  This paper presents how thermal mean field effects are incorporated
consistently in the hydrodynamical modelling of heavy-ion collisions. The
nonequilibrium correction to the distribution function resulting from a
temperature-dependent mass is obtained in a procedure which automatically
satisfies the Landau matching condition and is thermodynamically consistent.
The physics of the bulk viscosity is studied here for Boltzmann and
Bose-Einstein gases within the Chapman-Enskog and 14-moment approaches in the
relaxation time approximation. Constant and temperature-dependent masses are
considered in turn. It is shown that, in the small mass limit, both methods
lead to the same value of the ratio of the bulk viscosity over its relaxation
time. The inclusion of a temperature-dependent mass leads to the emergence of
the $\beta_\lambda$-function in that ratio, and it is of the expected
parametric form for the Boltzmann gas, while for the Bose-Einstein case it is
affected by the infrared cut-off. This suggests that the relaxation time
approximation may be too crude to obtain a reliable form of $\zeta/\tau_R$ for
gases obeying Bose-Einstein statistics.
