Can you melt a potato?

We put this to Dr Peter Wothers, from the Department of Chemistry at Cambridge University.

Peter - I'm currently sitting in a café, enjoying my last of a mouthful of jacket potato here. Very good! So, why don't potatoes melt? This is actually quite a tricky answer, but it depends on the complexity of what we're trying to melt. The things that generally will melt - and reversibly, so if you cool them down, it might form a solid again - would be very simple substances. Certainly, everyone knows that mercury is already a liquid [at room temperature], if you cool it down, you'll get a solid again. You can keep doing that forever, it will just reverse between solid, liquid and solid as you warm it and cool it. But if you get more complicated things, there are other possibilities. Things can start to change, start to break down. Molecules can actually decompose and this is what's going to happen.

So, if you think about sugar, this is a more complicated structure. It's made up of 3 different types of atoms - we've got carbon, hydrogen, and oxygen. And if we warm this up, well, we can get liquid sugar. We can melt the sugar, and this is where the molecules are all moving around, still in their sugar forms. But if you heat that up too much, again, different parts of the sugar molecules might start to interact with each other and form new chemical compounds. You can boil off water and you're going to be left with this sort of black gooey mess which is carbon.

Now, when you come to a potato, it's even more complicated. You haven't got one type of substance there. You've got complete array of all sorts of complexity. You've got water there, you've got starches in there, you've sugars in there, you've got fats in there. All of these can start interacting with each other. You're not just going to end up with one liquid mess of all of those. They're going to start interacting at much lower temperatures, recombining, giving out water vapour and decomposing what's left into this sort of black gooey carbon mess, and that's what you end up with - your burned potato.

Hannah - So, due to the chemical complexity of the simple starchy spud, it can't melt. Instead, cooking causes the constituent molecules in the potato to interact, evaporate and decompose, and at really high temperatures, form just black carbon. Marshmallow on the other hand is made of simpler smaller molecules including refined sugars which, if we carefully heat at low temperatures, we can melt the inside and keep the sugars intact.