On top of its valence quarks, the full nucleon ground state may contain appreciable admixture of ss¯ 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 singlequark 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.Pored valentnih kvarkova, cjelovito osnovno stanje nukleona može sadržavati znatnu primjesu parova ss¯ već pri malim prijenosima impulsa. Ovdje raspravljamo stranost u modelu nukleona sa srednjim poljem i provodimo eksplicitne račune u MIT modelu vreće uz prisutnost instantona. Računamo doprinos instantona stranosti u MIT vreći (povrh standardnog doprinosa stranosti u tom modelu). Iako se u biti primjenjuje račun smetnje, izlažemo podroban izvod izraza za nuklearne matrične elemente bilinearnih operatora stranih kvarkova, preko modelnog stanja valentnog nukleona i medudjelovanja koja tvore fluktuacije kvark-antikvark povrh onih od valentnog stanja. To se radi podrobno kako bismo objasnili da konačan izraz, dobiven u okviru kvarkovskih modela sa srednjim poljem (kada postoji Fockov razvoj i nukleonsko se stanje može složiti od jednokvarkovskih stanja), prelazi okvire računa smetnje. Primjenjuje se evolucijski operator koji sadrži izvore stranosti i FeynmanHellmannov teorem
