Céline BAJARD-SPARROW, Mylène CAUSSETTE, Céline FAUVEAU, Phil LATHAM, Patrice PELLERIN, Peter LANKHORST
DSM Food Specialties (www.dsm-oenology.com)

Potassium bitartrate is soluble under certain conditions of temperature and pressure. In wine, this salt is in an unstable state of super-saturation that can lead to crystal formation under certain conditions such as low temperatures. This phenomenon is known as tartaric precipitation or crystallisation.

Wine stability is influenced by a variety of factors including: tartaric acid content, concentration of potassium and calcium ions, pH, ethanol concentration, temperature and the presence of colloids. Nowadays several treatments are used to prevent the precipitation of tartrate salts. The oldest, described by Scazzola E. (1956), consists of metatartaric acid addition into the unstable wine. This molecule prevents the growth of KHT crystals. However, its action is limited in time because of its gradual hydrolysis into free tartaric acid. This actually results in an increase of the super-saturation state, which favours crystallisation of potassium tartrate (Carafa, 1958).

Cold stabilisation, another commonly used treatment, is characterised by long- term (weeks) conservation of the wine at low temperature. Cold induces the formation of the potassium tartrate crystals. This technique can be accelerated by the addition of cream of tartar (solution of potassium tartrate micro crystals) that plays the role of crystallisation initiator. Once formed, the crystals grow and can then be removed by filtration.

A more recent method is based on electrodialysis technology. The wine is recycled between plates. The electric potential difference applied between these plates, forces the migration of molecules through a selective membrane and thus removes ionic material from the wine (Saint Pierre et al., 1995).

Mannoproteins are natural components of the yeast cell wall (Figure 1). The cell wall of Saccharomyces cerevisiae is composed of 90 % polysaccharides (glucans and mannans) but also contains proteins, lipids, phosphates, chitin and minerals. The architecture of the yeast cell wall has been reviewed extensively by Kapteyn et al. (1999) and by Lipke et al. (1998). Mannoproteins are released into the wine, firstly during alcoholic fermentation and then during yeast autolysis as wine is aged on the lees. Naturally prevalent in wine, mannoproteins represent the second largest family of wine polysaccharides (Vidal et al., 2003).

Mannoproteins are highly branched structures of mannose residues, linked by various glycosidic bonds and a polypeptide chain. Their molecular weight can reach as high as 800 kDa. Though all mannoproteins have a common structure, they do present a large diversity in their composition, type of glycosidic linkages and organisation. These differences are responsible for their functional properties and effects in wine, from mouth feel improvement to induced tartaric stability.

The protective colloidal effect of mannoproteins is well known in winemaking. Increased stability of wine when aged on lees has been observed for many years. Subsequently other properties have been described, such as mannoproteins stabilising protein haze (Ledoux et al., 1992; Waters et al., 1994) and more evidence on potassium tartrate stability (Lubbers et al., 1993; Moine-Ledoux and Dubourdieu, 1997; Moine- Ledoux and Dubourdieu, 1999).

Mannoproteins appear to prevent the formation of crystals. It has been observed that a partial or complete elimination of protective colloids during the winemaking process generates a modification of the wine equilibrium leading to loss of tartaric stability. The mechanism of action is described to be based on a competitive inhibition, limiting crystal formation (Moutounet et al., 1999). It is generally agreed upon that mannoproteins do inhibit the nucleation (initial step of crystal formation) whilst their effect on the crystal growth is less important. As a consequence, wine protection against tartaric instability can only be achieved, when there are no existing crystals in the wine (Moutounet et al. 1999).

Mannoproteins are commercially available in a liquid or granulated form and are added to a wine just before bottling. It is very important to follow the precise instructions of the manufacturers.  

• “Un nuovo ingrediente per la stabilizzazione tartarica", Vignevini n°11- 2007. Delphine BOUISSOU, Alain SAMSON, Bernard SAINT-PIERRE, INRA - Unité expérimentale de Pech Rouge - Gruissan - France. Céline BAJARD-SPARROW, Mylène CAUSSETTE, Céline FAUVEAU, Phil LATHAM, Patrice PELLERIN, Peter LANKHORST, DSM Food Specialties.
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