Tannin

Tannins are a general term for polyphenols that contribute to the astringency in black tea. While astringency is often viewed negatively, it is the presence of tannins that provides depth, aftertaste, and sharpness to the tea. The reason milk tea works well is that the tannins provide a structure that supports the milk.

The concentration of tannins can vary significantly depending on the tea leaves; Assam and Uva tend to have higher levels, while Nuwara Eliya and first flush Darjeeling are milder. When cooled, tannins can bind with caffeine, causing a phenomenon known as 'cream down,' which results in a cloudy appearance. Therefore, for iced tea, it is preferable to use leaves with lower astringency.