Here's another great question from reader Pauline all the way over in New Hampshire, on a topic that probably doesn't get asked all that often;
“So, I bake infrequently, and I've got containers of baking soda and baking powder that have been in my pantry forever; do these things go bad? And while you're at it, was is this stuff anyway?
Glad to help, Pauline, and thanks for asking.
The quick and dirty answer to the former question is, yes, they can go bad. Baking soda and powder are chemical leavening agents that promote rising in baked good recipes that don't employ yeast. While the end result is much the same with all three leaveners, the primary benefit imparted by baking soda and powder is speed; they can and should be used right away after mixing, while yeast takes time and really can't be rushed much. The active constituents of both do have a shelf life, albeit a long one. Fortunately, there's a couple of quick test you can do to see if yours still makes the grade.
The first test is to find and read the expiration date; at the risk of being flippant, there are printed dates on the containers of both products, though they may take a bit of sleuthing to find. If yours is past its date, discard it and buy a fresh replacement. When you're at the store, check that expiration date on what you're about to buy; in a professional kitchen, we check the dates on every case that comes in, because it's not that uncommon to find expired product in your just-made delivery, and your local grocery is no exception to that rule.
The second test is for a chemical reaction, and is therefore a bit more definitive. Take a good pinch of baking soda and drop it into some fresh vinegar; if it fizzes actively, you're in business. For baking powder do the same thing into hot water. If either just sit there, toss them
On to the latter question;
Baking soda is pure sodium bicarbonate, an alkaline or base in chemical terms. Combined with moisture and an acidic ingredient like dairy, chocolate, or honey, you get a mild chemical reaction akin to the freshness test you just did. The resulting tiny bubbles of carbon dioxide, (CO2), remain trapped in the batter matrix. Exposed to baking temperatures in your oven, they expand and cause your baked goods to rise. baking soda is a pure chemical base, so it can impart a bitter taste note if you add too much; that said, a little extra is actually a very good thing, for a most interesting reason. Just a bit more baking soda than that needed to neutralize the acid in your recipe contributes in a very positive way to browning and flavor in your finished product. This has to do with the Maillard reaction, named after Louise-Camille Maillard, who first described it about a hundred years ago. What Maillard detailed was a complex set of reactions that lead to such culinary wonders as the luscious crust on your steak, the sweet beauty of caramelized onions, and the golden brown outside of a cream biscuit. On top of the lovely color added, the reaction also produces hundreds of aromatic compounds that add savoriness and complexity; in other words, it's a very good thing indeed. The key is moderation; an eighth of a teaspoon above a stated recipe amount is enough to hit the sweet spot.
Baking powder is a mixture of a base and an acid or acids; sodium bicarbonate is the base, while cream of tartar and sodium aluminum sulfate are the common acids. There's typically a bit of added starch as a carrier for the active ingredients as well. Baking powder is a more complex and balanced leavener than baking soda, since it contains both acid and base; it is completely inert when dry, but when introduced to moisture, the base and acid mix and generate CO2, and you're in business. The reason baking powder is called 'double acting' is the presence of the two different acids. When added to your recipe, the first acid, cream of tartar, mixes with the baking soda and goes to work right away. The second acid, sodium aluminum sulfate, is temperature activated; when your batter or dough hits roughly 175° F, that acid combines with the remaining base and contributes a bit more rise.
You can make basic baking powder at home by combining,
2 teaspoons Cream of Tartar
1 teaspoon Baking Soda
1 teaspoon Corn Starch
The obvious benefit is fresh product, assuming your constituents are, of course, but this will not be a double acting powder and as such, won't have quite the lifting power of the commercially prepared stuff.
If the aluminum makes you nervous, maybe it should. Aluminum has been found to adversely affect reproductive and nervous systems in animal studies. Some human studies have suggested a possible connection between aluminum and Alzheimer's Disease. The health effects of aluminum on humans are not definitive, but nonetheless, the Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives (JECFA) significantly lowered the tolerable intake of aluminum as a result of such studies.
Some recipes call for baking soda, others for baking powder, and some employ both. The leavener(s) called for is governed by the over-arching recipe. The straight base chemistry of baking soda dictates does well with more acidic ingredients, like buttermilk pancakes, or cake recipes that employ vinegar. Baking powder commonly gets paired with more neutral ingredients like plain milk or non-dairy alternatives.
So how about the interchangeability of these two? You can substitute baking powder in place of baking soda using a ratio of 3:1 powder to soda, but it's not a desirable substitute; the significant amount of added acid will impact the taste of your finished product. On the other hand, you cannot sub baking soda for baking powder, since baking soda lacks the acidity needed to make things rise.
With both these leavening agents, it's important to keep in mind that the reaction produced is relatively short lived and begins as soon as you mix ingredients. It's always best practice to have your oven preheated and to bake promptly, otherwise you'll miss the window of efficacy and your goodies will fall flat. Unlike a yeasted dough, which is relatively low in moisture and kneaded until a tough, elastic gluten network is produced that will trap massive amounts of carbon dioxide, quick batters and doughs are made with an extremely moist batter, because baking powder just doesn't generate enough gas to effectively leaven a thicker dough. Additionally, batters have relatively little gluten formation, so they're not that meaning that great at trapping and holding bubbles.
That's probably all you need to know about that.