Thermodynamic Principles of Ice Formation

The freezing temperature of a food is that temperature at which the first stable ice crystals form. The formation of an ice crystal first requires a nucleation.

 

This nucleation can be homogeneous or heterogeneous. The latter is more frequent in the case of food, where the nuclei are formed on suspended particles or on the cell wall.

 

The crystallization that originates during the freezing of food is the formation of a solid systematically organized from the solution. The process of crystallizing includes the phases of nucleation and the growth of crystals.

 

The ice crystallization occurs when the system is sufficiently subcooled. The subcooling is the difference in temperatures below the initial point of the freezing of the system. Nucleation is the combination of molecules in a particular order of sufficient size to survive serving as the place for crystal growth.

 

The nucleus of ice that is formed constitutes an embryo with the radius "r" where the Gibbs free energy is due to the surface contribution, as opposed to its crystal formation, and to its volumetric contribution, which is favorable to that formation. This is referred to in the following expression:

 

G = 4pr2g – (4pr3 Gv / 3Vm)

 

Where g is the surface free energy, Gv is the molar free energy change associated with the fluid-solid phase and Vm is the molar volume.

 

There will be a critical radius corresponding to the smallest embryo, whose free energy decreases as it grows, therefore becoming the minimum size of the stable core. The nucleation rate is highly dependent on the supercooling, which acts as the driving force for this process.

 

When the nuclei have formed, their growth occurs by addition of molecules in the solid-fluid. The rate of crystallization is controlled by the processes of heat and mass transfer. Water molecules move from the liquid phase to a stable place on the crystal surface. In crystallization, the removal of heat due to phase change mechanism is the determinant of all the growth of crystals.

 

The duration of supercooling depends on the characteristics of the food and the rate at which heat is removed. If subcooling is rapid, there will be a large number of nuclei that form small crystals. When less rapid, fewer nuclei will form and thus larger crystals will be formed.

 

For most of the freezing plateau (in section BC of the previous figure) the formation of ice crystals is controlled by heat transfer. The rate of mass transport controls the rate of crystal growth at the end of freeze period where solutions remaining are more concentrated.

 

As the temperature drops the different solutes become saturated, crystallizing the product.

 

The temperature at which the crystal of a solute in equilibrium with the liquid does not freeze and create ice crystals is called the eutectic temperature. Because food is a complex mixture of substances, the term eutectic temperature is used to describe that temperature which corresponds to the lowest eutectic temperature of the mixture of solutes of the food item. The maximum ice crystal formation is obtained at this temperature.