Re: pinhole Iron precipitate from fruit juice & tea

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From: Karen Kalumuck (karenk@exploratorium.edu)
Date: Wed May 24 2000 - 13:32:14 PDT 

Message-Id: <v01540b08b551ebd4ed44@[192.174.2.182]>
Date: Wed, 24 May 2000 12:32:14 -0800
From: karenk@exploratorium.edu (Karen Kalumuck)
Subject: Re: pinhole Iron precipitate from fruit juice & tea

Hi All!

I passed along Ben's question to my husband, who is a biochemist and has
worked with tannins in the past. To borrow a phrase from Paul Doherty,
it's "definitely more complicated than that"! Here's what Rob had to say:

I seem to recall trying to react the tannin we were studying at Shaman
Pharmaceuticals with iron (ferric
ammonium sulfate) and not getting a precipitate (there was a color change from
orange to very deep red). Precipitation of iron usually occurs either by the
formation of sulfides (from H2S) or from the formation of oxides. If anything,
tannins will reduce iron rather than oxidize it, so I am skeptical that the
precipitate formed from fruit juices and tea is due to iron. Anyway, how much
iron is in fruit juices?

I don't know the answer to this question. Tea tannins will form precipitates
with almost any protein, but I doubt that there is much protein in fruit
juices. Tannins also bind very tightly to polysaccharides, which could be
abundant in juices, but I don't know if this results in precipitation.

(End)

Karen E. Kalumuck, Ph.D.
Biologist
Exploratorium Teacher Institute
3601 Lyon St.
San Francisco, CA 94123
415-561-0313
karenk@exploratorium.edu

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