A tomato sauce stain soaked into a plastic container can largely be removed by leaving the container in direct sunlight for a few hours, or by scrubbing it with a paste of baking soda and water; the ultraviolet in sunlight chemically breaks down the stain's pigments and noticeably lightens the color.
Why does this stain sink in so easily?
The pigment in tomatoes and tomato paste, called lycopene, is fat-soluble and works its way easily into a plastic's porous surface, which is why containers that touch tomato-based sauces stain far faster and more permanently than glass or stainless steel does. Pouring a hot sauce into plastic while it is still hot speeds up that penetration too, since heat slightly widens the plastic's pores.
Sauces higher in oil help lycopene work its way deeper into the plastic, which is why an oily tomato sauce can leave a more stubborn stain than plain tomato juice would.
How do you remove the stain?
A stained container, washed first and then left on a sunny windowsill for a few hours, ideally half a day, will gradually have its stain lightened by the sunlight — a method many prefer for food-contact containers since it needs no chemicals. A thick paste of baking soda and a little water, spread over the stain and left for fifteen to twenty minutes before scrubbing with a sponge, noticeably reduces the stain through its mild abrasive action.
For stubborn stains, a little liquid dish soap mixed in with the baking soda gives extra help breaking down oil-based residue; letting that mixture sit on the container for a while before rinsing is the recommended approach.
How do you prevent staining next time?
Wiping the inside of a container with a thin layer of oil before adding tomato-based food reduces direct contact between the lycopene and the plastic, considerably delaying staining. Letting a hot sauce cool a little before pouring it into a container also cuts down on the heat that widens the plastic's pores, helping any stain that does form be less permanent.
Are glass or stainless steel containers a solution?
Glass and stainless steel do not have a porous surface, so pigments like lycopene barely penetrate them at all, making these materials more practical in the long run for kitchens that store tomato-based food often. Their one drawback compared with plastic is being heavier and breakable, so they may not be a full replacement in every kitchen.
Setting aside one plastic container just for tomato-based dishes is another simple fix, keeping any staining contained to a single piece rather than spreading across the whole set. Dark-colored plastic containers show stains less visibly than light-colored ones; deliberately setting one dark container aside for tomato-based dishes will not prevent staining but does make it far less noticeable, and that alone is often reason enough to make the switch.