The real fix is drying the gasket, not just washing it

The best way to keep mold out of lunch box corners and gaskets is to scrub them with a small brush and dish soap, then dry them completely before you close the lid.

The corners and rubber gasket are exactly where moisture and food bits hide. That’s all mold needs. A quick rinse under the tap does nothing. So take the gasket out if you can, then get into the crevices with a bottle brush, toothbrush, or even a pipe cleaner. Hot water and dish soap work, and a 50/50 water and white vinegar rinse will help kill spores.

Then dry it. That part matters more than the washing. Shake off the water, wipe the corners with a towel, and leave the lunch box open on the counter for a few hours if you have the time. Closing it up damp is basically rolling out a welcome mat for mold.

A few extra minutes of drying saves you from scraping black spots off rubber later.