Immersion chilling and freezing of a porous medium

An experimental apparatus, putting one face of a water-saturated glass bead bed in contact with a high-concentration (23% NaCl) aqueous freezant (the other face being insulated against heat and mass transfer), was constructed to simultaneous study heat and mass transfer at the product/solution interface and within the product more closely than is possible in real foods. The selected flow allowed uniform heat treatment (a constant heat transfer coefficient) over a large part of the product. The apparatus was used to monitor three elements within the product: the temperature profiles, the NaCl concentration profiles for the liquid phase and the freezing and thawing fronts. The first results showed that a surface layer of a highly impregnated non-frozen product was present at the end of freezing. If the frozen product was left in contact with the refrigerating solution, progressive thawing occurred from the surface inwards; thawed layer thickness seemed to be a linear function of the square root of time.