Why does a chocolate cake rise or swell as it bakes?

Chocolate cakes swell or rise during baking because of the interactions between its ingredients, primarily baking powder, baking soda, eggs, and steam. Here's the scientific explanation:

1. Chemical Leavening Agents (Baking Powder and Baking Soda):

- Chocolate cakes often contain baking powder or baking soda as leavening agents. These ingredients cause the cake batter to rise when heated.

- Baking powder contains both baking soda and an acid (usually cream of tartar). When moisture and heat are introduced during baking, the acid reacts with the baking soda to release carbon dioxide gas.

- Baking soda requires an acidic ingredient (like cocoa powder or buttermilk) to react and release carbon dioxide.

2. Eggs:

- Eggs contribute to the rising of a chocolate cake. The proteins in egg whites (albumen) coagulate and expand when heated, capturing air bubbles and expanding the cake batter.

- Egg yolks contain lecithin, an emulsifier that helps stabilize the air bubbles created by the egg whites and leavening agents.

3. Steam Production:

- As the cake bakes in the oven, the liquid ingredients (milk, water, eggs) turn into steam.

- This expanding steam gets trapped in the batter and causes it to swell or rise further.

4. Oven Heat:

- The heat in the oven is crucial for the cake's rise. It accelerates the chemical reactions, expanding the air pockets and causing the cake to grow.

5. Structure and Texture:

- The gluten in the flour provides some structure and helps trap the air bubbles created by the leavening agents, eggs, and steam.

- The type of chocolate used can also influence the cake's texture and rise. For instance, cakes made with dark chocolate may have a denser crumb compared to those made with milk chocolate.

Combining all these factors - leavening agents, eggs, steam, and heat - results in the characteristic rise or swell of a chocolate cake as it bakes.