Cream of Tartar from Wine Lees

Cream of tartar is potassium bitartrate, a by-product of winemaking. It forms naturally in grapes and, because it dissolves poorly, it crystallises in wine casks during fermentation and settles with the lees, the sediment left when the young wine is racked off. Its solubility rises steeply with temperature, so the lees are dissolved in hot water, filtered, and cooled: the potassium bitartrate crystallises out and is filtered off again. Purified, it is the white, odourless, acidic powder used in cooking.