A slow freezing process of tissues can allow sufficient time for water migration due to osmotic force from the inner region of a cell to the freeze-concentrated intercellular region. This can result in cell desiccation, cell wall disruption, loss of turgor and crushing of the dried cell by the large intercellular ice mass. These phenomena affect not only the texture of the frozen product but also a significant drip loss during thawing and cooking can occur, leading to a loss of nutrients.
During frozen storage, the existence of temperature gradients within a product creates water vapor pressure profiles resulting in moisture migration and relocation of the water, both within and from the product. There is an overall tendency for moisture to move into the void spaces around the foodstuff and to accumulate on the product surface and on the internal package surface. In packaged frozen food, moisture migration leads to ice formation inside the package. Temperature fluctuations (cooling-warming cycles) produce a net migration of moisture from the interior towards the surface of the foodstuff, or to the wrapping. The temperature of the packaging material follows the temperature fluctuations in the storage room faster than the product itself. As the surrounding temperature decreases, moisture inside the pores sublimes and diffuses to the packaging film; when ambient temperature increases, the ice on the wrapping tends to diffuse back to the surface of the food; however, reabsorption of water in the original location is impossible, and the process can be considered irreversible, producing undesirable weight losses.
Moisture migration can be minimized by maintaining small temperature fluctuations and small internal temperature gradients and by the inclusion of internal barriers within the product and within the packaging.
Weight losses during freezing and frozen storage have economic consequences unless the product is packaged in films of low water vapor permeability. Typical weight losses during meat processing are 1-2% during chilling, 1% during freezing, and about 0.5-1% per month during storage and transport unless the product is packaged in an impervious film; the rate of sublimation doubles for every 10 ℃ rises in temperature.
Freezer burn is a surface desiccation defect caused by sublimation that can occur when frozen tissues are stored without an adequate moisture barrier packaging. It manifests itself as an opaque dehydrated surface, produced by moisture losses in frozen foods. An excessive desiccation can accelerate oxidative alterations at the surface of the product. Freezer burn increases oxygen contact with the food surface area and raises oxidative reactions, which irreversibly alter color, texture, and flavor. It is caused by the sublimation of ice on the surface region of the tissue where the water pressure of the ice is higher than the vapor pressure of the environment. In cold storage rooms, the temperature of the freezing coil (evaporator) is always lower than the surrounding air, therefore ice forms and accumulates on the coil. As moisture is removed, the relative humidity of the air in the cold room drops. Since the water vapor pressure over the surface of the frozen product is higher than that of the air, a constant loss of water in the form of vapor is produced from unprotected materials; sublimation continues as long as this vapor pressure difference continues.
Glazing, dipping, or spraying a thin layer of ice on the surface of an unwrapped frozen product helps to prevent drying. Freezer burn is prevented if a product is packed in tight-fitting, waterproof, vapor-proof material, avoiding evaporation. Surface coating of prepared meals reduces the effect of this quality loss and may even add value to the product.