Chemistry of LeaveningBy: Maribeth Beyer

BackgroundAs you can see, bread, cakes, and cookies all look very different even though they are made from almost the exact same ingredients.

The cause of these differences is the leavening process.

Cakes and cookies are chemically leavened, but bread is leavened by yeast.

Bread Leavening

When people bake bread, they are also making a chemical reaction.

This chemical reaction, usually involving yeast, is called alcoholic fermentation.

The yeast ingests the sugar in the bread dough which breaks down into ethyl alcohol and carbon dioxide.

The ethyl alcohol and carbon dioxide are the most important part of the chemical reaction.

Bread LeaveningThe carbon dioxide creates air bubbles which make the “light, airy texture of bread.”

The carbon dioxide does not escape the bread because a gluten protein in the wheat of the flour forms a network of fibers that blocks the air bubbles from escaping.

Bread Leavening

The wheat used in bread dough contains enzymes that act as catalysts to the reaction.

Ethyl alcohol is equally important as it contributes to the taste and smell of the bread.

Organic acids, esters, and ketones are also products made during the chemical reaction.

Cake and Cookie LeaveningThe wheat used to make cakes and cookies does not contain as much gluten as the wheat used to make bread.

Because of the lack of gluten, the carbon dioxide is not easily trapped inside cakes or cookies, so an acid-base reaction is used.

In many recipes, baking soda (sodium bicarbonate) acts as the base in the reaction, and the acid can be a variety of ingredients.

The most common acid used is baking powder, which is actually a combination of acids that release carbon dioxide at different times during the baking procedure.

Cake and Cookie Leavening

Calcium biphosphate is the acid used in most baking powders, but sodium aluminum sulfate may also be used.

When the acid is dissolved in water, hydrogen ions are released and “react with the bicarbonate ions from the baking soda”.

This important acid-base reaction occurs so that the cake and cookies are light and fluffy, not dense and flat.

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"On the Rise." Chemistry World Oct. 2009: 54-57. RCS. Web. 7 Apr. 2013.

<http://www.rsc.org/images/BreadChemistry_tcm18-163980.pdf>.

Rudolph, Melissa. "Leavening: How Great Cooks Loaf." Chem Matters Apr. 1996: 4-6. Print.

Yeast Reaction Videohttp://www.youtube.com/watch?v=zaslZG5PLv4