Re: Tannic Acid

From: John Brewer <john@glossyormatt.com>
Subject: Tannic Acid
Date: Fri, 11 Nov 2005 21:06:37 +0000

> Cheaply and easily available is wine tannin. Is there a way this can
> be used to tone cyanotypes as tannic acid? My first foray today was
> hopeless. The method I used was to acidify a dry print in acetic
> acid, wash, bleach in ammonia, wash, soak in water with some tannin
> in. Googling doesn't seem to be much help. Is tannin also tannic
> acid? If not can I make tannic acid with tannin? The tannin I have
> is a fine brown powder.

Tannin is a very broad class of polyphenolic compounds, and tannic
acid does not mean a specific acid or any specific compound but it
also referrs to a broad class of polyphenolic compounds. (To make it a
bit more confusing, "tannic acid" is often used regardless of a
particular compound being proper acid or not.)

There are two major subclasses of tannin. One is condensed, and the
other is hydrolyzable.

Condensed tannins are the ones naturally precipitated as insoluble
polymer in fermentation and aging of wine. This class of compound is
characterized by 2 to 8 flavan-3-ol or flavan-3,4-diol groups in each
molecule. There, smaller polyphenolic groups are covalently bonded and
they are not readily hydrolizable by merely dropping the pH. This type
of tannin is not very reactive in aquaous media because they are not
very soluble.

Hydrolyzable tannin are gallotannins or ellagitannins. One common
example is pentagalloylglucose, where 5 gallic acids are bound to a
glucose. This is an ester and the binds can be readily broken down by
lowering the solution pH. This form of tannin does not naturally
precipitate in wine making so it is often removed by adding fining
agents (gelatin, albumen, and other forms of edible proteins, or
polyvinylpyrrolidone, particularly in the case of white wines. Yes,
the PVP is the same stuff as the one used in Kodak HC-110 developer as
the antistaining agent.) and filtering out the precipitated solids. But
those tannins are bound to the fining agents and they are not usable.

If you got cheap "wine tannin" mostly consisting of condensed tannins,
it makes sense to me that it is not very effective. If you want to
test if your tannin solution is active, you might want to do a quick
test by adding a few drops of weak gelatin solution in a beaker of
tannin solution (no warmer than room temp). The gelatin should
precipitate out.

Incidentally, if you make a cup of green/black tea and leave it out,
you'll probably see haziness in several hours. This is caused by
oxidation of tannins. Bottled tea products often contain ascorbic acid
to prevent this haziness. (Black tea is probably easier to cause this.)

Some of the very fine wine drinkers may find some wine also develop
haziness, called casse. The same oxidation of wine tannins can be
involved but presence of iron and cupper (from the grapes) can
complicate the thing. Adjusting tannin content in wine strictly by the
"art" is tricky as the chemical composition of wine varies. What's
worse, grapes that are infected by fungus cause more trouble due to
enzymes brought in with the infected grapes.

Vintners treat grapes with sulfur dioxide before crushing, use fining
agent to reduce the tannin content, and add metabisulfite, citric
acid, EDTA, etc. to inhibit the transition metals and enzymes, all to
prevent casse (and also improve other sensory qualities of the wine).
Received on Sat Nov 12 02:40:21 2005