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The effect on flavour oscillations of simple expanding backgroundspace-times, motivated by some D-particle foam models, is calculated for atoy-model of bosons with flavour degrees of freedom. The presence of D-particledefects in the space-time, which can interact non trivially (via particlecapture) with flavoured particles in a flavour non-preserving way, generatesmixing in the effective field theory of low-energy string excitations.Moreover, the recoil of the D-particle defect during the capture/scatteringprocess implies Lorentz violation, which however may be averaged to zero inisotropic D-particle populations, but implies non-trivial effects incorrelators. Both features imply that the flavoured mixed state sees anon-trivial flavour (Fock-space) vacuum of a type introduced earlier by Blasoneand Vitiello in a generic context of theories with mixing. We discuss theorthogonality of the flavour vacua to the usual Fock vacua and the effect onflavour oscillations in these backgrounds. Furthermore we analyse the equationof state of the Flavour vacuum, and find that, for slow expansion rates inducedby D particle recoil, it is equivalent to that of a cosmological constant. Someestimates of these novel non-perturbative contribution to the vacuum energy aremade. The contribution vanishes if the mass difference and the mixing angle ofthe flavoured states vanish.

Towards a microscopic construction of flavour vacua from a space-time foam model