Vinegar and sunlight beat anything else

The best way to kill mildew smell in a lunch box is a white vinegar wipe, followed by a thorough sun dry.

Soap and water gets the crumbs out, but it won’t kill the mold that’s actually causing the smell. White vinegar will. Dampen a cloth with undiluted white vinegar and wipe down the whole inside, including the corners and the lid seal. Let it sit for 10–15 minutes, then rinse with water.

After that, the key step: put the lunch box in direct sunlight for a few hours. UV light kills mold spores, and drying it out completely stops mildew from coming back. This works for plastic and fabric-lined boxes. A baking soda paste also works as a backup if you don’t have vinegar, but vinegar is faster.

If the lunch box is heavily moldy and made of something porous like fabric or cardboard, just throw it out. Not worth the fight.

And from now on, leave the lid open when you store it. Mildew needs moisture and darkness — take those away and you won’t have this problem again.