Astringency is an important characteristic of red wine quality. The sensation is generally thought to be produced by the interaction of wine tannins with salivary proteins and the subsequent aggregation and precipitation of protein-tannin complexes. From model studies them has been shown that tannins bind to proteins in three distinct stages. The first interactions involve both hydrophobic interactions and hydrogen bonding and result in the formation of protein-tannin complexes. The second interaction involves the formation of protein aggregates with bound tannins. Finally, the interaction occurs colloidal particles that lead to precipitation of protein-tannin complexes. These processes have been shown to involve hydrogen bonding. Greater concentrations of tannins have been shown to correlate directly with increases in perceived astringency and differences in tannin structures have been shown to have a substantial impact upon the efficacy of protein binding as well as the perception of astringency. Ultimately, the impact of ethanol concentration and pH on a comprehensive range of oral sensations by red wines concluded that variable ethanol concentration was responsible for differences in intensity of mouth feel attributes.