Measuring the Molecular Dimensions of Wine Tannins:
Comparison of Small-Angle X‑ray Scattering, Gel-Permeation
Chromatography and Mean Degree of Polymerization
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Abstract
The
molecular size of wine tannins can influence astringency, and
yet it has been unclear as to whether the standard methods for determining
average tannin molecular weight (MW), including gel-permeation chromatography
(GPC) and depolymerization reactions, are actually related to the
size of the tannin in wine-like conditions. Small-angle X-ray scattering
(SAXS) was therefore used to determine the molecular sizes and corresponding
MWs of wine tannin samples from 3 and 7 year old Cabernet Sauvignon
wine in a variety of wine-like matrixes: 5–15% and 100% ethanol;
0–200 mM NaCl and pH 3.0–4.0, and compared to those
measured using the standard methods. The SAXS results indicated that
the tannin samples from the older wine were larger than those of the
younger wine and that wine composition did not greatly impact on tannin
molecular size. The average tannin MWs as determined by GPC correlated
strongly with the SAXS results, suggesting that this method does give
a good indication of tannin molecular size in wine-like conditions.
The MW as determined from the depolymerization reactions did not correlate
as strongly with the SAXS results. To our knowledge, SAXS measurements
have not previously been attempted for wine tannins