Never try to whip egg whites in a plastic bowl either. It’s probably the one thing tupperware isn’t good for. Fat is chemically attracted to plastic, as anyone who’s ever washed out a plastic measuring cup that’s been filled with oil can attest. It can stay greasy even after several washings. In fact plastic that’s come in contact with fat never entirely gives it up, which means you start with a built-in disadvantage anytime you set out to whip egg whites in it. Some say wiping down plastic bowls with an acid like lemon juice or vinegar eliminates the fat, though I’m personally skeptical. Better to just use metal.