95 The Science of Baking-Mid

The Science of Baking FOCUS Question What are the physical and chemical changes that take place during baking?

Oven Chemistry Mmmmmm! Don't you love the smell of something delicious baking in the oven? Believe it or not, gooey cookies, fluffy cake, and warm bread are all made using chemistry. When a baker mixes together the right ingredients in the right order, the atoms inside start to interact. Each ingredient has a task, and once heat is added, a blob of batter transforms into a tasty baked treat. Let's find out why these kitchen chemical reactions taste so good!

Mixing It Up Let's bake a cake! Combine the dry ingredients, or reagents—flour, baking powder, and salt—into a mixture. Separately, mix the butter and sugar. The sugar doesn't dissolve, but it spreads evenly throughout the butter. You have created a colloid. Add the dry mixture to the colloid and then blend in the eggs, milk, and vanilla. Now you have a thick solution. Mixtures, colloids, and solutions are all the result of physical changes. Some physical changes are easy to undo, like picking raisins out of trail mix. But with cake batter, you can't easily separate the ingredients once they are combined. This kind of physical change is what allows bakers to create something new, like a cake!

A Sticky Framework Like the frame of a building, a cake needs a structure, or framework. Without it, the cake will collapse into a pile of crumbs. Gluten, a protein substance found in wheat flour, provides that structure. When the flour gets wet from the milk and eggs, two proteins join to form long strands of gluten. Mixing the batter creates even more gluten, which holds everything together. Gluten molecules have links between them that hold them together and make them elastic, or able to bounce back. This property allows cake and bread to rise, but too much gluten can result in chewy bread or dense cake. Not so yummy! To prevent this problem, bakers use flour with the right amount of protein for their bread or cake recipe, and they don't overstir the batter. glutenin + gliadin = gluten When the two proteins in flour—glutenin and gliadin—get wet. they unwind and join together to form gluten. Think About It If you have ever tried to stretch out pizza dough, you know that it springs back when you let go. Why do you think it does that?

Rising Up Now you have a solution of batter with gluten ready to provide a framework. But your batter is still a bowl of goo. To get an airy cake or other baked goods, you need bubbles in the batter in order to expand the elastic gluten framework. Cookie dough uses the properties of butter to create some of the bubbles. Butter is an emulsion—a glob of water fat and solid dairy products all stuck together. Inside the hot oven, the butter in your cookie dough melts. The dough flattens out, and the water in the butter separates from the fat and dairy. The water molecules turn to steam, forming bubbles that make the cookie dough rise slightly.

Heavy Lifting The real heavy lifter in baked goods is called a leavening (LEH-vuh-ning) agent. One of the chief leavening agents is baking soda, or sodium bicarbonate. Even before you place the cookie dough into the oven, the sodium bicarbonate begins to react with acids in the dough and creates carbon dioxide gas. In the oven, this reaction speeds up. The gas expands, opening air pockets in the gluten structure. This chemical change causes the dough to rise. It is impossible to reverse this step. Baking powder—baking soda mixed with an acid— is often used for cake batters that don't already have acid in them. It makes carbon dioxide gas as soon as it contacts the other ingredients, and it reacts further when heated in the oven. Lots of carbon dioxide gas bubbles create a fluffier cake!