Baking powder and salt.
the baking powder! when it reacts with the liquids, it expands and in almost every muffin recipie, there is baking powder
They would not raise as much.
Most are made with yeast, so they don't contain any baking powder.
Baking Powder
when using a self rising mix not much , when using a plain mix the muffins wont "rise"
Muffins are a type of "quick bread," which uses baking powder or baking soda as leavening. Yeast requires an hour or more to raise dough or batter, and produces a different crumb than desired in (American) muffins.
Yes, self-raising flour will help make the muffins rise, while plain flour won't unless you add baking powder to the muffins.
The amount of baking powder required depends entirely on what you intend to bake. Different types of cookies, muffins and other baked goods require different proportions of baking powder to flour.
Baking soda will cause things to rise also. Muffins are smaller so there is not as much need for lift. Baking powder will give more rise than soda. Whoever created the recipe chose to use soda instead of baking powder.
Bananas are not the best leavening agent for muffins. Baking powder is the best leavening agent for baking. Bananas do not rise to the same degree as baking powder.
you get chunks of flour and the other substances put in your muffin. When it's cooked the flour will not cook and you'll just bite into some flour or baking powder, not harmful.