Why is green tea bitter?
The bitter taste in green tea comes from catechins, which are a kind of tannins.
Catechin is a member of polyphenols. The catechins in tea is 100 times as strong as wine and 50 times as strong as cocoa. In fact it has the most potent antioxidant effect among natural foods.
There is also caffeine, which is a bitter substance and all together they cause the green tea to appear bitter and astringent taste.
Besides, there are two ways for tea to become unpleasantly bitter: steeping at a temperature too high, or steeping for too long. Generally speaking, the longer you brew tea, the more tannins will be released and the bitterer the tea will become.