  Fluctuations of conserved charges allow to study the chemical composition of
hadronic matter. A comparison between lattice simulations and the Hadron
Resonance Gas (HRG) model suggested the existence of missing strange
resonances. To clarify this issue we calculate the partial pressures of mesons
and baryons with different strangeness quantum numbers using lattice
simulations in the confined phase of QCD. In order to make this calculation
feasible, we perform simulations at imaginary strangeness chemical potentials.
We systematically study the effect of different hadronic spectra on
thermodynamic observables in the HRG model and compare to lattice QCD results.
We show that, for each hadronic sector, the well established states are not
enough in order to have agreement with the lattice results. Additional states,
either listed in the Particle Data Group booklet (PDG) but not well
established, or predicted by the Quark Model (QM), are necessary in order to
reproduce the lattice data. For mesons, it appears that the PDG and the quark
model do not list enough strange mesons, or that, in this sector, interactions
beyond those included in the HRG model are needed to reproduce the lattice QCD
results.
