WHAT IS A TANNIN?
Plants accumulate a wide variety of
"secondary" compounds, including alkaloids, terpenes and phenolics. Although
these compounds apparently do not function in "primary" metabolism
such as biosynthesis, biodegradation and other energy conversions of
intermediary metabolism, they do have diverse biological activities ranging
from toxicity to hormonal mimicry, and may play a role in protecting plants from
herbivory and disease.
Phenolic metabolism in plants is complex, and yields
a wide array of compounds ranging from the familiar flower pigments
(anthocyanidins) to the complex phenolics of the plant cell wall (lignin). However,
the group of phenolic compounds known as tannins is clearly distinguished from
other plant secondary phenolics in their chemical reactivities & biological
activities.
Tradition use of tannins as agents for converting
animal hides to leather ("tanning") is one manifestation of the most
obvious activity of the tannins: their ability to interact with and precipitate
proteins, including the proteins found in animal skin. The term
"tannin" comes from the ancient Celtic word for oak, a typical source
for tannins for leather making.
Bate-Smith defined tannins as "water-soluble
phenolic compounds having molecular weights between 500 and 3000...[giving] the
usual phenolic reactions...[and having] special properties such as the ability to
precipiate alkaloids, gelatin and other proteins".
Haslam has more recently substituted the term
"polyphenol" for "tannin", in an attempt to emphasize the multiplicity
of phenolic groups characteristic of these compounds. He notes that molecular
weights as high as 20,000 have been reported, and that tannins complex not only
with proteins and alkaloids but also with certain polysaccharides. I prefer to
use the term tannin, which emphasizes the character which sets tannins apart
from all other phenolics: the ability to precipitate proteins.
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