Double Acting Aluminum Free Baking Powder consists of baking soda, one or more acid salts (cream of tartar and sodium aluminum sulfate) plus cornstarch to absorb any moisture so a reaction does not take place until a liquid is added to the batter. Double Acting Baking Powder reacts to liquid and heat in two stages. The first reaction takes place when you add the baking powder to the batter and it is moistened. The second reaction takes place when the batter is placed in the oven.