h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How come sometimes when i put a mug of milk in the microwave it heats nicely and does nothing other than that, but other times the milk decides it would rather not be sat in a mug so it expands and begins pouring over the edge of the mug?

Microwaves operate on the principle of vibrating the H-O bond, found primarily in water and fats. This creates friction, hence heat Milk is an emulsion of fats, meaning it consists of lots of tiny fat globules (I love the word 'globules', don't you?) suspended in a watery base.

I'd guess that the reason that your milk suddenly gets explosive (tea and coffee have simlarly unpredictable problems) is due to the sympathetic knock-on effect of lots of fat molecule chains (fats are shaped like a comb, with a backbone and long side-branches, which can twist round to face the same way) vibrating as one. If lots of molecules start vibrating in synch, then inter-molecular forces (hydrogen bonding, in the most part) will induce neighbouring molecules to do the same. This then starts to spiral out of control.

The whole process is subject to chaos theory and unpredicability, which is why it doesn't happen every time. If we could predict what individual molecules would do, we'd have built a time machine by now.

B

Well, this is the way the milk boils. May be not every time it reaches the melting point in your microwave?

Sorry, not "melting point" but boiling.

It's all down to superheated liquids.
http://www.phys.unsw.edu.au/~jw/superheating.html

Not quite in the case described above in the first post. There is a difference between having a mug of milk boil out over the sides of a mug and a liquid vapourising from superheating.

I have been trying to remember the exact technical reasons but it is more to do with two phase mixtures i.e. liquid and vapour. Sometimes as the liquid boils the vapour does not disengage as quickly as it could and it pulls liquid up with it forming bubbles and foam that increases in void and overall volume.

I would then hazard a guess that the milk is able to heat up far quiker as you are essentially putting the same amount of heat energy into a smaller bulk volume and thus the temperature will rise more quickly.

Doing Chem Eng stuff on a Sunday - what is the world coming to!

The water doesn't vaporise - it boils explosively. It's happened to me occasionally when I've had to make tea in the microwave (oh the shame) at work. The water is perfectly still when you take it out of the microwave, but suddenly boils when you drop the tea bag in. What you're left with is half a cup of water on the counter and half a cup of water in the cup.

The phenomenon is documented as a warning in most (if not all) new microwave manuals these days.

I think it's because the water heats up uniformly throughout the container, and then the top layer cools, and the hotter water underneath rises, pushing the cooler water at the top over the sides.

Sorry, Pedro I disagree on that idea. If you have ever boiled milk in a saucepan it will boil over as well. As will cooking pasta or rice. Because you are putting starch into the water it is changing the properties of the liquid and making it more likely to create a multiphase 'liquid'.

I will see if I can drag out any info from my text books at work.

I put my bedtime cup of milk into the microwave in the same cup each time, and cook for 2 mins exactly. When I remove it, sometimes the handle is too hot for me to grasp, yet other times, it's ok and bearable to hold.

Always there is a skin on the top of the milk, which I remove with the base of the spoon before I add my chocolate powder.

Never has my milk boiled over though.

Perhaps you have a faulty microwave, it must be an awful mess to have to keep cleaning up

1st off, it's not O-H bonds which vibrate when you heat with microwaves. What you're actually doing is causing the molecules to move with respect to each other. The energy of the microwaves isn't high enough to cause molecular bond vibration.


Alfster, what if it were just superheating, but the fact that the milk contains fats, carbohydrates, etc. instead of just boiling like water would, it foams?

Is this 'problem' similar to when I melt butter in a small bowl in the microwave? (either for or Noggin's garlic bread). Sometimes the butter just melts and sizzles a bit on the sides of the bowl. Other times it totally geysers up and shoots butter all over the place. And it is always put in at the same temperature setting for the same amount of time. And it is always butter taken out of the fridge, so the butter is also always the same temperature.


az

Superheating is heating a liquid to above its boiling point that it would have at, say, atmospheric pressure. You now have latent heat of vapourisation being used as sensible heat. That is energy that would normal evaporate the liquid just raising the temperature. This quiet quite specific conditions to avoid causesing a nucleation site which would causes proper boiling. The fact that the the milk contains fats etc is not relevant. In fact, one of the most common injuries is that of people being serioulsy scalded by superheeated custard which is milk plus other powdered stuff in it. The very fact that it is gloopy is thatit sticks really nicely to skin once it is ejected from the smooth container it was in. What happens is that the liquid spontaneously turns to vapour ejecting the liquid above it outwards. When milk boils over it is quite a slow process in comparison. You can see it starting boil and foam over a few seconds. This would not happen by superheating. Also, you need a nucleation point to form and that does not happen on its own which is why putting a spoon into the cup after removing it from a microwave will seed the liquid and give it the nucleation points to boil off. THe milk is foaming up on its own in the microwave in these cases and I can pretty much guarante that the mugs I use and has hadthis happen in them have got enough nucleation points in to get boiling going.

I cannot remember hearing someone removing a mug of milk from a microwave for it to explode like that. It probably can happen but normally people talk about custard superheating. As more milk is heated in microwaves than custard we would have heard more about milk scalding people.

Milk seems capable of foaming without superheating, as when heated vigorously in a pan, where boiling is underway before the foaming starts, rather than it being a sudden process.

Would *any* milk foam if heated for long after it was all at boiling point? Possibly there's a case of the timing being borderline depending on the quantity of the milk - a little less milk than usual will heat faster, and could continue having energy dumped into it for longer after it was already boiling.

Is it not likely to be effected by the viscosity? Seems to me that slower convection is more likely to result in the lower parts being heated above boiling point, and thus eventually quite violent convection.

So like water in saucepans boiling over, but in a shorter time frame?

Thoughts?

Superheating in this case is not relevant. The faoming occurs precisely due to viscosity and surface tension. That is what forms a bubble.



You are getting your heat transfer theory a bit skewed here. A little less milk heats up at exactly the same rate as a lot of milk. It is the fact that there is only a little milk that makes the temperature of that milk rise quickly.

1kg of a liquid will require a certain amount of energy to raise it by 1degreeC. Its heat capacity. So, for a certain amount of heat the smaller the mass of liquid the quicker it will reach boiling. This is called sensible heat.

Once something is boiling that liquid will no longer hold the heat. THe heat will be used as latent heat. Heat that is used to transform a substance from one phase to another in this case from a liquid to a gas.

At this point under constant pressure conditions tyhe bulk liquid will remain at the same temperature as the heat is being used to disengage molecules of the substance from the liquid surface into the gas phase above.

What will make a difference if the amount of heat you pup inot the system at one time and that is all down to the wattage of your heat supply and the heat transfer coefficient of your container and latent heat of your substance. If you have aheat source that can pump in enough heat to vapourise a lot of liquid you will start to get foaming(the more viscous the liquid the more foaming) as the liquid will not have time to break its surface tension and drop under gravity back into the bulk of the liquid before the net bit of gas catches it.

Within a mass transfer column where you have got liquid spray falling down a tube(sometimes pacing is used to spread the liquid thinnly) and a gas going upwards(a portion of which will be absorbed by the liquid.

There will be a point at which the velocity of the gas going upwards is enough to counteract the mass and velocity of the liquid falling and the tube will flood. i.e. the liquid is held at a certain height by the gas until the mass of liquid is enough to over come the force of the gas upwards.

This is essentially what is happening when a viscous substance boils in a pan.But the overiding factor is the surface tension of the liquid.

Now I know why(sort of) my milk exploded when I put a spoonfull of ovaltine in it the other day. It was most perplexing

>>"A little less milk heats up at exactly the same rate as a lot of milk. It is the fact that there is only a little milk that makes the temperature of that milk rise quickly."

Isn't that rather what I said? Less milk + same power/time input -> milk reaches boiling point more quickly, and so spends longer actually boiling and generating bubbles.

If someone microwaved their milk for a fixed amount of time, and it usually just-about came to the boil, (ie the microwave stopped at or very shortly after the liquid was boiling, using a little less milk could result in that amount of milk having a relatively rather longer period of energy input after it reached boiling point, so bubbles of steam will be formed for an extended period.

It's possible that even a fairly short period of high-rate energy input will result in milk boiling over in a microwave in exactly the same way it does with vigorous heating in a pan.

One test could be to immediately set the microwave off for an extra heating period in the case when the milk doesn't froth up, and see if it froths up (in which case the original issue may come down to one of borderline timing), or if it doesn't froth up (in which case the issue may be related to subtle variation betwen differnet batches of milk).

(As a side-issue, how much would microwave output power vary with mains voltage - would there be a linear relationship, or something more complicated)

milk reaches boiling point more quickly, and so spends longer actually boiling and generating bubbles.>

Not quite old chap! By the very fact that it is not boiling will mean that it will not produce bubbles.



Would you believe that I actually work in a dairy that processes 1.5 million litres of milk per day?

There are subtle differences in milk qualitry but not enough to tip it over the edge (as it were)

>>"Not quite old chap! By the very fact that it is not boiling will mean that it will not produce bubbles."

???
Where did I say anything about the milk not being boiling in the case where it froths up - I was talking about a change in quantity possibly moving the situation from one where a portion of liquid just reached boiling point (and was possibly heatred for a very short while afterwards) to one where a smaller amount of liquid given the same time/power input would end up having a period of sustained boiling longer than the larger amount of liquid would have.

However, it seems to be my day for being misunderstood, so I'll bow out of this conversation before I consfuse anyone else.