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The Role Of Polysaccharides In Vinification And Their Contribution To Aspects Such As Body, Better Mouthfeel And Stability
LALLEMAND South Africa, PO Box 3542, Matieland, 7602
Polysaccharides constitute one of the main groups of macromolecules occurring in wine, and may derive from the grape as well as various micro-organisms. Some of the best-known polysaccharides are undoubtedly mannoproteins and mannanes which are formed and released by the yeast Saccharomyces cerevisiae during, and after, primary fermentation. Other forms of polysaccharides include glucanes formed by Botrytis cinerea.
This article attempts to inform winemakers about the exact nature of polysaccharides, the positive role which these molecules might play in the vinification process, and options available to the winemaker to maximise the concentrations of polysaccharides in wine.
POLYSACCHARIDES IN WINE
All wines contain polysaccharides and depending on their origin, these macromolecules may be divided into three groups.
Grape polysaccharides - These molecules are essentially pectins and polysaccharides such as arabinanes, galactanes and arabino-galactanes with a molecular weight ranging from 40 to 250 kDa. These molecules impart viscosity to wine, sometimes necessitating the use of pectolitic ensymes to clarify the wine and make filtration easier.
Fungi polysaccharides - Of these, the best-known examples are undoubtedly glucanes deriving from Botrytis cinerea. The molecule is linear, has a molecular weight of 1 000 kDa, and is known to be a possible cause of problems in the filtration of wine. However, new ensyme preparations are available to address this problem.
Yeast polysaccharides - This group of molecules is represented by the beta-glucanes that are linear molecules (molecular weight ranges from 25 and 270 kDa), and mannoproteins that are globular in shape. The molecular weight of the latter ranges from 10 to 450 kDa. Due to their three-dimensional structure, these mannoproteins do not cause any significant filtration problems.
THE IMPORTANCE OF YEAST POLYSACCHARIDES
Polysaccharides are released during alcoholic fermentation, as well as during the process of maturation on the lees, due to the autolysis of yeast cells. Rosi and co-workers indicated that different yeast strains varied in their ability to produce these polysaccharides (Rosi et al., 1998; Rosi et al., 1999). This is clearly demonstrated by the results summarised in Table 1. The amount of polysaccharides released by the yeast depends on the specific yeast strain, as well as the fermentation and maturation conditions. A higher concentration of polysaccharides is released by yeast at slightly higher temperatures. Ribéreau-Gayon et al., (2000b) indicated that the release of polysaccharides at 35oC was considerably higher than at 22oC. If a medium/wine is shaken or brought into motion (by means of "batonnage", for example), it can also result in an increased release of polysaccharides (Ribéreau-Gayon et al., 2000b).
The macromolecules produced by the respective yeasts as indicated in Table 1, consist mainly of polysaccharides (97,9%), as well as a small (1,3%) protein fraction. The biggest part (89,2%) of the polysaccharide fraction consists of the sugar mannose. This composition therefore confirms that the polysaccharide fraction that is released in the medium consists mainly of mannoproteins (Rosi et al., 1999).
A brief discussion of the role of these mannoproteins in the vinification process follows below.
Mannoproteins and tartrate stability - Lubbers et al., 1993 indicated that mannoproteins have a positive effect on tartrate stability. It seems as though the mechanism of activity is based on competitive inhibition, which limits crystal formation (Moutonnet et al., 1999). The findings above are confirmed by the fact that by leaving white wine on the lees for a considerable period of time (some months), it becomes relatively tartrate stable and does not therefore require cold stabilisation (Ribéreau-Gayon et al., 2000b).
Mannoproteins and protein stability - Ledoux et al., 1992 indicated that some mannoproteins had a noticeable influence on the protein stability of white and rosé wines. The presence, as well as the addition of such mannoproteins, will obviously necessitate less bentonite for the stabilisation of white and rosé wines. Therefore smaller volumes of aromatic molecules will subsequently bind and perhaps be removed from the wine by the bentonite.
Mannoproteins and tannins - For some time now there have been speculations that certain polysaccharides might well bind with tannins and by so doing, change the astringency of wines. Saucier et al., (1996) suggested a model that could possibly explain the polysaccharide-tannin interaction. This model sheds light on the reason why yeast strains that produce large quantities of polysaccharides, result in a wine with more body and better mouthfeel, as well as a wine with better colour stability.
Mannoproteins and aromatic components - Various studies, inter alia Lubbers et al., (1994) indicated that polysaccharides also have the ability to stabilise aroma components.
Mannoproteins and malolactic fermentation - Guilloux-Benattier et al. (1995), as well as Rosi et al. (1999) highlighted the positive influence of mannoproteins on the onset and development of malolactic fermentation. It was apparent that in a wine fermented primarily with a yeast strain that produced naturally higher levels of polysaccharides, the onset and completion of malolactic fermentation occurred more quickly than the malolactic fermentation of a wine fermented with a yeast strain that produced inherently lower concentrations of polysaccharides.
THE POSITIVE EFFECT OF POLYSACCHARIDES IN PRACTICE
It goes without saying that it is always important for the winemaker to have some or other practical application in his/her cellar so as to be able to provide the consumer with a better quality wine. From the above it is clear that polysaccharides are definitely able to make a positive contribution to the overall quality of wines, including a better mouthfeel, as well as better overall stability. Against this background it is therefore important for the winemaker to maximise the amount of polysaccharides in wine. This can be achieved in various ways.
1)The choice of yeast strain: This plays a very important role, since yeast strains differ in their ability to produce polysaccharides (Table 1). As mentioned, wines made with yeast strains that produce inherently higher levels of polysaccharides, are naturally softer, have more body and better mouthfeel. These strains include, inter alia, Lalvin ICV-D254, Lalvin CY3079 and Lalvin BM45. Yeast strains Lalvin D254 and Lalvin BM45 also impart good colour stability, inter alia, when used for the vinification of red wines. The Lallemand brochure can be consulted for more detail in this regard (Loubser, 2000).
2)The use of beta-glucanase ensymes: This type of ensyme acts on the yeast's cell walls and therefore contributes to the release of mannoproteins. This phenomenon also occurs naturally in the course of autolysis of yeast strains and is caused by the yeast's endo-glucanases, deriving from the vacuoles of the yeast strains. If commercial ensyme preparations such as Lallemand MMX are used, the autolysis effect can be accelerated slightly in order to obtain wines that have more body, are more complex and have greater tartrate stability (Izquierdo-Torres, 1999).
3)The maturation of wine on the lees: White wine can benefit enormously from being left on the lees after primary fermentation. Regular "batonnage" in this period can further stimulate the release of polysaccharides, which in turn will have a positive influence on the mouthfeel and body of the wine. The "batonnage" of red wines will have the same positive result.
4)Fermentation of red wine at higher temperatures: The tendency to ferment red wines at higher temepratures is already an established practice in the South African wine industry. For this reason much benefit is derived from the positive effect of polysaccharides, the release of which is encouraged by slightly higher temperatures. However, winemakers should take care to note that in the presence of high alcohol concentrations, fermentation temperatures exceeding 30oC are extremely detrimental to the functioning and viability of the yeast. Moreover, by leaving red wine on the skins for an additional period of time after fermentation, a further contribution will be made to the release of polysaccharides, which in turn will have a positive influence on aspects such as mouthfeel, body and overall quality.
A further advantage of polysaccharides is improved colour and lower tannin reactivity in white wine. The polysaccharides which are released during the maturation process of white wine (on the total lees) in barrels, are able to react and bind with certain phenolic compounds. The total polyphenolic index and the yellowish colour of the wine will gradually diminish as the maturation process progresses. Moreover, after several months wines that have undergone maturation on the total lees in barrels, will appear less yellow than the same wines that underwent maturation on fine lees in tanks. Some of the tannins, which derive from the wood, are also bound by the polysaccharides (mannoproteins). The result is a lower tannin concentration and consequently also a lower proportion of reactive tannins in a wine of this kind (Ribéreau-Gayon et al., 2000a). Wines such as these are therefore softer and more drinkable.
Furthermore, the maturation of white wine on total lees has the added advantage of reducing the sensitivity to oxidative pinking. This problem is characterised by a change in colour to a greyish-pink tint when wine becomes slightly oxidised due to processes such as stabilisation and bottling. Young white wines, and Sauvignon Blanc in particular, the must of which is reasonably protected against oxygen, is rather sensitive to this change in colour. However, the precise compound(s) involved in the protection of wine is (are) not yet known (Ribéreau-Gayon et al., 2000a).
CONCLUDING REMARKS
It is clear, therefore, that polysaccharides can play a very important role in the overall vinification process. Their contribution to better mouthfeel, inter alia, as well as the overall stability of wine, is apparent from the discussion above. Consequently the onus is on the winemaker to make the correct decision with regard to the yeast strain being used, as well as the vinification practice, in order to obtain maximum benefit from the activity and influence of polysaccharides in wine.
REFERENCES
1) GUILLOUX-BENANTIER, M., GUERREAU, J. & FEUILLAT, M. 1995. Influence of initial colloid content on yeast macromolecule production and on the metabolism of wine microorganisms. Am. J. Enol. Vitic. 46, 486-492.
2) IZQUIERDO-TORRES. 1999. Enzymatic preparations to improve the mouthfeel during sensory evaluation. In: booklet number 7, Colloids and Mouthfeel in Wines, Lallemand Technical Meeting, Montreal, 27-29 May, 27-29.
3) LEDOUX, V., DULAU, L. & DUBOURDIEU, D. 1992. Interpretation de l'amélioration de la stabilité proteique des vins au cours de l'élevage sur lies. J. Inten. Sci. Vigne Vin. 26, 239-251.
4) LOUBSER, P. A. 2000. Lallemand brosjure rakende wyngis, bakterieë en voedingstowwe. Lallemand Suid-Afrika, Posbus 3542, Matieland, 7602.
5) LUBBERS, S., CHARPENTIER, C., FEUILLAT, M. & VOILLEY, A. 1994. Influence of yeast walls on the behavior of aroma compounds in a model wine. Am. J. Enol. Vitic. 45, 29-33.
6) LUBBERS, S., LEGER, B., CHARPENTIER, C. & FEUILLAT, M. 1993. Essai colloides protecteurs d'extraits de parois de levures sur la stabilité tartrique d'un vin modêle. J. Inter. Sci Vigne Vin. 27, 13-22.
7) MOUTONNET, M., BATTLE, J. L., SAINT PIERRE, B. & ESCUDIER, J. L. 1999. Stabilisation tartrique. Détermination du degree d'instabilité des vins. Mesure de l'efficacité des inhibiteurs de cristallisation. In: Oenologie 1999, 6e Symposium International d'Oenologie. A. Lonvaud-Funel (Ed). Lavoisier Tec-Doc, Paris, 531-534.
8) RIBéREAU-GAYON, P., DUBOURDIEU, D., DONéCHE, B. & LONVAUD A. 2000a. Handbook of Enology. Vol. 1. The Microbiology of Wine and Vinifications. John Wiley & Sons Ltd.
9) RIBéREAU-GAYON, P., GLORIES, Y., MAUJEAN, A. & DUBOURDIEU, D., 2000b. Handbook of Enology. Vol. 2. The Chemistry of Wine Stabilization and Treatments. John Wiley & Sons Ltd.
10) ROSI, I., GHERI, A. & FERRARI, S. 1998. Effets des levures produisant des polysaccharides pariétaux sur certaines caractéristiques des vins rouges pendant la fermentation. Rev. Franc. Oenol. N.172, 24-26.
11) ROSI, I., GHERI, A., DOMIZIO, P. & FIA, G. 1999. Production of parietal macromolecules by Saccharomyces cerevisiae and their influence on malolactic fermentation. In: booklet number 7, Colloids and Mouthfeel in Wines, Lallemand Technical Meeting, Montreal, 27-29 May, 35-39.
12) SAUCIER, C., ROUX, D. & GLORIES, Y. 1996. Stabilité colloidale polymers catéchiques. Influence des polysaccharides. In: Oenologie 1995. 5e Symposium International d'Oenologie. A. Lonvaud-Funel (Ed.). Lavoisier Tec-Doc, Paris, 395-400.
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