Tannins: definition

The word tannin is very old and reflects a traditional technology. "Tanning" (waterproofing and preserving) was the word used to describe the process of transforming animal hides into leather by using plant extracts from different plant parts of different plant species.

• Plant parts containing tannins include bark, wood, fruit, fruitpods, leaves, roots, and plant galls.
• Examples of plant species used to obtain tannins for tanning purposes are wattle (Acacia sp.), oak (Quercus sp.), eucalyptus (Eucalyptus sp.), birch (Betula sp.), willow (Salix caprea), pine (Pinus sp.), quebracho (Scinopsis balansae) .
  

Tannins are phenolic compounds that precipitate proteins. They are composed of a very diverse group of oligomers and polymers. There is some confusion about the terminology used to identify or classify a substance as a tannin, In fact,

• not only tannins bind and precipitate proteins (other phenolics such as pyrogallol and resorcinol also have this property),
• not all polyphenols precipitate proteins or complex with polysaccharides.

One of the most satisfactory definition of tannins was given by Horvath (1981):

"Any phenolic compound of sufficiently high molecular weight containing sufficient hydroxyls and other suitable groups (i.e. carboxyls) to form effectively strong complexes with protein and other macromolecules under the particular environmental conditions being studied"

Tannins can complex with:

• Proteins
• Starch
• Cellulose
• Minerals