Mechanisms of ice recrystallization

Ice crystals formed after scraped-surface freezing and hardening of ice cream are unstable and will undergo recrystallization, the extent of which depends in part on how effectively the system has been stabilized. See also the discussion regarding ice cream shelf-life, where I have included some images to show the effects of recrystallization on ice crystals in ice cream. Recrystallization is the process of changes in number, size and shape of ice crystals during frozen storage, although the amount of ice stays constant with constant temperature throughout this process (dictated by the equilibrium freezing curve). Recrystallization basically involves small crystals disappearing, large crystals growing and crystals fusing together.

There are several types of recrystallization processes. Iso-mass recrystallization ("rounding off") refers to changes in surface or internal structure so that crystals with irregular shapes and large surface-to-volume ratios assume a more compact structure. In other words, sharper surfaces are less stable than flatter ones and will show a tendency to become smoother over time. Migratory recrystallization refers in general to the tendency of larger crystals to grow at the expense of smaller crystals. Ostwald ripening refers to migratory recrystallization that occurs at constant temperature and pressure due to differences in surface energy between crystals, most likely involving melting-diffusion-refreezing or sublimation-diffusion-condensation mechanisms. However, migratory recrystallization is greatly enhanced by temperature fluctuations (heat shock) inducing a melt-refreeze behavior due to ice content fluctuations. 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. 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" will occur because a high curvature surface like this has a natural tendency to become planar.