For example, you will often see instructions in a recipe to mix wet and dry ingredients in separate bowls, then to combine the two. But... what constitutes a wet (or dry) ingredient? Now, before you give me that "Duh!" look... it's not as obvious as you might think. Dry ingredients are things such as flour, baking powder, spices and such, while wet ingredients are things like milk, water, butter, eggs and sugar. Wait, what? That's right, in the world of baking, sugars (granulated white, brown and other sugars) are often considered 'wet' and should be mixed with other wet ingredients, not with dry ones. The exact reasons for this are a little murky and seem to depend heavily on what you're making (a cake or muffin as opposed to a yeast bread). The little bit of research I did into the matter turned up a great deal of complex explanations involving the formation of gluten... if you want to read up on it further, here is a good place to start. For the majority of baking projects you and I will usually tackle, such as cakes, muffins and quick breads, the reason is fairly straight-forward: mixing granulated sugars with other wet ingredients allows the sugar granules to dissolve, there by avoiding a gritty finished product.
What's the point of keeping wet and dry separate, anyway? Well, many recipes for things like breads and cakes include some sort of leavening, such as baking powder or baking soda. These cause a chemical reaction in the dough or batter that releases bubbles of carbon dioxide which then helps to lighten the finished product (ie: a nice, fluffy cake). There is, however, an important distinction between the two:
- Baking soda requires an acid (such as vinegar, lemon juice or buttermilk) to activate, so will be called for in recipes where one of these acids is also an ingredient.
- Baking powder is activated by moisture and heat so will be called for in recipes that do not include another acidic ingredient like above.
Hopefully that helps to de-mystify baking recipes a bit. For more handy info and tips to bake without fear, you might want to check out Tipnut.com: Handy Substitution Recipes for Baking.