Dry it fully open, every single day

Preventing mold in a compartment lunch box comes down to one habit: wash it the day you use it, then air-dry it completely with the lid open.

Mold needs moisture and food debris, and those little compartments are great at hiding both. So don’t just rinse it — actually scrub every corner, especially the seams between compartments. If the box has a rubber gasket or separate dividers, take those out and wash them too. Gaskets trap moisture like a sponge.

Dishwasher is fine for most plastic boxes, but the problem is the drying cycle. A dishwasher can leave water pooled in the corners if the box is stacked or oriented wrong. I’d hand-wash and then prop it upside down on a drying rack with the lid off or open. If you close it while any moisture is inside, you’ve basically built a mold terrarium.

If mold has already taken hold, wash with warm soapy water, then wipe down with white vinegar or a dilute bleach solution (1 tablespoon bleach per quart of water). Rinse, let it sun-dry for a few hours — sunlight kills the rest of the spores. Then start the daily dry-it-open habit.

Your lunch will taste better when the box doesn’t smell like an old basement.