Catechins are the source of the astringent flavour of tea and are a type of polyphenol found in tea leaves. When tea leaves are exposed to sunlight, more catechins will accumulate in the tea leaves. When you taste tea, the slight bitter taste and astringent flavour of the tea broth are caused by the catechins. As the concentration of catechins in tea increases, the bitterness will also increase.
Unfermented green tea retains a higher content of catechins. Semi-fermented teas such as Oolong and Bao Seed can also contain catechins, but the content is not as high as that of green tea. As for fully-fermented black tea, due to enzymatic oxidation, the catechins are replaced by the tea yolks and tea rosé, and the tea broth no longer shows a yellowish-green colour.