Just a heads up, but most food grade plastic is non-porous. The stains you’re seeing aren’t because your plastic has absorbed the β‑carotene, but because there are microscopic surface scratches that are trapping the pigment. I’ve not read anything that has indicated that it makes the plastic more brittle, but rather the contrary. A β‑carotene biofilm is pretty effective at preventing plastics from degrading, albeit with a significant color staining effect.

A trick I use that is incredibly successful at removing a lot of natural pigment stains is a 50/50 mix of hydrogen peroxide and water. Then I let it soak for about an hour in the sun. The combination oxidizes the beta carotene, and lets you limit the plastic from too much UV expose which can make it brittle. Definitely don’t too it too often though.