2. Gases Form and Expand.

The three most important leavening gases in baked goods are air, steam, and carbon dioxide. Heat from the oven affects these leavening gases in several ways. For example, heat causes water to vaporize into steam. Heat also increases the rate of fermentation in yeast-raised baked goods, so that carbon dioxide gas and alcohol are generated at a faster rate, at least until the yeast dies. Finally, heat helps to dissolve slow-acting baking powders to activate them. Once activated, baking powders release carbon dioxide into the liquid portion of the batter or dough. Depending on the formulation of the baking powder, this may start at room temperature and continue until the temperature reaches 170°F (75°C) or above.