Baking powder and baking soda both produce carbon dioxide,
which helps raise or "leaven" baked products. Baking soda
works best in conjunction with an acidic ingredient. In
the case of banana bread, this may be buttermilk, brown
sugar, molasses or the bananas themselves. Recipes generally
include just enough baking soda to balance the acidity
in the batter. For instance 1/4 teaspoon baking soda is
balanced with 1/2 cup buttermilk, applesauce or mashed
just-ripe banana (note that bananas become less acidic
as they ripen). This produces sufficient carbon dioxide
to raise one cup of flour.
This however, may not be sufficient to leaven the whole
recipe. Here's where baking powder comes in. Baking powder
contains both baking soda and a dry acidic ingredient.
Since it isn't dependant on acid ingredients in the batter,
it is used to add the extra leavening necessary to raise
the rest of the batter. Generally one teaspoon of baking
powder leavens one cup of flour. In the case of recipes
like banana bread which contain heavy ingredients, such
as bananas and sometimes heavy grains like wheat germ
or whole wheat flour, this may be increased to 1 1/2 or
2 teaspoons of baking powder per cup of flour.
