TARTARIC STABILIZATION OF WINE
Tartaric stabilization is a technical challenge in wine making and a growing concern, as wines are shipped to greater distances. The occurrence of tartrate crystals in wine bottles is a natural process that does not affect taste and is not a health concern, however it must be avoided for several reasons:
Purely aesthetically, crystals are often confused with broken glass, sugar, or chemical residues. This is frequently not accepted by consumers. Commercially, shipments can be returned, as it is especially the case for certain export markets. For sparkling wines, crystals can cause an excessive loss of product when disgorging or when the bottles are opened by the consumers.
Commercial pressures are therefore increasingly demanding wine stability,consequently it is important to introduce new reliable processes.
Eurodia / Ameridia is commercializing a new wine stabilization process that has been developed by INRA, the French National Agronomic Research Institute. This process has been recognized by the International Wine Office (OIV) as "good practice" and has been approved for commercial use by the European Union. Already several units are successfully operating in France and Italy for wines sold on the market.
The process is based on Electrodialysis. The process has been considered in the past for wine stabilization but the recent introduction of new ion exchange membranes has allowed the commercialization of this attractive process. The process works in combination with a new reliable test method based on conductivity to evaluate the extent of the required treatment and insure stability.
TABLE 1: ADVANTAGES OF TARTARIC STABILIZATION BY ELECTRODIALYSIS
A 100% reliability: the treated wines are always completely stable (the conventional refrigeration method is not). Gentle and gradual treatment that can be tailored to each specific wine thanks to an on-line test: a valuable tool for quality assurance. Electrodialysis stabilizes for potassium tartrate and also for neutral calcium tartrate. The process does not alter the wine characteristics (pH, acidity, sugar content, alcohol level,...) and has no effect on the taste, bouquet, and aspect (color) of the end product.
There are no temperature changes. There are no additives of any kind (it is a subtractive process). No extensive pretreatment is required (only filtration at 5 micron max.).
Reduction in consumables required (no filtration additives, no seeding material). Low energy cost in comparison with the refrigeration method. Competitive overall treatment costs. The equipment is compact and can be installed on automatic units that can be moved to provide service from vineyard to vineyard. Liquid effluent allows for the recovery of the tartaric values.
Why is there tartrate crystallization ?
Grapes contain a high level of tartaric acid (from 1 to 3 g/l of juice) as well as a high level of potassium (from 0.8 to 1.5 g/l of juice). Tartaric acid is a weak diacid that can be dissociated in the tartrate and bitartrate forms. It can precipitate as two main salts: either potassium bitartrate (THK) that is not very soluble or calcium tartrate (TCa) that is not soluble. Depending on the pH, the prevailing form can be one salt or the other, with more calcium tartrate at higher pH's. In the typical pH range of wine (3 to 4), THK is predominant and its solubility is the lowest between pH 3.4 and 4.
The solubility of the two salts varies with several parameters: temperature (it decreases at lower temperature), pH, as well as alcohol content (it decreases when the ethanol content increases). Some wine components (protecting colloids: polysaccharides, mannoproteins) slow down the crystal seeding process.
Conventional tartaric stabilization techniques
There are two types of techniques to stabilize wine;
Physical: to eliminate by refrigeration or by membrane separation a fraction of the ionic species that can precipitate. Chemical: requiring the use of additives to enrich the wine with metatartric acid and slow down the crystallization process. However, its effectiveness is limited in time. Inhibiting substances are also being tested such as carboxycelluloses and yeast mannoproteins.
The technique most commonly used until now is artificial refrigeration to bring the wine close to (but above) its freezing temperature to precipitate the potassium bitartrate and separate it before increasing the temperature. In some cases, "cream of tartar" is added to sursaturate the wine. The technique, besides being expensive and time-comsuming, requires two temperature changes and a good knowledge of the instability of the wine to treat. Most importantly, it is not completely reliable, as crystals can still precipitate during storage and transportation. In addition careful clarification is usually required before cooling and a separation step is needed to separate the crystals; this clarification can affect the nature of the wine.
In some wine producing regions, ion (cationic) exchange resins are sometimes used to remove cations (K+), but these remove all the cations in the wine to affect its character and its ionic balance. Therefore, it is usually done on a fraction of the product to be mixed with the rest. This is also an unreliable method that is not used for good quality wines and it is not authorized in the European Union.
Electrodialysis for tartrate stabilization:
Electrodialysis can be used to remove the postassium and calcium cations and the tartrate anions from wine. For a detailed description of electrodialysis, go to the Technologies Section of this website.
For the wine application, the electrodialysis stack has a wine and a brine compartments (see Figure 1). The potential applied to the electrodes leads to the migration of the ions in solution: the potassium and calcium cations migrate toward the cathode and the tartrates anions toward the anode. The tartrates can cross the anionic membranes and are removed from the wine because they cannot leave the brine compartments since the next membranes they reach are cationic membranes. Similarly, the cations are removed from the wine through the cationic membranes. With the alternate succession of cationic and anionic membranes in the stacks, the ions are removed from the wine compartments into the brine compartments. Two hydraulic circuits feed in parallel all the compartments of the same type. The reversible electrodes are protected thanks to a distinct circuit containing an electrolyte.
Membrane selection for the treatment of wine:
The idea to use electrodialysis for the stabilization of wine is not new but the development of suitable ion exchange membranes was necessary before it could become practical. This development had to take into account the following constraints: ·
Regulatory: membranes must satisfy the regulations relative to food additives and materials in contact with food products, and must satisfy migration tests in aqueous/ alcoholic solutions at acidic pH's.Technical: to achieve wine stabilization at competitive costs, membrane selection must be based on the evaluation of
- the kinetics of global deionization as well as specific to K+, Ca++, and tartrates.
- the mechanical resistance of the membranes.
- their resistance to fouling.
Quality of product: the selected pair of membranes must not affect the non-ionic components of the wines and have only a moderate impact on their physico-chemical balance. In the search for the optimal pair, the following limits were used as selection criteria:
- a maximum decrease of the ethanol content limited to 0.1 vol %.
- a pH decrease lower than 0.25.
- a decrease of the volatile acidity lower than 0.09 g/l (expressed as H2SO4).
The membranes that are currently used in commercial systems satisfy these three criteria. For other components of wine, the treatment has no impact on the concentrations of polyphenols (tannin), polysaccharides, amino acids, and volatile compounds. The impact on the wine color is comparatively lower than with the refrigeration method that requires clarification to also remove colloidal coloring substances.
A new test for tartaric stability:
In addition to the development of a practical process with the selection of suitable membrane pairs, one of the main contributions of INRA was the development of an integrated test to evaluate the wine (in)stability. The test is based on the conductivity variations over time of a sample of the wine that has been cooled to a negative temperature and seeded with calibrated potassium bitartrate crystals under constant agitation. This model of the instability phenomenon, followed over a period of about four hours (see Figure 2) should make it possible to determine the drop of conductivity for a theoretical infinite time (for a perfectly stable wine).
Mathematical formulae are used, depending on the type of wine to be treated, on the chosen temperature (-4°C to 0°C), and the addition of ethanol or other additives to accelerate precipitation. The conductivity reduction determined by this test gives the target conductivity of stability for a specific wine. It is therefore sufficient to treat the rest of the production until this conductivity level is reached (and not below) to achieve an absolute tartaric stability.
How does tartaric wine stabilization by electrodialysis work ?
Conductivity is an easy parameter to monitor automatically during electrodialysis. Once the test on a representative sample of the wine has been completed, the level of conductivity reduction to be obtained is the control point that can be monitored automatically (see Figure 3).
A sample of the wine is automatically taken before treatment and characterized by its pH and conductivity. The treatment of the rest of the production is only authorized if the wine is found to be unstable and if the pH and conductivity of each subsequent sample match the initial values. Thus, the process is automatically adapted to the specific instability of each wine. The extent of processing cannot exceed the optimum value for each wine, each being characterized by its conductivity and pH values. Therefore, the process can be run with limited manpower.
Process description:
A unit for tartaric stabilization of wine has an electrodialysis stack, with two tanks and circulation pumps for the fluids (wine and brine) in and out of the stack. The wine batch tank, is equipped with level gauges and both loops have a conductivity meter to control the wine circulation sequences and the brine concentration. The test device functions either in the measuring mode or in the process mode, with the conductivity as the control parameter. A pump feeds the wine from the storage tank into the batch tank from where it circulates through the stack. When the measured conductivity has reached the control point, the volume of stabilized wine is sent to the reception tank by a set of electric valves. Another batch of wine is then fed to the batch tank under the same conditions. Such an automatic batch operating mode processes small volumes of wine in a few minutes. Overall, the process is continuous and does not require a large volume of wine to be immobilized. The brine circuit, containing the ions that were extracted, is regulated by dilution (conductivity) and pH to avoid precipitation of potassium bitartrate in the stack. The waste brine volume is about 15 % of the wine volume.
Overall, a typical flow rate for the treatment is in the range of 100 l/h/m2 of cell, depending on the quantity of ions to be extracted to ensure stability. A satisfactory membrane life is achieved even if red wines tend to foul the membranes slightly faster because of higher surface tension reducing the ion transfer through the membranes. Anionic membranes are more prone to fouling but periodical (once per day) on-site rinsing and cleaning of the stack can reduce the effect of fouling and extend membrane life.
Based on experiments done at INRA with various wines from different regions, the drop of conductivity to be applied to red wines is between 5 and 20 %, while for white wines and young wines it can reach 30 %. Overall, all the wines that were treated by electrodialysis are and remain stable. Even more important, comparative sensory tests of control samples and of wines treated by electrodialysis showed no significant differences.
Technical benefits:
Reliable: due to the selection of suitable membranes and to the use of a stability test, integrated with the control of the process. This determines for each wine the usefulness and the extent of the treatment. Specific to each wine: this treatment can be adjusted to the degree of instability of each wine and to the specific risk of crystal precipitation in the bottled product. The process is adaptable to the characteristics of each wine by removing only the quantity of K+, Ca++, and tartrate necessary to achieve the tartaric stability level that has been defined in advance. Calcium tartrate removal: the treatment eliminates a fraction of the calcium, making the wine stable relative to calcium tartrate as well. Therefore, tartaric wine stabilization by electrodialysis has become a tool that can be integrated in the quality assurance strategy of wine producers. Other quality benefits: electrodialysis also can eliminate some of the clarification/ filtration steps that are essential to the refrigeration treatment. The precipitation of potassium bitartrate at lower temperature is more effective if the wine has been clarified. These clarifications can be detrimental to the quality of the wine, with potential loss of aroma.
Economic Considerations:
Savings: tartaric stabilization by electrodialysis allows for savings in the additives (filtration, seeding) required by the conventional refrigeration processes. Wastes will therefore be reduced as only one membrane cleaning per day is required, and only 10 to 20 liters of water are required in the brine loop per 100 liters of wine. This liquid effluent might allow the recovery of tartaric acid values, but this will have to be further developed. Low energy costs: power consumption is between 0.5 and 1 kWh per 1000 liters of processed wine, including pumping power. This is about ten times lower than the energy needed for refrigeration, depending on the overall process and the equipment used. Low labor costs: as discussed before, electrodialysis stabilization can be automated. Comparative evaluation: taking into account investment costs (depreciation over five years and interest) and operating expenses (power consumption, consumables, membrane replacement, labor, maintenance), the comparison is favorable to the electrodialysis treatment. It is possible to reduce the cost by as much as 50 to 70 % compared to refrigeration, depending on the size of the unit, the extent of the automatization the number of operating hours per day, etc. Example of costs: the typical investment for a (relatively large) production of 400,000 hl/year would be in the range of $400,000 with a typical operating cost of 57 cents per hectoliter. Mobile units: as the equipment is relatively compact and can easily be installed on a truck for smaller capacities (45,000 hl/year), it is possible to offer treatment services at the vineyards on a price per liter basis. It should be possible to offer this service for less than 10 cents per bottle.
Reference:
Electrodialysis, Adaptation to Tartric Stabilization of Wine, Moutounet M., Escudier J.L., Saint-Pierre B., Batlle J.L.
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