Astringency is a tactile sensation — a dryness and puckering on the palate — not a taste. It is caused by the binding of tea polyphenols (specifically theaflavins and catechins) to the glycoproteins in saliva, precipitating them and temporarily reducing lubrication. Higher steeping temperatures and longer times extract more tannins, increasing astringency. Milk famously reduces astringency by binding to the tannins before your saliva does. Green teas high in EGCG can be very astringent; pu-erh tends to be mellower due to fermentation.
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