I think they're just kept in whey.
(Sorry - I just thought that mature mozzarella would be old and in the whey ...)
But they are a bit slimy and unpleasant to handle (although that may simply be a male reaction, for reasons we needn't analyse).
(
Cassidy : Vissi d'arte,
Mon 5 May 2003, 9:05,
archived)