Question of the Week

How could I melt a potato?

Sat, 27th Oct 2012

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Sean Mooney asked:

Dear Naked Scientists,


On a recent camping trip we were baking potatoes and toasting a few marshmallows on a metal grid over the embers of a fire. Whilst drinking a beer I thought: if I could get my fire hot enough I would be able to melt the metal grid (perhaps not at campfire temperatures) but if I did melt the metal the potato would either burn away or turn into something which resembles a lump of coal; it would not melt like the marshmallows do if left too close to the heat for too long.


I'm pretty sure that most things can exist as a solid, liquid and gas so my question is: what would I have to do to melt my potato to a liquid like i can melt a marshmallow. ...and could I do it outside my tent and with a beer?






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

Peter -   I'm currently sitting in a caf, enjoying my last of a mouthful of jacket potato here.  Very good! 

Potatoes - the source of biodegradable plastics of the futureSo, why dont potatoes melt?  This is actually quite a tricky answer, but it depends on the complexity of what were 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.  Its made up of 3 different types of atoms weve 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, its even more complicated.  You haven't got one type of substance there.  Youve got complete array of all sorts of complexity.  Youve got water there, youve got starches in there, youve sugars in there, youve 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 thats 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.


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I often grill over wood.
If grilling potatoes, I like to wrap the potatoes in foil, then throw them into the middle of the fire as soon as I light it.  Then, once the coals get down enough to grill my meat, I'll pull the potato out, and put it on the grill (at some point removing the foil to get a bit of BBQ flavor.

So, your campfire may eventually burn the potato, but it doesn't really melt it.

However, the potato is fairly moist inside so it will protect from burning fairly well.  To bring it up to an extremely high temperature, you would certainly need to dry it first.  Perhaps practising with potato chips/crisps?

Pure carbon (diamond/graphite) will melt around 3500C. 

However, your potato will be formed of hydrocarbons, carbohydrates, and starches, along with water.
I'm seeing the melting point of starch is around 256C, and the melting point of cellulose is around 500C (pdf)

So, thinking about this.  The campfire may turn much of the potato to a gas (driving off the water, and burning it).  It would even tend to burn in a frying pan.

Perhaps you could get the desired temperatures/water content in a pressure cooker that allows high temperatures and pressures.

CliffordK, Sun, 21st Oct 2012

It would be cool to turn a potatoe into instant mash though :)
zakzeus, Mon, 22nd Oct 2012

I suspect the main problem is that it burns before it melts, so the trick might be to remove all the oxygen from around it to stop it burning. David Cooper, Mon, 22nd Oct 2012

i agree that the hotter you get it, you likely would not only be drying out your potato, but also changing the chemical structure of the potato.  If you carbonize it, then the melting point climbs quickly.

However, with your pressure cooker,possibly under Nitrogen or CO2, you should be able to get hotter temperatures while preserving the moisture content, and preventing the boiling water from cooling the potato.

At around 100 PSI, you should be able to bring the potato above 300C which is above the melting point of (pure) starch.  At around 700 PSI, you could get it to 500C which would be the melting point of cellulose.

Of course, these are much greater pressures and temperatures than most home pressure cookers are rated for, so don't try it in your kitchen.

Note, graph is on a logarithmic scale in PSI-absolute, about 14.7 PSI greater than regular PSI. CliffordK, Mon, 22nd Oct 2012

If you managed to get the metal grate hot enough (in the absence of oxygen), it would melt into a puddle. On cooling, you would find a puddle of solid metal, essentially the same chemical composition as when it started.

While some organic components of a potato may melt, many other organic compounds are likely to decompose or undergo chemical reactions first (indeed, this chemical change is one goal of cooking). So your potato puddle pancake would have a somewhat different chemical composition than a raw potato. evan_au, Tue, 23rd Oct 2012

Firstly, a potato would surely consist of components (water) that is already "melted" as such, whilst a metal grill is "frozen".

And secondly, would it be any easier to liquify it if it was frozen first? bizerl, Wed, 24th Oct 2012

A potato is made of many different elements, many of them volatile, like water. Volatile basically means it evaporates (turns to a gas) at low temperature.

The first thing that happens as you heat the potato is proteins in the vegetables begin to unfold and tangle. This process is called "cooking" and is used widely by humans to make food easier to digest and to kill harmful pathogens.

As the temperature climes the water in the potato will boil off along with most of the other volatile chemicals that make the potato good to eat. What you're left with is a lump of carbon. Carbon doesn't melt at atmospheric temperature. It has a triple point of ~4,330 C or 7,820 F at 10.8MPa. In an oxidizing environment (like your campsite) however the carbon will burn, combining with atmospheric oxygen to form carbon dioxide. krool1969, Wed, 24th Oct 2012

It's easy to melt a potato, but you have to freeze it first.
Also, while a good fraction of it is water, even when you thaw it, the potato will still not be a liquid.

Aside from the water potatoes contain a lot of starch.
That has long tangled chains of molecules. To "melt" then, you would have to heat them up until they were moving fast enough to slip past each other.
But there are bonds holding the starch molecules together. Those bonds are not very strong, but there area lot of them.
If you heat the starch molecules up until they have enough energy to overcome all those weak bonds they also have enough energy to overcome a few of the stronger bonds in the molecule so it starts to break down.
That's why the molecule decomposes before it melts.
Bored chemist, Thu, 25th Oct 2012

Put that potato into absolute zero level!
At absolute zero matter loses all of it's energy, even the energy that holds the atoms together. The atoms would effectively break down to their subatomic parts, and their sub-sub-atomic parts. In the fraction of the micro-split second that zero Kelvin was reached the matter would just be gone. If you put it in absolute zero you may be able to melt it down to EMR or a blackhole!We can approach it, but we cannot reach it. ::) Spacetectonics, Fri, 14th Dec 2012

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