  We extend a previous analysis concerning cosmological fluids with generalized
equations of state in order to study inflationary scenarios. In the framework
of the slow-roll approximation we find the expressions for the perturbation
parameters $\epsilon$, $\eta$ and the density perturbation spectra in terms of
the adiabatic index $\ga$ as a function of the universe scale factor. This
connection allows to find straightforwardly $\ga$ corresponding, for example,
to the simplest {\it chaotic} model and to the Harrison-Zeldovich potential and
shows its capability to be applied to more complicate situations. Finally, we
use this description to develop a new approach to the early universe dynamics,
based on a $1/N$ expansion, where $N$ is the e-fold number. To this aim, we
introduce a set of suitable dimensionless variables and show that at the
zero-th order in $1/N$, an improved slow-roll approximation is obtained.
