Leavening Agents

So, I've just been blindly throwing out baking recipes for three years without explaining why they work.  For those of you who try a recipe off the internet and can't figure out why you don't get a proper result, a little knowledge about leavening can help you fix the problems in most baked goods.  The kinds of leavening agents I use are, in order of complexity: air, steam, baking soda, baking powder, and yeast.

Air:  This refers to mechanically beating air into a batter, which then solidifies when the starches or proteins in the product set.  It is what makes meringue fluffy and sponge cakes light.  It makes whipped cream stiff.  When you force millions of tiny air bubbles to stay in suspension, they create volume.

Steam:  This is what puffs up cream puffs, Yorkshire pudding, and puff pastry.  It is responsible for the flakiness of pie crusts, biscuits, and croissants.  It's when you put something moist in your batter or dough (usually water or butter), then bake it at high heat for a short period of time.  The moisture creates steam quickly, which stretches out the dough.  The dough then gelatinizes.  When the steam bakes away, the solid shell of baked product remains.  It's like when you make a papier-mâché balloon, then pop the balloon inside once the paper has dried.

Baking Soda:  I'm not going into the chemistry lesson.  When mixed with moisture and a slight acid (buttermilk, lemon juice, vinegar, chocolate), baking soda foams into small bubbles of carbon dioxide which run their course fairly quickly.  You need to bake this product within a short period of time, 15 to 30 minutes.  It is what gives cookies their crumble and pancakes a fine grain.

Double-Acting Baking Powder:  Again, not boring you with the chemistry.  Baking powder combines baking soda with an acid and a stabilizing agent, so you don't need to add acid to the dough.  It activates when exposed to heat.  This means you don't have to bake it the same hour the dough is made.  The crumb is a bit larger and more solid.  It's used in cakes, biscuits, scones, and quick breads.  Often, you'll see it in combination with baking soda.  This is usually when there's an acidic batter that can take advantage of the soda to produce a slightly smaller crumb.

Yeast:  Yeast is an organism, specifically a fungus.  Ew, gross.  It munches on the sugar in dough and creates carbon dioxide and alcohol as a byproduct.  The "yeasty" smell we associate with bread is the alcohol.  Because you have to wait for the little yeasts to do their work, this method takes longer.  Generally, you let them go at it for an hour to create texture, punch down and shape the dough, then give them close to another hour to rise again.  The heat of the oven kills the yeast (it dies around 115º), so don't worry.  You're not eating live fungus.  Save that for raw mushrooms.  Because of the long gluten strands created during kneading and rising, yeast leavening is used for bread and certain pastries such as croissants and danish.  Some waffle batters are leavened by yeast, but the most common recipes use baking powder.  Sourdough and sponges are an offshoot of yeast leavening, when you let the yeasts work their magic over a much longer period of time to achieve a more acidic flavor.

Hope this clarifies some recipes and contributes to successful baking.