In the red velevet Cupcake recipe it says to use cider vinegar for the batter, but can you use lemon juice instead?
Nigella's Red Velvet Cupcakes (from Kitchen and on the Nigella website) use a combination of baking owder and bicarbonate of soda (baking soda) as raising agents. The bicarbonate of soda is added to help add extra lift to the cakes, as the cocoa powder can tend to slightly reduce the rise in cakes.
Bicarbonate of soda needs an acid to react with to make the carbon dioxide bubbles that cause the cakes to rise. And although the buttermilk is acidic and there is cream of tartar in the baking powder, the extra acid will help to make the bicarbonate of soda very active and make the cakes light. Cider vinegar is a mild-flavoured vinegar but lemon juice would also be fine to use as an alternative acid ingredient, as it should also not interfere with the flavour of the cakes.
It is worth noting that the cake batter will have quite a few bubbles when first mixed, but the bicarbonate of soda will expire fairly quickly. So make sure that you have the oven properly preheated and the muffin pan prepared so that you can put the cupcke batter into the oven as quickly as possible after it has been mixed.