The aim of this work was to verify whether the existence of an impregnated, non-frozen surface layer of a product was theoretically possible during immersion freezing. A simple model of heat and mass transfer in a porous medium was developed for this purpose. The equation system was solved analytically for the time interval before the product starts to freeze. Two dimensionless groups, representative of the interaction between heat and mass transport during the initial moments of treatment, were identified. These numbers estimated for real foods are close in value to those of a glass bead bed which for this point-of-view, can get the status of model food. Simulations carried out on the glass bead bed also clearly demonstrated that there is a surface layer which does not freeze. The existence of such a layer in foodstuffs treated by immersion freezing would explain the persistence of high impregnation observed even after the formation of a freezing front.