Gallic acid is a plant chemical found in most plants but especially abundant in tea, grapes and oak. It is in a group of chemicals called phenolics. Phenolics are chemicals based on the structure of phenol which is a molecule with a ring structure containing 6 carbons with a –OH (hydroxyl group) attached to one of the carbons. A molecule with more than one phenol ring is called a polyphenol. These phenols and polyphenols are known as powerful antioxidants and found in many plants.
Interestingly, this 6 member ring structure also referred to as an aromatic nucleus is only synthesized by plants and microorganisms, not animals or humans. Generally speaking phenolics are acidic meaning their pH is low (from 1-7).
Gallic acid has 3 –OH groups and one –COOH (carboxylic acid) group attached to the ring. Note that in these abbreviated structures shown that each of the 6 corners represents a carbon atom, so there are 6 carbon atoms in phenol.
Gallic acid can be found either alone or as part of plant tannins. Tannins are a family of high molecular weight, water soluble plant molecules. Another definition of tannin is a natural product containing phenolic structures that can precipitate proteins. They have an astringent taste (think tea) and have the ability to tan leather. When gallic acid becomes linked to a sugar such as glucose it can form a polygalloyl ester which is a simple type of tannin.