Evolution of Seed-Soluble and Insoluble Tannins during Grape Berry Maturation

Abstract

Condensed tannins (CT) in wine are derived from the seeds and skins of grapes, and their composition and content contribute to the bitterness/astringency characteristics and ageing potential of the wine. Global warming has accelerated the ripening process of grape berries, making them out of sync with seed ripening. To understand the influence of berry ripening on the seed CT composition and content, we analyzed the changes in the soluble and insoluble CT in the seeds of 'Cabernet Sauvignon' grapes from two vineyards over two years. The results showed that the seed-soluble CT presented a slight downward trend in fluctuation during grape berry development, while the insoluble CT increased continuously before the véraison and remained at a high level afterwards. Relatively speaking, a lower sugar increment in developing grape berries favored the conversion of seed CT towards a higher degree of polymerization. The terminal unit of soluble CT was dominated by epigallocatechin gallate, the content of which decreased as the seeds matured. It is suggested that the seeds should be fully matured to reduce this bitter component in tannins. This study provides a reference for us to control the grape ripening process and produce high-quality grapes for wine making.

Keywords: flavan-3-ol unit; grape seeds; insoluble tannin; mean degree of polymerization; soluble tannin.

Figure 1

Changes in total soluble solids, titratable acid content, and pH value during ripening of ‘Cabernet Sauvignon’ grapes from two vineyards in 2019–2020. In 2019, ‘Cabernet Sauvignon’ grapes from the GWT entered the véraison stage at 63 DAA, full coloring stage at 86 DAA, and commercial harvesting stage at 125 DAA. The grapes from the YSC started coloration at 75 DAA, full coloring at 86 DAA, and commercial harvesting at 138 DAA. In the 2020 vintage, grapes from both vineyards started véraison at 63 DAA, full coloring at 75 DAA and commercial harvesting at 123 DAA/The asterisk indicates that there is a statistical difference between the 2 vineyards in the same period at the 0.05 level: *, p < 0.05; **, p < 0.01; ***, p < 0.001; ns, not significant.

Figure 2

Color simulation plots of GWT and YSC grape seeds. $\Delta E^{*}{}_{ab}$ represents the color difference value between the seeds from the two vineyards in the same period.

Figure 3

Variation of soluble and insoluble CT content in GWT and YSC seeds in 2019 and 2020. The asterisk indicates that there is a statistical difference between the 2 vineyards in the same period at the 0.05 level:*, p < 0.05; **, p < 0.01; ns, not significant.

Figure 4

Nucleophilic reagent hydrolysis of condensed tannins under heated acid condition. Nuc: abbreviation for nucleophilic reagent.

Figure 5

Changes in the content of free flavan-3-ol monomers (a), terminal units (b), and extension units (c) in soluble CT of GWT and YSC seeds. The asterisk indicates that there is a statistical difference between the 2 vineyards in the same period at the 0.05 level: *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Figure 6

Changes in the content of terminal units (a) and extension units (b) in insoluble CT of GWT and YSC seeds. The asterisk indicates that there is a statistical difference between the two vineyards in the same period at the 0.05 level: *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Figure 7

Changes in the mean degree polymerization of soluble (a) and insoluble CT (b) in the GWT and YSC seeds. The asterisk indicates that there is a statistical difference between the 2 vineyards in the same period at the 0.05 level: *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Figure 8

Correlation of seed tannin content with seed coat color index and berry ripening index. mDP1 and mDP2 represent the mean polymerization degree of soluble and insoluble CT, respectively. TSS represents the total soluble solid and TA is the titratable acidity. L* represents the lightness indicator (L* = 0 for black, L* = 100 for white). a* and b* are two color indicators (a* > 0 for red, a* < 0 for green; b* > 0 for yellow, b* < 0 for blue). *** represents a significant correlation at p < 0.001, ** represents a significant correlation at p < 0.01, and * represent a significant correlations at p < 0.05.