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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.)

UPDATE:

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.

Anonymous
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3 Answers3

22

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.

Joonas Ilmavirta
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2

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.

Monopole
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0

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.

Anonymous
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