baking soda
Sodium bicarbonate; reacts with an acidic ingredient (buttermilk, vinegar, brown sugar) to release gas and make baked goods rise. It also raises the batter's pH, which promotes browning and can soften texture -- so even a small amount can noticeably change a baked good's color as well as its rise.
Buying guidance
Buy plain sodium bicarbonate sold for food use. It's inexpensive and stable dry, but don't reuse a box that's been sitting open in the refrigerator absorbing odors -- it can pick up smells and moisture that ruin a baked result.
Substitutions
- Baking powder -- only with a real quantity and formula change, since it's much weaker per teaspoon and doesn't raise pH the same way; not a clean 1:1 swap when a recipe depends on baking soda's browning and rise.
Tips & common mistakes
Baking soda needs an acidic ingredient in the batter to react with and neutralize -- without enough acid, the leftover alkalinity gives baked goods a faintly soapy or metallic aftertaste, which is the telltale sign of too much soda relative to what's there to react with, not too much soda in absolute terms.
Storage
Kept dry and sealed, baking soda doesn't really expire, but humidity is its enemy -- if it's clumped into hard lumps, moisture has already triggered part of the reaction, and its lifting power is diminished even though it still looks and tastes like baking soda.