How useless is a Chocolate Teapot?
|A very, very large amount of chocolate||Some tea|
Part of the reason that chocolate is so irresistible is the way that it is made up of a variety of fats, both from the cocoa bean and in many cases milk or even vegetable. These fats happen to melt at just below body temperature so the chocolate melts in your mouth. This is why chocolate is so obviously a bad material for making teapots, hence the phrase "as useless as a chocolate teapot"!
However, James called in to the show to ask how thick the walls of a chocolate teapot would need to be to be able to brew tea in it, so in the best tradition of Kitchen Science, we set out to find out the answer to this critical question.
The obvious way to find out how thick you would have to make a chocolate teapot is to make a series of chocolate teapots with walls of various thicknesses and test them by brewing tea. However this would take rather a long time and involve truly stupendous amounts of chocolate (as opposed to just ridiculous amounts). We decided to do a series of tests on a small amount of chocolate in order to find out roughly how much we would need for the teapot. Chocolate is a very complex substance and it doesn't melt in a simple way - it goes through a series of stages where it becomes gradually more fluid. The teapot could melt enough to become flexible and then the pressure of the tea could cause it to distort and empty its contents onto the floor, without actually melting through the walls. This means that just finding out whether it melts or not isn't very useful. So we needed an experiment that modelled being part of a chocolate teapot wall as closely as possible, while using a sane amount of chocolate.
After only about half a minute the lid started to melt, but otherwise the teapot survived its experience in one piece if not entirely unscathed. The tea was slightly unusual and sweet, but not unpleasant.
When chocolate melts it doesn't become totally liquid immediately, it remains quite viscous. Unless you apply a fairly large force to the melted chocolate, it seems to sit there. Chocolate is also mostly made of fat, which is a good thermal insulator (whales use blubber as a form of insulation). This means that the molten chocolate near the hot water protects the less molten chocolate below it, insulating it from the heat of the water.
Also, it takes a significant amount of energy to melt chocolate, so it will take a significant amount of time to move heat into the solid chocolate, thus slowing its melting.
|The construction of the teapot||The layer of molten chocolate is viscous, and so it doesn't flow away easily. It then sits as protective layer, insulating the solid chocolate below from the hot water above.|