Discoloured Foam pillows

[LIZS]

Washed some spare foam pillows which have been stored for a few years between use and there are still yellowish patches even though they are clean. Any ideas as to how/if they can be whitened again ?

Soak them in a solution of soda crystals and scalding hot water overnight then rinse them well with a bit of vinegar in the final rinse.
Let them drip dry outside then finish them off on a radiator.
They come up nicely.