Why do you add both baking powder and bicarbonate of soda to the Gluten-Free Banana Bread? I thought you could swap them, one for the other, so why use them together?
Baking powder and bicarbonate of soda (baking soda) are both leavening agents that are used in baking to help cakes, muffins and quick breads rise by producing bubbles of carbon dioxide. Bicarbonate of soda is one of the ingredients used to make baking powder. However they are not interchangeable. Bicarbonate of soda is an alkaline and to produce carbon dioxide it needs to be mixed with an acid ingredient, such as buttermilk or lemon juice. Not all batters and doughs have acid ingredients, so in some circumstances bicarbonate of soda on its own will not work. Bicarbonate of soda also acts very quickly and does not last for very long.
Baking powder is a mixture of bicarbonate of soda and one or two powdered forms of acid. When the two are mixed together and remain dry they do not react, but as soon as some liquid is added they will start to produce carbon dioxide. In addition, most baking powders have a "double action". This is where carbon dioxide is produced when liquid is added and then again when the mixture is heated, which gives a sustained rise and helps ensure that cakes are light after baking. So using baking powder means that the baked goods rise even if the recipe does not contain any acid ingredients. Some cakes tend not to rise as well if just baking powder is used, often those that contain cocoa powder or bananas such as Nigella's Gluten-Free Banana Bread, so a combination of baking powder and bicarbonate of soda is used to help to give extra lift.