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The scraped-surface freezer is at the core of ice cream production. Understanding and controlling this operation and the storage conditions of the final product are critical to creating the smooth and creamy treat consumers expect.
Ice cream is a multiphase soft solid composed of ice crystals, casein micelles, air cells, and emulsion droplets (both individual and clustered), all embodied within an unfrozen serum phase that contains dissolved components. These disparate parts interact as ice cream melts in the consumer's mouth, which defines the overall quality or acceptability of the product. Controlling the formation of these different phases as the ice cream freezes requires an understanding of advanced engineering principles.
Although frozen desserts have existed for centuries, modern manufacturing processes employ advanced control to ensure efficient operation that creates products with desirable attributes. Building on the rudimentary freezers of yesteryears, the modern scraped-surface freezer (SSF) efficiently creates the structures necessary for frozen desserts. However, the inner workings of the freezer (i.e., a scrapedsurface heat exchanger) remain somewhat elusive. This is in part because the inner workings are not visible for analytical measurement, but also because ice changes extremely rapidly as temperature varies.
In particular, controlling the creation of the ice phase is critical to product quality and shelf life. Numerous ice crystals smaller than about 50 pm produce a desirable smooth and creamy texture. The freezer must create these small ice crystals while also controlling the other phases (i.e., air and fat). The small ice crystals created in the freezer, however, are unstable; their high surface energy leads to recrystallization. To maintain the small ice crystals on the shelf, storage conditions must minimize recrystallization mechanisms, such as Ostwald ripening, in which small crystals dissolve and redeposit onto larger crystals.
Recent advances in the production of frozen desserts include developments of alternative processes and advances in modeling of freezers. This article provides a brief overview of ice cream production and the operation of the SSF and details the engineering behind these alternative processes and models of existing freezers.
The traditional ice cream process
The process of making ice cream starts with creating a mix from the component ingredients. Cream, or some other dairy ingredient, and sugar are the main components that make up the bulk of...