Dissecting Seed Proanthocyandin Composition and Accumulation under Different Berry Ripening Process in Wine Grapes

Abstract

Grape berry proanthocyandin (PA) mainly exists in the skin and seeds. Its content and composition determine the intensity of bitterness and astringency. Affected by global warming, the world’s wine-producing regions, in particular in dry-hot regions such as western China, are facing the problem of unsynchronized berry ripening and seed ripening. Therefore, it is urgent to understand the influence of berry ripening progression on the composition and accumulation of seed PA, ultimately providing strategies for grape harvest decisions. In this paper, Vitis vinfera L. cv. Cabernet sauvignon and Marselan grapes from four sub-regions with different maturation processes were used as experimental materials to study the changes of soluble and insoluble PA contents as well as differences in their composition and mean degree of polymers (mDP) in seeds. The results showed that compared with ‘Cabernet sauvignon’ seeds, the mDP of soluble and insoluble PA were higher in ‘Marselan’ seeds. Both varieties showed that the grape berry, with the fastest sugar accumulation, had relatively high soluble PA content in seeds and a high content of (-)-epigallocatechin-3-gallate and (-)-epicatechin in the seed PA composition units. In contrast, the ‘Cabernet sauvignon’ grapes from the YQ vineyard exhibited the slowest sugar accumulation speed among the four studied vineyards, and their seed PA had the highest mDP and the lowest proportion of (-)-epigallocatechin-3-gallate in the composition units when commercially harvested. According to the results, it is suggested that a faster maturation process would bring about higher levels of bitterness composition, such as (-)-epigallocatechin-3-gallate in seed PA, which is not conducive to the formation of good-tasting tannins.

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

“…The overall result suggests that slowing down the berry ripening process facilitates tannin polymerization and seed maturation. This result is consistent with our previous research [ 30 ]. It is known that wine bitterness is mainly contributed to by monomeric flavan-3-ols and low polymerization degrees of CT, whereas astringency is mainly contributed to by high polymerization CT [ 36 , 37 ].…”

“…Moreover, the insoluble CT of the seeds from both vineyards showed a rapidly increasing trend from the véraison stage (63 and 75 DAA in 2019 and 63 DAA in 2020), which seemed to be related to the decrease in soluble CT during this period. It was assumed that the soluble CT were further polymerized to insoluble CT with ripening, as reported previously [ 29 , 30 ].…”

“…The extraction, determination, and compositional unit analysis of soluble and insoluble CT in the seeds were essentially based on the method described in our recent publication [ 30 ]. Briefly, the grape seed powder of 0.100 g was thoroughly mixed with 1 mL of the tannin extraction buffer (70% acetone/water and 0.1% acetic acid) and treated ultrasonically for 30 min in an ice-water bath in the dark.…”

“…The lower solution containing the soluble CT was then lyophilized for 24 h, and the dry matter obtained was redissolved in 500 µL of 50% methanol/water solution. The absorbance value of the soluble CT reaction with 1% 4-(dimethylamino) cinnamaldehyde (DMACA) solution ( w / v ) dissolved in 1:1 methanol/6 mol/L; the hydrochloric acid was determined at 640 nm on the microplate reader (SpectraMax 190, Molecular Devices, San Jose, CA, USA), as described in our publication [ 30 ]. The soluble CT were quantified relative to the standard curve generated from the (−)-epicatechin concentration and the corresponding absorbance values.…”

“…To determine the concentration of insoluble CT, the insoluble CT sediment obtained above was lyophilized. The dried powder containing insoluble CT was then completely mixed with 1 mL of the butanol/HCL (95:5, v / v ) solution, vortexed, sonicated for 30 min in the dark, and incubated in a metal bath at 95 °C for 1 h. The absorbance of this reaction was measured at 550 nm on the microplate reader, as described previously [ 30 ]. The insoluble CT were quantified relative to the standard curve generated from the procyanidin B1 concentration and the corresponding absorbance values.…”