lifted from this page
https://en.wikipedia.org/wiki/Tannin on wikipedia.
Presence in water and woodThe leaching of highly water soluble tannins from decaying vegetation and leaves along a stream may produce what is known as a blackwater river. Water flowing out of bogs has a characteristic brown color from dissolved peat tannins. The presence of tannins (or humic acid) in well water can make it smell bad or taste bitter, but this does not make it unsafe to drink.
Tannins leaching from an unprepared driftwood decoration in an aquarium can cause pH lowering and coloring of the water to a tea-like tinge. A way to avoid this is to boil the wood in water several times, discarding the water each time. Using peat as an aquarium substrate can have the same effect. Many hours of boiling the driftwood may need to be followed by many weeks or months of constant soaking and many water changes before the water will stay clear. Adding baking soda to the water to raise its pH level will accelerate the process of leaching, as the more alkaline solution can draw out tannic acid from the wood faster than the pH-neutral water.
Softwoods, while in general much lower in tannins than hardwoods, are usually not recommended for use in an aquarium so using a hardwood with a very light color, indicating a low tannin content, can be an easy way to avoid tannins. Tannic acid is brown in color, so in general white woods have a low tannin content. Woods with a lot of yellow, red, or brown coloration to them (like cedar, redwood, red oak, etc.) tend to contain a lot of tannin.
Food items with tanninsPomegranates
Accessory fruitsStrawberries contain both hydrolyzable and condensed tannins.
Berries Most berries, such as cranberries, and blueberries, contain both hydrolyzable and condensed tannins.
NutsNuts that can be consumed raw, such as hazelnuts, walnuts, and pecans, contain high amounts of tannins. Almonds have a lower content. Tannin concentration in the crude extract of these nuts did not directly translate to the same relationships for the condensed fraction.
Herbs and spicesCloves, tarragon, cumin, thyme, vanilla, and cinnamon all contain tannins.
LegumesMost legumes contain tannins. Red-colored beans contain the most tannins, and white-colored beans have the least. Peanuts without shells have a very low tannin content. Chickpeas (garbanzo beans) have a smaller amount of tannins.
ChocolateChocolate liquor contains about 6% tannins.
Drinks with tanninsPrincipal human dietary sources of tannins are tea and coffee. Most wines aged in charred oak barrels possess tannins absorbed from the wood. Soils high in clay also contribute to tannins in wine grapes. This concentration gives wine its signature astringency.
Coffee pulp has been found to contain low to trace amounts of tannins.
Fruit juicesAlthough citrus fruits do not themselves contain tannins, orange-colored juices often contain food dyes with tannins. Apple juice, grape juices and berry juices are all high in tannins. Sometimes tannins are even added to juices and ciders to create a more astringent feel to the taste.
BeerIn addition to the alpha acids extracted from hops to provide bitterness in beer, condensed tannins are also present. These originate both from the malt and hops. Especially in Germany, trained brewmasters consider the presence of tannins as a flaw. In some styles, the presence of this astringency is acceptable or even desired, as, for example, in a Flanders red ale.
In lager type beers, the tannins can form a precipitate with specific haze-forming proteins in the beer resulting in turbidity at low temperature. This chill haze can be prevented by removing part of the tannins or part of the haze-forming proteins. Tannins are removed using PVPP, haze-forming proteins by using silica or tannic acid.
this page also has some interesting info.
https://en.wikipedia.org/wiki/Health_effects_of_natural_phenols_and_polyphenolstannins seem to have very little effect on the human body.
oak has been used for millennia in food production and if someone is concerned about bacteria getting into the pore structure then they should look at the size of bacteria and the smallest pores in timber. pores considered small (50 micrometers) are 50 times bigger than bacteria.
