# Chocolate problems

By Marcus Wilson 26/04/2011

Thinking back to last week’s MasterChef (the chocolate tower of terror – re-live it here), there were a couple of nice examples of cooking being a branch of physics. I’ve heard it said that cookery is all about managing the flow of heat into (or, in this case, out of) an object, which, of course, requires some physics.

The first example was the tempering of the chocolate. This is a problem in phase transitions and crystallization. The idea is to get the chocolate to crystallize into the most useful form, and to do that you need to get it to go through the right sequence of temperatures. First, hot enough to melt all the six different crystalline phases, then lower to get  phase IV and phase V crystals to grow. The latter are  the ones with the best properties for building things out of. Then the temperature is raised again to remove the phase IV, and letting the phase V crystals grow.  Wikipedia does a nice job of explaining the various phases of chocolate. I hadn’t a clue it was so complicated getting chocolate right.  I just eat it without thought towards the physics involved.

The second example was the cracking of the chocolate that had been cooled too quickly. This is quite likely a thermal expansion issue, similar to the way a glass can crack if boiling water is poured in. If the outside of the chocolate disc is much colder than the inside, there are stresses set up in the chocolate as the outside tries to contract, and suddenly our nicely tempered chocolate snaps. Oops.

The one thing I didn’t quite grasp about the episode was what one did with the chocolate tower of terror once all the cupcakes had been eaten.  Do you expect your guests to eat the tower, (in which case, how do you serve it?) or do you put it away in a cupboard for a later occasion?