To try to narrow the scope and help deduce an answer to this question, I'm wondering:
Why does tea become bitter if brewed too hot or too long?
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Tea contains a lot of flavinoid compounds notably tannins. Tannins are astringent and have a very strong bitter flavor.
The tannins are released much more slowly compared to the other flavor compounds. So when brewed for too long or too hot, much more tannins are released into the brew along and hence the resulting tea is much more bitter.