Introduction to calcium instability
Calcium-induced instabilities are a major cause of problems in bottled wines. The most common manifestation is in the form of crystalline calcium L-tartrate. Other less common occurrences of calcium instability are as calcium DL-tartrate and calcium oxalate. These last two have not been seen commonly since the early 1980s. 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 was the factor responsible for calcium oxalate instability.
Sources of calcium in wine
Vineyard soil is a natural source of calcium in musts, however, wines with a calcium level above 70-80 mg/L are considered to be at risk of instability. One source of excess calcium is the use of calcium carbonate in deacidification rather than the recommended potassium bicarbonate (Rankine 1989). 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, however, this cause is now uncommon because of the decreasing use of concrete vessels in contemporary winemaking.
Deposits of calcium tartrate usually appear as colourless or white, bipyramidal or rhomboid crystals. In some cases co-deposits are also present, eg phenolic and protein material, quercetin crystals, or yeast cells. The identification of a calcium tartrate deposit can be confirmed by the Institute through the use of IR spectroscopy.
Factors influencing calcium tartrate precipitation
The occurrence of calcium L-tartrate deposits is a most insidious problem in bottled wines because the crystals are slow to form and usually do not come out of solution for some time, often months, after bottling. For example, in the case of sparkling wines and white table wines, especially light bodied whites, this is often after commercial release. The few recorded investigations of occurrences in red wines probably do not reflect the true incidence of calcium instabilities in red wines because the crystalline deposit may not be observable in dark bottles or may be assigned as a potassium bitartrate deposit.
Calcium L-tartrate precipitation is favoured at higher wine pH values (McKinnon et al. 1994). Accordingly, winemaking operations that may increase the pH such as MLF and blending can increase the likelihood of instability.
Like potassium bitartrate, calcium L-tartrate will remain supersaturated in a wine, however, experience shows that in the case of calcium L-tartrate, supersaturation may be prolonged for extended periods. Many components of wine, including some of the natural acids and macromolecules, can greatly enhance a wine’s holding capacity for calcium L-tartrate. These inhibitory compounds may slow or even prevent nucleation by binding with (and so decreasing the amounts of) free calcium or tartrate and lowering the supersaturation. Alternatively, inhibitors may attach to the soluble calcium L-tartrate aggregates and block critical nucleus formation. In some wines, even those with excessive calcium concentrations, crystal growth inhibitors can slow growth to such an extent that crystals are prevented from developing to a detectable size for an extended period, perhaps as long as the lifetime of the wine. The inhibition of calcium L-tartrate precipitation is arguably the most important factor in calcium instability (McKinnon et al. 1995).
Unreliability of predictive tests or stabilising procedures
In contrast to the situation with potassium bitartrate, temperature has little effect on the rate of calcium L-tartrate precipitation. This fact means that simple cold tests are ineffective indicators of calcium L-tartrate instability and cold stabilisation cannot be employed as a reliable method of precipitation to remove the threat of instability. Indeed, wines that are potentially subject to calcium L-tartrate precipitation may prove to be impossible to stabilise even if kept at low temperature for long periods.
The associated problems of temperature independence and the lack of reliable cold stabilising procedures and tests to predict the instability, have led to close examination of factors that may help to stabilise at-risk wines against calcium L-tartrate precipitation. Among these factors are seeding to initiate precipitation and the use of concentration products to test for instability. However, empirical evidence has shown the concentration product method is of little value in predicting calcium L-tartrate instability. Seeding relies on the availability of calcium L-tartrate crystals of high quality and in a finely ground state. In the past, the unavailability of seed crystals in a suitable form and on an industrial scale meant that this technique had been applied with very little success in practical winemaking (McKinnon et al. 1995). However, with calcium tartrate crystals of a suitable size now available commercially, seeding with micronised calcium L-tartrate to promote calcium tartrate stabilisiation is an option available to winemakers.
The effects of particular wine components on calcium tartrate instability
In studies into the influence of wine components on the precipitation of calcium L-tartrate it was found that malic acid in particular is highly inhibitory to the crystallisation process. This fact has important implications for the stability of some table and sparkling wines. Those wines that undergo MLF during tirage or in the post fermentation period of table wine making become more vulnerable to calcium L-tartrate precipitation. This is because of the resulting pH increase and because an efficient calcium L-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 containing a sub-critical concentration of calcium in the presence of malic acid, can become unstable following MLF. Typically in these wines the characteristic delay in precipitation of calcium L-tartrate results in the instability not showing itself until after disgorgement of the sparkling wine or following bottling of the table wine (McKinnon et al. 1995).
Other natural wine components like the polyuronic acids of grape pectins are also efficient inhibitors of calcium L-tartrate crystal growth. These macromolecules are at lower levels in sparkling wines than table wines which may account for the frequent occurrence of calcium L-tartrate instability in sparkling wines (McKinnon et al. 1996).
Scrupulous elimination of sources of calcium from winemaking procedures appears to be the most practical method of avoiding calcium instability problems.
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