Raising Agents: Physical
Can you imagine what a cake would taste and look like if it had no air in it? It would be flat, the sponge would be dense.
What are physical raising agents?
Raising agents act by adding gas into the mixture to give cakes and bread, lightness. As well as chemical and biological raising agents, raising agents such as air and steam can be added to the mixture through mechanical actions.
How do physical raising agents work?
Air can be added to food mechanically using whisking, sieving, creaming, beating, rubbing in, rolling and folding.
During creaming (where fat and sugar are creamed together), air is added to the mixture, and is enclosed by the fat. When the cake is cooked, the air expands causing the cake to rise and the sponge to be light.
Whisking is the same as beating, however a whisk is used. The ingredients are whisked together, driving air into the mixture. For example think about when you make meringues. Egg whites are whisked with sugar. As the air is incorporated into the mixture, it creates a foam and the volume of the mixture increases, giving lightness.
You may have also sieved flour to make cakes and doughs and though this is only to get rid of lumps, sieving flour can also add air to a product. The air becomes trapped between the flour particles.
Folding and steam
Have you made pastry dough? Folding the pastry dough into layers, traps the air between the layers. This happens every time the dough is folded. With puff pastry (where the dough is layered with fat), the butter in the layers of the puff pastry, during cooking, lets off steam (another natural raising agent). The steam (water vapour) causes the pastry to rise. Once the steam has evaporated, the pastry is set in place from the heat during the baking process. The same process can be applied when you make batters, as well as choux pastry (for profiteroles etc.). The water in these mixtures turns to steam when the food is cooked in a very hot oven. This causes the food to rise and as the water is removed the food bakes and becomes more solid. For steam to be an effective raising agent, you need the mixture to contain a large amount of water and a high baking temperature.
AQA video: Investigating how raising agents work