  On top of its valence quarks, the full nucleon ground state may contain
appreciable admixture of s-\bar{s} pairs already at small momentum transfers.
This paper discusses strangeness in the mean-field type of nucleon models, and
exemplifies this by explicit calculations in the MIT bag model enriched by the
presence of instantons.
  We calculate the instanton contribution to the strangeness in the MIT bag (on
top of the standard contribution to strangeness found in that model). Although
we do it in an essentially perturbative way, we present a detailed derivation
of the formula expressing nucleon matrix elements of bilinear strange quark
operators, in terms of a model valence nucleon state and interactions producing
quark-antiquark fluctuations on top of that valence state. We do it in detail
to clarify our argument that in the context of the mean-field type of quark
models (where a Fock state expansion exists and where the nucleon state can be
constructed out of single-quark states), the resulting formula acquires a
significance beyond perturbation theory.The derivation combines the usage of
the evolution operator containing a strangeness source, and Feynman-Hellmann
theorem.
