Bananas and baking powder

Dan Lepard’s column in Saturday’s Guardian featured banana cake, one of my favourites. Lepard advocates using very ripe bananas, with which I agree, but also mentions something interesting that I’d not known before. Apparently, bananas become more alkaline as they ripen, which interferes with the ability of the baking powder to do its job. As a result, the recipe contains more baking powder than usual.

Lepard doesn’t explain why the alkalinity of the banana affects the raising power of the baking powder – understandably, since he has limited space. For anyone who has never worried too much about what baking powder is and how it works, this might seem rather mysterious.

Baking powder contains two main ingredients: bicarbonate of soda, and some form of acid, which may vary between brands. In the presence of moisture, bicarb reacts with acids to produce carbon dioxide, which is what causes cakes to rise.

If you were paying any attention during school chemistry lessons, you will know that acids and alkalis are opposites on the pH scale, and that if you mix an acid and an alkali together, they will (in equal amounts and at equal strengths) cancel each other out.

If the banana in the cake mixture is alkaline, it will have the effect of reducing the strength of the acid in the baking powder. This in turn will weaken the strength of the reaction between the bicarb and the acid, so less carbon dioxide will be produced, and the cake will not rise as well as we would expect.

Ok, I’ll admit it’s not the most captivating revelation you’re ever likely to hear, but it’s worth knowing these things…