The sweet science of cake

What makes cake taste so good? If you're the one doing the eating, the answer is easy - sugar! But if you're the one doing the baking, the answer is more complicated.

That's because a recipe isn't just a grocery list of ingredients. It's more like a science experiment. Flour, water, and butter must be mixed together in specific amounts and in the right order to create the chemical reactions necessary to turn liquid batter into soft, spongy cake.

One of the most important ingredients in a cake is gluten (GLOO-tin), says Jacquy Pfeiffer. He's an instructor and owner of the French Pastry School in Chicago. Gluten does what it sounds like: It glues all the parts of a recipe together. "Just a small amount is all you need," Mr. Pfeiffer says.

Don't bother looking for gluten in your kitchen - it's there, but it's not in a can or a bottle. That's because gluten is formed when you mix flour and water together. Inside every speck of flour are proteins called glutenin and gliadin. Usually, they exist separately. Until they get wet, that is.

"When you add water to flour and mix it together," Pfeiffer says, "you make the glutenin and gliadin stick together. This is called gluten." Pastry chefs call this process "activating the gluten."

Gluten doesn't just exist in cake. It's in nearly every kind of food that contains flour and water, from piecrusts and cookies to bread and tortillas. The more gluten a food has, the sturdier and firmer it can become.

Wheat bread has a lot of gluten in it. The gluten is what makes bread slices strong enough to support the weight of whatever you put in your sandwich. That sandwich wouldn't work so well if you made it with something that contained less gluten - that's why you'll never see a ham and cheese on cake!

So how do you control the amount of gluten that forms when you mix flour and water? Some flour, like pastry flour, has fewer gluten-forming proteins in it than others. Another way to control the gluten is to add an ingredient that contains fat, like butter or vegetable shortening.

To understand how fats work, run your finger lightly across a stick of room-temperature butter. (Make sure this is OK with a grown-up first.) It feels slick and slippery, like a raincoat. Mixed with flour, it works like a raincoat, too.

"When you mix a fat like butter with flour before adding water," Pfeiffer says, "the butter coats the flour proteins with a thin layer of fat, giving them a thin layer of protection against the water." Some water still gets through to activate the gluten, but not as much as without the butter.

Eggs keep flour proteins from forming gluten, too, because the yolks contain fat. Eggs also serve as an emulsifier, which means they help distribute the fat evenly throughout the batter to give the final product a uniform texture.

Gluten doesn't just make food stronger, it can make food lighter, too. It does this by trapping air bubbles inside the dough or batter. When air is baked inside something, it helps make that food taste and feel fluffy.

Consider a slice of birthday cake. If you look closely at the side of a slice of cake, you'll see the surface has lots of little holes in it. These were formed in part by air bubbles that got trapped inside the batter by gluten. When pastry chefs add air to a batter or dough, they say they are "incorporating" it.

When Chicago pastry chef Sheila Auslander makes cake for her customers at the Bucktown Baker Inc., she often incorporates air using a three-step process. First, she creates air pockets in her batter by using an electric mixer to cream together butter and sugar. Sugar crystals have jagged edges, so they pull air into the mixture when the mixer whirls them around. You can see this happening: The butter becomes lighter in color and texture as it's creamed with the sugar.

The second step is a lot like the first: She continues to use the mixer to stir the batter as she adds the other ingredients to the creamed butter and sugar mixture, including flour. The gluten formed in the moistened flour traps even more air as the mixer stirs the batter.

The third step is the addition of baking powder. Baking powder may look like flour, but it's much different. It is a "leavener" (LEV-uh-ner). Leaveners lighten the texture and increase the volume of baked goods. How?

"When baking powder gets wet," Ms. Auslander says, "it starts a chemical reaction that produces bubbles of carbon dioxide. The carbon dioxide will eventually cause the air pockets in the batter to expand."

It's hard to tell what baking powder is doing when you're mixing it into cake batter, but you will definitely see it working when you pop that batter into the oven.

As the batter is heated, the baking powder starts releasing tiny carbon dioxide bubbles like crazy. (Note: Carbon dioxide is what gives soda its "fizz," too.) Remember those air pockets you made in the batter with the electric mixer? The carbon dioxide bubbles are drawn to these air pockets, creating a system of bubbles that looks like a honeycomb. Once the two types of bubbles are together, the oven's heat causes them to expand even more. This makes a cake rise and gives it a light, tender texture.

Last but certainly not least, there's sugar. Sugar's most obvious job is to make things taste sweet. But it has other uses, too, like helping cake turn golden brown on top once it's in the oven.

"When sugar gets very hot, it melts," Auslander says. "This is called 'caramelizing.' " If you think that word sounds familiar, you're right - the candy kind of caramel is essentially melted sugar mixed with butter and milk. No wonder the kitchen smells so good when a cake is in the oven! And understanding kitchen science will ensure that the cakes you bake will taste as good as they smell.

• There are many types of flour, and each produces a different amount of gluten. To learn what these flours and gluten levels are like, see: www.exploratorium.edu/cooking/ bread/activity-gluten.html. This is part of a fun site about food and cooking sponsored by The Exploratorium, a children's museum in San Francisco.

If you like kitchen science, look for 'The Science Chef: 100 Fun Food Experiments and Recipes for Kids,' by Joan D'Amico and Karen Eich Drummond (Wiley, 1994). It is full of facts, explanations, and hands-on experiments you can eat.

2 cups all-purpose flour (no need to use cake flour)
2 teaspoons baking powder
1 teaspoon salt
1/2 cup unsalted butter (one stick) at room temperature
1-1/4 cups sugar
3 eggs
2 teaspoons vanilla extract
1 cup milk
Butter and flour to coat two 8- or 9-inch round cake pans
Frosting (buy at the store, use a cookbook recipe - or try whipped cream!)

NOTE: You'll need an adult's help. You'll also need an electric mixer, cake pans, two mixing bowls, and things like measuring spoons and cups.

With the help of an adult, make sure the oven rack is in the middle of the oven. Heat the oven to 350 degrees F.

Rub a thin layer of butter all over the inside of two cake pans (use a paper towel or a butter wrapper). Add a little flour to each pan and shake it around until it coats the butter.

In one bowl, stir together the flour, baking powder, and salt. Set aside. Put the butter and sugar in a second bowl, and use an electric mixer to blend them until they are light and fluffy. Add the eggs to the butter-sugar mixture and beat well. Add the vanilla. Beat.

Add half of the flour mixture and beat until it's blended. Add half the milk and blend. Add the rest of the flour mixture and blend. Add the rest of the milk and (you guessed it) blend.

Once the last of the flour and milk is blended into the batter, stop mixing immediately! Pour half the batter into each of the two pans - use a spatula to spread it around evenly. Bake 30 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Let the cakes cool thoroughly in the pans, then invert them onto plates. (Run a butter knife around the inside of the pans to loosen the cake.) Top with your favorite frosting.

- Courtesy of Sheila Auslander, Bucktown Baker, Inc.

• Butter is easier to cream when it's at room temperature. If you forgot to take the butter out of the refrigerator before starting, thinly slice the stick and let it sit for 10 minutes. Don't warm butter in the microwave - you might overheat it. That will cause the milk proteins in the butter to turn brown and burn, ruining the taste of the butter.

• To avoid lumps in your batter, sift or gently whisk together powdery ingredients like flour, baking powder, baking soda, and spices.

• Shiny pans are best for cake baking because they reflect heat, creating a light finish and a tender crust. Dark metal and glass pans absorb heat. If you use them, reduce the oven heat by 25 degrees F.

• Fill the cake pans only halfway. The batter needs room to rise.

• Don't open the oven door until you're three-fourths through the cooking time. Why? A sudden rush of cold air, or an accidental slam of the oven door, might collapse your cake. That's because those actions could cause the air bubbles trapped in the batter to deflate.