Chapter 10

Tartrate Stabilization of Wine

Tartrate Instability in Wine

Tartrate instability is the phenomenon that occurs at a specific temperature when tartaric acid salts become supersaturated: their concentration is higher than the quantity theoretically soluble. Under cool conditions this state leads to the formation of crystals. The tartaric acid composition of grape musts ranges from 2.0 to 10 g/L, and varies according to growing region (e.g., climate, soil), variety, stage of grape maturity, and viticultural practice. Ripe grapes from cooler (more northerly) vineyards often have tartaric acid levels in excess of 6 g/L. In wine, tartaric acid (H2T) ionizes to yield bitartrate (HT+) and tartrate (T2-).

Potassium Bitartrate

A wine's tendency to precipitate potassium bitartrate (KHT) crystals depends on the concentration of potassium ions (K?) and bitartrate ions (HT-), but also on other wine compositional parameters, particularly pH, temperature, alcohol, and the presence of polymeric and colloidal substances. Depending on pH, the ratios of H2T/HT-/ T2- = can vary greatly and thus significantly influence the potential for precipitation of insoluble salts. As Figure 10.1 shows, the bitartrate ion (HT-) is at its highest concentration at pH 3.65, and at this point maximum precipitation of KHT will normally occur.

Calcium Tartrates

Calcium tartrates (CaT) do not saturate the wine unless calcium levels in the wine are high and therefore, they are not usually a component of tartrate instability. When the concentration-product of calcium tartrate in the wine is greater than its solubility-product, it will precipitate, although its precipitation rate is very slow and may take months to achieve stability.

Testing Wine Calcium Tartrate Stability

Stability tests used to verify KHT stability (cold test and mini contact test, Section 10.3) do not confirm if wine is calcium unstable. The reason is that cold temperatures have little effect on the rate of CaT precipitation and the tests are not long enough. In fact, wine calcium content is very low compared to potassium and this is one of the reasons why the spontaneous formation of nuclei of crystallization can take very long, even years, making CaT precipitation unpredictable.

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