Re: What is the chem. rxn. between flour and water that occurs when baking?

Hi Kelly.

Let me start by saying that I hold the opinion that every good chemist should bake, brew and cook.

The chemistry of baking is a complicated area. Multiple things are going on, all of which are necessary to get something that tastes good out of the oven (as opposed to a brick!).

Let's use bread as an example.

1) baking powder. Baking soda (sodium bicarbonate) is different from baking powder. Baking soda is a substance known as a "leavening agent" in baking circles. It produces CO2 (sodium bicarbonate is NaHCO3), which creates the air bubbles that make bread fluffy. It needs an acidic medium to operate, and needs to be dissolved. Upon dissolving, the baking soda and acid exist as ions, which can react to make CO2 (remember...acids aren't always a bad thing. Phosphoric acid is an ingredient in soda pop).

NaHCO3 in solution is Na+ and HCO3-
acid is solution is H+ and acid-

so H+ reacts with HCO3- to give H2O and CO2

Baking powder has a dry acid added, usually cream of tartar (potassium hydrogentartrate) or tartaric acid.

Do you see one of the reasons for the water?

2)salt (NaCl) this is what is known as a dough conditioner. I don't know exactly how it works, but it is necessary for the proper dough texture.

3)flour. Flour is basically ground up wheat. It contains various starches, one of which is called gluten. The starches are long threadlike structures, which tangle up tightly when they become hydrated (when you add water)...you have probably noticed how sticky dough gets, huh? This makes the dough like a big balloon so that it holds the CO2 bubbles in.

4)sugar. I think that sugar is used mostly for flavor, generally to balance out the salty flavor.

I'd guess that the reason for punching down the rising bread several times is to get the CO2 mixed evenly (by allowing all of the leavening agent to come "up to speed").

5)yeast. I am going to stretch on this a little. Yeasts are generally used in fermentation to create CO2 and ethanol (alcohol) from sugars...this is how wines and beers are made. The flour contains chemicals called enzymes, which when mixed with water can convert starches to sugars (this is what happens in beer production, where ground barley is converted to sugars and fermented). Yeasts cannot convert starches, but can convert sugars to alcohol and CO2. When you mix the flour with water and yeast, I think that you produce a little alcohol and a lot of CO2, which also helps the bread rise. I do not know why you typically need both yeast and baking soda to make bread.

I hope that this answered your questions! Please email me if you have any further questions.

Best Regards,
Mike Weibel
weibel@chemistry.chem.utah.edu