I made NIgella's Chocolate Guinness Cake at the weekend but it hardly rose in the oven at all. Should there be baking powder in the recipe as well as the bicarbonate of soda? Still a nice cake but I was just disappointed with the size of it!
Nigella's Chocolate Guinness Cake (from Feast) only uses bicarbonate of soda (baking soda) and not baking powder. Bicarbonate of soda is an alkali and needs an acid to react with to create carbon dioxide bubbles. In baking powder the bicarbonate of soda is combined with an acid called cream of tartar and the two react when mixed with liquid.
However in the Chocolate Guinness Cake acidity is provided by the sour cream, and also a little by the Guinness. Therefore only the bicarbonate of soda is needed in the cake batter to make it rise. You should start to see the batter bubbling slightly as soon as it has been mixed as bicarbonate of soda reacts very quickly.
There are a few common reasons for cakes with bicarbonate of soda not rising. The first is a lack of acid but we know that this recipe contains acid ingredients so this would not the cause. The second is that the cake batter may have been left standing too long before baking which means that the bicarbonate of soda has finished creating bubbles by the time it is baked. Make sure that the oven is fully preheated before you mix the cake and put it into the oven as soon as the batter is in the cake pan.
Finally, the bicarbonate of soda may be old or have expired. Although it has a relatively long shelf life, bicarbonate of soda will become less active over time and the packaging usually has a "best before" date on it. If the bicarbonate of soda is older than this date then the cake may not rise. To test your bicarbonate of soda you can put 1/2 teaspoonful into 60ml/1/4 cup/4 tablespoons very hot water that has had 1/4 teaspoon of lemon juice or vinegar added to it. The bicarbonate of soda should fizz up quite vigourously as soon as it is added to this liquid. If it doesn't then you should discard it and buy a new packet.