I just got home from my departmental Christmas party. It was fun! One of the strange questions I got was “what is Cream of Tartar and what is it for?”
Yes. I have this in my spice cabinet. Chemically, it is potassium bitartrate:
From the chemical structure, you can see a three half OH groups (the half is the O–). The bitartrate ion shown here is therefore fairly alkaline. When you throw this molecule into water, those 3 hydrogens pop right off (apparently), forming H+ ions. That makes it an acid, and according to the illustrious Wikipedia entry, it creates a pH of about 3.557 in water. Thus, tartrate and tartaric acid form a conjugate acid/base pair.
Some uses for cream of tartar in cooking:
- Often used in meringue. This helps to stabilize the egg whites as they foam during beating. Note that Oregon State University has the pH of egg whites listed as about an 8, so they’re naturally alkaline. Adding the cream of tartar shifts the pH and causes the egg proteins (albumin, mostly) to denature (solidify, in this case), so you get stiffer peaks that last longer. You could get the same effect with vinegar or lemon juice, but that wouldn’t taste very good.
- Also used in creating icing and smooth sugar solutions. This chemically helps keep the sugar from forming regular crystals and solidifying. It’s a geometry thing.
- Ingredient in baking powder: the third ingredient of baking powder along with baking soda and corn starch. Kind of makes you wonder why recipes call for both–usually, that has to do with pH again. Baking soda neutralizes acids, including cream of tartar, when they’re mixed in liquid form (not as much happens in the dry, crystalline powder forms).
- Leavening: this means it’s an ingredient that can contribute to rising/fluffiness. Did you ever make a vinegar and baking soda volcano as a kid? It’s the same chemistry here: acid + base = bubbles! Those bubbles cause breads and other baked goods to rise without using yeast. Unleavened breads usually refer to ones made without yeast, although some may still use these chemical leaveners (pita is a good example).
- Sometimes used in whipped cream: what a waste! Whipped cream is best enjoyed fresh. However, it will denature milk proteins just as well as it will egg proteins.