Red velvet cake is a dessert consisting of a red chocolate layer cake with a cream cheese or buttercream icing. The cake is made red using cocoa powder and a significant amount of water-based red food colouring, usually about 28 to 56 g (1 to 2 oz). The cocoa powder helps deepen the red colour and is used in a small quantity compared to chocolate cake recipes. Some bakers show surprise at cake ingredients such as vinegar, which is common in red velvet cake to produce the leavening process.
Baking soda produces carbon dioxide when it combines with certain acidic ingredients. Just as yeast helps bread dough rise, cakes require leavening to produce the final product. With yeast products, a significant amount of time is set aside for rising before preparing the dough for baking. Cakes, however, need a leavening process that will act quickly to produce the necessary rise in dough while baking, but not so quickly that the cake will fall (collapse) after it has risen.
Baking soda and baking powder
Baking soda and baking powder are both used in baking and help the leavening process. Baking soda consists of sodium bicarbonate, which is an alkaline substance commonly used for acid indigestion and heartburn treatment. Baking powder is usually available in two forms: with and without aluminium. Baking powder contains baking soda, but the two are not exactly interchangeable. Baking powder contains cream of tartar and other reactive ingredients that produce a double-acting effect, causing the cake to form gas bubbles in the batter while mixing, then expand when baking to give a light, risen final product. Baking powder contains ingredients that react together when moistened, whereas baking soda requires an acidic component to react. This is why many recipes that call for baking soda, but not baking powder, also call for vinegar, buttermilk, sour milk or lemon juice.
Vinegar in cake
The vinegar -- an acid -- reacts with the baking soda -- an alkaline -- to release carbon dioxide gas in the form of bubbles that expand and cause the batter to rise in the pan while baking. In red velvet cake, the baking soda reacts with vinegar to expand the cake while it bakes in the oven, and generally there is no need for baking powder. Recipes may include both vinegar and another acidic liquid such as buttermilk or sour milk, or just the vinegar and baking soda. Likewise, some recipes may call for baking powder as well.
Buttermilk and sourmilk
Buttermilk is fermented and reacts with baking soda and baking powder to produce the necessary leavening in baking. In general, if a recipe calls for 1/2 teaspoon of baking soda, a cup of buttermilk is required to react with the baking soda completely. Using buttermilk or sour milk and baking soda produces a leavening that is four times greater in volume than simply using baking powder. In other words, baking powder may contain alkaline and acid components that react when they become moist in the batter, but the use of buttermilk and baking soda is chemically more efficient. If buttermilk is unavailable, or you don't have any on hand, you can create sour milk by adding a tablespoon of plain white vinegar or lemon juice to a liquid measuring cup and filling to the 237 ml (1 cup) mark with regular milk. Alternatively, you can add 1 1/4 teaspoons of cream of tartar to your recipe for every cup of milk.