In the spirit of "reuse or recycle," many of us feel good when we microwave leftovers in plastic margarine tubs we've saved or when we reuse plastic grocery bags as food-storage containers. Both are ecologically sound practices, but are they safe practices?
According to United States Department of Agriculture's Meat and Poultry Hotline, the answer is No!
Cold storage food containers are intended for those uses only. They have not been tested or approved for any other use. Margarine tubs and cottage cheese containers are not heat stable, and chemicals from the plastic may migrate into the food during heating, especially in the microwave. Use only microwave safe plastic containers.
Be careful about microwaving foods in packaging materials other than ones that specifically designate use for microwaving and then only use them one time. Materials suitable for microwaving include oven bags, wax paper, and plastic wrap. However, make sure the plastic wrap doesn't touch the food and then don't reuse it! A waxed paper liner from a cereal box will work nicely as a cover or wrap for microwaving foods, but it should only be used one time.