Have you ever worked with a recipe that called for baking powder, but all you have is baking soda? You sit there for a minute debating whether you really need to go all the way back out to the store to buy baking powder. How different are they, anyway? They look the same; they're used in similar recipes in similar amounts; they're both supposed to leaven batters, and their names even sound the same. But don't be fooled! Despite their similarities, using the wrong one can impact your food's color, texture and flavor.
Baking soda is comprised of sodium bicarbonate, a chemical with a basic (high) pH. This high pH allows baking soda to react with other foods in special ways. For example, when baking soda is mixed with an acid (low pH), they'll instantly react to produce carbon dioxide (CO2) gas. Maybe you remember mixing baking soda and vinegar to make a volcano in grade school? The same reaction is occurring in your food, but on a more diluted scale. The CO2 released from this reaction is what helps leaven batters for foods like biscuits and pancakes. If baking soda alone is added to a recipe, there must also be a complimentary acidic ingredient in order to produce CO2. Some common acidic ingredients include: buttermilk, sour cream, citrus juice, vinegar and cream of tartar. Ideally, the amount of acid present in the recipe is balanced to perfectly neutralize the amount of base (baking soda) added. However, that can be difficult to guarantee, since food acids from natural products like sour cream and citrus juices can vary based on growing, storage, and manufacturing conditions. So, subtly different results might be seen at different times of year, by using an older lemon, or by switching to a new brand of sour cream. Distilled white vinegar and cream of tartar are standardized, so they won't fluctuate like agricultural products. Keep in mind that this acid-base reaction occurs quickly and is usually complete in about two minutes. So, if you leave your batter on the counter for a long time before cooking, the CO2 in your batter will escape, leaving your batter flat. Because of these variables, baking soda + acid may not be the most reliable leavening system in the world, but baking soda has lots of other important effects on food too, which I'll get into shortly.
Baking powder is like baking soda's more sophisticated big sister. Whereas baking soda is made up only of sodium bicarbonate, baking powder contains both sodium bicarbonate and the acids needed to react with it. In this case, those acids are monocalcium phosphate (MCP) and sodium aluminum sulfate (SAS). Why two acids? Because most retail baking powders are "double acting." That means the acid-base reaction occurs in two stages: one at room temp while you mix your batter, and the other at high temps once your food is cooking. The first stage is just like what happens with baking soda + acid systems. MCP is the fast-acting acid in most baking powders, and it releases a small burst of CO2 to give your batter body before cooking begins. The delayed second stage is made possible by using an acid, like SAS, that isn't soluble (can't dissolve) at room temp. SAS becomes more soluble at higher temperatures, so during cooking it begins to dissolve and produce CO2. This second stage accounts for the lion's share of CO2 produced by baking powder. Because it's produced later in the cooking process, CO2 from this stage has less time to escape before the batter sets permanently. The more bubbles in the batter when it finally sets, the fluffier the finished product! Cornstarch is also added to baking powders to prevent any acid-base reactions from occurring during storage. Despite this, the reaction will inevitably occur slowly over time. That's why baking powders have an expiration date. Beyond that date, the powder may be less potent. Humidity can speed up the deterioration of baking powder, so store it in a cool dry place.
Even though the main purpose of baking soda and baking powder is leavening, they can also have an impact on flavor and color. The high pH of baking soda, if not neutralized by an acid, enhances browning by the Maillard reaction. The Maillard reaction occurs between amino acids (from proteins) and (most) sugars (except sucrose). This reaction is responsible for the browning of toast, meats, french fries, baked goods and most other foods that brown when you cook them. This reaction is complex and mysterious, but it produces rich brown colors and toasty, nutty, caramel-y, cooked flavors and aromas that are important to the enjoyment of food! Most foods are neutral or acidic, so it's rare to see the effects of high pH on browning, but using baking soda in a recipe that actually calls for baking powder will show you just that.
These two muffins were cooked side-by-side in the same oven; as you can see, the extra browning effect can be intense! The muffin on the left was cooked according to the recipe, with baking powder. It's our control sample -- our example of how a normal muffin would turn out under experimental conditions. Muffin recipes typically call for a good bit of baking powder, so when I replaced that with the same amount of baking soda, the effect was severe. In fact, the test muffins were so dark that they looked almost burnt, and they tasted incredibly bitter. (Inedible, actually.) But this single experiment isn't enough to demonstrate the effects of swapping baking powder and baking soda. We need to look in the reverse order as well. What happens if we find a recipe that typically calls for baking soda and replace it with baking powder?
Chocolate chip cookies typically call for a small amount of baking soda. I wanted to see what would happen if I replaced that with baking powder. I used the tried-and-true Nestlé® Toll House® recipe. I made a control batch with baking soda and a test batch with baking powder. I cooked the test and control cookies together on the same cookie sheet, rotating the pan halfway through, to ensure that they were subjected to identical cooking conditions. As you can see, the baking powder cookie (on the right) came out lighter in color. That's no surprise, given what we learned from the muffins, but there was another interesting effect. Because the cookies only used a small amount of baking soda/baking powder, neither batch was subject to the bitter off-notes of the test muffins. This allowed me to evaluate the flavor differences caused by the differences in browning. The control cookies tasted like a classic homemade chocolate chip cookies with lots of toasty caramel-y notes that come from browning. However, those made with baking powder tasted surprisingly doughy. In the absence of browning flavors, the flavor of the raw ingredients, like flour, came through strongly! Several of my friends mentioned that the test cookies reminded them of Entenmanns® chocolate chip cookies. We weren't able to do a side-by-side comparison, but it's an interesting thought. Might baking powder (or the absence of baking soda) help achieve a soft-baked flavor profile? That being said, almost everyone that tried them preferred the classic recipe with baking soda, and I agree. If nothing else, these experiments certainly suggest that swapping these two ingredients WILL impact your results, even if it's subtle. So, even if it's raining and you don't feel like it, it's probably worth the trek back to the store to get the right ingredient. Happy baking!
Notes
1. Amy Brown, Understanding Food: Principles and Preparation, 2nd Edition (Belmont: Wadsworth/Thomson Learning, 2004), 410.