Category : Science and Education
Archive   : TANNIN.ZIP
Filename : TANN13
Using commercially available standards, relative amounts of
tannin can easily be determined. The assay is standardized with
the commercial standard, and the tannin in the plant extracts is
then reported in terms of the weight of the selected standard.
Nontannin phenolics are sometimes adequate standards. For
example, the rhodanine assay and the various redox assays can be
standardized with reagent grade gallic acid. The vanillin assay
can be standardized with commercial catechin, and the acid
butanol assay with commercial cyanidin
GROUP ASSAYS.
Tannic acid is probably the best commercially available tannin
standard. Although the commercial preparation contains variable
amounts of low molecular weight galloyl esters and of nontannin
material, the material is easily purified with Sephadex LH20
(Hagerman and Klucher, 1986). This purified material would be
an appropriate standard for the rhodanine assay, for any of the
redox assays, or for any precipitation assay.
(
the acid butanol assay.
Although tannic acid is an appropriate standard for analysis of
tannins, it may be an inappropriate material to use for in vivo
tests of the biological activity of tannin. Recent work
(Robbins and Hagerman, unpublished) suggests that tannic acid
does not affect nitrogen utilization in ruminants whereas other
hydrolyzable tannins diminish nitrogen utilization
(
material might be appropriate as a chemical standard but quite
inappropriate for bioassay.
Sorghum tannin is an appropriate standard condensed tannin, but
is not available commercially, and is rather tedious to purify
(Hagerman and Butler, 1980b). Quebracho tannin is the only
readily available condensed tannin. It must be purified
(Asquith and Butler, 1985) because it contains large amounts of
nontannin materials. There are probably substantial differences
among the materials provided by various suppliers, and even
among various lots from a single supplier. Since a single
commercial standard is not available, we suggest that all
workers start to use purified quebracho standard prepared from a
single lot of the crude commercial mixture. We will provide the
crude material from a single lot of commercial quebracho and
instructions for preparation of the purifed standard to any
interested laboratories
standard should significantly reduce confusion about analysis of
condensed tannin.
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References
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