Dr. Carien Coetzee
31 January 2023
Calcium-induced instabilities is a most insidious problem in bottled wines mainly due to the unpredictable and slow formation of crystals which usually do not come out of solution for some time after bottling (often after commercial release).
Forms of tartrate instabilities
Tartrate instability can manifest as the precipitation of two salts: potassium bitartrate (KHT) and calcium tartrate (CaT). Even though both are tartrate salts, the two salts form under very different conditions and at different rates.1
Types of calcium instabilities
The most common manifestation is in the form of crystalline calcium L-tartrate. Other less common occurrences of calcium instability are calcium DL-tartrate and calcium oxalate. When problems involving calcium DL-tartrate did occur, investigation revealed the instability resulted from the use of DL-tartaric acid as an acidulant. Oxalic acid present in some commercial batches of L-tartaric acid and can be responsible for calcium oxalate instability. 1
Calcium tartrate crystals morphology
Calcium tartrate forms crystals that are very different from potassium bitartrate and therefore easily recognizable under the microscope. Deposits of calcium tartrate usually appear as colourless or white, bipyramidal or rhomboid crystals.1 In some cases co-deposits are also present such as phenolic and protein material, quercetin crystals, or yeast cells.1
Calcium tartrate crystal formation and main influencing factors
The occurrence of calcium tartrate deposits is a most insidious problem in bottled wines because the kinetics of crystallization are very slow and the crystals usually do not come out of solution for some time, often months, after bottling. The limiting factor for calcium tartrate crystal formation is the initial nucleation which requires a lot of energy.
The main factors promoting calcium precipitation are calcium, pH and tartaric acid. Like potassium bitartrate, calcium tartrate will remain supersaturated in a wine. However, for calcium tartrate, supersaturation may be prolonged for extended periods before crystallisation occurs. Many components in wine, including some of the natural acids and macromolecules, can greatly enhance a wine’s holding capacity for calcium tartrate. In some wines, calcium tartrate crystals are prevented from developing to a detectable size for an extended period due to the presence of inhibitory compounds. The different concentrations and types of inhibitory compounds present in wines are also the reason why some wines may be able to inhibit calcium tartrate crystallisation for longer periods of time while other wines will undergo crystal formation earlier even though both wines may have the same starting concentration of calcium.
In particular, pH has a tremendous impact as calcium tartrate precipitation is favoured at higher wine pH values.2 The pH plays a key role in the formation of calcium tartrate because it regulates the dissociation equilibrium of the tartaric acid: the higher the pH, the higher the percentage of tartrate ion present and, consequently, the more likely it is that calcium tartrate will precipitate.3 A pH increase of only 0.1 has a dramatic effect on the speed and intensity of calcium precipitation. Winemaking operations that may increase the pH such as malolactic fermentation and blending can increase the likelihood of instability.1
Sources of calcium
Vineyard soil is a natural source of calcium and the amount of calcium absorbed by the vine depends largely on the characteristics of the soil. In general, the more alkaline the soil, the greater the calcium accumulation by the vine. Calcareous soils and the high availability of nitrates promote calcium uptake, while the presence of some metals decreases it. Other contributors are high transpiration of the acid before vériason and the mobilization of calcium in response to thermal and water stress.
Oenological sources of excess calcium include the use of calcium carbonate during deacidification rather than potassium bicarbonate.4 Another source has been the use of casein or other milk products for fining. Fermentation and wine storage in unlined or inadequately coated concrete tanks has also been a cause of calcium instability in the past.
Testing for calcium instability
The stability of calcium tartrate is often estimated based on the calcium concentration found in wine. Many publications indicate 80 mg/L for white and rosé wines and 60 mg/L for red wines as threshold values above which the wine is considered unstable. These references, although insightful, are not always appropriate indicators of calcium tartrate instability due to the presence of unknown types and concentrations of crystal inhibitors as well as the influence of wine pH. Calcium concentration is, therefore, not a reliable indicator in isolation and should rather be used in conjunction with stability tests or other factors to determine the risk of crystal formation.
Conductivity analysis tools, so useful in defining the stability of potassium bitartrate in wine, are of no help in determining calcium tartrate instabilities.
Unlike potassium bitartrate, the precipitation of calcium tartrate is little affected by low temperatures. The conventional freeze test is therefore not suitable to determine calcium tartrate instabilities.
Calcium precipitation test
In a relatively new test a wine is subjected to seeding (the addition of micronized calcium tartrate) after which the wine is cooled. This is done to encourage calcium tartrate crystal formation and precipitation. The calcium concentration in the wine before seeding is compared to the calcium concentration in the wine after seeding (and precipitation). The difference in calcium concentration is used as a predictive indicator of calcium tartrate instabilities. If the difference is low, then the wine can be considered as calcium tartrate stable. Large differences between calcium concentration in the wine at the beginning vs the end of the test can be an indicator that the wine is calcium tartrate unstable.
Multifactorial statistical calculation
A multifactorial statistical calculation method based on the Yates algorithm uses wine parameters to calculate the level of instability of wine. These parameters include pH, tartaric acid concentration and calcium concentration.3
Calcium tartrate or potassium bitartrate?
Both calcium tartrate and potassium bitartrate form white (or red in the case of red wine) crystals and a sandy precipitate. To distinguish one salt from the other a small trial can be done. The crystals are added to a flask or beaker after which clean water is added. The water is then heated to between 80 and 100°C and stirred occasionally. If the crystals dissolve, it is likely potassium bitartrate. If the crystals do not dissolve, then it is likely calcium tartrate. Further tests can be applied for more specific identification.
Calcium tartrate solubility is only three times lower at -4°C compared to the solubility at 20°C. Therefore, the precipitation of calcium tartrate is only slightly affected by low temperatures.1 Wines that are potentially calcium tartrate unstable may prove to be impossible to stabilise even if kept at low temperatures for long periods.1 Therefore, cold stabilisation cannot be used as a reliable method to remove the threat of instability by precipitation. However, the use of low temperatures may, in some cases, accelerate the sedimentation kinetics of calcium tartrate formation and is applied during the calcium precipitation test where seeding initiated crystal formation.
Protective colloids are generally ineffective in preventing calcium tartrate crystal formation.3 However, certain colloids are able to partially modify the shape of the crystal.3 Metatartaric acid seems to be effective in inhibiting crystal formation, however, its effects are not stable over time as it undergoes hydrolysis, leading to an increase in acidity due to the release of tartaric acid.5 Consequently, metatartaric acid loses its inhibitory effect over time, depending on wine storage temperature, with more rapid hydrolysis occurring at higher temperatures.
Exchange resins and electrodialysis
Ion exchange resins, if specific for bivalent cations, can improve the stability of calcium tartrate by removing calcium and, indirectly, by lowering the pH.3 Electrodialysis can reduce the concentration of calcium and tartaric acid, two critical factors contributing to calcium instability. Both techniques are not specific to calcium tartrate stabilization but can help reduce the risk of calcium tartrate precipitation.3
Seeding (addition of crystallization nuclei) can be used to initiate and accelerate the natural calcium tartrate precipitation thus making the crystallization process predictable and controlled. This technique relies on the availability of high-quality, micronized calcium tartrate crystals which provides millions of crystallizing germs and helps to overcome the main limiting factor of the crystallization process: germ formation. The added germs can grow and form larger crystals without the need to cool the wine and without being hindered by the presence of suspended particles making it suitable for use in conjunction with clarification processes. It should be noted that potassium bitartrate precipitation does not induce calcium tartrate precipitation.
The advantage of this treatment includes the fact that the added micronized calcium tartrate nuclei are insoluble and not consumed by microorganisms. Together with the effectivity, the easibility of use and its respect of the wine’s sensorial properties the addition of micronized calcium tartrate can be a suitable solution to reduce the risk of calcium tartrate instabilities.
As mentioned earlier, many components in wine can greatly enhance a wine’s holding capacity for calcium tartrate. Inhibitory compounds such as gluconic acid, malic acid, citric acid, potassium and magnesium may slow or even prevent nucleation by binding with free calcium or tartrate thereby lowering the supersaturation. Alternatively, inhibitors may attach to the soluble calcium tartrate aggregates and block critical nucleus formation.
Malic acid in particular is highly inhibitory to the crystallisation process.1 Wines that undergo malolactic fermentation become more vulnerable to calcium tartrate precipitation due to the resulting pH increase and because an efficient calcium tartrate crystallisation inhibitor (malic acid) is replaced by a less efficient one (lactic acid). This means that a sparkling wine or a full-bodied white wine containing a sub-critical concentration of calcium in the presence of malic acid, can become unstable following malolactic fermentation. Typically, in these wines the characteristic delay in precipitation of calcium tartrate results in the instability not showing itself until sometime after the pH change took place.6
Other natural wine components like the polyuronic acids of grape pectins are also efficient inhibitors of calcium tartrate crystal growth. These macromolecules are at lower levels in sparkling wines than table wines which may account for the frequent occurrence of calcium tartrate instability in sparkling wines.7
The occurrence of precipitated calcium tartrate in wine is becoming more frequent all over the world. The temperature independence of the crystal formation, the absence of a reliable correlation between calcium concentration and calcium tartrate instability and the lack of reliable stabilising procedures and tests to predict the instability makes calcium instability an insidious and serious problem.
The monitoring and management of potential calcium instabilities has not yet become part of the routine activity of many oenologists. Scrupulous elimination of sources of calcium from viticultural and winemaking procedures appears to be the most practical method of avoiding calcium instability problems. However, the inhibition of calcium tartrate precipitation is arguably the most important preventative factor.6
(1) THE AUSTRALIAN WINE RESEARCH INSTITUTE. Calcium Instabilities. https://www.awri.com.au/industry_support/winemaking_resources/fining-stabilities/hazes_and_deposits/calcium_instability/ (accessed 2023-01-27).
(2) McKinnon, A. J.; Scollary, G. R.; Solomon, D. H.; Williams, P. J. The Mechanism of Precipitation of Calcium L(+)-Tartrate in a Model Wine Solution. Colloids Surf A Physicochem Eng Asp 1994, 82 (3), 225–235. https://doi.org/10.1016/0927-7757(93)02636-S.
(3) Quinterno, G.; Triulzi, G.; Scotti, B. CALCIUM TARTRATE INSTABILITY – A New and Increasingly Widespread Oenological Challenge That Can Be Managed by Safe and Easy-to-Apply Means. Enartis. 2021.
(4) Rankine, B. C. Making Good Wine: A Manual of Winemaking Practice for Australia and New Zealand.; Sun Books: South Melbourne, 1989.
(5) Marchal, R.; Jeandet, P. Use of Enological Additives for Colloid and Tartrate Salt Stabilization in White Wines and for Improvement of Sparkling Wine Foaming Properties. In Wine Chemistry and Biochemistry; Springer New York: New York, NY; pp 127–158. https://doi.org/10.1007/978-0-387-74118-5_7.
(6) Mckinnon, A. J.; Scollary, G. R.; Solomon, D. H.; Williams, P. J. The Influence of Wine Components on the Spontaneous Precipitation of Calcium L(+)-Tartrate in a Model Wine Solution. Am J Enol Vitic 1995, 46 (4), 509–517. https://doi.org/10.5344/ajev.19184.108.40.2069.
(7) McKinnon, A. J.; Williams, P. J.; Scollary, G. R. Influence of Uronic Acids on the Spontaneous Precipitation of Calcium-(+)-Tartrate in a Model Wine Solution. J Agric Food Chem 1996, 44 (6), 1382–1386. https://doi.org/10.1021/jf950111v.
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