Cream of tartar (potassium bitartrate or potassium hydrogen tartrate) is a by product of winemaking. It is the potassium acid salt of tartaric acid.
In food, potassium bitartrate is used for:
- Stabilizing egg whites, increasing their heat tolerance and volume;
- Preventing sugar syrups from crystallising;
- Reducing discolouration of boiled vegetables;
- Frequent combination with baking soda (which needs an acid ingredient to activate it) in formulations of baking powder.
- Commonly used in combination with potassium chloride in sodium-free salt substitutes
A similar acid salt, sodium acid pyrophosphate, is confused with cream of tartar due to their similar function in baking powder.
Occurance in wine
Potassium bitartrate crystallises in wine casks during the fermentation of grape juice. In wines bottled before they are fully ripe, it can precipitate on the side of the bottle in a sort of crust, thus forming what is called "crusted wine".
This crude form (known as beeswing) is collected and purified to produce the white, odorless, acidic powder used for many culinary and other household purposes.
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