Do you know of anyone who turns beet red after drinking a glass of red wine? I do – and the reason for this is simple. Some people are allergic to tannin – a common constituent of berries and fruits from plants.
In our case, tannin is part of the wine making process. The tannin is carried by the "skin" of red grapes. To make red wine, these grapes are crushed (not pressed) and the resulting juice steeps with the skins and the tannic acid leaches its colour into the juice. Then the juice/skin mix is pressed to get the juice out. The great majority of grapes (red + white) have a clear juice within them. Grapes are pressed immediately without crushing for white wine. Tannin is also present in abundance in oak (hello rye whiskey) and various nuts.
Tannic acid also has many medicinal + industrial uses. Leather is made from hides which have been "tanned" just to cite one example. So even if you don’t drink red wine you could be wearing some or sitting on some. In the medical field, tannic acid is used in balms and ointments to treat cold sores (a type of herpes – I wonder if it works on other forms of herpes…). An apple a day keeps the doctor away – and who knows maybe a glass a day keeps the herpes away.