Gallic acid is a phytochemical found in many plant tissues, both as a free compound and as a component of the plant polymer tannin. This phenolic molecule is an antioxidant, and appears to have antimicrobial and anticancer activities in animal studies. Gallic acid is also part of the structure of other, larger polyphenolic compounds, the molecules made up of multiple phenolic groups. An example is epigallocatechin-3-gallate (EGCG), a green tea phenol that appears to have strong anti-carcinogenic abilities.
There are thousands of phenolic compounds known, and many more are identified each year. These compounds are only made by plants and microorganisms, but many offer tremendous health benefits to humans. Phenolics are often powerful antioxidants, and can have such properties as protecting against oxidative damage. This can protect against diseases as diverse as dementia and cancer. Many of these compounds, including gallic acid, also have antimicrobial properties.
Many larger polyphenols contain gallic acid as part of their structure. When the compound is connected to another molecule by an ester linkage it is known as a gallate. Examples include the green tea phenols EGCG and epicatechin gallate. These compounds are known as catechins. EGCG in particular is a powerful antioxidant and has been the subject of numerous studies on its ability to protect against heart disease and cancer.
ECGC is considered a green tea phenol because of its high concentrations in green tea. It is much less prevalent in black tea because it is destroyed during the processing that makes black tea. The manner of preparation of green tea can greatly affect the concentration of available polyphenols. Adding milk will keep them from being absorbed, and adding water that is too hot will cause them to chemically change. To get the most health benefits out of green tea, it is best to prepare it with water that has not reached the boiling point and to drink it without milk.
Gallic acid is common in plants. Some have particularly large concentrations, including blueberries, witch hazel, oak bark, tea leaves, and grapes. The technical term for this compound is 3,4,5-trihydroxybenzoic acid.
One substance that has large amounts of this molecule is a type of tannin, a complex molecule made out of phenolic compounds. When there are many gallate molecules linked together, it forms a substance called a hydrolysable tannin. There are several classes of this type of astringent polymer, once used to tan leather. This astringency gives tea its bite. Tannin is found in plants and is also a component of some soils.
Gallic acid is also used in research laboratories as part of the standard test to measure the concentration of phenolics and polyphenols in plant extracts and other substances. A compound called the Folin-Ciocalteau reagent reacts with the phenol group but does not indicate the concentration of phenolic compounds. Varying amounts of gallic acid are prepared to test at the same time with the Folin-Ciocalteau reagent to be able to provide a measurement of the number of molecules in the mixture. Results of the phenolic samples tested are usually displayed as gallic acid equivalents.