Astringency

The reaction of tannins with protein is responsible for their astringency—a property of considerable importance in human and animal feeding. It can be described as the complex of sensations due to shrinking, drawing, or puckering of the epithelium in the buccal cavity. Astringency is not one of the basic taste sensations, as its effect is noticed away from the site of the primary taste receptors. It is a tactile sensation arising from reduced lubrication of the tongue and soft palate (15). The primary reaction that leads to astringency is the precipitation of proteins and mucopolysaccharides in saliva. The astringency of tannins is their most noticeable organoleptic property and is correlated with their protein complexation ability. The simple glucose-gallic acid esters bind more strongly to protein than do the proanthocyanidins because of conformational restraints imposed by the interflavan bonds of the latter group. The gallotannins are, accordingly, more astringent than the proanthocyanidins. It has been shown that proteins that are rich in the amino acid proline, and that possess open, flexible structures have the highest affinity for polyphenols. The presence of these proteins in human saliva is protective against the entrance of excessive quantities of polyphenols into the digestive tract. There is evidence that the prevalence of tannins in the human diet tends to increase the formation of the proline-rich proteins (16).

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