Slow freezing results in a low rate of nucleation and the production of a small number of large ice crystals, whereas fast freezing causes a high rate of nucleation leading to the formation of a large number of small ice crystals. However, during frozen storage ice crystals undergo metamorphic changes. Recrystallization reduces the advantages of fast freezing and includes any change in the number, size, shape, orientation, or perfection of crystals following completion of initial solidification. In frozen aqueous solutions, recrystallization is the process by which the average ice crystal size increases with time. Small ice crystals are thermodynamically unstable, having a high surface/volume ratio and therefore a large excess of surface free energy. The net result of minimizing free energy is that the number of crystals decreases at constant ice phase volume but their mean size increases. Recrystallization basically involves small crystals disappearing, large crystals growing and crystals fusing together and affects the quality of the product because small ice crystals make the product quality better, large crystals often cause damage during freezing.
There are different types of recrystallization processes: iso-mass, migratory, accretive, pressure-induced, and irruptive.
SURFACE ISO-MASS RECRYSTALLISATION
This includes changes in the shape or internal structure of a crystal and reduction of the defects as the crystal tends to a lower energy level maintaining a constant mass of ice. This “rounding off” process may be produced by surface diffusion of the water molecules. Ice crystals of irregular shape and large surface-to-volume ratio adopt a more compact configuration with a smaller surface-to-volume ratio and a lower surface energy. Sharper surfaces are less stable than flatter ones and show a tendency to become smoother over time.
MIGRATORY RECRYSTALLISATION OR GRAIN GROWTH
This refers to the tendency of large crystal in a polycrystal system to grow at the expense of the smaller ones. Ostwald ripening refers to migratory recrystallization that occurs at constant temperature and pressure due to differences in surface energy between crystals, which is proportional to the crystal curvature. Melting-diffusion-refreezing or sublimation-diffusion-condensation are possible mechanisms leading to an increase in average crystal size, a decrease in the number of crystals, and a decrease in surface energy of the entire crystalline phase.
At constant temperature and pressure, migratory recrystallization is the result of differences in the surface energies of large and small crystals. The small crystals, with very small radii of curvature, cannot bind the surface molecules as firmly as larger crystals, thus, small crystals exhibit lower melting points than large ones. Migratory recrystallization is enhanced by temperature fluctuations inducing a melt-refreeze behavior can lead to complete disappearance of smaller crystals during warming and growth of larger crystals during cooling, or to a decrease in size of crystals during partial melting and regrowth of existing crystals during cooling. Melt-refreeze should occur to a greater extent at higher temperatures and more rapidly for smaller crystals.
This is produced when contacting crystals join together increasing crustal size and decreasing the number of crystals and surface energy of the crystalline phase. The proposed mechanism of crystal aggregation is surface diffusion. Accretion refers to a natural tendency of crystals in close proximity to fuse together, the concentration gradients in the areas between them are high, thus, material is transported to the point of contact between crystals and a neck is formed. Further, “rounding off” occurs because a high-curvature surface like this has a natural tendency to become planar. The number of molecules leaving a curved surface is larger than the number of molecules arriving on that surface. The continues exchange of molecules at the interface serves to reduce the curvature of a single crystal (forming a sphere) or to reduce the number of small crystals by adding to the larger crystals.
If force is applied to a group of crystals, those crystals that have their basal planes aligned to the direction of force grow at the expense of those in other orientations. This type of recrystallization is found not very frequently in foods.
Under conditions of very fast freezing, aqueous specimens solidify in a partially non-crystalline state and not all the freezable water is converted to ice. Upon warming to some critical temperature, crystallization of ice occurs abruptly. This phenomenon is called ”irruptive recrystallization”; however “devitrification” is also used when the frozen specimen is totally non-crystalline after initial solidification.
Besides, it should be taken into account that the stable shelf life of the product is influenced by the ice crystal size. This is important in products that are consumed in the frozen state (e.g. ice cream). In these cases, a coarse or sandy texture is normally observed when large ice crystals are present.