@zuzig I don’t think you can judge the antioxidant capacity or potency of a cosmetic product with any foods (or any easily doable-at-home experiments).
the antioxidant function of ascorbic acid in apples is by virtue of pH denaturing the enzyme (grated apples brown more quickly because more enzyme is released compared to cut apples).
I would suggest doing a test for UVB induced oxidative stress in vitro to see If the product worked.
I do however think the apple test would make for an app(l)ealing (sorry for the pun) marketing story, however then you would need the active to be oil soluble with a partition coefficient favoring the water phase. I am not in marketing and could be wrong on that part.