Ice cream is the most popular frozen dairy product. It is defined as sweetened product composing of milk fat and milk nonsolid fat, and it is frozen by agitation to be aerated. Composition of ice cream should be compatible with local legal requirements. Freezing of ice cream divides into two basic categories: high shear application for extensive ice crystallization and air incorporation, which is satisfied by rapid freezing. Rapid freezing should promote small-sized ice crystal due to consumer quality acceptance.
Ice industry is dominated by continuous freezers, which have scraped surface heat exchangers. Rotating knife blades remove the ice layers and incorporate them into the mixture while satisfying aeration. Mixing, cooling, and aeration processes take place simultaneously. The mechanisms that lead to ice formation in an ice cream freezer are quite complex. Ultimately, the product exiting the freezer contains numerous small ice crystals. The dissolved sugar lactose, salts, and other components make initial freezing temperature about -2.5°C. The ice crystals in ice cream at the exit of the freezer are somewhat block shaped and vary in size from a few microns to over 100 mm. Very small ice crystals gives ice cream its smooth, cool character. When crystals become larger, the ice cream may be considered coarse. The dynamic whipping and freezing allows fat network formation in the product. It was noted that ice cream is composed of emulsion and foam. Recrystallization is the other concern about frozen products. It happens after melting, growth, and ripening that occur after the initial ice crystal phase has been developed. Although recrystallization occurs with no change in ice phase volume, it leads to changes in the distribution of ice crystals within the system, based on the thermodynamic difference in melting points between large crystals and small ones.
Olson et al. (2003) studied properties of frozen dairy desserts processed by microfluidization of their mixes. Their study investigated sensory properties and meltdown rates of ice creams having different composition processed either by homogenization or microfluidization. They found that microfluidization produced nonfat and low-fat ice creams usually had a slower meltdown without affecting sensory properties.