Effect of vine training system on the phenolic composition of red grapes ( Vitis vinifera L. cv. Xinomavro)

摘要

Aims: In order to investigate the effect of vine training system on grape phenolic composition, a divided canopy system (Lyre) and a vertical shoot positioned trellis with two different pruning systems (Royat and Guyot) were applied in a commercial vineyard of cv. Xinomavro (Vitis vinifera L.). The quality of the produced grapes was determined with respect to three analytical parameters: anthocyanin content, tannin composition and proanthocyanidin structure.Methods and results: Monomeric anthocyanins, flavan-3-ol monomers and oligomers, tannin mean degree of polymerization (mDP), galloylation percentage (%G), and extension and terminal subunit structure were determined in seeds and skins by HPLC and LC/MS. Total phenolics, anthocyanin content, tannin concentration and antioxidant activity were measured in whole berries. Grapes and wines of the Lyre system were characterized by increased concentration of total and individual anthocyanins and total phenols. Individual flavan-3-ol monomers and oligomers were highest in Royat grape skins and Guyot seeds. Concerning proanthocyanidin structure, Royat grapes had significantly higher mDP and lower %G in skin and seed polymeric tannin fractions, compared to the other two systems, possibly leading to a lower grape astringency potential.Conclusion: Training system may affect grape anthocyanin concentration, total flavan-3-ol content as well as skin and seed proanthocyanidin structure (mDP, %G, extension and terminal subunits).Significance and impact of the study: The results of the present study could be of high importance to both grape growers and winemakers. Xinomavro grapes are rather poor in anthocyanins while being characterized by dry and astringent tannins. By adopting Lyre as a training system, higher anthocyanin contents could be achieved in grapes, resulting in wines with more attractive colour when adopting a longer pre-fermentation skin contact process. On the contrary, grapes of the Royat system might be more appropriate to produce full-bodied wines with higher ageing potential, due to their more appropriate tannin structure, by using longer maceration periods. IntroductionPhenolic compounds represent one of the most important groups of compounds for red wine as they affect directly wine quality. Proanthocyanidins (condensed tannins) are polymers composed of flavan-3-ol subunits. They are located in grape skins and seeds and are responsible for the stabilization of the colour and the sensory characteristics of the wines due to their astringent and bitter properties (Ribéreau-Gayon et al., 1999; Chira et al., 2009; Sun et al., 2013).The level of astringency is related not only to the concentration of tannins but also to their degree of polymerization. Astringency increases with increasing tannin size, at least up to 8 or 10 flavanol units (polymerized procyanidins are increasingly reactive with proteins and, therefore, have an increasingly astringent character), and then decreases since they become either no longer soluble or too bulky to bind with the proteins (Chira et al., 2009; Sun et al., 2013). On the contrary, tannin polymerization mediated by ethanol softens their flavour (Chira et al., 2012). In addition, the molecular size of proanthocyanidins affects their bitterness, since monomers are more bitter than polymers (Peleg et al., 1999; Ribéreau-Gayon et al., 1999). Astringency is also reported to increase with the degree of tannin galloylation (Cosme et al., 2009; Chira et al., 2011; ?urko et al., 2014). Galloylation has been shown to increase tannin interactions with various proteins, suggesting that it could be responsible for increased astringent sensation (Cheynier et al., 1997).In order to characterize tannin structure, extraction from grapes with the use of specific solvents should be applied, following tannin fractionation in accordance to their molecular size. Since tannin isolation and individual analysis are rather difficult procedures, depolymerization is often employed to facilitate their characterization. Treatment of condensed tannins with acid, in the presence of a nucleophile such as phloroglucinol (Prieur et al., 1994; Souquet et al., 1996), allows subunit profiling by HPLC and calculation of the average molecular mass, expressed as mean degree of polymerization (mDP).Extensive research has been conducted in order to determine tannin composition and structure using the depolymerization method. The results showed that grape tannins derived from skins and seeds vary in length, subunit composition and sensory properties (Peleg et al., 1999). Seed tannins are shorter, with a lower mDP, and display a higher percentage of subunits bearing gallic acid esters, which is expressed as percentage of galloylation (%G) (Prieur et al., 1994). Skin tannins are generally larger with a higher mDP (Souquet et al., 1996). However, seed and skin proanthocyanidins were found to be equally astringent when tasted at the same concentration in a wine or bu