Science Questions

Sun, 5th Mar 2006

Part of the show Recycling, Water Use and Problem Plastic


Jim in Houston asked:

Why do eggs make things rise when they're baked, and why does yeast make dough rise?


When you make a cake, you mix together fat, sugar, flour and eggs. To make the batter, eggs have a very important property that means they can emulsify things. In other words, eggs contain lipoproteins that can stick fat into a liquid. So you mix together fat, flour and sugar and make a cake batter. When you put it in the oven, the fat melts and the air that's been beaten into the cake expands into the gaps. This makes the cake rise. The protein that's in the egg whites solidifies and holds the whole cake structure as a stable thing. This means that when it comes out the oven, it doesn't just flop down. If you've got baking powder in there as well, that releases carbon dioxide which is another gas that expands and makes the nice big gaps in your lovely fluffy cake. That's why eggs make things rise. They make everything stick together and then hold it together as a solid protein structure. The yeast story is similar. Yeast is a type of fungus and when you warm up your bread mixture of flour and water, they create carbon dioxide. That creates the holes in your nice fluffy bread and a little bit of alcohol too!


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