Cream of tartar, variously called potassium bitartrate or potassium hydrogen tartrate, is a powdery, acidic byproduct of grape fermentation that occurs during wine making. Tartaric acid is naturally present in grapes and forms as a crystalline substance inside the barrels when grape juice is fermented. Nifty, hey? Originally, it was used medicinally and as a cleaning agent in Roman times, but then some genius discovered that it worked really well in the kitchen. (They probably got bored with cleaning, and started chucking it randomly into egg whites. Who knows). And yes, cream of tartar is particularly excellent at stabilising and increasing the volume of whisked egg whites, when making meringues, angel food cake, macarons and soufflés. It’s also used to prevent sugar crystallising when making syrups, frostings and other confectionery, and even for preventing the discolouration of veggies during cooking. 

Sometimes it’s used in baking in conjunction with baking soda plus an acidic ingredient (like buttermilk or yoghurt) to help activate the soda. In fact, combined in a ratio of 2 parts cream of tartar to 1 part baking soda, you can fashion your own baking powder. Handy if you ever get caught short. (Although let’s be real. If you’ve run out of baking powder how likely is it you’ll also have cream of tartar, just randomly?) This needs to be used promptly though as it doesn’t contain stabilisers like commercial baking powder does, so it will lose potency quickly. Lemon juice or vinegar are often used as substitutes for c. of t. which is why pavlova recipes, for example, often call for a dash of vinegar. And for all you DIY mums out there, cream of tartar is also an essential ingredient in play dough recipes; the official line is that it enhances the elasticity but we reckon it’s to stop the little angels eating it.