astringency is the dry sensation experienced in the mouth on consumption of plant-based polyphenols (catechins) found in wine and tea as well as certain fruits and vegetables. It is commonly explained as arising from the loss of lubricity owing to the precipitation of proteins from the salivary film that coats and lubricates the oral cavity. Here, we investigate this hypothesis directly by probing the impact of astringent compounds on the lubricating properties of saliva. By preadsorbing saliva onto an elastic hydrophobic substrate to form a highly lubricating and robust film under conditions designed to mimic the low pressure rubbing contacts experienced in the oral cavity (Bongaerts, Rossetti, & Stokes, 2007), we probe the interaction of this film on exposure to solutions containing tea catechins. We examine the response of the adsorbed salivary film to polyphenol structure, concentration and temperature, as well as the influence of astringency modulating solutions consisting of a thickener (maltodextrin) and milk. We find that a significant increase in friction coefficient occurs upon exposure with epigallocatechin gallate (EGCG) solutions due to a depletion of the lubricating proteins from the elastic substrates. The friction coefficient increases more rapidly with increasing EGCG concentration, this is in line with a corresponding increase in astringency perception. In addition, the inclusion of a hydrocolloid thickener in EGCG solutions caused a decrease in astringency perception probably due to specific EGCG–maltodextrin interaction and to an increase in viscosity, which lowers the friction coefficient between the elastic substrates. These findings show that the physical interaction of saliva proteins with EGCG molecules, which we probe through the loss of saliva lubricity, can be advantageously used to predict the astringent acuity of EGCG using our simple oral mimetic technique, supporting the hypothesis that astringency is related to a loss of lubrication. However, epicatechin (EC) did not alter the lubricating properties of the salivary film, although the EC solutions were perceived to be astringent, an observation which seems to question a simple causal dependence on oral lubrication. In addition to that, milk mitigated the astringency perception of EGCG solutions although considerably reducing saliva lubricity. We conclude that the depletion of saliva lubricating proteins is not necessary to obtain an astringent perception, and that astringency is unlikely to be a purely tactile percept.