During baking the volume of the bread and cake increases because of usuage of typical leavening agent and five levels of constant baking temperatures (200, 225, 250, 275, 300oC).
The volume of the bread also increases or expands because of the following reasons:
1. Air incorporation (While mixing the dough, the air mixes with the batter)
2. Carbon dioxide (CO2) production which is because of either chemical agent or by fermentation.
3. Water vaporization (vapourization). This happens during baking.