Two easy ways to open a vacuum sealed container.
I'd like to say I've never done it, microwaved a container with the lid on, but I have. As you might expect, I ended up with a lid that had been vacuum sealed to the container and felt impossible to remove. Hungry and at a loss for what to do next, I got to thinking. Vacuum seals are created between your tupperware and its lid when they are heated together and then cooled. As you heat your tupperware, the contents inside expand as they warm up. To release the building pressure from the contents expanding, some air is released, escaping between the lid and the container itself. Then, when your container and its contents begin to cool, it tries to equalize the pressure again. Your tupperware is rigid though, and cannot flex with the pressure necessarily, so the lid dents inward instead to account for that pressure change.
So....to release the lid, we'll have to increase temperature of the contents inside the tupperware so that they expand again, exerting a pressure force on the lid allowing it to release. Follow along for two methods of removing the vacuum sealed lid from your tupperware.
Step 1: Method 1: Heat Source
For method 1, we will be reheating the contents of the container to get them to expand again. To do this, find a pot with a diameter larger than your tupperware lid. Fill this pot with water and place on the stove to boil. Once boiled, allow your water to cool just enough so it is not actively boiling or steaming* anymore.
*NOTE: It is important to wait until the water is no longer steaming so that you do NOT get a steam burn while holding your tupperware over the water. You can also put an oven mitt over your hand if you are still worried about hot air rising from the water.
Step 2: Heat It Up
Invert your container, don't worry about the lid--remember it's sealed on! Dip your container far enough into the hot water so that just the lid is submerged. You'll feel the pressure release when the seal is broken. It took me about two minutes of soaking the lid in the hot water.
If your container is heavy, feel free to take breaks or switch hands. The contents just have to heat up, it doesn't have to be a consecutive two minutes. It will just be faster if it is.
And again, if you are worried about burning your hands from the steam coming off the water, I would recommend wearing an oven mitt.
Step 3: Open Sesame!
Place your heated container on a dish towel. This towel will contain the contents of your container if they splatter on the immediate opening.
Using your fingers, or a dish towel, pry the lid off of the container. It should be relatively easy if the seal has released. If it is not easy go back to the last step and begin heating the lid in the water bath again.
Step 4: Method 2: Patience
This method is an alternative to the first if you do not mind losing the contents of your container to spoiling.
Leave your container out at room temperature for a week to two weeks. As your container sits out on your counter, the food inside will begin to spoil and the air inside the container will begin to expand. After 1-2 weeks, the air will have expanded enough with the growth of bacteria on your food that the seal will break. Test every couple of days after one week until your lid comes off easily.
It is NOT recommended to eat this food as it has most likely spoiled. This method is simply to separate the lid and container base so that you can use them again.