Heating frozen meals question.

[MontyVeda]

The issue with frozen food in the microwave is that it might not properly reheat the centre of the food if it's particularly thick and/or wide. ...

that's why i use the defrost setting... when it's not microwaving the heat is conducting through the food

 

[presta]

But there is a benefit to gentle defrosting at room temperature

The benefit to defrosting at room temperature is food poisoning, do it in the fridge. The reason fridges exist in the first place is that bacteria grow fast at room temperature

that's why i use the defrost setting... when it's not microwaving the heat is conducting through the food

Same here. The 30 minutes it takes to cook brown rice is plenty to thaw and heat a portion of curry on defrost. It tends to boil and spoil the flavour if you're not careful, which is why I bought a food thermometer so that I can get it as hot as possible without boiling.

 

[lazybloke]

The benefit to defrosting at room temperature is food poisoning, do it in the fridge. The reason fridges exist in the first place is that bacteria grow fast at room temperature

If someone cares enough about food quality to avoid/minimise microwaving, they probably have a fairly good grasp of food poisoning risks.

Personally, I find it far too slow defrosting anything in the fridge. Far quicker to do it at room temperature; once the food has melted through, it can then go in the fridge until I'm ready to reheat and serve it.

TL;DR Defrosting at room temperature doesn't mean letting it reach room temperature.

 

[presta]

If someone cares enough about food quality to avoid/minimise microwaving, they probably have a fairly good grasp of food poisoning risks.

Personally, I find it far too slow defrosting anything in the fridge. Far quicker to do it at room temperature; once the food has melted through, it can then go in the fridge until I'm ready to reheat and serve it.

TL;DR Defrosting at room temperature doesn't mean letting it reach room temperature.

If it's a large, slow thawing piece, the risk is that the outside will reach room temperature before the middle thaws. The larger the food, and the faster you thaw it, the greater the temperature gradient between centre and surface.

 

[chathomas]

If it's a large, slow thawing piece, the risk is that the outside will reach room temperature before the middle thaws. The larger the food, and the faster you thaw it, the greater the temperature gradient between centre and surface.

That explanation makes perfect sense! I've always put frozen food straight in the microwave oven, sometimes stirring it and putting it back in to even out the heat. But I never thought about why this might be necessary. Now I understand a lot better!

 

[presta]

That explanation makes perfect sense! I've always put frozen food straight in the microwave oven, sometimes stirring it and putting it back in to even out the heat. But I never thought about why this might be necessary. Now I understand a lot better!

Microwaves heat quicker than an oven because they heat the food directly instead of by convection and radiation, but they also only heat the outside layer of the food, so the same applies: if the food is large enough the outside will warm quicker than the middle. I never cook or thaw anything in the microwave unless it's a liquid that can be stirred.

 

[Electric_Andy]

I rarely buy frozen food, but I have always defrosted it first before cooking if the instructions say so. I think the reason why they sometimes state "defrost first" is because maybe there is too much variation in people's microwaves/ovens so they are just covering their back. If it's defrosted, 4 mins is a safe bet that it'll be heated properly. If frozen, 20 minutes on defrost setting may differ between appliances so they can't be sure that you're eating it piping hot?

 

[presta]

There are several effects at work:

1. Thermal resistance and heat capacity combine to make a low-pass filter that slows down the conduction of heat from the outside to the middle. All the energy required to warm the middle has to pass through the resistance of the outside layers, so the faster you try to make the temperature change the steeper the temperature gradient will be. This is the reason why it's possible to cook the meringue on the outside of a Baked Alaska without melting the ice cream inside. The surface area for absorbing heat is proportional to the square of size, but the volume that needs heating is proportional to the cube, so the bigger the object the slower it will warm up. A big turkey can take days, a pea a minute or two.

2. Microwaves are absorbed by the food, that's why it gets hot, but that means the more food the microwave has already passed though the less unabsorbed energy it has left remaining. If the microwaves got all the way to the centre of the food unattenuated they wouldn't have heated anything on the way. The depth to which they penetrate varies according to the type of food, but it's typically 1-2cm, so anything beyond that is dependent on thermal conduction, and the rate that can happen is still limited as in 1 above. If I put a 'block' of frozen stew in the microwave, the outside 15mm thaws, but then nothing much else happens quickly, other than the outside boiling, unless I scrape the thawed layer away.

3. If you put an electrical conductor in an electromagnetic field it will distort the field, so even if the field was evenly distributed to start with, it won't still be by the time you've put the food in it. Have you ever noticed that if you try to thaw a chicken breast in the microwave you get small spots that are cooked whilst the rest is still cold or frozen? The rotating turntable is supposed to prevent them, but it doesn't because the interference pattern that caused the hotspots rotates with the chicken breast that's creating the interference pattern.

If you want food to heat up evenly your options are: do it slowly, do it in small pieces, or stir it, and if the heating isn't even enough, you end up with one bit too hot, another bit too cold, or both.

(I stopped using Tupperware for thawing & reheating in the microwave because the 'tide mark' around the top could become so superheated that it would melt the plastic, and embed itself in the bowl.)

 

[lazybloke]

As soon as i read 'low pass filter' i knew it was an excellent post @presta , but the interference pattern rotates too? What? How?
The turntable and rollers are glass and plastic.... you mean the food becomes electrically conductive? And hence creates its own interference?

Or are you super-heating your food to create plasma?! 🔥

 

[presta]

the food becomes electrically conductive

It doesn't become conductive, it is conductive. Not as good as copper, but if you plug yourself into the mains it kills you because meat conducts a current. That said, it was my mistake to suggest it has to be a conductor, insulators distort electromagnetic fields too: that's how a camera lens focuses light, bending of the wave where its velocity changes.

I remember putting a bowl of tomato soup in once, and when I took it out the bubbles where it had been boiling had formed a perfect standing wave pattern on the surface, like a Chladni plate.

 

[lazybloke]

More nice science there but i'm still struggling.
Can understand say, thin parts of food that will heat quicker than 'thicker' parts, but curious how a perfect (or even imperfect) standing wave will rotate with the food....
I need a diagram and video

 

[fossyant]

It's all got scientific. Stick food in for a few minutes, stop, stir, stick back in and repeat. I'll sometimes add water if its a home made meal (i.e. left over sunday lunch).