  We show that mixing transformations for Dirac fields arise as a consequence
of the non-trivial interplay between rotations and Bogoliubov transformations
at level of ladder operators. Indeed the non-commutativity between the
algebraic generators of such transformations turns out to be responsible of the
unitary inequivalence of the flavor and mass representations and of the
associated vacuum structure. A possible thermodynamic interpretation is also
investigated.
