Why Do Crystals Form in Wine over Time

A wine bottle with sediment or crystals at the bottom is not flawed. While many consumers find such crystals to be unattractive, vintners know that crystal formation is commonplace in wine production. What chemistry explains the origin of the crystals in wine  

The complexity of wine composition is a central reason for the vast variety of wines in the marketplace. In addition to water and ethanol, the major components, a variety of organic acids as well as metal ions from minerals in the skin of the grape are present. Initially, all of these substances remain dissolved in the bottled grape juice. As the fermentation process occurs, the increasing alcohol concentration in the wine alters the solubility of particular combinations of acid and metal ions. Unable to remain in solution, the insoluble substances settle as crystals. Since the process of red-wine making involves extended contact of the grape juice with the skins of the grapes (where the minerals are concentrated), wine crystals are more common in red wines than in white wines. 

Grapes are one of the few fruit crops that contain a significant amount of the weak organic acid known as tartaric acid, HOOC-(CHOH)2-COOH. More than half of the acid content of wine is ascribed to tartaric acid. As a weak acid, tartaric acid partially ionizes in water to yield the bitartrate or hydrogen tartrate ion  

The bitartrate ion can combine with potassium ion, also present in high concentrations in grapes, to form the soluble salt potassium bitartrate (also known as cream of tartar). In water sodium bitartrate is fairly soluble 1 g dissolves in 162 ml of water at room temperature. In alcohol solution (formed as fermentation of the wine yields ethanol), the solubility of potassium bitartrate is significantly reduced 8820 ml of ethanol are required to dissolve 1 g of the saltJ l As a consequence deposits of potassium bitartrate form as the salt precipitates out of solution. 

To prevent the formation of wine crystals during the bottling process, winemakers use a method known as cold stabilization. By lowering the temperature of the wine to 19-23°F for several days or weeks, the solubility of tartrate crystals is lowered, forcing the crystals to sediment. The resulting wine is then filtered off the tartrate deposit. The temperature dependence of the solubility of potassium bitartrate is readily apparent in the following comparison while 162 ml of water at room temperature dissolves 1 g of the salt, only 16 ml of water at 100°C are needed to solubilize the same amount of saltJ Recent developments employ a technique known as electrodialysis to remove tartrate, bitartrate, and potassium ions from newly fermented wine at the winery before potassium bitartrate crystals form. 

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Question 3.2 Why Do Crystals Form in Wine over Time ... 

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