Continous freezers in icecream
The continuous freezer is the defining piece of equipment in an ice cream line. It freezes and churns the mix in a single continuous pass, and it is this combination of rapid heat extraction and mechanical agitation that produces the smooth, creamy texture a finished product is judged on. In practice the freezer sets ice crystal size, air incorporation and discharge consistency — three variables that downstream packaging and hardening cannot correct.
Operation follows six steps. The ice cream mix, containing dairy, sweeteners, flavourings and other additives, is fed into the freezer. A refrigerant such as ammonia or freon is circulated around the freezing chamber, extracting heat and rapidly chilling the product. As the mix travels through the cylinder it is agitated and mixed by a dasher — a stainless steel shaft carrying a set of blades rotating at 150-200 rpm inside the freezing cylinder — which distributes the cold evenly and promotes the formation of small ice crystals. The freezer holds a controlled freezing temperature typically around -20 to -30 degrees Celsius, and partial freezing of the mix takes about 30 sec, reducing ice crystal size to 45-55 µm with air voids in the range of 100-150 µm diameter.
The freezer also controls overrun, the amount of air incorporated into the product. A higher overrun creates a lighter, more aerated product; a lower overrun gives a denser, creamier texture. Once the desired consistency is reached, the product is discharged and directed to packaging equipment or to further processing, depending on the application.
For commercial production the advantages are high production capacity, consistent product quality from continuous operation, time efficiency because less viscosity is needed and incorporation of air is less, flexibility in recipe and flavour variation, precise air incorporation through overrun control, and an efficient, automated process. There is also less tendency towards sandiness, because rapid freezing favours small lactose crystals.