Cooks in today's hurried world often find themselves without a key ingredient, and they are tempted to find substitutions. Some substitutions work. Corn syrup can fill in for honey, for example. Each is sweet with a similar, syrupy consistency. Lemon juice could be substituted for vinegar. Each has a sour, acidic flavor. Some substitutions, like baking powder for corn starch, ought never to be made no matter how desperate the situation.

Baking powder serves the cook as a leavening agent. Through a chemical reaction, it produces carbon dioxide gas and makes dough and batter rise to produce light, fluffy pancakes or biscuits. The water and heat activate the baking powder, and a cook watching pancakes on the griddle will see bubbles form in the batter as the reaction takes place and the cooking pancake rises.

As a thickener, pure corn starch is hard to beat. Thanks to its profound blandness, it has no effect on the flavor of a gravy or pudding. With a little practice, a cook can control the amount of thickening with precision as she slowly pours a slurry of corn starch and water into a boiling liquid. Sauces thickened with corn starch are translucent. Sweet and sour sauce is a good example of a sauce thickened with corn starch.

Perhaps the most common and readily available substitute for corn starch in a home kitchen is regular white flour. Flour will see a cook through any thickening emergency. The best one-for-one substitute for corn starch as a thickener for sauces, gravies and puddings is arrowroot. If the recipe calls for 2 tablespoons of cornstarch, 2 tablespoons of arrowroot will do the job. More exotic substitutions would be tapioca and sahlab, which is made from orchid tubers.

Baking powder does contain corn starch, and it will thicken a boiling liquid. Curious cooks can conduct an experiment to see the chemical reaction and the thickening happen. Pour 1/4 cup of water into a pan and bring it to a boil. Add a couple spoonfuls of baking powder. The mixture gets foamy and thickens. While the bubbles may be stirred out of the mixture, the flavors will remain. The baking powder's chemical reaction gives the paste a salty, bitter and unpleasant taste.