Factors Affecting Skin Tannin Extractability in Ripening
Grapes
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Abstract
The
acetone-extractable (70% v/v) skin tannin content of Vitis vinifera L. cv. Cabernet Sauvignon grapes was
found to increase during late-stage ripening. Conversely, skin tannin
content determined following ethanol extraction (10, 20, and 50% v/v)
did not consistently reflect this trend. The results indicated that
a fraction of tannin became less extractable in aqueous ethanol during
ripening. Skin cell walls were observed to become more porous during
ripening, which may facilitate the sequestering of tannin as an adsorbed
fraction within cell walls. For ethanol extracts, tannin molecular
mass increased with advancing ripeness, even when extractable tannin
content was constant, but this effect was negligible in acetone extracts.
Reconstitution experiments with isolated skin tannin and cell wall
material indicated that the selectivity of tannin adsorption by cell
walls changed as tannin concentration increased. Tannin concentration,
tannin molecular mass, and cell wall porosity are discussed as factors
that may influence skin tannin extractability