Here’s one for the chemistry, chemical engineering, and mechanical engineering graduate students out there…
What’s a good way to measure the uniformity of heating of milk in a plastic cup in a microwave oven?
Explanation: I have watched a new mother pour milk into a plastic cup with measurements on the side to determine the quantity, then pour that cold milk into a ceramic coffee mug to be heated in the microwave for 30 seconds, then pour the warm milk into a plastic sippy cup to be served to a toddler (3 items to wash). I have observed day care workers perform the same task by heating the milk in the plastic server cup (1 item to wash).
When I have inquired of mothers dirtying either 2 or 3 dishes in serving a cup of milk what their reasoning was, the answer was “It is bad to microwave food or drink in plastic.” Why? “Hot or warm liquid will pull dangerous chemicals out of the plastic.” What about the fact that they were going to then take the warm liquid and immediately dump it into the plastic? If the temperature was a problem, wouldn’t the warm milk hitting the sippy cup leach out whatever dangerous chemicals were in there just about as badly as if the milk had become warm in the microwave? Blank stare in response.
An acquaintance who works for Harvard University managing engineers building scientific experiments opined that the mothers were likely right. “The milk right up against the cup might get super hot while it is in the microwave while the milk in the center remains cold.” I pointed out that this didn’t make a lot of sense given the relatively long wavelengths of microwave cooking energy compared to the size of a sippy cup (Wikipedia says standard frequencies for microwave ovens are .9 to 2.45 GHz, resulting in wavelengths of about 5-12 inches). And wouldn’t convection and conduction within the liquid also make the heating pretty uniform? Hot spots seemed more likely when microwaving solid foods. We put some milk into a sippy cup and tried an on-the-spot experiment. Using sophisticated temperature probes (i.e., our fingers) we couldn’t discern any differences between the edge and center temperatures.
How would one construct an experiment to do this in an actual microwave oven and measure the edge and center temperatures continuously as the milk is heated? (we stuck our fingers in only once the milk was done) There are plenty of infrared temperature probes, I think, that don’t require contact with the item being measured, but I don’t know how well they would operate if stuffed into an operating microwave oven.
1) Take a series of IR photographs through the window while it’s getting zapped and rotating.
2) Wire the thing with thermocouples placing the junctions where you want to measure. Shield the readout electronics attached to the TC wires.
Microwaves destroy B12. Milk (and formula and other B12 rich foods) should ideally not be heated using a microwave as you lose the B12. Do nuke yer spinach though as stove top cooking destroys B9.
Some type of temperature probe that only measures the temperature at tip. Another subjective way is to drink the milk with a straw from different locations. I’ve found it’s not neccesarily edge vs. interior but just different parts. Heat up a glass of milk for ~75s at 1000W and drink the milk quickly with a straw and you’ll notice a difference in temperature that is enough to make it uncomfortable to drink quickly.
I would use a suspension of thermochromic beads in water (So you can see them change color – milk is too.. milky), or a solution of thermochromic ink. You can source TC beads/inks with different transition temperatures across your range of interest (Effectively between 20 ‘C and 70 ‘C). Three or four different beads will let
you capture a number of datapoints sufficient to get both spacial and temporal info.
By the way, you can easily model the scenario using a FEM package like COMSOL, if you have access to it. See for example: http://www.comsol.com/model/microwave-oven-1424
But the experiment with the beads is more fun!
Not your whole answer, but microwaving fax paper can show where the standing waves (and therefore hot spots) are, e.g.
http://maartenrutgers.org/fun/microwave/microwave.html#fax
(Does superheating water in clean mugs in a microwave imply the temperature in the mug isn’t even? I thought I remembered that but can’t find anything…)
Phil,
Couldn’t the mother just purchase one of the many bottle warmers on the market? They quickly heat a small amount of water and plastic bottles/ sipppy cups can be used, hence there is no need for three separate containers.
I’m sure you are familiar?
Edit
I meant to say the bottle warmers heat a small amount of water rapidly, thus providing the necessary heat to warm any liquid i.e. milk in a plastic bottle. They use less electricity than a microwave, no?
put a cup inside a cup
measure the temperature difference between cups
I don’t think you want any electronics on this one. I’ve seen an experiment where somebody takes a wet page of old-style fax paper and sticks it in the microwave. The paper blackens according to the hot-spots in the oven. I think the best way would be using some substance that melts at a temperature that you would still consider safe. Ideally this substance should be less opaque to MW than water (otherwise it would heat faster). Candle wax, maybe?
Andrea: I don’t think that your idea of using a solid material will answer the question. Convection and conduction within milk are very different from those processes within a solid. It is well-known that microwaving a solid food will result in some temperature non-uniformity, albeit not typically hot enough anywhere to result in burning.
Les: Thanks for the reference (“Physics inside a Microwave Oven”; http://maartenrutgers.org/fun/microwave/microwave.html ). Regarding your question about superheating of water, the end of the page that you cited talks about it: “I personally feel that boiling water in a microwave is probably the most dangerous everyday thing you can do. Since microwaves can heat water so uniformly, and since the containers we put in microwaves are often very smooth, it is not unusual that the temperature goes above the boiling point without actually boiling the water. This water is now called ‘superheated.’ A very slight disturbance can trigger the water into a roiling boil, making it practically jump out of the cup.”
(I.e., the author seems to think that the superheating is due in part to uniform heating, not a lack of uniform heating. This is consistent with http://www.animations.physics.unsw.edu.au/jw/superheating.htm talking about water, but the same article talks about uneven heating as well (presumably of solid food rather than water)).
I had a similar idea about thermochromic liquid crystals, but I’d just grab a sheet of them already printed on plastic:
http://www.hallcrest.com/ts.cfm
They are black below 25C, go through a color change from red through the spectrum and up to blue, then finally back to black at about 35C.
Cut out a piece that’s the right size to split a transparent sippy cup in half vertically, fill the cup with water cooler than 25C, then watch as the water heats up in the microwave. If there really is a sharp temperature gradient at the wall of the sippy cup you’ll see a vary narrow red through blue band at the edges of the film touching the wall. If the water heats up more uniformly the whole sheet of TLCs will go through the color change at the same time.
I’m assuming that the thin film will not greatly affect the water circulation in the cup. I suppose you could come up with a way to insert a smaller strip spanning the width of the cup but not for the entire height of the cup.
I don’t have any of this stuff on hand any more or I’d try it myself and video tape it for you.
– Rob
My engineering is a bit rusty, but a theoretical analysis of this problem would be complex and involve viscosity (low for water/milk), size of the vessel (small), temperature gradient (moderate). Convective versus conductive/diffusive mixing involves formulas such as Reynolds Number, Prandtl number, etc.
Wikipedia mentions liquid metals and engine-oils as being extreme examples (compared to water). Obviously honey, eggs or butter do not work well in microwave ovens!
In short I don’t think heating non-uniformity is an issue with milk. Prolonged exposure to containers at high temperature would be a different matter altogether (plasticizers are considered as nasty chemicals by some).
Milk should either be heated in a hot water bath or using a hot plate with a magnetic stirrer.
Lisa’s right imo.
( + Didn’t Dr. Spock teach us, that mothers are always right? )
The discussion brings up the idea to make do without take-away food,
because it is hot when put into polystyrene. Zapping only adds to it.
Being hungry’s worse, though.
“… And wouldn’t convection and conduction within the liquid also make the heating pretty uniform? …”
If there is convection, that fact alone proves that the liquid was heated unevenly.
If there is conduction, that fact alone proves that the liquid was heated unevenly.