  QCD thermodynamics is investigated by means of the Polyakov-loop-extended
Nambu Jona-Lasinio (PNJL) model, in which quarks couple simultaneously to the
chiral condensate and to a background temporal gauge field representing
Polyakov loop dynamics. The behaviour of the Polyakov loop as a function of
temperature is obtained by minimizing the thermodynamic potential of the
system. A Taylor series expansion of the pressure is performed. Pressure
difference and quark number density are then evaluated up to sixth order in
quark chemical potential, and compared to the corresponding lattice data. The
validity of the Taylor expansion is discussed within our model, through a
comparison between the full results and the truncated ones.
