Polyphenolic, polysaccharide and oligosaccharide composition of Tempranillo red wines and their relationship with the perceived astringency
Introduction
Astringency is one of the most important attributes of red wine and it is close related to its overall quality, being widely acknowledged that high quality level red wines have a balanced level of astringency (Gawel, 1998). Astringency has been described as an oral sensation which causes the drying, roughing and puckering of the mouth epithelia and a complete terminology has been developed to describe this complex sensation in red wines (Gawel, Oberholster, & Francis, 2000). It has been classically attributed to the interaction between tannins and salivary proteins (Bate-Smith, 1954) leading to precipitation, although more recently other mechanisms, including the influence of colloidal particles that remain in solution and the involvement of laminin receptor (Schwarz & Hofmann, 2008) had been proposed.
Phenolic compounds present in wine, and especially tannins have been widely related to astringency perception (Kennedy et al., 2006, Brossaud et al., 2001, Preys et al., 2006). Regarding proanthocyanidins, several variables, such as total concentration and the average polymerisation degree (aDP) (Preys et al., 2006), and their subunit composition and their distribution highly correlate with the astringency perception (Quijada-Morin et al., 2012).
Salivary proteins–tannin interactions appear to be the main astringency mechanism in red wines. Polysaccharides could inhibit this interaction in red wines, as it has been previously proposed for the loss of astringency in ripening fruits (Ozawa, Lilley, & Haslam, 1987). Those authors observed a release of small pectin soluble fragments as the cellular structure of fruit softens up during ripening, and they suggested that there is a competition with salivary proteins for polyphenolic substrates which lead to a modification in astringency perception and response. Several in vitro studies had been developed, showing that complex polysaccharides can disrupt protein–tannin interaction by different mechanisms; inhibiting protein–tannin interaction (Carvalho et al., 2006, Escot et al., 2001) or inhibiting the precipitation of the protein–tannin complexes (de Freitas et al., 2003, Mateus et al., 2004), thereby polysaccharides would limit the concentration of available proanthocyanidins, and thus astringency would be reduced. In addition to this, several polysaccharide families had been described as compounds able to interact with tannins (Poncet-Legrand et al., 2007, Riou et al., 2002) or with proanthocyanidin aggregates to yield soluble complexes (Riou et al., 2002), so the presence of available proanthocyanidins in the medium would also be reduced by these mechanisms. These studies used different indirect approaches to show the influence of complex carbohydrates on protein–tannin interactions; such as nephelometry studies (Carvalho et al., 2006, de Freitas et al., 2003, Mateus et al., 2004), light scattering (Carvalho et al., 2006), determination of gelatin index (Escot et al., 2001) or SDS–PAGE (Soares, Mateus, & de Freitas, 2012). Sensory analysis has been also used to study the influence of polysaccharides on astringency perception in model wine solutions (Vidal et al., 2004, Vidal et al., 2004), showing that all polysaccharide families reduced the perception of astringency in some degree (McRae & Kennedy, 2011). In addition to this, the above-mentioned sensory studies in model wine revealed that acidic polysaccharides have a greater impact on the reduction of astringency perception. RG-II is the main acidic polysaccharide in wines (Vernhet, Pellerin, Prieur, Osmianski, & Moutounet, 1996); isolated fractions of this polysaccharide caused a significant decrease of overall astringency in model solution, which has been attributed mainly to changes in mouth lubrication and the formation of complexes with astringent compounds. Neutral polysaccharides also tend to decrease the intensity of astringency attributes, nevertheless in that study the differences between model wine and the fraction containing a mixture of mannoproteins and type II arabinogalactan proteins isolated from wine, were not statistically significant (Vidal et al., 2004).
The aim of this work was to study the proanthocyanidic, polysaccharide and oligosaccharide composition of Tempranillo red wines and to establish relationships with perceived astringency.
Section snippets
Wine samples
Thirteen commercial wines made from Tempranillo grapes were purchased from selected Spanish wineries. The wines belonged to four vintages (2006, 2008, 2009 and 2010), and three Spanish protected designations of origin, Ribera de Duero, Toro and Rioja (Table 1). All the samples are Tempranillo wines that have evolved during the ageing time. They were stored under cellar conditions before the analyses were conducted. Oenological parameters as pH or ethanol content were similar across the studied
Proanthocyanidin composition
Acid-catalysed depolymerisation in the presence of phloroglucinol was perform in order to obtain information about the proanthocyanidin subunit composition (Fig. 1a), average degree of polymerisation (aDP) (Fig. 1b), average molecular weight (aMW) and total proanthocyanidin concentration of these wines (Fig. 1c), as previously described (Quijada-Morin et al., 2012).
Polysaccharide composition
The concentration (mg/L) of Mannoproteins (MPs), Polysaccharides Rich in Arabinose and Galactose (PRAGs,) type II
Conclusions
The results of this study confirm the influence of polysaccharides and oligosaccharides on perceived astringency in Tempranillo red wines. These compounds are able to modulate overall astringency.
The smoothing capacity of the different polysaccharides families is confirmed in wines for the first time. This effect is especially important for mannoproteins and RG-II. Glycosyl residues in the oligosaccharide fraction did not express a clear trend against astringency perception. Taking into account
Acknowledgements
Thanks are due to the Spanish MICINN (Ref. AGL2011-30254-C02) and to Consolider-Ingenio 2010 Programme (Ref. CSD2007-0063) for financial support. N. Quijada-Morín also thanks the Spanish MICINN for the F.P.I. predoctoral scholarship.
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