In short raising (or leavening) agents make your cakes and breads rise, your mousse and soufflé light and fluffy. There are three main rising agents used in cooking; eggs, baking soda and baking powder. Baking powder is probably the most common of the three. Basically the raising agent is anything that is added to introduce gas to the mixture that lightens and softens the final product.
Baking powder and baking soda both introduce carbon dioxide gas into the mixture when reacted with moisture and temperature (heat from your oven). Both are a white, dry powders that are chemical raising agents. So what’s the difference?
Baking soda (also known as bicarbonate of soda or bicarb soda) is pure sodium bicarbonate (a chemical compound). Baking soda, in the presence of moisture (which most cake batters have) and an acid (like chocolate, fruit and milk) form carbon dioxide. The carbon dioxide gas bubbles get trapped in the mixture as it is cooked and gives the food its light, fluffy texture. Baking soda also produces a slightly tangy taste (although in most foods this is very minimal).
Baking powder is a mix of baking soda, cream of tartar (an acid) and a drying agent (like starch). The combination of the three means that the mixture requires moisture and heat. The same reaction happens, moisture and heat react with the baking powder producing carbon dioxide gas which gets trapped and expands during cooking. Interesting tid bit: self-raising flour contains baking powder, so if you have a recipe that calls for plain flour but don’t have it just use self-raising and omit the baking powder. Equally if you run out of self-raising flour and need it just add 1-2 teaspoons per cup of flour to the flour and mix before using it the recipe as normal.
Oh and no you can’t substitute baking soda with baking powder. If you have to you can use baking soda and cream of tartar instead of baking powder but it will change the taste of the final product.
Eggs are a completely different matter. First off they are a mechanical raising agent (you have to do something to them for them to work as a raising agent) and secondly they don’t produce a gas, they simply introduce air into the mixture. Mechanical raising agents, like eggs, produce similar results to chemical raising agents (like baking powder and baking soda), by trapping gas in the mixture. When you beat eggs with a whisk (the hard way) or an electric mixer (the easy way) you are incorporating air to get into the eggs. It is this air that raising the mixture.
Tangent time… eggs (at least the ones I’m talking about) come from chickens, which makes them an animal bi-product. The parts of animals we eat and animal bi-products, like eggs and milk, are protein based. The protein, beside from being an important mineral is the vital factor of eggs ability to be a raising agent.
Ok back to mechanical raising agents. Beating the eggs incorporates air, which you can see with an increase in volume. The beating also denatures the protein in the eggs, which changes the shape and allows air to be trapped between the protein. This type of raising is especially important in soufflé and mousse where the egg formed shape is very important in the final texture. Egg whites (albumen) are especially good as a mechanical raising agent and for both soufflé and mousse the egg white and air structure formed by beating creates the texture of the product.
So if you want a light, air filled texture then mechanical raising agents (like beaten eggs) are important. If you are going for a moist cake then you want to look more at your chemical raising agents (like baking soda and baking powder).
If you want to put what you have just learnt to good use then check out my chocolate mousse recipe below (on video) or have a look at my recipe page (for the chocolate mousse and many more written recipes).