How do you remove stains from plastic containers used to reheat foods (like tomato sauce) in the microwave?
These stains are permanent. Nothing will remove them, as they are now part of the plastic, which has been modified on a molecular level.
This change is caused by the high temperatures experienced in isolated areas on the perimeters of containers used to heat or re-heat food in a microwave oven. The worst part is not the tomato sauce becoming part of the container, but the plastic container becoming part of the food!
There are ongoing studies on the effects of chemicals that mimic estrogen, which are found in most plastic packaging and plastic containers. Researchers are currently trying to draw the link between this, increased in infertility in men, and increased breast cancer in women, caused by these xenoestrogens. Yikes!
Not to fear monger, but it might be best to err on the side of caution by tossing those plastic containers and only use glass or ceramic dishes in the microwave.