Tartrate Stabilization of Wine
Methods of Tartrate Stabilization of Wine
A lengthy period of post-fermentation processing (e.g., wood maturation for reds and for some full-bodied whites) will lead to a natural precipitation of KHT, and these wines will, therefore, enjoy some degree of natural self-stabilization. Nevertheless, tests should be done to ensure the stability of these wines. Other wines, such as light-bodied whites, which do not spend as much time after fermentation and before bottling, will retain most or all their unstable KHT.
Stabilization by Chillproofing
The aim of cold stabilization processes is to provoke the crystallization of tartaric salts. The wines are placed in refrigerated tanks between -4 and 0 degrees C (24.8-32°F) to reduce the solubility of tartaric salts; usually up to three weeks, under stirring to allow nucleation and crystal growth (Figure 10.3). In the initial stages of chilling, KHT crystals rapidly precipitate out of solution but over time, precipitation slows due to the reduction in KHT saturation level.
Stabilization by Contact Seeding
Contact seeding involves the direct addition of potassium bitartrate powder to a wine to encourage the formation of nucleation sites to enhance crystal growth rates. Nucleation is the process of initiating crystal growth by generating sites to form crystals. Formation of crystals is influenced by the quantity of potassium bitartrate powder, crystal size, contact time, and temperature of seeding (Zoecklein et al., 1995).
Agitation, Temperature, and Filtration
Constant stirring or mixing the seeding crystals for several hours is essential to ensure maximum surface contact. Ninety minutes is considered to be the minimum contact time with 4 hours being optimal. The latest development is the continuous contact process, where the crystals are packed into the conical base of a vertical tank and the cooled wine is pumped upwards through the crystal bed.
Stabilization by Electrodialysis
Electrodialysis is the separation of ions using a charged membrane to enhance diffusion. This process is often applied to desalinate salt water, but can also be used to remove bitartrate ions from wine using the same principles. With electrodialysis, the wine passes through an electrical field. Charged ions are then removed as the wine passes through anionic and cationic membranes, one to tartrate anions, and the other to potassium and calcium.
Sensory Effects
When wine is treated with electrodialysis, there were no significant differences in color, aroma, and flavor when compared with cold stabilized wine. However, there have been contradicting studies that state there is a slight loss in aroma and flavor when treated with electrodialysis relative to cold stabilization, but still at an acceptable level. Overall, the sensory evaluations for wine treated by electrodialysis are satisfactory and are backed up by the fact that The International Organisation of Vine and Wine (OIV) has accepted it as a practice.
Inhibition Techniques by Product Addition
As mentioned, there are various methods that can be used during the winemaking process to reduce KHT precipitation in bottled wines. Some techniques are considered subtractive and involve reducing the concentration of tartaric acid and/or potassium in wine (e.g., chilling the wine, contact seed stabilization, electrodialysis). Alternatively, there are additive techniques that make use of protective colloids or crystallization inhibitors which can be added to wine. The primary action of crystallization inhibitors is to eliminate nucleation sites or coat the KHT to prevent crystal growth.
Metatartaric Acid
If the wine is expected to be consumed shortly after bottling, treatment with metatartaric acid is an inexpensive means of establishing short-term tartrate stability (Zoecklein et al., 1995). When added to wine, metatartaric acid restricts KHT crystallization, and interferes with the growth of potassium bitartrate and calcium tartrate crystals. The effectiveness of different metatartaric acid products may vary, depending on the average esterification rate - reaction between an alcohol and the acid (Ribereau-Gayon et al., 2006b).
Mannoproteins
Previous research has shown wines matured on lees tend to have greater tartaric stability. One possible explanation to this observation is the production of mannoproteins, a major component of yeasts - cell walls, which is released during yeast autolysis.
Carboxymethyl Cellulose
Carboxymethyl cellulose (CMC) is another colloidal substance that interacts with tartrate crystals and inhibits their growth. Carboxymethyl cellulose functions as an inhibitor of crystal growth by eliminating nucleation sites, restricting further crystal growth. In wine it acts like metatartaric acid in stabilizing the wine from bitartrate precipitation, but it has the advantage that it is very stable and does not lose its efficiency with time and higher temperatures.
Potassium Polyaspartic Acid
Potassium polyaspartic acid (KPA), a new product on the market, is purported to be effective in inhibiting tartrate crystallization in white, rose, and red wines and in maintaining long-term tartrate stability.
Wine Suitability with Crystallization Inhibitors
When crystallization inhibitors are used for KHT stabilization, wine must meet certain requirements. Metatartaric acid, CMC, and KPA, unlike mannoproteins, are quite reactive with wine proteins due to their high negative charge. For this reason, it is imperative to check wine protein stability and be sure wine is well below the maximum stability limit, whatever the analytical method used.
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