As anyone who’s taken high school chem knows, acids taste sour, and bases taste bitter. And yet, when you brew your tea too strong, or make something with unwashed acorn flour, or otherwise get a high concentration of tannic acid, the taste is quite definitely bitter, not sour. Why is this? Is tannic “acid” improperly named? Are there other, stronger, bases commonly associated with tannic acid, so we’re not really tasting the acid at all? Is this just an exception to the “bitter = base” rule? Or is there some other explanation?
FWIW, in wine-tasting a bitter dry wine is often described as “tannic”.
quote:“As anyone who’s taken high school chem knows, acids taste sour, and bases taste bitter”
The first thing our chemistry teacher emphasized was *not to drink the chemicals!!![i/] 
Tea contains tannins, which are acidic, but it also contains caffeine, which is basic. (And, of course, other organic compounds, e.g. chlorophyll, etc.) Most of the caffeine in teas leaves is readly extracted in hot water so that bitter taste is probably the caffiene.
All my chemistry references describe tannic acid as having an “astringent taste,” not sour or bitter. When I brew my tea too strong, it’s the astringency I notice first, but I wouldn’t go so far as to call it an “astringent taste”.
I found what appears to be an abstract of a paper entitled INTERACTION OF ASTRINGENT AND BASIC TASTE COMPOUNDS If I’m reading it right, they say astringent substances like tannin and alum increase the apparent taste intensity of sweet, sour, and salty foods. Alum decreases the perception of the bitterness of caffeine but tannin does not.
When I was a kid the best chemistry book I had was one published by – of all places – Golden Press (usually known for their kid-friendly but lightweight books). It had wonderfully involved details and experiments. Among other things, it told you how to make plastic. It would never be published today. For one thing, there were a lot of experiments using carbon tetrachloride and similar things you’d rather not have adulkts messing around with, let alone kids. One of the things that would keep this book from being published today was a very good, cautious experiment about tasting acids and bases. The book gave proper directions for producing an extremely dilute acid and base, which you were to taste one drop of to show that acids taste “sour” and bases taste “brackish”. You just know that someone doing this today would be slapped by a lawsuit. (Incidentally, this is the same book that the “radioactive boy scout” touted in the pages of Reader’s Digest used. Figures.)
As for the OP – is it possible that the bitter taste is not due to tannic acid, but to another substance? Tannin, maybe?
In India they would. The West has long recognized only 4 tastes, based on analysis of the taste buds. (A 5th one with a Japanese name has recently been admitted, although taste buds are not sensors for it, but the whole inner surface of the mouth.)
But according to Ayurveda there are 6 tastes (Shat rasa): in addition to the usual 4 there are astringent and pungent. The extra 2 are not sensed with taste buds but with the entire inner surface of the mouth. In Ayurveda good health is seen in terms of balance; ideally a complete meal should contain all 6 tastes. For bitter, they have something really bitter: the bitter gourd karela. Ayurveda classifies lentils (dâl) as astringent; for me strong astringency is present in persimmons and in Red Delicious apple skins.
I ate a few spoonfuls of tannic acid from a chemistry set once many years ago. I don’t recall it being either bitter or sour. (By the way, it didn’t work. I passed my physical and was drafted.)
Yes,caffeine is bitter.Coffee also tastes bitter but actually coffee is acidic since they are present in much larger quantities.
However there are also exceptions to the sour/bitter rule.Especially when dealing with inorganic substances,many acidic substances taste bitter as well.