Whether added to snickerdoodle cookies to make them wonderfully soft, to whipped egg whites to make them stable, or to simple syrup to prevent sugar crystals from forming, cream of tartar is an all-around good thing to have on hand.
Just exactly what is cream of tartar that makes it so magical? Technically, it’s an acid—specifically, tartaric acid.
It’s a byproduct of wine production, the residue left on the barrels, actually.
Most commonly, cream of tartar is used as a leavener, because when it’s combined with baking soda, together they produce carbon dioxide gas.
That’s the same gas that’s produced by yeast in bread baking.
When it’s added to egg whites, it boosts the strength of the individual air bubbles and slows down their natural tendency to deflate.
And when added to simple syrup, it prevents sugar’s natural tendency to re-bond and form crystals.