Professor Yoriyuki Nakamura of the University of Shizuoka wrote the original article in Japanese, which CHAMART translated into English.

Catechins
Tea catechins mainly include epicatechin (EC), epicatechin gallate (ECg), epigallocatechin (EGC), and epigallocatechin gallate (EGCg). Of these, epigallocatechin gallate accounts for more than half. Un-fermented tea, like green tea, contains a high proportion of catechins. Catechins in black and oolong tea, which are fermented, are lesser because they are oxidized and converted to theaflavins. Catechins in post-fermented tea, such as Pu’er tea, are lost because they are decomposed by filamentous fungus during sedimentation fermentation.

Sencha (medium-steamed green tea) and its used tea leaves

Catechins are the components that bring an astringent or bitter taste in tea. Gyokuro requires umami taste. Therefore, the tea gardens of gyokuro are covered by komo (straw mat) or man-made black mat to block sunlight so that amino acids increase, but catechins become less in the tea leaves.
On the other hand, black tea requires a large amount of catechins for its refreshing astringency. However, catechins are polymerized and converted to theaflavins or thearubigins by the enzyme polyphenol oxidase, which decreases the catechin content.

Catechins have the strongest antioxidant effect among polyphenols, and exhibit a variety of functions, including anti-mutation, anti-cancer, anti-oxidation, anti-arteriosclerosis, suppression of blood cholesterol, suppression of fat absorption, antibacterial, anti-viral, caries prevention, improvement of intestinal flora, deodorization, and suppression of blood pressure.