  In the context of a microscopic model of string-inspired foam, in which foamy
structures are provided by brany point-like defects (D-particles) in
space-time, we discuss flavour mixing as a result of flavour non-preserving
interactions of (low-energy) fermionic stringy matter excitations with the
defects. Such interactions involve splitting and capture of the matter string
state by the defect, and subsequent re-emission. Quantum fluctuations of the
D-particles induce a non-trivial space-time background; in some circumstances
this could be akin to a cosmological Friedman-Robertson Walker
expanding-Universe, with weak (but non-zero) particle production. Furthermore
the D-particle medium can induce an MSW type effect. We have argued previously,
in the context of bosons, that the so-called flavour vacuum is the appropriate
state to be used, at least for low-energy excitations, with energies/momenta up
to a dynamically determined cutoff scale. In this work we evaluate the
flavour-vacuum expectation value (condensate) of the stress-energy tensor of
the (1/2)-spin fields with mixing in an effective low-energy Quantum Field
Theory in this foam-induced curved space-time. We demonstrate, at late epochs
of the Universe, that the fermionic vacuum condensate behaves as a fluid with
negative pressure and positive energy, but alone it cannot lead to present-day
accelerating Universes. One needs flavoured boson contributions for this
purpose.
