Chocolate Science!

I melt 3 spoons of dark chocolate in microwave oven in low. It melts in 3 minutes and it's just mildly warm. I add half a spoon of milk which makes it a bit cold again. So I microwave it again and in 10 seconds it BOILS!! In 20 seconds the whole thing is burnt. WHY ???

(Same thing happens if I use almond milk instead of cow milk.)


I already know that liquids heat better than solids in microwaves because the oven emits in the water resonance frequency causing water molecules to move rapidly. However I feel like the degree in which the chocolate-milk mixture gets hot, combined with the 10sec/3min figures is disproportionate to the quantity of the milk contained in the mixture. If I put the same quantity of milk alone in the microwave and I heat it for 10 seconds it does get pretty hot...But the chocolate-milk mixture gets even hotter than that. That is the part that I don't understand.

  • $\begingroup$ You need more data points. What does milk alone do in 20 seconds? Since the microwave heats the milk, mixing it with chocolate first gives you a higher starting temperature. $\endgroup$ – Robert DiGiovanni Dec 21 '20 at 2:00
  • $\begingroup$ What is the shape of your bowl? A picture after you add the milk but before you start to microwave it again would help. $\endgroup$ – RBarryYoung Dec 21 '20 at 15:03
  • $\begingroup$ This is not seasoned advice stackexchange, granted. But for deity's sake, don't microwave chocolate. Nothing good comes out of it. Use a pan on a range, or a double boiler. Or even just microwave the water and make a makeshift double boiler with a smaller cup. $\endgroup$ – Jeffrey Dec 21 '20 at 22:41
  • $\begingroup$ @Jeffrey Unless you want to check wave length of the microwaves. Chocolate seems to be a superb material. $\endgroup$ – luk32 Dec 22 '20 at 11:34

As already pointed out, microwaves in the oven have just the right frequency to heat water molecules. But this alone does not explain why chocolate with a tiny bit of milk heats up so much quicker than a glass of milk. The key is heat capacity.

With no milk the chocolate is almost transparent to the microwaves. Even a little bit of milk makes the mixture capable of absorbing a substantial portion of the energy the microwaves bring in. This energy is then spread out through the mixture of chocolate and milk. The heat capacity of the mixture is essentially that of chocolate if there is little milk, and the specific heat capacity of chocolate is much lower than that of water or milk. Therefore it takes less energy to heat chocolate to such temperatures that it burns.

If there is too little milk, you don't catch enough of the energy of the microwaves. If there is too much milk, it increases the heat capacity and slows down the heating. Somewhere in between there is a sweet spot where the milk acts as a microwave antenna for the chocolate but does not take up a substantial portion of the total heat.

I am not commenting on what the burning of the chocolate actually means. This answer only concerns the energy transfer process that leads to temperature change at different rates. The temperature around the boiling point of milk (≈water) is not enough for combustion but is enough for other processes that change the color and flavor of the chocolate. Also, sensitivity to microwave heating is not binary; plain chocolate heats up too but less so than milk, and it also depends on the variety of chocolate in use.

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    $\begingroup$ Are you sure? It might just be that when you mix in pieces of chocolate, you're creating a bunch of nucleation sites for bubbles to form in the milk, allowing it vigorously boil much more easily. $\endgroup$ – nick012000 Dec 21 '20 at 13:41
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    $\begingroup$ @nick012000 There could be several mechanisms at play, of course, but my experience confirms that the amount of water has that effect: the quickest heating is at a smallish amount. And vigorous boiling is only possible if there is enough heat to bring the milk to boiling temperature, which takes more time if there is more milk. I'm not saying other mechanisms are impossible, just that I find this one most likely. $\endgroup$ – Joonas Ilmavirta Dec 21 '20 at 13:49
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    $\begingroup$ Note: I was spreading incorrect "folk science" by repeating the misconception that microwave heating is resonant: see physics.stackexchange.com/q/71834, physics.stackexchange.com/q/38251, physics.stackexchange.com/q/385231. Thanks to MartinPrikryl for correcting me. $\endgroup$ – rob Dec 21 '20 at 15:25
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    $\begingroup$ @abligh: Not as in "combust", but in the cooking sense of the chocolate hardening up and darkening, and tasting bad. This can happen even while it's wet, I think. (Also related is "seizing": Chocolate is a delicate emulsion of mostly fats, and water itself can cause it to harden up into a grainy non-smooth mess if you add a small amount while it's hot. food24.com/…. Google for burnt chocolate finds mostly articles about seized chocolate and how to fix it; terminology is used casually.) $\endgroup$ – Peter Cordes Dec 21 '20 at 18:46
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    $\begingroup$ @abligh Indeed, you will see separation of the fats that the cocoa butter consists of somewhere between $46-54\,^\circ \mathrm C$. And actual melting of these fats and initiation of burning of the cocoa dry-particles can happen after $93\,^\circ \mathrm C$. Any chocolatier will thus always avoid temperatures above $45\,^\circ \mathrm C$ during his chocolate tempering. acselementsofchocolate.typepad.com/elements_of_chocolate/… $\endgroup$ – Steeven Dec 21 '20 at 21:17

I'm not an expert in this field and definitely not a good cook but I think when you heat up your milk, after some point it's proteins cluster and sticks to your cup because there are imperfections in the material even if they are not visible. (that's why when people boil milk, they add small amount of water for it to serve as an insulating layer between the pan and milk) Even in case of chocolate, if you heat it up to 50°C it will burn.


Very good points were made in the answers. However the point of my question and the thing that baffled me is that the milk alone was not getting nearly as hot as the chocolate-milk mixture.

What I think happens is that when the milk is being heated alone its heat is being quickly dissipated into the air because the mass of the milk is to little to hold it. Therefore the heat from its vibration escapes the liquid very fast.

However when mixed with the chocolate the mixture mass is 10 times higher. And the chocolate due to its higher viscosity does not release heat into the air that quickly. Therefore it acts as an insulator and the heat produced by the vibrating milk accumulates in the chocolate.

Also I think a key factor is bubbles. When milk boils it forms bubbles which release a lot of heat helping the liquid to cool down. But chocolate (thick as it is) forms bubbles a lot harder. By the time it does form bubbles it has already reached a burning point.

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    $\begingroup$ I addressed that very point of milk alone not heating as much in my answer. (I don't think the heat is substantially dissipated into the air unless the milk is boiling hot.) The crucial quantity is not mass but heat capacity, and those two have a very different ratio for chocolate and milk. $\endgroup$ – Joonas Ilmavirta Dec 21 '20 at 12:38
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    $\begingroup$ You re right. Upon reading your answer more carefully I see that you indeed covered that point $\endgroup$ – Anonymous Dec 21 '20 at 13:40

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