Structural Modification of Wine Arabinogalactans during Aging on Lees

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The effect of aging red wine on lees is under development as a winemaking technique. During aging on lees, yeast components, in particular polysaccharides, ...
150 – Doco et al.

Structural Modification of Wine Arabinogalactans during Aging on Lees Thierry Doco,1* Patrick Vuchot,1,2 Véronique Cheynier,1 and Michel Moutounet1 The effect of aging red wine on lees is under development as a winemaking technique. During aging on lees, yeast components, in particular polysaccharides, are released to the wine by autolysis. Evolution of polysaccharides originating from the cell walls of grape berry (arabinans, AGPs, RG-II, and RG-I) or released during fermentation or after autolysis of yeasts (mannoproteins) was followed during aging on lees of a red wine from Madiran. The analyses of composition and structure of the macromolecular fraction of the wines aged on lees with or without stirring showed degradation of polysaccharides from grape. This modification consisted of a 75% dearabinosylation of arabinogalactan-proteins (AGPs) and a total degradation of the pectic arabinans. Partial degradation of the ß-(1→3)-galactan chains of the backbone of the AGP was demonstrated by structural analysis. The arabinose to galactose molar ratio of AGPs decreased from 0.8 for a control wine to 0.3 for a wine aged on lees. The stirring of lees during the aging of wine allowed a slight increase of mannoprotein content. This release may be sufficient to alter organoleptic qualities of wines aged on lees over time. Key words: Wine, aging, lees, arabinogalactans, AGPs, mannoproteins, arabinans Abbreviations: Arabinogalactan type II (AG); arabinogalactan-protein type II (AGP); rhamnogalacturonan type I (RG-I); rhamnogalacturonan type II (RG-II); mannoprotein (MP).

Polysaccharides are a main group of macromolecules in wine. They come from grape berries, yeasts, or other microbial flora. Grape polysaccharides arise from degradation of native polysaccharide originating from cell walls and include arabinogalactan-proteins (AGPs) and arabinogalactans (AGs) (Brillouet et al. 1990, Pellerin et al. 1995, Pellerin and Cabanis 1998), arabinans (Villetaz et al. 1981, Belleville et al. 1993), rhamnogalacturonan type I (RG-I) (Vidal et al. 2003), and rhamnogalacturonan type II (RG-II) (Doco and Brillouet 1993, Pellerin et al. 1996, Vidal et al. 2000a).

AGs are side chains of the RG-I backbone that arise from the degradation of the hairy (high density of side chains) region of pectins by pectinases. In wine, AGPs and AGs are present (Pellerin et al. 1995) at concentrations of ~100 to 250 mg/L for red wines and 50 to 150 mg/L for white wines (Pellerin and Cabanis 1998). AGPs can have interesting physicochemical properties. In particular, an AGP isolated from a red wine has been shown to inhibit protein haze (Waters et al. 1993). An acidic arabinogalactan was shown to reduce self-aggregation of tannins in model wine solutions (Riou et al. 2002).

The core structure of AGPs and AGs consists of ß- D galactopyranose chains linked in position 3 and highly substituted in position 6 by other (1→3)-linked chains of ß-Dgalactopyranoses (Brillouet et al. 1990). This core carries side chains of (1→6)-ß-D-galactopyranoses, themselves substituted in positions 3 and 4 by arabinofuranosyl residues, glucuronic acid, and the following terminal disaccharides [Rhap-(1→4)GlcpA] (Pellerin et al. 1995). The molar ratio of arabinose to galactose is between 0.8 and 1.0 for all AGPs previously isolated from wines. AGPs are localized in soluble form within the cell walls (Saulnier and Brillouet 1989) and thus are easily extracted during pressing of grape berries (Vidal et al. 2000b).

The main chain of arabinans consists of linear (1→5)-linked backbone of arabinofuranosyl residues, substituted by a terminal residue of α-L-arabinofuranose linked in the O-2 or O-3 position (Villetaz et al. 1981). Arabinan concentration is ~100 mg/L at the end of alcoholic fermentation. Arabinans are linked to the rhamnose residues of side chains of the RGI structure. They are rarely present in wine (Pellerin and Cabanis 1998). However, an insoluble linear arabinan has been isolated from a red wine (Belleville et al. 1993). RG-I, which consists of repeated [→2)-α-L-Rhap-(1→4)α-D-GalpA-(1→] disaccharide units, is relatively abundant in grape berry cell walls (Vidal et al. 2001). However, only small quantities of RG-I (