Baking powder goes to work immediately when mixed with a liquid and provides a second lift when it hits the heat of the oven.
The formulas used to make baking powders today are much more carefully calibrated than when this leavener was first packaged for home cooks nearly 200 years ago, but the chemistry remains the same. Baking powders depend on the inclination of an alkaline substance (sodium bicarbonate, or baking soda) and an acid (originally cream of tartar, but today there are more options) to react and produce carbon dioxide gas bubbles, thereby leavening batters for muffins, cakes, and other baked goods that need a quicker rise than can be provided by yeast.
Baking soda has always been a pretty good buy, while cream of tartar, a byproduct of wine making, has never come cheap. It was first used in prepared baking powders in 1835, but by the 1850s a cheaper alternative, monocalcium phosphate (MCP), was introduced, and it continues to be used in baking powders today. MCP is similar to cream of tartar in that it reacts with baking soda immediately when the two are combined with water. (Cornstarch is a component of all baking powders. It absorbs moisture, thereby helping to keep the acid and the baking soda from interacting during storage; it also helps to disperse the acid and baking soda evenly throughout a batter.) What this immediate reaction means in professional baker's terms is that MCP gives a batter more bench rise (the leavening that takes place before a batter goes into the oven) than oven rise (the leavening that takes place in the oven). This is not necessarily a bad thing, but it does require the cook to get the batter into the oven fairly quickly; if not, the baking powder will exhaust much of its leavening power on the bench, and the muffin or cake will not rise as much as it could or should in the oven.
Enter the acidic leavener sodium aluminum sulfate (SAS), added to many baking powders since the beginning of the 20th century. SAS and a compound used interchangeably with it today, sodium aluminum phosphate (SALP), don't react with baking soda and water at room temperature. It's only in the oven, when the temperature rises above 120 degrees Fahrenheit, that their leavening power goes to work. Several top-selling brands of baking powder make use of both MCP and one of the aluminum compounds as a kind of insurance so the home cook gets both good bench rise and good oven rise. (Another popular brand uses only MCP with no aluminum compounds.) The MCP goes to work as soon as liquid is added to the dry ingredients, and the SAS or SALP kicks in when exposed to the heat of the oven. In most such formulations, about one-third of the leavening takes place on the bench and the balance in the oven. These baking powders are all referred to as double-acting, in reference to the fact that the leavening action takes place twice-once outside the oven, once inside the oven.
Does any of this make any difference in the kitchen? To answer this question, we made biscuits, scones, and yellow cake with the aluminum-free brand and baking powders containing SAS or SALP. We then compared the results. Both types of baking powders performed well in terms of creating a good rise; thus, the brands containing aluminum do not guarantee more oven rise as long as the mixed batter made with aluminum-free baking powder isn't left to sit around before baking. The other issue is taste. Critics of baking powders containing SAS or SALP state that these compounds give baked goods a slight but unpleasant metallic flavor. A couple of our tasters could indeed detect a very slight metallic flavor in each baked good made with the baking powders containing aluminum, but most tasters could not discern a difference.
The answer? Aluminum-free baking powders work just as well as brands made with aluminum compounds. If you have a keen palate that is highly sensitive to metallic flavors or if you wish to limit your ingestion of aluminum, choose an aluminum-free powder.