Potassium bitartrate, also known as potassium hydrogen tartrate, with formula KC4H5O6, is a byproduct of winemaking. In cooking it is known as cream of tartar. It is the potassium acid salt of tartaric acid, a carboxylic acid.
Potassium bitartrate crystallizes in wine casks during the fermentation of grape juice, and can precipitate out of wine in bottles. The crystals will often form on the underside of a cork in wine-filled bottles that have been stored at temperatures below 10 °C, and will seldom, if ever, dissolve naturally into the wine.
These crystals also precipitate out of fresh grape juice that has been chilled or allowed to stand for some time. To prevent crystals forming in homemade grape jam or jelly, fresh grape juice should be chilled overnight to promote crystallisation. The potassium bitartrate crystals are removed by filtering through two layers of cheesecloth. The filtered juice may then be made into jam or jelly. In some cases they adhere to the side of the chilled container, making filtering unnecessary.
The crude form (known as beeswing) is collected and purified to produce the white, odorless, acidic powder used for many culinary and other household purposes.
In food, potassium bitartrate is used for:
Stabilizing egg whites, increasing their heat tolerance and volume
Stabilizing whipped cream, maintaining its texture and volume
Preventing sugar syrups from crystallizing
Reducing discoloration of boiled vegetables
Additionally it is used as a component of Baking powder, as an acid ingredient to activate baking soda
Sodium-free salt substitutes, in combination with potassium chlorideA similar acid salt, sodium acid pyrophosphate, can be confused with cream of tartar because of their common function as a component of baking powder.
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