Reader Adrian wants to know why people in China use alum instead of potassium bitartrate, a.k.a. cream of tartar, to create leavening reactions. The answer I expect has to do with availability. Potassium bitartrate, a salt of tartaric acid, is a by-product of the winemaking process. It collects on the insides of casks as wine ferments. It’s also commonly found on the underside of wine corks where the crystals are frequently mistaken for bits of broken glass (for that reason it’s the bane of winemakers, since a large portion of those bottles get returned).
Of course Western cultures aren’t the only ones that make wine, but we tend to be the ones that make it from grapes, and grapes are unusually high in tartaric acid. It’s for this reason that I expect cream of tartar is much less common in places like China where most wines are made from fermented starches (rice, sorghum) or fruits like lychee and plum.