Head on over to your kitchen cabinet and yank out the baking powder, then find the ingredient listings. If you were not paying attention when you bought your box or can, there is a good chance you will see “sodium aluminum sulfate” listed.
Baking soda, baking powder, and yeast all react to create carbon dioxide bubbles (pockets) in a baked good. Baking powder is closely related to baking soda; the main difference, and the reason it is used quite often in recipes, is that baking powder gives the cook much more power in controlling when the product expands.
As you examined your baking powder package, you may have also noticed the phrase “double-acting”. Most commercial baking powder is double-acting, meaning that it works both as it is added to a recipe and again when it is heated. Double-acting is where sodium aluminum sulfate comes in; it is an acid used during the heating side of the expansion reaction.
It just so happens that sodium aluminum sulfate is an easy compound to work with, which is why it has become pervasive in commercial baking powder.
Are there any side effects? Baking powder with aluminum may have a slightly metallic (tinny) taste. I noticed this before I switched to aluminum-free baking powder. Aside from that, and possible (but unproven) medical risks, aluminum-free is just much more natural.
My kitchen cabinet contains Rumford Brand baking powder. Rumford is a bit more expensive than conventional baking powder, but one can lasts a while.