Polysaccharides in wine

Solubility whereas monosaccharides are very soluble in water and in polar solvents such as alcohols, pyridine and dimethylsulfoxide, solubility decreases when molecular weight increases thus some polysaccharides are totally insoluble. Formation of some polysaccharides in wines can produce defects (e.g. dextrans appearing in wines cause ropiness). 

Increased attention has been paid in the last few years to the smdy of polysaccharides in wine and great advances in the knowledge of their structures and intrinsic properties have been achieved (Vidal et al. 2003 Ayestaran et al. 2004). 

After their isolation by chromatographic techniques (anion-exchange chromatography, size exclusion, etc.), different analytical methodologies have been used to identify and quantify the polysaccharides in wine the most commonly used being the traditional methylation analysis followed by GC-MS (Doco and Bril-louet 1993). Polysaccharides have also been determined after solvolysis with anhydrous methanol containing HCl by GC-MS of their per-G-trimethylsilylated methyl glycosides (Vidal et al. 2003). Other techniques such as Fourier transform infrared spectroscopy (FTIR) have been more recently proposed (Coimbra et al. 2002,2005 Boulet et al. 2007). 

MPs are constituted by 90% of mannose, protein and phosphoric acids and represent the 35% of total polysaccharides in wine (Vidal et al. 2003). MPs combined with phenolic compounds have shown an indirect effect on astringency, although their stabilizing effect on protein precipitation in white wine and tartrate crystallization in both red and white wines are their main function (Ribereau-Gayon et al. 2006). Glucomannoproteinshscve been also detected in wines in lower amount than MPs (Ribereau-Gayon et al. 2006). 

Dufour and Bayonove (1999a) reported two criteria for polysaccharide discrimination acidity and protein content. Neutral peptic substances (type II arabinogalac-tans and arabinogalactans-proteins) represent 40% of the polysaccharides in wine and acidic pectic polysaccharides, (e.g. homogalacturonans and rhamnogalacturo-nans) account for 20% of them. Because of the difficulty in purifying wine polysaccharides, most of the studies on interactions between wine polysaccharides and aroma compounds have been carried out with exocellular and cell wall mannoproteins (thus mainly glycoproteins) of Saccharomyces (see effect of yeast and derivatives in the next section). 

Yeasts are the second major source of polysaccharides in wine. A great deal of research has been devoted to the structure of yeast cell walls (Volume 1, Section 1.2), but much less to the type of carbohydrate colloids released into wine. 

A sensory study based on an incomplete factorial design allowed to demonstrate that astringency of procyanidins was reduced in the presence of rhamnogalaturonan II added at levels encountered in wine but was modified neither by anthocyanins nor by the other wine polysaccharides (mannoproteins and arabinogalactan proteins). Increase in ethanol level resulted in higher bitterness perception but had no effect on astringency. 

Some MP fractions obtained from wine also had the ability to inhibit protein-tannin aggregation (unpublished results). Polysaccharides showed effects at concentrations at which they are present in wine, which means that they could have an influence on wine astringency. 

Karadjova and coworkers [90] in a detailed and comprehensive investigation established a scheme for fractionation of wine components and Cu, Fe, and Zn determination in the different fractions. Like Fe, the other two metals may analogously exist in wines as free ions, as complexes with organic acids and as complexes with proteins, polyphenols and polysaccharides. The resin XAD-8 was used for the separation of wine polyphenols. Dowex ion exchange resins were used for the separation of cationic and anionic species of metals that were subsequently quantified off-line in Bulgarian and Macedonian wines by FAAS or ET-AAS (depending on their concentration levels). 

The mechanism of protein haze formation in wines is not fully understood. Slow denaturation of wine proteins is thought to lead to protein aggregation, flocculation into a hazy suspension and, finally, formation of visual precipitates. The importance of non-proteinaceous factors in white wine protein haze formation such as proan-thocyanidins (Koch and Sajak 1959 Waters et al. 1995a Yokotsuka et al. 1991) have been suspected for some time. Other factors such as polysaccharides, alcohol levels and pH have also been implicated (Mesquita et al. 2001 Siebert et al. 1996a). It has been observed that grape protein added to model wine does not precipitate or produce haze when heated, whereas visually obvious hazes occur when the same protein is added to a commercial wine (Pocock 2006). 

D-Mannose is present in polysaccharides of plants (mannans) and also in combination with other sugars. Mannoproteins are considered one of the main polysaccharides of wines as it will be described later. 

Carbohydrates are minor components of wine which contribute to sensorial properties and play and important role in the different reactions occurring during fermentation and aging. Whereas monosaccharides and polyalcohols are rather well-known, more research is necessary on disaccharides, oligosaccharides and non-phenolic glycosides. The presence of cyclitols in wines and wine derivatives can constitute a useful tool to characterize their origin. Among all the studies related to carbohydrates in wine, polysaccharides have been the most important focus in the last years. 

The interaction between aroma compounds and other wine micro-organisms (e.g. lactic acid bacteria) or with metabolites produced during malolactic fermentation has been studied to a limited extent. Interactions between polysaccharides produced by the most common wine lactic bacteria (Oenoccocus oeni) during malolactic fermentation have been shown to be responsible for the reduced volatility of some aroma compounds in wines (Boido et al. 2002). The possibility of direct interactions between the surface of the bacteria cells and aroma compounds should also be considered since this type of interaction has been found for other food lactic bacteria (Ly et al. 2008). 

Among wine polysaccharides, mannoproteins play an important role in protein haze stabilisation (Waters et al. 1994 Dupin et al. 2000). Gelatin fining of a wine phenolic extract in wine-like solution resulted in a much higher precipitation rate than when the same treatment was applied on the original wine. After addition of wine polysaccharides at the concentration normally encountered in wines, precipitation was reduced back to the level measured in wine, confirming the stabilizing effect of polysaccharides (Cheynier et al. 2006). 

Crystal appearance and growth are slower in red wines than in white wines and also differ within red wines. Arabinogalactan-proteins and mannoproteins were the major polysaccharides in the precipitates while rhamnogalaturonan II could not be detected. The average degree of polymerisation of proanthocyanidins in the deposit was higher that that of wine proanthocyanidins, indicating that polymers were selectively associated with the tartrate crystals. A preferential association of apolar fiavonols was similarly observed, presumably as their lower solubility favours adsorption on surfaces. 

Recently, Carvalho et al. (2006b) studied the influence of wine polysaccharides (AGP, RGll and MP) on salivary protein-tannin interactions. The results showed that the most acidic fractions of AGPs and MPs have the ability to inhibit the formation of aggregates between condensed tannins and two different salivary proteins (a-amylase and lB8c). The concentrations tested are below to those present in wine which means that they could have an influence in wine astringency. 

On the other hand, it has also been found that polysaccharides present in wines interfere with the self-aggregation of proanthocyanidins (Riou et al. 2002). The change in the proanthocyanidin colloidal state in wines could also affect their ability to complex with salivary proteins and thereby their sensory properties. 

As the free sugar only in small amount in wine and fermented milk products (cheese and yogurt). Constituent of oligosaccharides and polysaccharides... 

As the free sugar in small amounts in wine and in the extract of the leaves of poison ivy (Rhus toxidendmn), and in some microbial polysaccharides... 

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