Dear Home Ec 101,
Why is it sometimes when I heat foods in the microwave the bowl is screaming hot and the food is still a block of ice?
Scalded in Scranton*
Here on Home Ec 101 I’ve written previously about how microwaves work, if you missed it the first time, here’s the short version.
Think of your microwave as a box built to hold in waves of energy. These microwaves bounce all over the inside of this box when the appliance is running. When items are placed in the microwave three things can happen: the waves can either pass through object, bounce off the object, or be absorbed by the object. When conditions in the microwave oven are perfect the dish is pretty much ignored by the microwaves which reach food and excite the water molecules which heats the food.
What do we know about real life and optimal conditions? I know, I know! The two rarely occur at the same time outside of a laboratory. You don’t live in a lab, do you?
Of course you don’t, your bowl is hot and food is cold. Why?
In your case the microwaves are being absorbed by the dish which is pretty much shielding your food from the microwaves. If you used a plate, the shielding effect would be less noticeable, as less of the surface area of the food would be blocked by microwave absorbing material.
So, what kind of dishware can be used in the microwave?
Skip metal which may cause arcing or the buildup of charge that jumps from one point to another. It’s like lightning on a domestic scale.
Glassware should be safe, but it isn’t always. The best test is to check the manufacturer’s suggested use.
Engineers and other sticklers, I know I’m using some terms a little loosely, we’re just working on basic concepts today.
It’s important to remember that not all ceramic is created equally. Dry, unglazed ceramic is usually microwave safe, but there is a caveat. The material is porous and can absorb water. When this happens the microwaves heat the water in the ceramic dish. If the water gets hot enough it can cause uneven thermal expansion (some things get bigger when they are heated) which can break your dish.
When it comes to ceramic glazes a few other factors come into play. Some ceramic glazes may may contain dangerous chemicals like lead that can leach into food. These containers should not be labelled food safe. Caveat emptor is the rule of the day. Most retail establishments would not knowingly sell consumers items not intended for food use without a clear label. However it has happened in the past. Use extra caution with decorative bowls, that label not for food use is VERY important.
With food safe glazes, no dangerous chemicals should leach into your food. That doesn’t mean your dish should be used in the microwave. If the bowl gets hot, before the food, the microwaves are exciting molecules in the glaze. While this may not be dangerous from a chemical standpoint, burns still hurt. Throw in the the fact that you’re not getting the most efficient use of your microwave and I’d quit using that container to reheat food.
Plastics are a whole ‘nother ball of wax. Personally, I sidestep the issue and stick to microwave safe glass and ceramic. Your mileage may vary, but please ensure the plastic used is rated microwave safe. Once the plastic-ware shows signs of wear, it should be recycled.
*Today’s question actually came from a tweet by @TJeffrey: Don’t you hate it when you open the microwave and pull out a bowl that’s 1,000 degrees, but the food inside is ice cold? I just couldn’t think of a good synonym for hot or burnt that went with a local town. It happens.
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