Tomorrow, Fourth of July, heralds one of those holidays on which we cook and eat too much food; so much that we must save some of it to reheat and eat in the days to follow. As the proportion of people who actually like to cook keeps dwindling, most people are eating leftovers more and more for their meals.
We all use microwave ovens to warm food; it is such a convenient way to re/heat dishes. But leftovers become unappetizing and dry after microwaving (not to add that it tends to come out heated in patches rather than uniformly). Why?
Microwave heats with broadband electromagnetic waves; so EVERYTHING in the oven absorbs the energy almost equally: including, especially, moisture. Food comprises mostly of water: meat, for instance, is up to 60% water. But that is not “free” water: it is chemically bonded in the food tissue structures. Microwave radiation easily expels that water, drying up the food and thus altering its properties (flavor, taste, texture, etc.)! Hence “microwave-safe” plastic lids are used to cover the food in the oven; the lids have holes in the roof to vent the released steam.
One poor remedy applied by some people to minimize water loss from microwave-heated food is to sprinkle water on it before insertion into the oven. That works poorly for three reasons:
- The amount you can sprinkle is minute compared to the water content that’s expelled by irradiation.
- Sprinkled water is loose on the food and cannot replace water that is chemically bonded in the food.
- Instead, the sprinkled water is most easily vaporized; so it does nothing for you.
When I realized what was happening, I threw away my fancy microwave covers. I put my food in a non-permeable ceramic dish and cover it with a similar bowl or plate that mates well. Thus moisture is trapped in a cavity within which there is dynamic equilibrium as the food heats up. That reduces moisture loss from the food. (You’d be surprised by the amount of vapor released when you open the cover.) The trapped steam also causes more uniform heating of the food by convection. Finally, the dynamic equilibrium ensures that my leftovers can heat up to higher temperatures and for longer times without being roasted to tasteless dryness. (I like to heat leftovers for many minutes to ensure that any bacteria etc. are dead!)
As a result the reheated dish stays succulent and retains more flavor that when I used to employ the holey covers!
There’s much amateurish guesswork on the internet suggesting that microwave heats water molecules through resonance. That is incorrect. True, resonance is a highly efficient way to amplify forces or motion; but it is not the explanation here because the frequency of microwave does NOT coincide with the resonance frequency of water. What happens is that the water molecule is more easily affected than other constituents of food because it is the most loosely bonded in the tissue structure; hence it is the most easily expelled.