Felix Klempka is right about lemon juice.  May I repeat my MMD post of
June 1999?

"The late Lionel Box, Bechstein's top man in London between the wars
and Boesendorfer's in the 70s, taught me to clean and polish white key
tops with the traditional paste made of pure lemon juice and "whiting"
i.e.  fine gypsum plaster powder.

This can be done with the keys in place quite easily if necessary. Use
a finger covered in a soft rag dipped into the paste. For very yellowed
keys apply a thin coating and leave for a few minutes. Rub gently but
firmly until the natural colour is regained, remembering that real
ivory is never very white. Finish by buffing with a clean, dry soft
rag.  No wax or other polishes please -- pianists prefer the velvety
feel of ivory finished this way, and the feel seems to improve with use
(natural oils from the fingertips?) and very occasional cleaning by
this method.

The citric acid does most of the bleaching and the whiting acts as a
polish. There is little danger of wearing away the tops or of detaching
them, and the whiting disguises any fine joint lines.

To keep keys white and moths at bay leave the fall open whenever
possible -- sunlight whitens and it's easier to vacuum out dust from
a key bed than to polish keys.

By the way, in Britain key tops were traditionally stuck on with
a paste made of hot glue thickened with whiting -- i.e. gesso -- to
ensure overall bonding and thus prevent finger noise."

Patrick Handscombe
Wivenhoe, Essex, UK