The mouth-feel properties of polysaccharides and anthocyanins in a wine like medium
Introduction
The macromolecular fraction of red wines is mainly composed of polysaccharides and polyphenolic compounds, such as proanthocyanidins (condensed tannins). The contribution of proanthocyanidins to the astringency of wine has been investigated by a number of groups (Brossaud et al., 2001, Kallithraka et al., 2001, Peleg et al., 1999, Vidal et al., submitted for publication) and these compounds are also believed to contribute to colour stability in red wines (Somers & Verette, 1988). The importance of anthocyanins and derived pigments to wine colour has already been established (Singleton, 1972, Somers, 1971); however, the extent to which these pigments contribute to the mouth-feel properties of wine is not clear. Anthocyanins have been reported, from an informal assessment, to have only a mild indistinctive flavour (Singleton & Noble, 1976). It has been suggested that anthocyanins could modulate the astringency perception in wine, either directly or through reactions with astringent proanthocyanidins that occur during ageing (Singleton & Noble, 1976). More recently, it has been observed that an anthocyanin fraction complemented grape proanthocyanidin astringency and did not contribute bitterness (Brossaud et al., 2001).
In comparison to wine polyphenolic compounds, wine polysaccharides, the other main macromolecular fraction in red wines, have been less studied. In the past decade, their structures have been characterised (Pellerin et al., 1996, Pellerin et al., 1995, Vidal et al., 2000) and some of their properties determined (Riou et al., 2001, Vernhet et al., 1999, Vernhet et al., 1996). The main polysaccharides in wine are grape-derived type II arabinogalactan-proteins (AGP), and rhamnogalacturonans (RG-II), and yeast-derived mannoproteins (MP) (Pellerin & Cabanis, 1998).
Wine polysaccharides are regarded as probably being responsible for ‘mellowness’ (Semichon & Flanzy, 1927) but, to our knowledge, no rigorous sensory studies on purified wine polysaccharides have been carried out to determine their organoleptic properties in wine. Like that of anthocyanins, the role of polysaccharides in mouth-feel properties of wine may possibly be direct, by contributing mellowness, or indirect, through modulating astringency due to proanthocyanidins. There is currently no firm evidence for a direct effect of wine polysaccharides on mouth-feel properties of wine. One study demonstrated that removal of mannoproteins and other high molecular mass compounds from a Riesling wine by ultrafiltration had no impact on the sensory properties of that wine (Will, Pfeifer, & Dietrich, 1991). An indirect role for wine polysaccharides may be more likely. Smith and Noble (1998) showed that the extent and duration of the astringency sensation from a grape seed extract containing, inter alia, proanthocyanidins, was decreased in the presence of carboxymethyl cellulose. They speculated that this was because the viscosity of carboxymethyl cellulose compensated for the loss of lubrication caused by salivary protein precipitation by proanthocyanidins.
It was recently shown that wine RG-II favoured the self-aggregation of grape seed proanthocyanidins in wine-like solutions and indeed may have co-aggregated with the proanthocyanidins whereas wine MP and AGP tended to inhibit proanthocyanidin aggregation (Riou et al., 2001). Thus, in addition to wine polysaccharides providing increased viscosity, it is possible that the presence of polysaccharide–proanthocyanidin complexes in wine may also reduce astringency. This scenario appears to occur in persimmon fruit and juice, natural products relatively high in astringency, like red wine. Soluble persimmon pectins reduced the astringency of persimmon juice and isolated persimmon proanthocyanidins in solution (Taira & Ono, 1997) and this was considered to be due to the formation of soluble proanthocyanidin–pectin complexes that led to the precipitation of fewer salivary proteins than the persimmon proanthocyanidins alone. The existence of different classes of polysaccharides has been postulated, some being inhibitors of proanthocyanidin precipitation by proteins and others having no influence on that precipitation (Haslam, 1998).
In the present study, we isolated polysaccharide fractions and anthocyanins in sufficient amounts to carry out their sensory descriptive analysis. The aim of this work was to determine the intrinsic sensory properties of these classes of compounds in a model wine system before investigating whether these compounds modulated the astringency of grape proanthocyanidins in this medium. Two polysaccharide fractions were prepared, one containing the two main neutral polysaccharides in wine, AGP and MP, and the other containing the main acidic wine polysaccharide, RG-II.
Section snippets
Purification of anthocyanins
Grapes (50 kg) of Vitis vinifera cv. Shiraz harvested at commercial maturity at the INRA experimental station of Pech-Rouge, France were manually peeled. Skins were immediately frozen in liquid nitrogen and ground using a Dangoumau blender (Prolabo, France). The resulting powder was sequentially extracted with ethanol/water (12:88, v/v), methanol and acetone/water (60:40, v/v), as described by Vidal et al. (2003). The acetone extract (2 l) was recovered by coarse filtration, acetone removed
General
The aim of this study was to determine the influence that wine polysaccharides and anthocyanins could have on the mouth-feel properties of wine. Anthocyanins were purified from grape skin extract rather than wine to avoid, as far as possible, the presence of anthocyanin adducts that can form during fermentation and wine ageing (Mateus et al., 2001, Somers, 1971). A neutral polysaccharide fraction, containing the two major neutral wine polysaccharides, mannoproteins (MP) and type II
Conclusions
This study has provided evidence that wine polysaccharides probably play an important role in the mouth-feel properties of wine. Both neutral and acidic polysaccharide fractions examined are likely to contribute a ‘fullness’ sensation to wine. The observation that the acidic polysaccharide, RG-II, significantly decreased the astringency of the model wine solution, whereas the neutral polysaccharide fraction had less effect, strongly suggests that the composition and structure of polysaccharide
Acknowledgements
The authors wish to thank Patrick Iland, George Skouroumounis, Alan Pollnitz, James Kennedy, Peter Costello, Ken Pocock, Mark Gishen, Renata Ristic, Ella Robinson, Peter Valente, and Matt Holdstock, for taking part in the sensory analysis. We thank Peter Høj for his encouragement and Markus Herderich and Peter Høj for critical reading of the manuscript. This work was supported by Australia's grapegrowers and winemakers through their investment body, the Grape and Wine Research and Development
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