These situations happen a lot around me since I always bring my lunch to the school. It is not just plastic warp, but my plastic container lid too. Unlike Why does plastic wrap grow, then shrink, then grow again when microwaved?, it happens even when I am only microwaving my lunch, which doesn't contain a lot of liquid. It also happened when I microwaved my steaks, which are partly submerged in meat juices and other sauces, in a glass bowl with a plastic wrap on the top. The plastic warp "wrapped" against the steaks, but when I started to remove the wrap, there was no hot gas shooting out.
I am not getting the answer the aforementioned question provided since I don't understand how can the air inside the microwave and inside the container decrease so rapidly, and the "superheating" they mentioned is only about liquid at around boiling temperature. So could any one help me out here?
Best Answer
All food contains some water - it's mostly the water molecules that absorb the microwave energy.
As the water heats up, some will evaporate. Since there is already some air in the space above the food, the addition of water vapor will increase the pressure - and this will result in some "venting", with part of the air-vapor mixture escaping from the container.
Once you stop the microwave, the production of vapor stops (at least it slows down a lot). The air above the food cools down, leading to condensation of the vapor. But since a lot of the air escaped during the heating phase (see previous paragraph), now you have less volume of gas left - and the gas that is left will be cooler and occupy a smaller volume. Both these thinks lead to a "shrinking down" of the plastic cover.