The amount of baking powder used in muffins varies between 2% and 6% based on flour at 100% with lower amounts in muffins with ingredients that increase acid.
Baking soda is used in addition to double-acting baking powder when muffins contain acidic ingredients such as sour cream, yoghurt, buttermilk, light sour cream, molasses and some fruits and fruit juices.
Since baking soda begins to release carbon dioxide as soon as it is moistened by the liquid acids in recipe , it is important to thoroughly blend all the dry ingredients together and to make sure the soda is evenly mixed before adding the liquid ingredients.
Gases released by a leavening agent influence volume and cell structure. When the mixture is heated, the gas will expand, causing the mixture to rise. The gas aerates and leavens the batter before and or during baking.
Stretching of the cell walls during baking improves texture and promotes tenderness. It will results in the tenderizing of the product, providing a pleasurable mouthfeel.