When Karyn introduced me a couple of weeks ago, Mary asked me about the difference between baking powder and baking soda. I gave her a nutshell answer, but promised a full posting with some more info and clarification.
What I initially wrote was, “Baking soda is bicarbonate of soda. In the presence of an acid (vinegar, citrus, etc), it releases carbon dioxide gas causing bubbles. These bubbles leaven whatever it is that you’re baking. Baking powder is basically baking soda mixed with an acid in powdered form. When a liquid is added, you get the same reaction and the same leavening.”.
So let’s back things up a bit. In my last post about beer, we talked about yeast metabolism. When yeast are given a food source in the presence of oxygen, they produce energy and carbon dioxide gas (CO2). That carbon dioxide gas being trapped within your dough is what causes bread to rise.
Of course, that process generally takes a couple of hours, so isn’t a good option for everything. If you’re making batters or cookies, you might not want to wait that long and you might not want the yeastiness of baking bread in your chocolate cake.
Baking soda and baking powder are both chemical leaveners. They undergo a chemical reaction in the presence of the right ingredients or conditions to produce the same CO2 gas. And you don’t have to wait for hours…
Chemically, baking soda is NaHCO3. In the presence of an acid, it reacts to produce our carbon dioxide gas and some kind of side product. In the case of the classic science demo the ‘kitchen volcano’, vinegar (acetic acid) is mixed with baking soda, to produce CO2, liquid water, and sodium acetate* to produce lots of frothy, foamy ‘lava’. It’s the carbon dioxide gas that causes the bubbles to form.
Common acidic ingredients that will produce this reaction are vinegars, buttermilk, yogurt, sour cream, fruit juices, honey, and molasses.
As I mentioned in my short answer, baking powder is a mixture of baking soda and an acid in solid form. It also has a ‘filler’ component, often cornstarch. When you mix baking powder in the presence of a liquid (or heat, for slow-acting BP), the crystalline acid will dissolve and react with the baking soda, giving you the same reaction as above. This is great for when you’re making a recipe without an acidic liquid ingredient.
The filler serves two purposes, it provides more volume to the powder so that it’s easier to measure (would you want to try to measure 1/16th tsp?), and keeps things dry inside the container. Imagine what might happen if there was no filler and it was humid inside your pantry…
The dry acid ingredient can vary based on the kind of baking powder we’re talking about. There are actually three types of BP: fast-acting, slow-acting, and double acting.
• Fast-acting begins to react to form CO2 as soon as the liquid is added.
• Slow-acting only starts to react in the presence of heat, so nothing happens until you pop whatever you’re making into the oven.
• Double-acting has both types of acids, so you get a hit of reaction upon addition of your liquid ingredient and then another once the temperature starts to rise. Most likely, the BP in your kitchen is double-acting.
If you’re out of baking powder, you can improvise using the following ratios to make the amount of BP the recipe calls for: 1 part baking soda, 1 part cornstarch, and 2 parts cream of tartar (the acid). For example, if your recipe calls for 1 t. baking powder, you’d use 1/4t. baking soda, 1/4t. cornstarch, and 1/2t. cream of tartar.
If you’re out of baking soda, go to the store and buy some more baking soda. Unfortunately, you can’t use BP to replace the soda because of the additional acid that it contains. Too many acid byproducts can affect taste, texture, and color.
I don’t know about you, but I couldn’t tell you when I bought the baking powder or soda in my pantry. Because they rely on a chemical reaction to cause leavening, and chemicals can break down over time, you might want to test them out next time you reach for them (especially if company’s coming). Here’s how:
• Baking soda: ¼ t + 2t white vinegar should cause immediate foaming
• Baking powder: 1t + ½ c hot water should do the same.
If they don’t, or if they’re sort of lethargic about it, you might want to think about replacing them.
Mary (and everyone else!), I hope this was more helpful in answering your question!
*Sodium acetate is edible in the quantities we’re talking about, and is even used as a flavoring agent. It’s also what’s commonly used in heating pads to give off heat, but that’s another story altogether!