Interesting facts from the world of organic chemistry
Derivatives - functional derivatives of carboxylic acids:
Tannic acid
Although this substance is called tannic acid, the pKa value of this acid is very low (approximately around 10). Her sour nature is however undeniable. This is a consequence of the large number of phenolic groups in its structure as you can see down below.
Picture above: Tannic acid is the commercial form of tannin.
Tannic acid can practically only be obtained by extraction. Caesalpinia spinosa, Rhus semialata, Quercus infectoria or Rhus coriaria plants are used. The reason is the definition introduced by international pharmaceutical companies, FAO-WHO and some others. Therefore, extracts from chestnut or oak wood are sometimes not considered as a source of tannic acid, even though they may contain it (e.g. in a more or less modified form).
Resources:
[1] Moravcová, in Biologically active natural substances, Prague, VŠCHT Prague, 2006.
[2] iGEM 2008, “California Institute of Technology,” 2008. [Online]. Available: http://2008.igem.org/Team:Caltech/Project/Vitamins. [Accessed 8 6 2013].
[3] E. Hagerman, “The Tannin Handbook,” 1998, 2002, 2011. [Online]. Available: http://www.users.muohio.edu/hagermae/. [Accessed 8 6 2013].