Technical Yearbook 2024

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, supersaturation may be prolonged for extended periods before crystallisation occurs for calcium tartrate. Many components in wine, including some of the natural acids and macromolecules, can significantly 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 why some wines may be able to inhibit calcium tartrate crystallisation for extended periods 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 dramatically affects 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 The vineyard soil is a natural source of calcium. The amount of calcium absorbed by the vine depends largely on the characteristics of the soil. Generally, 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 the high transpiration of the acid before vériason and calcium mobilisation in response to thermal and water stress. Oenological sources of excess calcium include using calcium carbonate rather than potassium bicarbonate during deacidification. 4 Another source has been using casein or other milk products for fining. Fermentation and wine storage in unlined or inadequately coated concrete tanks have also been a cause of calcium instability in the past. Testing for calcium instability Calcium concentration 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. Therefore, calcium concentration is not a reliable indicator in isolation and should be used with stability tests or other factors to determine the risk of crystal formation. Conductivity test Conductivity analysis tools, so useful in defining the stability of potassium bitartrate in wine, are of no help in determining calcium tartrate instabilities. Freeze test Unlike potassium bitartrate, calcium tartrate precipitation 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 micronised 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. The wine can be considered calcium tartrate stable if the difference is low. Large differences between calcium concentration in the wine at the beginning versus the end of the test can indicate 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 wine’s instability level. 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. A small trial can be done to distinguish one salt from the other. 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. Treatment Cold stabilisation Calcium tartrate solubility is only three times lower at -4°C compared to the solubility at 20°C. Therefore, calcium tartrate precipitation is only slightly affected by low temperatures. 1 Wines that are potentially calcium

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TECHNICAL YEARBOOK 2024