Ice cream is considered a colloidal system. It is composed by ice cream crystals and aggregates, air that does not mix with the ice cream by forming small bubbles in the bulk and partially coalesced fat globules. This dispersed phase made from all the small particles is surrounded by an unfrozen continuous phase composed by sugars, proteins, salts, polysaccharides and water. Their interactions determine the properties of ice cream, whether soft and whippy or hard.
Ostwald ripening
Chocolate-glazed Magnum ice cream bar
Ostwald ripening is the explanation for the growth of large crystals at the expense of small ones in the dispersion phase. This process is also called migratory recrystallization. It involves the formation of sharp crystals. Theories about Ostwald recrystallization admit that after a period of time, the recrystallization process can be described by the following equation:
r = r(0) + Rt exp (1/n)
Where r (0) is the initial size, n the order of recrystallization, and t a time constant for recrystallization that depends on the rate R (in units of size/time).
To make ice cream smooth, recrystallization must occur as slowly as possible, because small crystals create smoothness, meaning that r must decrease.
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