  Neutrino processes in dense matter play a key role in the dynamics,
deleptonization and early cooling of hot protoneutron stars formed in the
gravitational collapse of massive stars. Here we calculate neutrino mean free
paths from neutrino-hyperon interactions in dense matter containing hyperons.
Significant contributions to the neutrino opacity arise from scattering and
absorption reactions involving the sigma minus particle. The estimates given
here emphasise the need for (a) opacities which incoparate many-body effects in
a multi-component mixture, and (b) new calculations of thermal and leptonic
evolution of protoneutron stars with neutrino transport and equations of state
with strangeness-rich matter.
