Baking Soda vs Baking Powder

I've had these two boxes as staples in my baking supplies forever as do most of homes. We follow the instructions to use one or the other (or both) in our recipes but do we know why?

I knew that baking soda makes great volcanoes when mixed with vinegar but why do some recipes use baking soda and some baking powder?

I read that the same ingredient, sodium bicarbonate, is in both but there is something more in baking powder. I did some experiments (as all ex-chemistry students like to do – or is it just me?)

I added a tablespoon (or 15 mls) of baking soda to a glass (left) and did the same with the baking powder (right).

Then I added about 50 mls (¼ cup) of water to each. I did not see much difference except the baking soda had larger clear bubbles than the baking powder but not much more action.

Then I tried the same experiment with vinegar! Wow!

The baking soda erupted! The baking powder did bubble a bit but very little in comparison.

Next, I decided to do a simple baking experiment – biscuits. The recipe normally uses baking powder.

Basic biscuits*

Preheat oven to 425°F

Sift together

2 cups flour

3 teaspon baking powder

½ teaspoon salt

Cut in

¼ cup shortening

Add all at once, stirring until soft ball is formed:

¾ cup milk

Turn dough onto floured board, knead lightly 20-25 times. Roll or pat dough ½ inch thick. Cut with floured biscuit cutter or glass. Place on ungreased baking sgheet and bake 10-12 minutes. Serve hot. Makes 18-20.

*Doris Janzen Longacre, More-with-Less Cookbook, Scottsdale: Herald Press, 1976, p.72

Now a scientific experiment includes experimental details (a recipe) and the parameters (ingredients) are changed to determine how the results will differ.

I followed the recipe above exactly for the one batch of biscuits and just substituted baking soda for the baking powder in the the second batch and used a double-acting baking soda in the third batch.

 Now an comparative experiment must have the exact same conditions for all batches so I put all the biscuits on the same cookie sheet for the oven.

Then I baked the three batches to see what the small change in ingredients made to the final product.

Wow! Quite the difference! The baking soda biscuits look wonderful but actually tasted quite bitter. The paler golden biscuits in the middle tasted the way biscuits should taste and the double acting ones were a bit taller and more speckled and tasted very similar to the baking powder ones but a tiny bit bitterer.

So why the variety? Baking soda is pure sodium bicarbonate which is a chemical base and needs the addition of an acid to create a reaction.

Sodium bicarbonate

(IUPAC name: Sodium hydrogen carbonate)

The original test with the vinegar results in the following reaction:

NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂(g)

You can see that the mixture of the sodium bicarbonate, NaHCO_3, and acetic acid, CH₃COOH, creates sodium acetate and water and carbon dioxide - it is the carbon dioxide that creates  the explosion of bubbles!

So recipes using baking soda alone also must include something like buttermilk to play the role of the acid in the reaction above.  The milk in the biscuits reacted with some of the sodium bicarbonate but not all of it so the biscuits were bitter which is the natural taste of the baking soda.

The first baking powder I used contains sodium acid pyrophosphate, sodium bicarbonate, corn starch, and mono calcium phosphate.

Sodium acid pyrophosphate

(IUPAC name: Disodium dihydrogen diphosphate)

Monocalcium phosphate

(IUPAC name: Calcium dihydrogen phosphate)

Calcium dihydrogen phosphate is also used in the food industry as a leavening agent to cause baked goods to rise. Because it is acidic, when combined with an alkali ingredient – commonly sodium bicarbonate (baking soda) or potassium bicarbonate – it reacts to produce carbon dioxide and a salt. The carbon dioxide gas is what leavens the baked good. When combined in a ready-made baking powder, the acid and alkali ingredients are included in the right proportions such that they will exactly neutralize each other and not significantly affect the overall pH of the product.  http://en.wikipedia.org/wiki/Monocalcium_phosphate

The double action baking powder contains corn starch, sodium bicarbonate, sodium aluminum sulphate and acid calcium phosphate (mono calcium phosphate by another name).

The main use for SODIUM ALUMINUM PHOSPHATE, is as a leavening agent or acid for mixing baking powders, this is a new product in the baking industry. The SODIUM ALUMINUM PHOSPHATE, has a different performance profile than other leavening agents; it reacts slowly with the Sodium Bicarbonate in the mixing stage, there is only a 20 to 30 % Carbon Dioxide delivery from available. The difference is released during the oven stage.  http://www.answerbag.com/q_view/411610

This reaction is:NaAl(SO₄)₂ + 3 NaHCO₃ ----> Al(OH)₃ + 2 Na₂SO₄ + 3 CO₂ 

Finally, the thermal decomposition  reaction of sodium bicarbonate is:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

So even in the oven, any unreacted baking soda will continue to produce carbon dioxide in smaller amounts.

So basically you need both the acid and the base to create the optimum reaction in any of the variations!

Reference:
TLC Cooking: What is baking powder, and how does it work?