  At very large densities and/or temperatures a quark-hadron phase transition
is expected to take place. Simulations of QCD on lattice at zero baryon density
indicate that the transition occurs at $T_c \sim 150-170$ MeV. The calculations
indicate that transition is likely to be second order or a cross over
phenomenon. Although the lattice simulations have not given any indication on
when the transition occurs at nonzero baryon density, the transition is
expected to occur around the densities of few times nuclear matter density.
Also, there is a strong reason to believe that the quark matter formed after
the phase transition is in colour superconducting phase. The matter densities
in the interior of neutron stars are expected to be several times the nuclear
matter density and therefore the neutron star cores may possibly consist of
quark matter. One then expects that this quark matter is formed during the
collapse of supernova. Starting with the assumption that the quark matter, when
formed consists of predominantly u and d quarks, we consider the evolution of
strange quarks by weak interactions in the present work. The reaction rates and
time required to reach the chemical equilibrium are computed here. Our
calculations show that the chemical equilibrium is reached in about $10^{-7}$
seconds. Further more during and immediately after the equilibration process
enormous amount of energy is released and copious numbers of neutrinos are
produced. We show that for reasonable models of nuclear equations of state the
amount of energy released could be as high as $10^{53}$ ergs and as many as
$10^{58}$ neutrinos may be emitted during the quark matter formation.
