  The thermodynamic potential and thermal dependence of low lying mass spectra
of scalars and pseudoscalars are evaluated in a generalized Nambu --
Jona-Lasinio model, which incorporates eight-quark interactions. These are
necessary to stabilize the scalar effective potential for the light and strange
quark flavors, which would be otherwise unbounded from below. In addition it
turns out that they are also crucial to i) lower the temperature of the chiral
transition, in conformity with lattice calculations, ii) sharpen the
temperature interval in which the crossover occurs, iii) or even allow for
first order transitions to occur with realistic quark mass values, from certain
critical values of the parameters. These are unprecedented results which cannot
be obtained within the NJL approaches restricted to quartic and six-quark
interactions.
