Why re-boiling milk doesn't boil over again?
When I heat milk, at some point it begins to boil over. When I reheat the same milk after it has reached boiling point once before, it doesn't boil over again, it just boils "normally". Why is this?
I haven't done extensive testing, and I can't seem to find information about this anywhere; be it in English or in my native language. But here is what has happened to me:
- I make a homemade hot beverage with low fat milk, cocoa, sugar and coffee (a pseudo-Mocha). Enough for 2 cups.
- I boil the pseudo-Mocha, and hope to pay enough attention to prevent it boiling over, but there is still the "rising bubble" phenomenon occurring.
- I pour out one cup, the rest of the liquid stays in the pan.
- A few hours later, I boil the pseudo-Mocha again.
- This time, no bubbly phenomenon occurs, just "classical" boiling.
Best Answer
If you heat food high enough, you can cause chemical changes. Letting it cool back off doesn't necessarily reverse these changes.
Typical changes include:
- deactivation of enzymes
- denaturing (breaking down) proteins
- converting sugars into more complex forms
- evaporation of part of the liquid
In general, we call this 'cooking', but there's specifically a term in English for that you're doing to the milk: scalding
I suspect that you've denatured the proteins in the milk, and proteins can create foams (such as what happens when you whip egg whites or the liquid from canned beans), but there might be other processes that have happened as well.
I also know that scalding isn't called for typically in modern cookbooks. Part of this is because today's milk is homogenized and pasteurized, and so some of the chemical changes that we get from scalding have already happened. But there are still quite a few people who insist that you need to scald milk for certain types of bread making, or you won't get the proper rise and final texture from the loaves.
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Quick Answer about "Why re-boiling milk doesn't boil over again?"
When heated close to the boiling temperature, these gasses will form bubbles and cause the milk to boil over. There will also be chemical changes, and it's entirely possible that the proteins will denature to such an extent that they will no longer form a foam as easily.Can the boiled milk be boiled again?
Some people like removing the creamy layer formed on the surface of the milk and some just like consuming it hot. There are no harmful effects in boiling the milk again and again; it wouldn't lose its nutritional value.\u201dWhat happens if we boil milk again and again?
A large number of people heat and re-heat the same milk again and again, and that too at a high temperature for a long time, thus, killing the nutrients. According to experts, milk subjected to less heating retains its nutrient value.Why does milk not boil over water?
The rising water vapour gets trapped below this layer. On further heating of milk, the water vapour expands and creates enough pressure to raise the thick creamy layer and escapes out causing the milk to spill down. In case of water, there is no such layer on its surface that traps water vapor.Can you double boil milk?
If your stove runs hot and you can't boil milk slowly enough directly over a flame, you could use the double boiler method.Why does milk spill over while boiling ? (Why milk boils over?) I English
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Answer 2
I believe there are two processes involved here, one physical and the other one chemical.
When you heat milk (or any other liquid) dissolved gasses are released. Milk can contain up to 10% dissolved gasses, mostly CO? and O?. When heated close to the boiling temperature, these gasses will form bubbles and cause the milk to boil over.
There will also be chemical changes, and it's entirely possible that the proteins will denature to such an extent that they will no longer form a foam as easily.
You could easily make an experiment to determine which effect is the greatest. Boil up some milk, in order to cause both degassing and denaturing. Let it cool down to room temperature, and sit until the next day. Whisk it, in order to reabsorb gasses, and try to boil it again. If it will boil over now, then the dissolved gasses was the dominant factor. If it doesn't, then you will have shown that the denaturing of proteins is dominant. (Then discard the milk - you probably shouldn't drink milk that's been sitting out overnight, even after boiling it.)
Let us know what you find!
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