Tannins, also known as tannins, are a class of compounds with polyphenolic hydroxyl groups. They are widely found in plants and play an important role in plant physiological functions.

Tannins have unique characteristics, mostly amorphous powders, with astringent and astringent taste. Its chemical structure is complex and consists of different amounts of phenolic hydroxyl groups connected to each other. According to the chemical structure, tannins can be divided into two categories: hydrolyzable tannins and condensed tannins.

Hydrolyzable tannins are connected by phenolic acids and polyols through ester bonds. Under the action of acids, bases or enzymes, they are easily hydrolyzed into small molecules of phenolic acids and sugars or polyols. Condensed tannins are formed by catechins and other monomers through condensation reaction. There are no ester bonds in the structure. When exposed to acids, bases or enzymes, they are not easy to hydrolyze, but they are easy to shrink into polymer insoluble products, called tanned red.

Tannins are widely used in many fields. In medicine, they have the functions of astringent, hemostatic, and antibacterial. The astringent property can coagulate local proteins, reduce exudation, and achieve the effect of astringent and antidiarrheal; the hemostatic effect can promote platelet coagulation and speed up the coagulation process; the antibacterial properties inhibit the growth of a variety of bacteria and fungi. In the field of leather manufacturing, tannins can be combined with proteins in leather to transform it into a flexible, water-resistant and microbial-resistant leather. This process is called "tanning". In the food industry, tannins, as natural antioxidants, can delay the oxidative deterioration of food and prolong the shelf life; they also affect the flavor of some foods, such as the appropriate amount of tannins in wine, which gives it a unique astringent taste and mellow taste.

Tannins are widely distributed in the plant kingdom, and are rich in tannins in tea, pomegranate, and gallnut. Humans have a long history of cognition and application of tannins. With the development of science and technology, the research on tannins is becoming more and more in-depth, and its potential value is expected to be explored and utilized.