Forgot your password?
typodupeerror
Idle Science

The Physics of Hot Pockets 222

Posted by samzenpus
from the when-you-absolutely-have-to-eat dept.
StartsWithABang (3485481) writes "You've all had the experience: you're all excited to microwave your favorite snack. So you pull it out of the freezer, you throw it in, and you let it rip. A minute or two later, you pull it out, and there it is: boiling on the outside, frozen in the middle. Finally, a physicist answers the eternal question: why do microwaved foods remain frozen on the inside when they reach scalding temperatures on the outskirts? Starts With A Bang explains the whole phenomenon. Bonus for the crisping sleeve explanation!"
This discussion has been archived. No new comments can be posted.

The Physics of Hot Pockets

Comments Filter:
  • by Bin_jammin (684517) <Binjammin@gmail.com> on Friday May 16, 2014 @01:20AM (#47015279)
    a microwave with more than 300 watts of power. I've never had the issue of hot outside/cold inside, my problems have always been of the hot outside/nuclear inferno/solar coronal mass ejection on the inside variety, regardless of where I've microwaved them. I don't even follow the instructions on the package very closely, just pull it out of the wrapper, put it in the sleeve, toss it in, slap the door shut, 3 or so minutes, and out comes an external breading hot to the touch with napalm in the center. Maybe there are just a lot of broken microwaves, or even more likely, people that don't know how to use them properly?
  • by toejam13 (958243) on Friday May 16, 2014 @02:14AM (#47015423)

    Agreed. I rarely microwave my food with a power duty cycle level higher than 70%. You need those few seconds of rest for the heat to evenly distribute inside your food.

    For frozen stuff, I usually set it to 50% so that the outside doesn't overcook. Takes longer, but not as long as a regular oven.

  • by TapeCutter (624760) on Friday May 16, 2014 @05:01AM (#47015787) Journal
    Hmmm, my MW is 1100 watts. For frozen meat pies (Australia's national dish), heat for 1min, stand for 5min, heat for 1min, stand for 2min, it comes out like a warm pie from the bakery. However if I heat for 2min straight, the outside is hot, the centre is frozen, and the pastry has turned into something that would be suitable for re-treading tyres. Thermal inertia explains the frozen centre, but I'm neither a cook or chemist so I have no idea why the pastry turns to rubber?
  • Re:Microwave trays (Score:5, Interesting)

    by Anonymous Coward on Friday May 16, 2014 @06:07AM (#47015937)

    Of course there are microwaves made with competent stirrers and well-placed feeds. Panasonic makes microwave ovens that feature both (the feed is at the bottom of the cavity), so no rotation is necessary. They also use variable fast pulse width modulation to drive the magnetron, resulting in a smooth output power, thus avoiding another common contribution made by on-off duty cycles to burnt skin / frozen middle problems especially at lower power settings. (Triac controlled magnetrons can only go on and off; most PWM microwave ovens not driven by an inverter supply do a cycle on the order of 30 seconds.) There is work on continuous phase shifting, which will avoid heating the same islands in food even in pessimal cases (like poorly conducting food put in the centre of a rotating tray). Finally, there is still substantial research going into probes inserted into food so as to provide feedback to the driver logic (dynamically or for capture into programs which take food type mass as variables) and whether non-invasive probes can provide useful dynamic feedback every time the oven is in use.

    The main problem is that the cheapest microwave ovens are just good enough to follow recipes that call for some number of seconds on the high setting while being unreliable at other settings, and the food industry targets its instructions accordingly. This is a global problem, not unique to the USA. However, as large-cavity combination microwave/grill/convection ovens become more popular in densely populated areas (why waste space having two or three ovens? why not coat your microwave with pyrolytic surfaces that you clean by simply baking or roasting something? why not cook with microwaves and brown with the grill simultaneously?) this is likely to change faster than patents expire, especially as several key manufacturers (Panasonic, GE) will not be cannibalizing conventional oven products. The critical path and most visible extra cost to the first time buyer is mainly in the design of trays and dishes which work well under arbitrary conditions in a combo oven, and avoiding damage when someone uses the wrong tray, dish or tool for a given programme.

  • Re:Microwave trays (Score:3, Interesting)

    by jasonataylor (787485) <jasontaylorNO@SPAMrcn.com> on Friday May 16, 2014 @01:04PM (#47018835) Homepage
    Anonymous Coward wrote,

    Of course there are microwaves made with competent stirrers and well-placed feeds. ... The main problem is that the cheapest microwave ovens are just good enough to follow recipes that call for some number of seconds on the high setting while being unreliable at other settings, and the food industry targets its instructions accordingly. This is a global problem, not unique to the USA.

    I think your comment is misleading, for it incorrectly implies that the main problem is cheap microwave ovens and directions to "the masses" about how to use them on food packages. In my opinion, this is wrong for the following reasons.

    Firstly, the stirrer technology is you refer to is just a some rotating metal near the magnetron. It is more than 20 years old. By and large, it sucks. That's why it has been replaced by the rotating carousel, which is far superior and why you cannot, actually, buy it easily today in a new microwave-capable oven unless rotation is ruled out for a different reason. However, field unevenness in no way is the main cause of the problems discussed in the article, which is not one-sided heating, as can be the case in non-rotating ovens, but that heating is concentrated in already defrosted areas and food edges/corners.

    In other words, even with a perfect EM radiative bath the edges and corners of a hotpocket will still burn or be overcooked if heated rapidly enough while the center is still frozen. That's why even if you buy the most expensive microwave oven on the planet, it will still have this same problem of producing food that is overcooked on the edges and the corners, which experience the highest EM radiation. The cause is not any unevenness of EM modes in an unloaded oven cavity. Rather, it is the shielding effect of food and the relatively small field skin depths to food size ratios.

    Your comment about pyrolytic surfaces is interesting, but also misleading, for it implies that convection microwave ovens solve the problems discussed in the blog post. The problem is that microwave ovens use steam cooling of the corners, which boil and splatter food all over the oven as the steam turns things like beans into tiny bombs. Even with simultaneously convective and microwave ovens, of which I own one, one still has a frozen center burnt corner issue, since convective heating is even worse at edge heating. The reason is of course that the convective oven relies upon thermal conduction, which is entirely skin heating, although in truth one gets some infrared radiation which can penetrate a little bit.

    You asked,

    why not cook with microwaves and brown with the grill simultaneously?

    Great question. Ceramic would work for that, but it is expensive. Traditional stainless steel grills are conductive, so they would reflect the EM radiation back into the magnetron, which will shorten it's life. So here is where your comment is spot on; it is indeed a cost and consumer ignorance issue, although grills have other issue I won't go into here. Regardless, dual mode ovens do not fully fix the underlying issue anyway, which, again, is over the overheating of food portion edges and corners.

    The primary reason one probably sees less burning in dual-mode ovens may come as a surprise. This just my hunch, but, in my opinion, it is primarily because dual-mode microwave/convection ovens tend to have a larger cavity. The FCC regulates how much leakage can occur in a microwave oven, and there is more cavity area. So larger ovens have to have less maximum magnetron power, due to the FCC. Therefore, dual mode ovens tend to use less magnetron power, which gives more time for conductive heating to defrost the food centers. Another alternative reason has to due with the amperage limitations of most nema (or other) sockets. If there is power going to a heating coil, that's less power for the magnetron. Same effect, however; longer time to cook = more time to thaw cent

The economy depends about as much on economists as the weather does on weather forecasters. -- Jean-Paul Kauffmann

Working...