Also called sodium bicarbonate and bicarbonate of soda, baking soda is an alkaline compound that produces carbon dioxide gas when it reacts with an acid (see “An explosive experiment,” page 18). The acidic ingredients most often used to react with baking soda are buttermilk (or milk soured with vinegar or lemon juice), yogurt, sour cream, honey, molasses and puréed fruit (bananas and apples, for example). But since not every recipe includes an acidic ingredient, an alternate leavening agent is needed: baking powder.
Baking powder is simply baking soda preblended with just the right amount of acid in the form of a dry compound. When mixed with liquids, the baking powder dissolves, allowing the baking soda and acid to react and produce carbon dioxide. To keep the two active ingredients dry and separate, baking powder also contains cornstarch.
Why do some recipes call for baking soda and baking powder?
Often a recipe will include both baking soda and baking powder. This is because cakes, muffins and quick breads require a certain ratio of leavening agent to flour to produce the right amount of carbon dioxide and rise properly. If the recipe doesn’t contain enough acid to react with the baking soda, baking powder is used to make up the difference.
Two types of baking powder
Two types of baking powder are sold in supermarkets : single-acting and double-acting. Although labels don't always specify the type, you can easily figure out which by reading the ingredient list.
1. Single-acting baking powder
This type of baking powder contains sodium bicarbonate and a single acidic compound, almost always monocalcium phosphate. That’s the formula for Canada’s popular Magic brand baking powder. Single-acting baking powders give off carbon dioxide as soon as they are mixed with a liquid. Dough and batters made with them begin rising when the dry and liquid ingredients are mixed together. That’s why you’re best off getting the dish into the oven as quickly as possible.
2. Double-acting baking powder
Besides baking soda and monocalcium phosphate, this type of baking powder contains one more ingredient. Usually it’s sodium aluminum sulphate, a compound that releases carbon dioxide only when heated to 50°C (120°F) or higher. That’s why the baking powder is called double acting: it acts a first time as soon as it becomes wet (like single-acting baking powder) and a second time in the oven. This means that doughs and batters don’t have to be baked as soon as they are mixed and that they rise continuously.
Which type of baking powder is better?
Although single-acting powders react only once, they are every bit as effective as double-acting powders. Why? Because manufacturers use a special process to coat the monocalcium phosphate, which slows the reaction speed and ensures the powder produces much of its carbon dioxide while in the oven. This is the baking powder I prefer, for the simple reason that it contains no aluminum.
The perfect dose
While it may seem counterin¬tui¬tive, using too much leavening agent can cause baked goods to collapse. When there’s an oversupply of carbon dioxide, the gas bubbles that form in the batter become so big they coalesce, rise to the top and burst, leaving insufficient gas to raise the dough or batter and weakening the cake’s structure. But there are other good reasons for using a light hand, especially with baking soda. Any bicarbonate that doesn’t react with the acids is transformed by the heat into an unpleasant, soapy-tasting compound. Too much bicarbonate also makes pastries darker (browning, also known as the Maillard reaction, occurs more easily when dough is alkaline). And it can change the colour of fruits and nuts: cherries, blueberries and raspberries may turn blue while sunflower seeds may become green.
How much is enough?
Generally speaking, you can rely on the quantities of leavening agents given in recipes. Sometimes, however, they may contain too much. How can you tell? Here are some guidelines to help you decide:
Allow 5 to 10 ml (1 to 2 teaspoons) of baking powder or 1 ml (1/4 teaspoon) of baking soda (used in combination with an acidic ingredient) for each 250 ml (1 cup) of flour.
Know that 2.5 ml (1/2 teaspoon) of baking soda used in combination with 250 ml (1 cup) of buttermilk or soured milk produces as much carbon dioxide as 10 ml (2 teaspoons) baking powder. To sour milk, combine 15 ml (1 tablespoon) vinegar or lemon juice with 250 ml (1 cup) milk and let rest for 5 minutes.
Is your baking powder still active?
Indefinitely. Baking powder, on the other hand, loses its potency over time, especially if exposed to high humidity. Baking powder manufacturers recommend using their product in the year after the container is opened. To check whether your baking powder is still potent, stir a spoonful into approximately 125 ml (1/2 cup) hot water. If the mixture bubbles, your baking powder is still active.
In a pinch...
Fresh out of baking powder? Then make your own by mixing 2.5 ml (1/2 teaspoon) cream of tartar with 1 ml (1/4 teaspoon) baking soda. This amount will replace 5 ml (1 teaspoon) of commercial baking powder.
What is ammonium carbonate?
This lesser-known leavening agent—also called baking ammonia—is used to make traditional cookies like Lebkuchen (gingerbread cookies), Springerle and ammonia cookies. Unlike baking powder and baking soda, ammonium carbonate doesn’t need to react with an acidic ingredient to produce carbon dioxide. In the presence of heat and humidity, it simply breaks down into ammonia (which can definitely be smelled during cooking), carbon dioxide and water. Baking ammonia can be found in bulk grocery stores and some ethnic markets.
An explosive experiment
Here’s a simple experiment that lets you compare the principle behind baking soda and baking powder. Put a spoonful of baking soda in a tall glass and a spoonful of baking powder in another. Add a little water to the glass with the baking powder; after a few seconds the mixture will begin to fizz and form carbon dioxide bubbles. Then add some water to the glass that contains the baking soda. What happens? Nothing! That’sbecause the baking soda needs to react with an acid in order to produce carbon dioxide. Now add a little vinegar to the baking soda glass: bubbles will appear immediately and in impressive quantities. That’s why, in cooking, baking soda is used in conjunction with an acidic ingredient such as buttermilk, lemon juice, yogurt or puréed fruit. Try this experiment with your kids—they’ll love it!