  We investigate the general properties of Unified Dark Matter (UDM) fluid
models where the pressure and the energy density are linked by a barotropic
equation of state (EoS) $p = p(\rho)$ and the perturbations are adiabatic. The
EoS is assumed to admit a future attractor that acts as an effective
cosmological constant, while asymptotically in the past the pressure is
negligible. UDM models of the dark sector are appealing because they evade the
so-called "coincidence problem" and "predict" what can be interpreted as
$w_{\rm DE} \approx -1$, but in general suffer the effects of a non-negligible
Jeans scale that wreak havoc in the evolution of perturbations, causing a large
Integrated Sachs-Wolfe effect and/or changing structure formation at small
scales. Typically, observational constraints are violated, unless the
parameters of the UDM model are tuned to make it indistinguishable from
$\Lambda$CDM. Here we show how this problem can be avoided, studying in detail
the functional form of the Jeans scale in adiabatic UDM perturbations and
introducing a class of models with a fast transition between an early
Einstein-de Sitter CDM-like era and a later $\Lambda$CDM-like phase. If the
transition is fast enough, these models may exhibit satisfactory structure
formation and CMB fluctuations. To consider a concrete case, we introduce a toy
UDM model and show that it can predict CMB and matter power spectra that are in
agreement with observations for a wide range of parameter values.
