Baking powder is a chemical leavener that is used primarily in cake baking to enlarge the air bubbles in the batter, which gives volume and tenderness to the cake crumb. In Europe, most cakes are leavened with beaten egg white or whole eggs whereas in North America, most cakes use baking powder, baking soda, or a combination of the two. Baking powders are mixtures of dry acid or acid salt and baking soda, with starch or flour added to stabilize and standardize the mixtures.
Most baking powders are "double acting," meaning that they will react or liberate carbon dioxide when they come in contact with moisture during mixing of the batter and again when exposed to heat during baking. (A "Single acting" leavener, such as baking soda alone, reacts fully when it comes in contact with moisture.)
We also, tested the two baking powders by activating 1/2 teaspoon of each in custard cups with hot water. Within less than a minute the original Rumford had activated, fizzing furiously to completely dissipate. The reduced sodium Rumford only activated partially with dry, non-activated powder nestled on top of the foamy activated powder (even after 10 minutes). Doing this hot water test is good method for verifying that your baking powder is still activated.
We recommend that you always mark the date upon opening a can of baking powder and store it airtight to avoid humidity. Baking powder can lose a substantial amount of its strength after about a year.
We have also tested Argo's baking powder, which also activates more during the heating phase. We tested it against our baking powder of choice, Rumford's original Aluminum-Free Baking Powder. We found it especially effective in cakes baked in fluted tube pans as we could use the same amount of baking powder, but the Argo resulted in a less domed top which, when inverted, sat flatter on the plate. (To get similar results with the Rumford would require such a minute amount of extra baking powder it would be hard to measure accurately.) When using the Argo in a layer cake, however, it needed to be decreased to keep the cake from dipping.
Here are the ingredients listed for each baking powder: Rumford Original Aluminum-Free (red background can) Monocalcium Phosphate, Sodium Bicarbonate, Non-GMO Cornstarch Rumford Reduced Sodium Aluminum-Free (silver background can) Monocalcium Phosphate, Calcium acid Phosphate, Sodium Bicarbonate, Non-GMO Cornstarch, Potassium Bicarbonate Argo Aluminum-Free Sodium Acid Pyrophosphate, Sodium Bicarbonate, Corn Starch And Monocalcium Phosphate. The White Velvet Butter Cake recipe is in The Cake Bible and Rose's Heavenly Cakes.